Method for measuring formation properties with a time-limited formation test

Abstract

 An apparatus and method for determining at least one downhole formation property is disclosed. The apparatus includes a pretest piston (118a) positionable in fluid communication with the formation, and a series of flowlines pressure gauges (120a,123a,130a), and valves (121a,124a,128a) configured to selectively draw into the apparatus for measurement of one of formation fluid and mud. The method includes performing a first pretest to determine an estimated formation parameter; using the first pretest to design a second pretest and generate refined formation parameters whereby formation properties may be estimated.

Description

BACKGROUND OF THE INVENTION

Field of the Invention

[0001] The present invention relates generally to the field of oil and gas exploration. More particularly, the invention relates to methods for determining at least one property of a subsurface formation penetrated by a wellbore using a formation tester.

Background Art

[0002] Over the past several decades, highly sophisticated techniques have been developed for identifying and producing hydrocarbons, commonly referred to as oil and gas, from subsurface formations. These techniques facilitate the discovery, assessment, and production of hydrocarbons from subsurface formations.

[0003] When a subsurface formation containing an economically producible amount of hydrocarbons is believed to have been discovered, a borehole is typically drilled from the earth surface to the desired subsurface formation and tests are performed on the formation to determine whether the formation is likely to produce hydrocarbons of commercial value. Typically, tests performed on subsurface formations involve interrogating penetrated formations to determine whether hydrocarbons are actually present and to assess the amount of producible hydrocarbons therein. These preliminary tests are conducted using formation testing tools, often referred to as formation testers. Formation testers are typically lowered into a wellbore by a wireline cable, tubing, drill string, or the like, and may be used to determine various formation characteristics which assist in determining the quality, quantity, and conditions of the hydrocarbons or other fluids located therein. Other formation testers may form part of a drilling tool, such as a drill string, for the measurement of formation parameters during the drilling process.

[0004] Formation testers typically comprise slender tools adapted to be lowered into a borehole and positioned at a depth in the borehole adjacent to the subsurface formation for which data is desired. Once positioned in the borehole, these tools are placed in fluid communication with the formation to collect data from the formation. Typically, a probe, snorkel or other device is sealably engaged against the borehole wall to establish such fluid communication.

[0005] Formation testers are typically used to measure downhole parameters, such as wellbore pressures, formation pressures and formation mobilities, among others. They may also be used to collect samples from a formation so that the types of fluid contained in the formation and other fluid properties can be determined. The formation properties determined during a formation test are important factors in determining the commercial value of a well and the manner in which hydrocarbons may be recovered from the well.

[0006] The operation of formation testers may be more readily understood with reference to the structure of a conventional wireline formation tester shown in Figures 1 A and 1B. As shown in Figure 1A, the wireline tester 100 is lowered from an oil rig 2 into an open wellbore 3 filled with a fluid commonly referred to in the industry as "mud." The wellbore is lined with a mudcake 4 deposited onto the wall of the wellbore during drilling operations. The wellbore penetrates a formation 5.

[0007] The operation of a conventional modular wireline formation tester having multiple interconnected modules is described in more detail in U.S. Patent Nos. 4,860,581 and 4,936,139 issued to Zimmerman et al. Figure 2 depicts a graphical representation of a pressure trace over time measured by the formation tester during a conventional wireline formation testing operation used to determine parameters, such as formation pressure.

[0008] Referring now to Figures 1A and 1B, in a conventional wireline formation testing operation, a formation tester 100 is lowered into a wellbore 3 by a wireline cable 6. After lowering the formation tester 100 to the desired position in the wellbore, pressure in the flowline 119 in the formation tester may be equalized to the hydrostatic pressure of the fluid in the wellbore by opening an equalization valve (not shown). A pressure sensor or gauge 120 is used to measure the hydrostatic pressure of the fluid in the wellbore. The measured pressure at this point is graphically depicted along line 103 in Figure 2. The formation tester 100 may then be "set" by anchoring the tester in place with hydraulically actuated pistons; positioning the probe 112 against the sidewall of the wellbore to establish fluid communication with the formation, and closing the equalization valve to isolate the interior of the tool from the well fluids. The point at which a seal is made between the probe and the formation and fluid communication is established, referred to as the "tool set" point, is graphically depicted at 105 in Figure 2. Fluid from the formation 5 is then drawn into the formation tester 100 by retracting a piston 118 in a pretest chamber 114 to create a pressure drop in the flowline 119 below the formation pressure. This volume expansion cycle, referred to as a "drawdown" cycle, is graphically illustrated along line 107 in Figure 2.

[0009] When the piston 118 stops retracting (depicted at point 111 in Figure 2), fluid from the formation continues to enter the probe 112 until, given a sufficient time, the pressure in the flowline 119 is the same as the pressure in the formation 5, depicted at 115 in Figure 2. This cycle, referred to as a "build-up" cycle, is depicted along line 113 in Figure 2. As illustrated in Figure 2, the final build-up pressure at 115, frequently referred to as the "sandface" pressure, is usually assumed to be a good approximation to the formation pressure.

[0010] The shape of the curve and corresponding data generated by the pressure trace may be used to determine various formation characteristics. For example, pressures measured during drawdown (107 in Figure 2) and build-up (113 in Figure 2) may be used to determine formation mobility, that is the ratio of the formation permeability to the formation fluid viscosity. When the formation tester probe (112 Figure 1B) is disengaged from the wellbore wall, the pressure in flowline 119 increases rapidly as the pressure in the flowline equilibrates with the wellbore pressure, shown as line 117 in Figure 2. After the formation measurement cycle has been completed, the formation tester 100 may be disengaged and repositioned at a different depth and the formation test cycle repeated as desired.

[0011] During this type of test operation for a wireline-conveyed tool, pressure data collected downhole is typically communicated to the surface electronically via the wireline communication system. At the surface, an operator typically monitors the pressure in flowline 119 at a console and the wireline logging system records the pressure data in real time. Data recorded during the drawdown and buildup cycles of the test may be analyzed either at the well site computer in real time or later at a data processing center to determine crucial formation parameters, such as formation fluid pressure, the mud overbalance pressure, ie the difference between the wellbore pressure and the formation pressure, and the mobility of the formation.

[0012] Wireline formation testers allow high data rate communications for real-time monitoring and control of the test and tool through the use of wireline telemetry. This type of communication system enables field engineers to evaluate the quality of test measurements as they occur, and, if necessary, to take immediate actions to abort a test procedure and/or adjust the pretest parameters before attempting another measurement. For example, by observing the data as they are collected during the pretest drawdown, an engineer may have the option to change the initial pretest parameters, such as drawdown rate and drawdown volume, to better match them to the formation characteristics before attempting another test. Examples of prior art wireline formation testers and/or formation test methods are described, for example, in U.S. Patent Nos. 3,934,468 issued to Brieger; 4,860,581 and 4,936,139 issued to Zimmerman et al.; and 5,969,241 issued to Auzerais. These patents are assigned to the assignee of the present invention.

[0013] Formation testers may also be used during drilling operations. For example, one such downhole tool adapted for collecting data from a subsurface formation during drilling operations is disclosed in U.S. Patent No. 6,230,557 B1 issued to Ciglenec et al., which is assigned to the assignee of the present invention.

[0014] Various techniques have been developed for performing specialized formation testing operations, or pretests. For example, U.S. Patent Nos. 5,095,745 and 5,233,866 both issued to DesBrandes describe a method for determining formation parameters by analyzing the point at which the pressure deviates from a linear draw down.

[0015] Despite the advances made in developing methods for performing pretests, there remains a need to eliminate delays and errors in the pretest process, and to improve the accuracy of the parameters derived from such tests. Because formation testing operations are used throughout drilling operations, the duration of the test and the absence of real-time communication with the tools are major constraints that must be considered. The problems associated with real-time communication for these operations are largely due to the current limitations of the telemetry typically used during drilling operations, such as mud-pulse telemetry. Limitations, such as uplink and downlink telemetry data rates for most logging while drilling or measurement while drilling tools, result in slow exchanges of information between the downhole tool and the surface. For example, a simple process of sending a pretest pressure trace to the surface, followed by an engineer sending a command downhole to retract the probe based on the data transmitted may result in substantial delays which tend to adversely impact drilling operations.

[0016] Delays also increase the possibility of tools becoming stuck in the wellbore. To reduce the possibility of sticking, drilling operation specifications based on prevailing formation and drilling conditions are often established to dictate how long a drill string may be immobilized in a given borehole. Under these specifications, the drill string may only be allowed to be immobile for a limited period of time to deploy a probe and perform a pressure measurement. Due to the limitations of the current real-time communications link between some tools and the surface, it may be desirable that the tool be able to perform almost all operations in an automatic mode.

[0017] Therefore, a method is desired that enables a formation tester to be used to perform formation test measurements downhole within a specified time period and that may be easily implemented using wireline or drilling tools resulting in minimal intervention from the surface system.

SUMMARY OF THE INVENTION

[0018] A method for determining formation parameters using a downhole tool positioned in a wellbore adjacent a subterranean formation is provided. The method comprises the steps of establishing fluid communication with the formation; performing a first pretest to determine an initial estimate of the formation parameters; designing pretest criteria for performing a second pretest based on the initial estimate of the formation parameters; and performing a second pretest according to the designed criteria whereby a refined estimate of the formation parameters are determined.

[0019] Methods for determining formation properties using a formation tester are also provided. A method for determining at least one formation fluid property using a formation tester in a formation penetrated by a borehole includes collecting a first set of data points representing pressures in a pretest chamber of the formation tester as a function of time during a first pretest; determining an estimated formation pressure and an estimated formation fluid mobility from the first set of data points; determining a set of parameters for a second pretest, the set of parameters being determined based on the estimated formation pressure, the estimated formation fluid mobility, and a time remaining for performing the second pretest; performing the second pretest using the set of parameters; collecting a second set of data points representing pressures in the pretest chamber as a function of time during the second pretest; and determining the at least one formation fluid property from the second set of data points.

[0020] Methods for determining a condition for terminating a drawdown operation during a pretest are also provided. A method for determining a termination condition for a drawdown operation using a formation tester in a formation penetrated by a borehole includes setting a probe of the formation tester against a wall of the borehole so that a pretest chamber is in fluid communication with the formation, a drilling fluid in the pretest chamber having a higher pressure than the formation pressure; decompressing the drilling fluid in the pretest chamber by withdrawing a pretest piston at a constant drawdown rate; collecting data points representing fluid pressures in the pretest chamber as a function of time; identifying a range of consecutive data points that fit a line of pressure versus time with a fixed slope, the fixed slope being based on a compressibility of the drilling fluid, the constant drawdown rate, and a volume of the pretest chamber; and terminating the drawdown operation based on a termination criterion after the range of the consecutive data points is identified.

[0021] Methods for determining formation fluid mobilities are provided. A method for estimating a formation fluid mobility includes performing a pretest using a formation tester disposed in a formation penetrated by a borehole, the pretest comprising a drawdown phase and a buildup phase; collecting data points representing pressures in a pretest chamber of the formation tester as a function of time during the drawdown phase and the buildup phase; determining an estimated formation pressure from the data points; determining an area bounded by a line passing through the estimated formation pressure and curves interpolating the data points during the drawdown phase and the buildup phase; and estimating the formation fluid mobility from the area, a volume extracted from the formation during the pretest, a radius of the formation testing probe, and a shape factor that accounts for the effect of the borehole on a response of the formation testing probe.

[0022] Methods for estimating formation pressures from drawdown operations during pretests are provided. A method for determining an estimated formation pressure from a drawdown operation using a formation tester in a formation penetrated by a borehole includes setting the formation tester against a wall of the borehole so that a pretest chamber of the formation tester is in fluid communication with the formation, a drilling fluid in the pretest chamber having a higher pressure than the formation pressure; decompressing the drilling fluid in the pretest chamber by withdrawing a pretest piston in the formation tester at a constant drawdown rate; collecting data points representing fluid pressures in the pretest chamber as a function of time; identifying a range of consecutive data points that fit a line of pressure versus time with a fixed slope, the fixed slope being based on a compressibility of the drilling fluid, the constant drawdown rate, and a volume of the pretest chamber; and determining the estimated formation pressure from a first data point after the range of the consecutive data points.

[0023] In another aspect, the invention relates to a method for determining downhole parameters using a downhole tool positioned in a wellbore adjacent a subterranean formation. The method includes establishing fluid communication between a pretest chamber in the downhole tool and the formation via a flowline (the flowline has an initial pressure therein), moving a pretest piston positioned in the pretest chamber in a controlled manner to reduce the initial pressure to a drawdown pressure, terminating movement of the piston to permit the drawdown pressure to adjust to a stabilized pressure and repeating the steps until a difference between the stabilized pressure and the initial pressure is substantially smaller than a predetermined pressure drop. One or more downhole parameters may then be determined from an analysis of one or more of the pressures. An initial estimate of the formation parameters from an analysis of one or more of the pressures and pretest criteria for performing a second pretest based on the initial estimate of the formation parameters may be determined, and a pretest of the formation according to the designed pretest criteria whereby a refined estimate of the formation parameters is determined may be performed.

[0024] In yet another aspect, the invention relates to a method for estimating a formation pressure using a formation tester disposed in a wellbore penetrating a formation. The method comprises measuring a first pressure in a flowline that is in fluid communication with the subterranean formation, moving a pretest piston in a controlled manner in a pretest chamber to create a predetermined pressure drop in the flowline, stopping the pretest piston after a selected movement of the pretest piston, allowing the pressure in the flowline to stabilize and repeating the steps until a difference between the stabilized pressure in the flowline and the first pressure in the flowline is substantially smaller than the predetermined pressure drop. The formation pressure may then be determined based on a final stabilized pressure in the flowline.

[0025] Finally, in another aspect, the invention relates to a method of determining mud compressibility using a downhole tool positioned in a wellbore adjacent a subterranean formation. The method includes capturing wellbore fluid in the formation tester (the wellbore fluid is in fluid communication with a pretest chamber having a movable piston therein), selectively moving the piston in the pretest chamber to alter the volume of captured fluid in the downhole tool, measuring the pressure of the captured fluid and estimating mud compressibility from the measured pressure.

[0026] Other aspects and advantages of the invention will be apparent from the following description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] 

Figure 1A shows a conventional wireline formation tester disposed in a wellbore.

Figure 1B shows a cross sectional view of the modular conventional wireline formation tester of Figure 1A.

Figure 2 shows a graphical representation of pressure measurements versus time plot for a typical prior art pretest sequence performed using a conventional formation tester.

Figure 3 shows a flow chart of steps involved in a pretest according to an embodiment of the invention.

Figure 4 shows a schematic of components of a module of a formation tester suitable for practicing embodiments of the invention.

Figure 5 shows a graphical representation of a pressure measurements versus time plot for performing the pretest of Figure 3.

Figure 6 shows a flow chart detailing the steps involved in performing the investigation phase of the flow chart of Figure 3.

Figure 7 shows a detailed view of the investigation phase portion of the plot of Figure 5 depicting the termination of drawdown.

Figure 8 shows a detailed view of the investigation phase portion of the plot of Figure 5 depicting the determination of termination of buildup.

Figure 9 shows a flow chart detailing the steps involved in performing the measurement phase of the flow chart of Figure 3.

Figure 10 shows a flow chart of steps involved in a pretest according to an embodiment of the invention incorporating a mud compressibility phase.

Figure 11 A shows a graphical representations of a pressure measurements versus time plot for performing the pretest of Figure 10. Figure 11B shows the corresponding rate of change of volume.

Figure 12 shows a flow chart detailing the steps involved in performing the mud compressibility phase of the flow chart of Figure 10.

Figure 13 shows a flow chart of steps involved in a pretest according to an embodiment of the invention incorporating a mud filtration phase.

Figure 14A shows a graphical representation of a pressure measurements versus time plot for performing the pretest of Figure 13. Figure 14B shows the corresponding rate of change of volume.

Figures 15 shows the modified mud compressibility phase of Figure 12 modified for use with the mud filtration phase.

Figures 16A-C show flow chart detailing the steps involved in performing the mud filtration phase of the flow chart of Figure 13. Figure 16A shows a mud filtration phase. Figure 16B shows a modified mud filtration phase with a repeat compression cycle. Figure 16C shows a modified mud filtration phase with a decompression cycle.

Figure 17A shows a graphical representation of a pressure measurements versus time plot for performing a pretest including a modified investigation phase in accordance with one embodiment of the invention. Figure 17B shows the corresponding rate of change of volume.

Figure 18 shows a flow chart detailing the steps involved in performing the modified investigation phase of Figure 17A.

Figure 19A shows a graphical representation of a pressure measurements versus time plot for performing a pretest including a modified investigation phase in accordance with one embodiment of the invention. Figure 19B shows the corresponding rate of change of volume.

Figure 20 shows a flow chart detailing the steps involved in performing the modified investigation phase of Figure 19A.

Figure 21 shows a fluid compressibility correction chart which may be used to provide corrected mud compressibility when the original mud compressibility is performed at a different temperature and/or pressure.

DETAILED DESCRIPTION

[0028] An embodiment of the present invention relating to a method 1 for estimating formation properties (e.g. formation pressures and mobilities) is shown in the block diagram of Figure 3. As shown in Figure 3, the method includes an investigation phase 13 and a measurement phase 14.

[0029] The method may be practiced with any formation tester known in the art, such as the tester described with respect to Figures 1A and 1B. Other formation testers may also be used and/or adapted for embodiments of the invention, such as the wireline formation tester of U.S. Patent No. 4,860,581 and 4,936,139 issued to Zimmerman et al. and the downhole drilling tool of U.S. Patent No. 6,230,557 B1 issued to Ciglenec et al. the entire contents of which are hereby incorporated by reference.

[0030] A version of a probe module usable with such formation testers is depicted in Figure 4. The module 101 includes a probe 112a, a packer 110a surrounding the probe, and a flow line 119a extending from the probe into the module. The flow line 119a extends from the probe 112a to probe isolation valve 121a, and has a pressure gauge 123a. A second flow line 103a extends from the probe isolation valve 121a to sample line isolation valve 124a and equalization valve 128a, and has pressure gauge 120a. A reversible pretest piston 118a in a pretest chamber 114a also extends from flow line 103a. Exit line 126a extends from equalization valve 128a and out to the wellbore and has a pressure gauge 130a. Sample flow line 125a extends from sample line isolation valve 124a and through the tool. Fluid sampled in flow line 125a may be captured, flushed, or used for other purposes.

[0031] Probe isolation valve 121 a isolates fluid in flow line 119a from fluid in flow line 103a. Sample line isolation valve 124a, isolates fluid in flow line 103a from fluid in sample line 125a. Equalizing valve 128a isolates fluid in the wellbore from fluid in the tool. By manipulating the valves to selectively isolate fluid in the flow lines, the pressure gauges 120a and 123a may be used to determine various pressures. For example, by closing valve 121a formation pressure may be read by gauge 123a when the probe is in fluid communication with the formation while minimizing the tool volume connected to the formation.

[0032] In another example, with equalizing valve 128a open mud may be withdrawn from the wellbore into the tool by means of pretest piston 118a. On closing equalizing valve 128a, probe isolation valve 121a and sample line isolation valve 124a fluid may be trapped within the tool between these valves and the pretest piston 118a. Pressure gauge 130a may be used to monitor the wellbore fluid pressure continuously throughout the operation of the tool and together with pressure gauges 120a and/or 123a may be used to measure directly the pressure drop across the mudcake and to monitor the transmission of wellbore disturbances across the mudcake for later use in correcting the measured sandface pressure for these disturbances.

[0033] Among the functions of pretest piston 118a is to withdraw fluid from or inject fluid into the formation or to compress or expand fluid trapped between probe isolation valve 121a, sample line isolation valve 124a and equalizing valve 128a. The pretest piston 118a preferably has the capability of being operated at low rates, for example 0.01 cm³/sec, and high rates, for example 10 cm³/sec, and has the capability of being able to withdraw large volumes in a single stroke, for example 100 cm³. In addition, if it is necessary to extract more than 100 cm³ from the formation without retracting the probe, the pretest piston 118a may be recycled. The position of the pretest piston 118a preferably can be continuously monitored and positively controlled and its position can be "locked" when it is at rest. In some embodiments, the probe 112a may further include a filter valve (not shown) and a filter piston (not shown).

[0034] Various manipulations of the valves, pretest piston and probe allow operation of the tool according to the described methods. One skilled in the art would appreciate that, while these specifications define a preferred probe module, other specifications may be used without departing from the scope of the invention. While Figure 4 depicts a probe type module, it will be appreciated that either a probe tool or a packer tool may be used, perhaps with some modifications. The following description assumes a probe tool is used. However, one skilled in the art would appreciate that similar procedures may be used with packer tools.

[0035] The techniques disclosed herein are also usable with other devices incorporating a flowline. The term "flowline" as used herein shall refer to a conduit, cavity or other passage for establishing fluid communication between the formation and the pretest piston and/or for allowing fluid flow there between. Other such devices may include, for example, a device in which the probe and the pretest piston are integral. An example of such a device is disclosed in U.S. Patent No. 6,230,557 B1 and U.S. Patent Application Serial No. 10/248,782, assigned to the assignee of the present invention.

[0036] As shown in Figure 5, the investigation phase 13 relates to obtaining initial estimates of formation parameters, such as formation pressure and formation mobility. These initial estimates may then be used to design the measurement phase 14. If desired and allowed, a measurement phase is then performed according to these parameters to generate a refined estimate of the formation parameters. Figure 5 depicts a corresponding pressure trace illustrating the changes in pressure over time as the method of Figure 3 is performed. It will be appreciated that, while the pressure trace of Figure 5 may be performed by the apparatus of Figure 4, it may also be performed by other downhole tools, such as the tester of Figures 1A and 1B.

[0037] The investigation phase 13 is shown in greater detail in Figure 6. The investigation phase comprises initiating the drawdown 310 after the tool is set for duration T_i at time t₃, performing the drawdown 320, terminating the drawdown 330, performing the buildup 340 and terminating the buildup 350. To start the investigation phase according to step 310, the probe 112a is placed in fluid communication with the formation and anchored into place and the interior of the tool is isolated from the wellbore. The drawdown 320 is performed by advancing the piston 118a in pretest chamber 114a. To terminate drawdown 330, the piston 118a is stopped. The pressure will begin to build up in flow line 119a until the buildup 340 is terminated at 350. The investigation phase lasts for a duration of time T_IP. The investigation phase may also be performed as previously described with respect to Figures 1B and 2, the drawdown flow rate and the drawdown termination point being pre-defined before the initiation of the investigation phase.

[0038] The pressure trace of the investigation phase 13 is shown in greater detail in Figure 7. Parameters, such as formation pressure and formation mobility, may be determined from an analysis of the data derived from the pressure trace of the investigation phase. For example, termination point 350 represents a provisional estimate of the formation pressure. Alternatively, formation pressures may be estimated more precisely by extrapolating the pressure trend obtained during build up 340 using techniques known by those of skill in the art, the extrapolated pressure corresponding to the pressure that would have been obtained had the buildup been allowed to continue indefinitely. Such procedures may require additional processing to arrive at formation pressure.

[0039] Formation mobility (K/µ)₁ may also be determined from the build up phase represented by line 340. Techniques known by those of skill in the art may be used to estimate the formation mobility from the rate of pressure change with time during build up 340. Such procedures may require additional processing to arrive at estimates of the formation mobility.

[0040] Alternatively, the work presented in a publication by Goode at al entitled "Multiple Probe Formation Testing and Vertical Reservoir Continuity", SPE 22738, prepared for presentation at the 1991 Society of Petroleum Engineers Annual Technical Conference and Exhibition, held at Dallas, Texas on October 6 through 9, 1991 implies that the area of the graph depicted by the shaded region and identified by reference numeral 325, denoted herein by A, may be used to predict formation mobility. This area is bounded by a line 321 extending horizontally from termination point 350 (representing the estimated formation pressure P₃₅₀ at termination), the drawdown line 320 and the build up line 340. This area may be determined and related to an estimate of the formation mobility through use of the following equation:

where (K/µ)₁ is the first estimate of the formation mobility (D/cP), where K is the formation permeability (Darcies, denoted by D) and µ is the formation fluid viscosity (cP) (since the quantity determined by formation testers is the ratio of the formation permeability to the formation fluid viscosity, ie the mobility, the explicit value of the viscosity is not needed); V₁ (cm³) is the volume extracted from the formation during the investigation pretest, V₁ = V(t₇ +T₁) - V(t₇-T₀) = V(t₇) - V(t₇ -T₀) where V is the volume of the pretest chamber; r_p is the probe radius (cm); and ε_K is an error term which is typically small (less than a few percent) for formations having a mobility greater than 1 mD/cP.

[0041] The variable Ω_S , which accounts for the effect of a finite-size wellbore on the pressure response of the probe, may be determined by the following equation described in a publication by F. J. Kuchuk entitled "Multiprobe Wireline Formation Tester Pressure Behavior in Crossflow-Layered Reservoirs", In Situ, (1996) 20, 1,1:

where r_p and r_w represent the radius of the probe and the radius of the well, respectively; ρ = r_p / r_w , η = K_r / K_z ; ϑ = 0.58 + 0.078log η+ 0.26log ρ+ 0. 8ρ²; and K_r and K_z represent the radial permeability and the vertical permeability, respectively.

[0042] In stating the result presented in equation 1 it has been assumed that the formation permeability is isotropic, that is K_r = K_z = K, that the flow regime during the test is "spherical", and that the conditions which ensure the validity of Darcy's relation hold.

[0043] Referring still to Figure 7, the drawdown step 320 of the investigation phase may be analyzed to determine the pressure drop over time to determine various characteristics of the pressure trace. A best fit line 32 derived from points along drawdown line 320 is depicted extending from initiation point 310. A deviation point 34 may be determined along curve 320 representing the point at which the curve 320 reaches a minimum deviation δ₀ from the best fit line 32. The deviation point 34 may be used as an estimate of the "onset of flow", the point at which fluid is delivered from the formation into the tool during the investigation phase drawdown.

[0044] The deviation point 34 may be determined by known techniques, such as the techniques disclosed in U.S. Patent Nos. 5,095,745 and 5,233,866 both issued to Desbrandes, the entire contents of which are hereby incorporated by reference. Debrandes teaches a technique for estimating the formation pressure from the point of deviation from a best fit line created using datapoints from the drawdown phase of the pretest. The deviation point may alternatively be determined by testing the most recently acquired point to see if it remains on the linear trend representing the flowline expansion as successive pressure data are acquired. If not, the drawdown may be terminated and the pressure allowed to stabilize. The deviation point may also be determined by taking the derivative of the pressure recorded during 320 with respect to time. When the derivative changes (presumably becomes less) by 2-5%, the corresponding point is taken to represent the beginning of flow from the formation. If necessary, to confirm that the deviation from the expansion line represents flow from the formation, further small-volume pretests may be performed.

[0045] Other techniques may be used to determine deviation point 34. For example, another technique for determining the deviation point 34 is based on mud compressibility and will be discussed further with respect to Figures 9-11.

[0046] Once the deviation point 34 is determined, the drawdown is continued beyond the point 34 until some prescribed termination criterion is met. Such criteria may be based on pressure, volume and/or time. Once the criterion has been met, the drawdown is terminated and termination point 330 is reached. It is desirable that the termination point 330 occur at a given pressure P₃₃₀ within a given pressure range ΔP relative to the deviation pressure P₃₄ corresponding to deviation point 34 of Figure 7. Alternatively, it may be desirable to terminate drawdown within a given period of time following the determination of the deviation point 34. For example, if deviation occurs at time t₄, termination may be preset to occur by time t₇, where the time expended between time t₄ and t₇ is designated as T_D and is limited to a maximum duration. Another criterion for terminating the pretest is to limit the volume withdrawn from the formation after the point of deviation 34 has been identified. This volume may be determined by the change in volume of the pretest chamber 114a (Figure 4). The maximum change in volume may be specified as a limiting parameter for the pretest.

[0047] One or more of the limiting criteria, pressure, time and/or volume, may be used alone or in combination to determine the termination point 330. If, for example, as in the case of highly permeable formations, a desired criterion, such as a predetermined pressure drop, cannot be met, the duration of the pretest may be further limited by one or more of the other criteria.

[0048] After deviation point 34 is reached, pressure continues to fall along line 320 until expansion terminates at point 330. At this point, the probe isolation valve 121a is closed and/or the pretest piston 118a is stopped and the investigation phase build up 340 commences. The build up of pressure in the flowline continues until termination of the buildup occurs at point 350.

[0049] The pressure at which the build up becomes sufficiently stable is often taken as an estimate of the formation pressure. The buildup pressure is monitored to provide data for estimating the formation pressure from the progressive stabilization of the buildup pressure. In particular, the information obtained may be used in designing a measurement phase transient such that a direct measurement of the formation pressure is achieved at the end of build up. The question of how long the investigation phase buildup should be allowed to continue to obtain an initial estimate of the formation pressure remains.

[0050] It is clear from the previous discussion that the buildup should not be terminated before pressure has recovered to the level at which deviation from the flowline decompression was identified, ie the pressure designated by P₃₄ on Figure 7. In one approach, a set time limit may be used for the duration of the buildup T₁. T₁ may be set at some number, such as 2 to 3 times the time of flow from the formation T₀. Other techniques and criteria may be envisioned.

[0051] As shown in Figures 5 and 7, termination point 350 depicts the end of the buildup, the end of the investigation phase and/or the beginning of the measurement phase. Certain criteria may be used to determine when termination 350 should occur. A possible approach to determination of termination 350 is to allow the measured pressure to stabilize. To establish a point at which a reasonably accurate estimate of formation pressure at termination point 350 may be made relatively quickly, a procedure for determining criteria for establishing when to terminate may be used.

[0052] As shown in Figure 8, one such procedure involves establishing a pressure increment beginning at the termination of drawdown point 330. For example, such a pressure increment could be a large multiple of the pressure gauge resolution, or a multiple of the pressure gauge noise. As buildup data are acquired successive pressure points will fall within one such interval. The highest pressure data point within each pressure increment is chosen and differences are constructed between the corresponding times to yield the time increments Δt_i(n). Buildup is continued until the ratio of two successive time increments is greater than or equal to a predetermined number, such as 2. The last recorded pressure point in the last interval at the time this criterion is met is the calculated termination point 350. This analysis may be mathematically represented by the following:

[0053] Starting at t₇, the beginning of the buildup of the investigation phase, find a sequence of indices {i(n)} ⊂{i}, i(n) > i(n -1), n = 2,3, ....., such that for n ≥ 2,i(1) =1, and

where n_P is a number with a value equal to or greater than, for example, 4, typically 10 or greater, δ_P is the nominal resolution of the pressure measuring instrument; and ε_P is a small multiple, say 2, of the pressure instrument noise - a quantity which may be determined prior to setting the tool, such as during the mud compressibility experiment.

[0054] One skilled in the art would appreciate that other values of n_P and ε_P may be selected, depending on the desired results, without departing from the scope of the invention. If no points exist in the interval defined by the right hand side of equation (3) other than the base point, the closest point outside the interval may be used.

[0055] Defining Δt_i(n) ≡ t_i(n) - t_i(n-1), the buildup might be terminated when the following conditions are met: p_i(n) ≥ p(t₄) = P₃₄ (Figure 7) and

where m_P is a number greater than or equal to, for example, 2.

[0056] The first estimate of the formation pressure is then defined as (Figure 7):

In rough terms, the investigation phase pretest according to the current criterion is terminated when the pressure during buildup is greater than the pressure corresponding to the point of deviation 34 and the rate of increase in pressure decreases by a factor of at least 2. An approximation to the formation pressure is taken as the highest pressure measured during buildup.

[0057] The equations (3) and (4) together set the accuracy by which the formation pressure is determined during the investigation phase: equation (3) defines a lower bound on the error and m_P roughly defines how close the estimated value is to the true formation pressure. The larger the value of m_P, the closer the estimated value of the formation pressure will be to the true value, and the longer the duration of the investigation phase will be.

[0058] Yet another criterion for terminating the investigation phase buildup may be based on the flatness of the buildup curve, such as would be determined by comparing the average value of a range of pressure buildup points to a small multiple, for example 2 or 4, of the pressure gauge noise. It will be appreciated that any of the criteria disclosed herein singly, or in combination, may be used to terminate the investigation phase buildup (ie. 340 on Figure 5), measurement phase buildup (ie. 380 on Figure 5 and described below) or, more generally, any buildup.

[0059] As shown in Figure 7, the termination point 350 depicts the end of the investigation phase 13 following completion of the build up phase 340. However, there may be instances where it is necessary or desirable to terminate the pretest. For example, problems in the process, such as when the probe is plugged, the test is dry or the formation mobility is so low that the test is essentially dry, the mud pressure exactly balances the formation pressure, a false breach is detected, very low permeability formations are tested, a change in the compressibility of the flowline fluid is detected or other issues occur, may justify termination of the pretest prior to completion of the entire cycle.

[0060] Once it is desired that the pretest be terminated during the investigation phase, the pretest piston may be halted or probe isolation valve 121 closed (if present) so that the volume in flow line 119 is reduced to a minimum. Once a problem has been detected, the investigation phase may be terminated. If desired, a new investigative phase may be performed.

[0061] Referring back to Figure 5, upon completion of the investigation phase 13, a decision may be made on whether the conditions permit or make desirable performance of the measurement phase 14. This decision may be performed manually. However, it is preferable that the decision be made automatically, and on the basis of set criteria.

[0062] One criterion that may be used is simply time. It may be necessary to determine whether there is sufficient time T_MP to perform the measurement phase. In Figure 5, there was sufficient time to perform both an investigation phase and a measurement phase. In other words, the total time T_t to perform both phases was less than the time allotted for the cycle. Typically, when T_IP is less than half the total time T_t, there is sufficient time to perform the measurement phase.

[0063] Another criterion that may be used to determine whether to proceed with the measurement phase is volume V. It may also be necessary or desirable, for example, to determine whether the volume of the measurement phase will be at least as great as the volume extracted from the formation during the investigation phase. If one or more of conditions are not met, the measurement phase may not be executed. Other criteria may also be determinative of whether a measurement phase should be performed. Alternatively, despite the failure to meet any criteria, the investigation phase may be continued through the remainder of the allotted time to the end so that it becomes, by default, both the investigation phase and the measurement phase.

[0064] It will be appreciated that while Figure 5 depicts a single investigation phase 13 in sequence with a single measurement phase 14, various numbers of investigation phases and measurement phases may be performed in accordance with the present invention. Under extreme circumstances, the investigation phase estimates may be the only estimates obtainable because the pressure increase during the investigation phase buildup may be so slow that the entire time allocated for the test is consumed by this investigation phase. This is typically the case for formations with very low permeabilities. In other situations, such as with moderately to highly permeable formations where the buildup to formation pressure will be relatively quick, it may be possible to perform multiple pretests without running up against the allocated time constraint.

[0065] Referring still to Figure 5, once the decision is made to perform the measurement phase 14, then the parameters of the investigation phase 13 are used to design the measurement phase. The parameters derived from the investigation phase, namely the formation pressure and mobility, are used in specifying the operating parameters of the measurement phase pretest. In particular, it is desirable to use the investigation phase parameters to solve for the volume of the measurement phase pretest and its duration and, consequently, the corresponding flow rate. Preferably, the measurement phase operating parameters are determined in such a way to optimize the volume used during the measurement phase pretest resulting in an estimate of the formation pressure within a given range. More particularly, it is desirable to extract just enough volume, preferably a larger volume than the volume extracted from the formation during the investigation phase, so that at the end of the measurement phase, the pressure recovers to within a desired range δ of the true formation pressure p_f. The volume extracted during the measurement phase is preferably selected so that the time constraints may also be met.

[0066] Let H represent the pressure response of the formation to a unit step in flow rate induced by a probe tool as previously described. The condition that the measured pressure be within δ of the true formation pressure at the end of the measurement phase can be expressed as:

where T'_t is the total time allocated for both the investigation and measurement phases minus the time taken for flowline expansion, ie T_t' = T_t- (t₇ - t_f) = T₀ + T₁ + T₂ +T₃ in Figure 5 (prescribed before the test is performed - seconds); is the approximate duration of formation flow during the investigation phase (determined during acquisition - seconds); T₁ is the duration of the buildup during the investigation phase (determined during acquisition - seconds); T₂ is the duration of the drawdown during the measurement phase (determined during acquisition - seconds); T₃ is the duration of the buildup during the measurement phase (determined during acquisition - seconds); q₁ and q₂ represent, respectively, the constant flowrates of the investigation and measurement phases respectively (specified before acquisition and determined during acquisition - cm³/sec); δ is the accuracy to which the formation pressure is to be determined during the measurement phase (prescribed - atmospheres ), ie, p _f - p(T_t) ≤ δ, where p_f is the true formation pressure; φ is the formation porosity, C_t is the formation total compressibility (prescribed before acquisition from knowledge of the formation type and porosity through standard correlations - 1/atmospheres);

where n = t, 0, 1, 2 denotes a dimensionless time and τ≡φµC_tr²_* / K_r represents a time constant ; and, r_* is an effective probe radius defined by

where k is a complete elliptic integral of the first kind with modulus m ≡

. If the formation is isotopic then r_*=2r_p/(πΩ_S).

[0067] Equivalently, the measurement phase may be restricted by specifying the ratio of the second to the first pretest flow rates and the duration, T₂, of the measurement phase pretest, and therefore its volume.

[0068] In order to completely specify the measurement phase, it may be desirable to further restrict the measurement phase based on an additional condition. One such condition may be based on specifying the ratio of the duration of the drawdown portion of the measurement phase relative to the total time available for completion of the entire measurement phase since the duration of the measurement phase is known after completion of the investigation phase, namely, T₂ + T₃ = T

- T_o - T₁. For example, one may wish to allow twice (or more than twice) as much time for the buildup of the measurement phase as for the drawdown, then T₃ = n_TT₂, or, T₂ = (T'_t - T_o - T₁) /(n_T+ 1) where n_T ≥ 2 . Equation (6) may then be solved for the ratio of the measurement to investigation phase pretest flowrates and consequently the volume of the measurement phase V₂ = q₂T₂.

[0069] Yet another condition to complete the specification of the measurement phase pretest parameters would be to limit the pressure drop during the measurement phase drawdown. With the same notation as used in equation (6) and the same governing assumptions this condition can be written as

where Δp_max (in atmospheres) is the maximum allowable drawdown pressure drop during the measurement phase.

[0070] The application of equations (6) and (7) to the determination of the measurement phase pretest parameters is best illustrated with a specific, simple but non-trivial case. For the purposes of illustration it is assumed that, as before, both the investigation and measurement phase pretests are conducted at precisely controlled rates. In addition it is assumed that the effects of tool storage on the pressure response may be neglected, that the flow regimes in both drawdown and buildup are spherical, that the formation permeability is isotropic and that the conditions ensuring the validity of Darcy's relation are satisfied.

[0071] Under the above assumptions equation (6) takes the following form:

where erfc is the complementary error function.

[0072] Because the arguments of the error function are generally small, there is typically little loss in accuracy in using the usual square root approximation. After some rearrangement of terms equation (8) can be shown to take the form

where λ ≡ T₂ + T₃, the duration of the measurement phase, is a known quantity once the investigation phase pretest has been completed.

[0073] The utility of this relation is clear once the expression in the parentheses on the left hand side is approximated further to obtain an expression for the desired volume of the measurement phase pretest.

[0074] With the same assumptions made in arriving at equation (8) from equation (6), equation (7) may be written as,

which, after applying the square-root approximation for the complementary error function and rearranging terms, can be expressed as:

Combining equations (9) and (12) gives rise to:

Because the terms in the last two bracket/parenthesis expressions are each very close to unity, equation (13) may be approximated as:

which gives an expression for the determination of the duration of the measurement phase drawdown and therefore, in combination with the above result for the measurement phase pretest volume, the value of the measurement phase pretest flowrate. To obtain realistic estimates for T₂ from equation (14), the following condition should hold:

Equation (15) expresses the condition that the target neighborhood of the final pressure should be greater than the residual transient left over from the investigation phase pretest.

[0075] In general, the estimates delivered by equations (10) and (14) for V₂ and T₂ may be used as starting values in a more comprehensive parameter estimation scheme utilizing equations (8) and (11). While equations (8) and (11) have been used to illustrate the steps in the procedure to compute the measurement phase parameters, it will be appreciated that other effects, such as tool storage, formation complexities, etc., may be readily incorporated in the estimation process. If the formation model is know, the more general formation model equations (6) and (7) may be used within the parameter estimation process.

[0076] The above described approach to determining the measurement phase pretest assumes that certain parameters will be assigned before the optimal pretest volume and duration can be estimated. These parameters include: the accuracy of the formation pressure measurement δ; the maximum drawdown permissible (Δp_max); the formation porosity φ - which will usually be available from openhole logs; and, the total compressibility C_t - which may be obtained from known correlations which in turn depend on lithology and porosity.

[0077] With the measurement phase pretest parameters determined, it should be possible to achieve improved estimates of the formation pressure and formation mobility within the time allocated for the entire test.

[0078] At point 350, the investigation phase ends and the measurement phase may begin. The parameters determined from the investigation phase are used to calculate the flow rate, the pretest duration and/or the volume necessary to determine the parameters for performing the measurement phase 14. The measurement phase 14 may now be performed using a refined set of parameters determined from the original formation parameters estimated in the investigation phase.

[0079] As shown in Figure 9, the measurement phase 14 includes the steps of performing a second draw down 360, terminating the draw down 370, performing a second build up 380 and terminating the build up 390. These steps are performed as previously described according to the investigation phase 13 of Figure 6. The parameters of the measurement phase, such as flow rate, time and/or volume, preferably have been predetermined according to the results of the investigation phase.

[0080] Referring back to Figure 5, the measurement phase 14 preferably begins at the termination of the investigation phase 350 and lasts for duration T_MP specified by the measurement phase until termination at point 390. Preferably, the total time to perform the investigation phase and the measurement phase falls within an allotted amount of time. Once the measurement phase is completed, the formation pressure may be estimated and the tool retracted for additional testing, downhole operations or removal from the wellbore.

[0081] Referring now to Figure 10, an alternate embodiment of the method 1 incorporating a mud compressibility phase 11 is depicted. In this embodiment the method 1b comprises a mud compressibility phase 11, an investigation phase 13 and a measurement phase 14. Estimations of mud compressibility may be used to refine the investigation phase procedure leading to better estimates of parameters from the investigation phase 13 and the measurement phase 14. Figure 11A depicts a pressure trace corresponding to the method of Figure 10, and Figure 11B shows a related graphical representation of the rate of change of the pretest chamber volume.

[0082] In this embodiment, the formation tester of Figure 4 may be used to perform the method of Figure 10. According to this embodiment, the isolation valves 121a and 124a may be used, in conjunction with equalizing valve 128a, to trap a volume of liquid in flowline 103a. In addition, the isolation valve 121a may be used to reduce tool storage volume effects so as to facilitate a rapid buildup. The equalizing valve 128a additionally allows for easy flushing of the flowline to expel unwanted fluids such as gas and to facilitate the refilling of the flowline sections 119a and 103a with wellbore fluid.

[0083] The mud compressibility measurement may be performed, for example, by first drawing a volume of mud into the tool from the wellbore through the equalizing valve 128a by means of the pretest piston 118a, isolating a volume of mud in the flowline by closing the equalizing valve 128a and the isolation valves 121 a and 124a, compressing and/or expanding the volume of the trapped mud by adjusting the volume of the pretest chamber 114a by means of the pretest piston 118a and simultaneously recording the pressure and volume of the trapped fluid by means of the pressure gauge 120a.

[0084] The volume of the pretest chamber may be measured very precisely, for example, by measuring the displacement of the pretest piston by means of a suitable linear potentiometer not shown in Figure 4 or by other well established techniques. Also not shown in Figure 4 is the means by which the speed of the pretest piston can be controlled precisely to give the desired control over the pretest piston rate q_p. The techniques for achieving these precise rates are well known in the art, for example, by use of pistons attached to lead screws of the correct form, gearboxes and computer controlled motors such rates as are required by the present method can be readily achieved.

[0085] Figures 11A and 12 depict the mud compressibility phase 11 in greater detail. The mud compressibility phase 11 is performed prior to setting the tool and therefore prior to conducting the investigation and measurement phases. In particular, the tool does not have to be set against the wellbore, nor does it have to be immobile in the wellbore in order to conduct the mud compressibility test thereby reducing the risk of sticking the tool due to an immobilized drill string. It would be preferable, however, to sample the wellbore fluid at a point close to the point of the test.

[0086] The steps used to perform the compressibility phase 11 are shown in greater detail in Figure 12. These steps also correspond to points along the pressure trace of Figure 11 A. As set forth in Figure 12, the steps of the mud compressibility test include starting the mud compressibility test 510, drawing mud from the wellbore into the tool 511, isolating the mud volume in the flow line 512, compressing the mud volume 520 and terminating the compression 530. Next, the expansion of mud volume is started 540, the mud volume expands 550 for a period of time until terminated 560. Open communication of the flowline to wellbore is begun 561, and pressure is equalized in the flowline to wellbore pressure 570 until terminated 575. The pretest piston recycling may now begin 580. Mud is expelled from the flowline into the wellbore 581 and the pretest piston is recycled 582. When it is desired to perform the investigation phase, the tool may then be set 610 and open communication of the flowline with the wellbore terminated 620.

[0087] Mud compressibility relates to the compressibility of the flowline fluid, which typically is whole drilling mud. Knowledge of the mud compressibility may be used to better determine the slope of the line 32 (as previously described with respect to Figure 7), which in turn leads to an improved determination of the point of deviation 34 signaling flow from the formation. Knowledge of the value of mud compressibility, therefore, results in a more efficient investigation phase 13 and provides an additional avenue to further refine the estimates derived from the investigation phase 13 and ultimately to improve those derived from the measurement phase 14.

[0088] Mud compressibility C_m may be determined by analyzing the pressure trace of Figure 11A and the pressure and volume data correspondingly generated. In particular, mud compressibility may be determined from, the following equation:

where C_m is the mud compressibility (1/psi), V is the total volume of the trapped mud (cm³), p is the measured flowline pressure (psi), ṗ is the time rate of change of the measured flowline pressure (psi/sec), and q_p represents the pretest piston rate (cm³/sec).

[0089] To obtain an accurate estimate of the mud compressibility, it is desirable that more than several data points be collected to define each leg of the pressure-volume trend during the mud compressibility measurement. In using equation (16) to determine the mud compressibility the usual assumptions have been made, in particular, the compressibility is constant and the incremental pretest volume used in the measurement is small compared to the total volume V of mud trapped in the flowline.

[0090] The utility of measuring the mud compressibility in obtaining a more precise deviation point 34a is now explained. The method begins by fitting the initial portion of the drawdown data of the investigation phase 13 to a line 32a of known slope to the data. The slope of line 32a is fixed by the previously determined mud compressibility, flowline volume, and the pretest piston drawdown rate. Because the drawdown is operated at a fixed and precisely controlled rate and the compressibility of the flowline fluid is a known constant that has been determined by the above-described experiment, the equation describing this line with a known slope a is given by:

where V (0) is the flowline volume at the beginning of the expansion, C_m is the mud compressibility, q_p is the piston decompression rate, p⁺ is the apparent pressure at the initiation of the expansion process. It is assumed that V(0) is very much larger than the increase in volume due to the expansion of the pretest chamber.

[0091] Because the slope a is now known the only parameter that needs to be specified to completely define equation (17) is the intercept p⁺, ie., b. In general, p⁺ is unknown, however, when data points belonging to the linear trend of the flowline expansion are fitted to lines with slope a they should all produce similar intercepts. Thus, the value of intercept p⁺ will emerge when the linear trend of the flowline expansion is identified.

[0092] A stretch of data points that fall on a line having the defined slope a, to within a given precision, is identified. This line represents the true mud expansion drawdown pressure trend. One skilled in the art would appreciate that in fitting the data points to a line, it is unnecessary that all points fall precisely on the line. Instead, it is sufficient that the data points fit to a line within a precision limit, which is selected based on the tool characteristics and operation parameters. With this approach, one can avoid the irregular trend associated with early data points, i.e., those points around the start of pretest piston drawdown. Finally, the first point 34a, after the points that define the straight line, that deviates significantly (or beyond a precision limit) from the line is the point where deviation from the drawdown pressure trend occurs. The deviation 34a typically occurs at a higher pressure than would be predicted by extrapolation of the line. This point indicates the breach of the mudcake.

[0093] Various procedures are available for identifying the data points belonging to the flowline expansion line. The details of any procedure depend, of course, on how one wishes to determine the flowline expansion line, how the maximal interval is chosen, and how one chooses the measures of precision, etc.

[0094] Two possible approaches are given below to illustrate the details. Before doing so, the following terms are defined:





where, in general, N(k) < k represents the number of data points selected from the k data points (t_k, p_k) acquired. Depending on the context, N(k) may equal k . Equations (18) and (19) represent, respectively, the least-squares line with fixed slope a and the line of least absolute deviation with fixed slope a through N(k) data points, and, equation (20) represents the variance of the data about the fixed slope line.

[0095] One technique for defining a line with slope a spanning the longest time interval is to fit the individual data points, as they are acquired, to lines of fixed slope a. This fitting produces a sequence of intercepts {b_k}, where the individual b_k are computed from: b_k = p_k + at_k. If successive values of b_k become progressively closer and ultimately fall within a narrow band, the data points corresponding to these indices are used to fit the final line.

[0096] Specifically, the technique may involve the steps of: (i) determining a median,

, from the given sequence of intercepts {b_k}; (ii) finding indices belonging to the set

where n_b is a number such as 2 or 3 and where a possible choice for ε_b is defined by the following equation:

where the last expression results from the assumption that time measurements are exact.

[0097] Other, less natural choices for ε_b are possible, for example, ε_b = S_p,k; (iii) fitting a line of fixed slope a to the data points with indices belonging to I_k; and (iv) finding the first point (t_k,p_k) that produces p_k - b

+ at_k > n_SS_p,k, where b

= b̂_k or

_k depending on the method used for fitting the line, and n_S is a number such as 2 or 3. This point, represented by 34a on Figure 11 A, is taken to indicate a breach of the mudcake and the initiation of flow from the formation.

[0098] An alternate approach is based on the idea that the sequence of variances of the data about the line of constant slope should eventually become more-or-less constant as the fitted line encounters the true flowline expansion data. Thus, a method according to the invention may be implemented as follows: (i) a line of fixed slope, a, is first fitted to the data accumulated up to the time t_k. For each set of data, a line is determined from p(t_k) =

_k - at_k, where

_k is computed from equation (18); (ii) the sequence of variances

is constructed using equation (20) with N(k)=k; (iii) successively indices are found belonging to the set:

(iv) a line of fixed slope a is fitted to the data with indices in J_k. Let N(k) be the number of indices in the set; (v) determine the point of departure from the last of the series of fixed-slope lines having indices in the above set as the first point that fulfills p_k-

_k +at_k > n_SS_p,k, where n_S is a number such as 2 or 3; (vi) define

(vii) find the subset of points of J_k such that

(viii) fit a line with slope a through the points with indices in N ; and (ix) define the breach of the mudcake as the first point (t_k, p_k) where p_k -

_k +at_k > n_SS_p,k. As in the previous option this point, represented again by 34a on Figure 11A, is taken to indicate a breach of the mudcake and the initiation of flow from the formation.

[0099] Once the best fit line 32a and the deviation point 34a are determined, the termination point 330a, the build up 370a and the termination of buildup 350a may be determined as discussed previously with respect to Figure 7. The measurement phase 14 may then be determined by the refined parameters generated in the investigation phase 13 of Figure 11 A.

[0100] Referring now to Figure 13, an alternate embodiment of the method 1c incorporating a mud filtration phase 12 is depicted. In this embodiment the method comprises a mud compressibility phase 11a, a mud filtration phase12, an investigation phase 13 and a measurement phase 14. The corresponding pressure trace is depicted in Figure 14A, and a corresponding graphical depiction of the rate of change of pretest volume is shown in Figure 14B. The same tool described with respect to the method of Figure 10 may also be used in connection with the method of Figure 13.

[0101] Figures 14A and 14B depict the mud filtration phase 12 in greater detail. The mud filtration phase 12 is performed after the tool is set and before the investigation phase 13 and the measurement phase 14 are performed. A modified mud compressibility phase 11a is performed prior to the mud filtration phase 12.

[0102] The modified compressibility test 11a is depicted in greater detail in Figure 15. The modified compressibility test 11a includes the same steps 510-580 of the compressibility test 11 of Figure 12. After step 580, steps 511 and 512 of the mud compressibility test are repeated, namely mud is drawn from the wellbore into the tool 511a and the flowline is isolated from the wellbore 512a. The tool may now be set 610 and at the termination of the set cycle the flowline may be isolated 620 in preparation for the mud filtration, investigative and measurement phases.

[0103] The mud filtration phase 12 is shown in greater detail in Figure 16A. The mud filtration phase is started at 710, the volume of mud in the flowline is compressed 711 until termination at point 720, and the flowline pressure falls 730. Following the initial compression, communication of the flowline within the wellbore is opened 751, pressures inside the tool and wellbore are equilibrated 752, and the flowline is isolated from the wellbore 753.

[0104] Optionally, as shown in Figure 16B, a modified mud filtration phase 12b may be performed. In the modified mud filtration phase 12b, a second compression is performed prior to opening communication of the flowline 751, including the steps of beginning recompression of mud in flowline 731, compressing volume of mud in flowline to higher pressure 740, terminating recompression 741. Flowline pressure is then permitted to fall 750. Steps 751-753 may then be performed as described with respect to Figure 16A. The pressure trace of Figure 14A shows the mud filtration phase 12b of Figure 16B.

[0105] In another option 12c, shown in Figure 16C, a decompression cycle may be performed following flowline pressure fall 730 of the first compression 711, including the steps of beginning the decompression of mud in the flowline 760, decompressing to a pressure suitably below the wellbore pressure 770, and terminating the decompression 780. Flowline pressure is then permitted to fall 750. Steps 751 - 753 may then be repeated as previously described with respect to Figure 16A. The pressure trace of Figure 14A shows the mud filtration phase 12c of Figure 16C.

[0106] As shown in the pressure trace of Figure 14A, the mud filtration method 12 of Figure 16A may be performed with either the mud filtration phase 12b of Figure 16B or the mud filtration phase 12c of 16C. Optionally, one or more of the techniques depicted in Figures 16A-C may be performed during the mud filtration phase.

[0107] Mud filtration relates to the filtration of the base fluid of the mud through a mudcake deposited on the wellbore wall and the determination of the volumetric rate of the filtration under the existing wellbore conditions. Assuming the mudcake properties remain unchanged during the test, the filtration rate through the mudcake is given by the simple expression:

where V_t is the total volume of the trapped mud (cm³), and q_f represents the mud filtration rate (cm³/sec); C_m represents the mud compressibility (1/psi) (where C_m is determined during the modified mud compressibility test 11a or input); p represents the rate of pressure decline (psi/sec) as measured during 730 and 750 in Figure 14. The volume V_t in equation (22) is a representation of the volume of the flowline contained between valves 121a, 124a and 128a as shown in Figure 4.

[0108] For mud cakes which are inefficient in sealing the wellbore wall the rate of mud infiltration can be a significant fraction of the pretest piston rate during flowline decompression of the investigation phase and if not taken into account can lead to error in the point detected as the point of initiation of flow from the formation, 34 Figure 7. The slope, a , of the fixed slope line used during the flowline decompression phase to detect the point of initiation of flow from the formation, ie the point of deviation, 34 Figure 7, under these circumstances is determined using the following equation:

where V (0) is the flowline volume at the beginning of the expansion, C_m is the mud compressibility, q_p is the piston decompression rate, q_f is the rate of filtration from the flow line through the mudcake into the formation, and p⁺ is the apparent pressure at the initiation of the expansion process which, as previously explained, is determined during the process of determining the deviation point 34.

[0109] Once the mudcake filtration rate q_f and the mud compressibility C_m have been determined, it is possible to proceed to estimate the formation pressure from the investigation phase 13 under circumstances where filtration through the mudcake is significant.

[0110] Preferably embodiments of the invention may be implemented in an automatic manner. In addition, they are applicable to both downhole drilling tools and to a wireline formation tester conveyed downhole by any type of work string, such as drill string, wireline cable, jointed tubing, or coiled tubing. Advantageously, methods of the invention permit downhole drilling tools to perform time-constrained formation testing in a most time efficient manner such that potential problems associated with a stopped drilling tool can be minimized or avoided.

[0111] Another embodiment of performing investigation phase measurements will be described with reference to Figures 17A, 17B, and 18. Prior to setting the formation tester 805, the mud compressibility is preferably determined as described above (not shown). Subsequent to the determination of the mud compressibility and prior to setting the formation tester, the pressure measured by the tool is the wellbore fluid, or mud hydrostatic, pressure 801. After the tool is set 805, the pretest piston 118a, as shown in Figure 4, is activated 810 to withdraw fluid at a precise and fixed rate to achieve a specified pressure drop 814 in a desired time t_pi 815. It is preferred that the desired pressure drop (Δp) be of the same order but less than the expected overbalance at that depth, if the overbalance is approximately known. Overbalance is the difference in pressure between the mud hydrostatic pressure and the formation pressure. Alternatively, the desired pressure drop (Δp) may be some number (e.g., 300 psi) that is larger than the maximum expected value of the "flow initiation pressure" (e.g., 200 psi). Whether the actual formation pressure is within this range is immaterial to the embodiments of the invention. Therefore, the following description assumes that the formation pressure is not within the range.

[0112] In accordance with embodiments of the invention, the piston drawdown rate to achieve this limited pressure drop (Δp) may be estimated from

where C_m is the compressibility of the flowline fluid, which is assumed to be the same as the wellbore fluid; V_t is the volume of the trapped fluid within the flowline 103a between the valves 121a, 124a and 128a shown in Figure 4; Δp is the desired pressure drop and t_pi is the duration of the pretest drawdown.

[0113] Referring to Figures 17A, 17B, and 18, a method of performing an investigation phase 13b in accordance with embodiments of the invention comprises the step of starting the drawdown 810 and performing a controlled drawdown 814. It is preferred that the piston drawndown rate be precisely controlled so that the pressure drop and the rate of pressure change be well controlled. However, it is not necessary to conduct the pretest (piston drawdown) at low rates. When the prescribed incremental pressure drop (Δp) has been reached, the pretest piston is stopped and the drawdown terminated 816. The pressure is then allowed to equilibrate 817 for a period t

, 818 which may be longer than the drawdown period t_pi 817, for example, t

= 2 t_pi. After the pressure has equilibrated, the stabilized pressure at point 820 is compared with the pressure at the start of the drawdown at point 810. At this point, a decision is made as to whether to repeat the cycle, shown as 819 in Figure 18. The criterion for the decision is whether the equalized pressure (e.g., at point 820) differs from the pressure at the start of the drawdown (e.g., at point 810) by an amount that is substantially consistent with the expected pressure drop (Δp). If so, then this flowline expansion cycle is repeated.

[0114] To repeat the flowline expansion cycle, for example, the pretest piston is re-activated and the drawdown cycle is repeated as described, namely, initiation of the pretest 820, drawdown 824 by exactly the same amount (Δp) at substantially the same rate and duration 826 as for the previous cycle, termination of the drawdown 825, and stabilization 830. Again, the pressures at 820 and 830 are compared to decide whether to repeat the cycle. As shown in Figure 17A, these pressures are significantly different and are substantially consistent with the expected pressure drop (Δp) arising from expansion of the fluid in the flowline. Therefore, the cycle is repeated, 830-834-835-840. The "flowline expansion" cycle is repeated until the difference in consecutive stabilized pressures is substantially smaller than the imposed/prescribed pressure drop (Δp), shown for example in Figure 17A as 840 and 850.

[0115] After the difference in consecutive stabilized pressures is substantially smaller than the imposed/prescribed pressure drop (Δp), the "flowline expansion" cycle may be repeated one more time, shown as 850-854-855-860 in Figure 17A. If the stabilized pressures at 850 and 860 are in substantial agreement, for example within a small multiple of the gauge repeatability, the larger of the two values is taken as the first estimate of the formation pressure. One of ordinary skill in the art would appreciate that the processes as shown in Figure 17A, 17B, and 18 are for illustration only. Embodiments of the invention are not limited by how many flowline expansion cycles are performed. Furthermore, after the difference in consecutive stabilized pressures is substantially smaller than the imposed/prescribed pressure drop (Δp), it is optional to repeat the cycle one or more times.

[0116] The point at which the transition from flowline fluid expansion to flow from the formation takes place is identified as 800 in Figure 17A. If the pressures at 850 and 860 agree at the end of the allotted stabilization time, it may be advantageous to allow the pressure 860 to continue to build and use the procedures described in previous sections (see the description for Figure 8) to terminate the build up in order to obtain a better first estimate of the formation pressure. The process by which the decision is made to either continue the investigation phase or to perform the measurement phase, 864-868-869, to obtain a final estimate of the formation pressure 870 is described in previous sections. After the measurement phase is completed 870, the probe is disengaged from the wellbore wall and the pressure returns to the wellbore pressure 874 within a time period 895 and reaches stabilization at 881.

[0117] Once a first estimate of the formation pressure and the formation mobility are obtained in the investigation phase 13b shown in Figures 17A and 18, the parameters thus obtained may be used to establish the measurement phase 14 pretest parameters that will produce more accurate formation parameters within the allotted time for the test. The procedures for using the parameters obtained in the investigation phase 13b to design the measurement phase 14 pretest parameters have been described in previous sections.

[0118] In the embodiments shown in Figures 17A, 17B, and 18, the magnitude of the pressure drop (Δp) during the flowline expansion phase is prescribed. In an alternative embodiment, as shown in Figures 19 and 20, the magnitude of the volume increase (ΔV) during the flowline expansion phase is prescribed. In this embodiment, a fixed and precisely regulated volume of fluid (ΔV) is extracted at each step at a controlled rate to produce a pressure drop that may be estimated from:

[0119] The procedures used in this embodiment are similar to those described for embodiments shown in Figures 17A, 17B, and 18. Prior to setting the formation tester, the mud compressibility is preferably determined (not shown). Subsequent to the determination of the mud compressibility and prior to setting the formation tester, the pressure measured by the tool is the wellbore or mud hydrostatic pressure 201.

[0120] Referring to Figures 19A, 19B, and 20, after the tool is set 205, the pretest piston 118a shown in Figure 4 is activated. In accordance with one embodiment of the invention, a method for performing an investigation phase 13c comprises the steps of starting the drawdown 210, withdrawing fluid at a precise and fixed rate 214 until the volume of the pretest chamber 114a is increased by the prescribed amount ΔV. The incremental change in volume of the pretest chamber may be on the order of 0.2 to 1 cubic centimeter, for example. One of ordinary skill in the art would appreciate that the amount of the prescribed volume increase (ΔV), is not limited to these exemplary volumes and should be chosen according to the total volume of the trapped fluid. The resulting expansion of the flowline fluid induces a pressure drop in the flowline.

[0121] When the prescribed increment in pretest chamber volume has been achieved, the pretest piston 118a is stopped and the drawdown is terminated 215. The pressure in the flowline is then allowed to equilibrate 217 for a period t_oi 218 that is longer than the drawdown period t_qi 216, for example, t_oi = 2 t_qi. After the pressure has stabilized (shown at point 220 in Figure 19A), a decision is made as to whether to repeat the "flowline expansion" cycle 219 (shown in Figure 20). The criterion for making the decision is similar to that described for the embodiments shown in Figures 17A and 18. That is, if the pressure after stabilization or equalization (e.g., at point 220) is significantly different from that at the start of the drawdown (e.g., at point 210) and the pressure difference is substantially consistent with the expected pressure drop arising from the expansion of the fluid in the flowline, then the "flowline expansion" cycle is repeated.

[0122] To repeat the "flowline expansion" cycle, for example, the pretest piston is re-activated 220, the flowline is expanded by precisely the same volume ΔV 224, and the pressure is allowed to stabilize 230. Again, if the pressures at 220 and 230 are significantly different and are substantially consistent with the expected pressure drop arising from the expansion of the fluid in the flowline, the cycle is repeated, for example 230-234-235-240. The "flowline expansion" cycle is repeated until the difference in consecutive stabilized pressures, e.g., pressures at 230 and 240 as shown in Figure 19A, is substantially smaller than the expected pressure drop due to the expansion of fluid in the flowline.

[0123] After the difference in consecutive stabilized pressures is substantially smaller than the expected pressure drop, the "flowline expansion" cycle may be repeated one more time, shown as 240-244-245-250 in Figure 19A. If the stabilized pressures at 240 and 250 substantially agree, the larger of the two values is taken to represent the first estimate of the formation pressure. One of ordinary skill in the art would appreciate that the processes as shown in Figures 19A, 19B, and 20 are for illustration only. Embodiments of the invention are not limited by how many "flowline expansion" cycles are performed. Furthermore, after the difference in consecutive stabilized pressures is substantially smaller than the expected pressure drop, it is optional to repeat the cycle one or more times.

[0124] The point at which the transition from flowline fluid expansion to flow from the formation takes place is identified as 300 in Figure 19A. If the pressures at 240 and 250 agree to within a selected limit (e.g., a small multiple of the gauge repeatability) at the end of the allotted stabilization time, it may be advantageous to allow the pressure at 250 to continue to build and use the procedure disclosed in the previous section (see Figure 8) to terminate the build up in order to obtain a better first estimate of the formation pressure. The process by which the decision to continue the investigation phase or whether to execute the measurement phase, 250-258-259-260, to obtain a final estimate of the formation pressure 260 is as described in previous sections. After the measurement phase is completed 260, the probe is disengaged from the wellbore wall and the pressure returns to the wellbore pressure 264 within a time period 295 and reaches stabilization at 271.

[0125] Once a first estimate of the formation pressure and the formation mobility are obtained in the investigation phase 13c, shown in Figures 19A and 20, the parameters thus obtained may be used to establish the measurement phase 14 pretest parameters that will produce more accurate formation parameters within the allotted time for the test. The procedures for using the parameters obtained in the investigation phase 13c to design the measurement phase 14 pretest parameters have been described in previous sections.

[0126] In a previous section, methods for determining mud compressibility are outlined. The mud compressibility is dependent on its composition and on the temperature and the pressure of the fluid. As a result, the mud compressibility often changes with depth. Therefore, it is desirable to measure the mud compressibility in situ at a location near where the testing is to be performed. If the tool configuration does not allow the mud compressibility to be determined as described above, the in-situ mud compressibility may be estimated by alternate methods as described in the following.

[0127] In a method according to embodiments of the invention, the formation tester may be set in casing, for example near the casing shoe, to establish a fluid seal with the casing. A compression and decompression of the well fluid trapped in the tester flowline is performed by means of the pretest piston 118a shown in Figure 4. Procedures for performing the mud compressibility test are described above with reference to Figures 11 A and 11B. Once the pretest piston rate q_p, the rate of pressure change ṗ and the trapped volume V are known, the mud compressibility may be estimated from C_m = -q_p/(Vṗ).

[0128] In this particular embodiment, the true vertical depth (hence, the temperature and pressure) at which the compressibility measurement is performed may be significantly different from the depth where the formation pressure is to be measured. Because the compressibility of drilling fluids is affected by temperature and pressure, it would be necessary to apply a correction to the compressibility thus measured in order to estimate the compressibility of the drilling mud at the depth where the testing is to be performed.

[0129] In a method in accordance with the present invention, the wellbore pressure and temperature information are acquired before the measurement begins, e.g., at point 801 as shown in Figure 17A, using conventional pressure and temperature sensors. Based on known drilling mud properties and in-situ temperature and pressure measurements, charts as shown in Figure 21 may be constructed for the purpose of conducting temperature and pressure corrections. Alternatively, analytical methods known in the art may be used to compute correction factors which when applied to the original compressibility measurement will provide the in-situ flowline fluid compressibility at the depth at which the formation pressure is to be measured. See e.g., E. Kartstad and B.S. Aadnoy, "Density Behavior of Drilling Fluids During High Pressure High Temperature Drilling Operations," IADC/SPE paper 47806, 1998.

[0130] In another method according to embodiments of the invention, the compressibility of a surface-derived (e.g., mud-pit) sample over the range of expected downhole temperature and pressure conditions are measured. An estimate of the in-situ mud compressibility under the downhole conditions may then be estimated from known relationships between the mud density and mud pressure and mud temperature according to methods known in the art. See, e.g., Figure 21 and E. Kartstad and B.S. Aadnoy, "Density Behavior of Drilling Fluids During High Pressure High Temperature Drilling Operations," IADC/SPE paper 47806, 1998.

[0131] Figure 21 depicts a typical relationship between fluid compressibility (C_m) and fluid pressure (p) for oil based and water based muds. Solid line 10 depicts the variation in mud compressibility with wellbore pressure for a typical oil based mud. Dashed line 11 depicts the corresponding variation in mud compressibility for a typical water based mud. The compressibility of the oil based mud at the surface is represented by reference number 7. The compressibility of the oil based mud at the casing shoe is represented by reference number 8. The compressibility of the oil based mud at a given measurement depth below the casing shoe is represented by reference number 9. The compressibility correction ΔC represents the difference between the compressibility of the oil based mud at the casing shoe 8 and that at the measurement depth 9. The compressibility measurement made at the casing shoe 8 may be adjusted by the compressibility correction ΔC to determine the compressibility at the measurement depth 9. As indicated by the dashed line 11, the change in compressibility and corresponding compressibility correction for water based muds may be less significant than the correction depicted by the solid line 10 for oil based muds.

[0132] As noted above, mud compressibility under the downhole conditions, either measured directly in situ or extrapolated from other measurements, may be used in embodiments of the invention to improve the accuracy of the estimates of formation properties from the investigation phase and/or measurement phase as shown, for example, in Figure 11A.

[0133] While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.

Claims

1. A method for determining downhole parameters using a downhole tool positioned in a wellbore adjacent a subterranean formation, the method comprising:

(a) establishing fluid communication between a pretest chamber in the downhole tool and the formation via a flowline, the flowline having an initial pressure therein;

(b) moving a pretest piston positioned in the pretest chamber in a controlled manner to reduce the initial pressure to a drawdown pressure;

(c) terminating movement of the piston to permit the drawdown pressure to adjust to a stabilized pressure;

(d) repeating steps (a) to (c) until a difference between the stabilized pressure and the initial pressure is substantially smaller than a predetermined pressure drop; and

(e) determining one or more downhole parameters from an analysis of one or more of the pressures.

2. The method of claim 1, wherein the pretest piston is moved at a fixed rate.

3. The method of claim 1, wherein the pretest piston is moved such that a predetermined change in volume in the flowline occurs.

4. The method of claim 1, wherein the movement of the pretest piston is controlled by controlling one of reduction of pressure in the flowline, rate of pressure change in the flowline, incremental volume change the pretest chamber and combinations thereof.

5. The method of claim 1, wherein the duration of step (c) is longer than step (b).

6. The method of claim 1, further comprising determining when to terminate step (c).

7. The method of claim 1, further comprising setting the downhole tool.

8. The method of claim 1, wherein the step of determining comprises determining one of mud compressibility, formation pressure, wellbore pressure, mobility and combinations thereof.

9. The method of claim 1, further comprising measuring one of a wellbore pressure, a formation pressure and combinations thereof.

10. The method of claim 9, further comprising determining the difference in pressure between the formation pressure and the wellbore pressure.

11. The method of claim 1, wherein an estimation of the formation pressure is determined from the initial and stabilized pressures.

12. The method of claim 11, wherein the larger of the initial and stabilized pressures is an estimation of the formation pressure.

13. The method of claim 1, further comprising determining whether to perform a measurement phase.

14. The method of claim 13, wherein the parameters are used to design a measurement phase pretest.

15. The method of claim 14, further comprising performing a measurement phase pretest.

16. A method for determining formation parameters using a downhole tool positioned in a wellbore adjacent a subterranean formation, the method comprising:

(a) measuring a first pressure in a flowline that is in fluid communication with the subterranean formation;

(b) moving a pretest piston in a controlled manner in a pretest chamber to create a predetermined pressure drop in the flowline;

(c) stopping the pretest piston after a selected movement of the pretest piston;

(d) allowing the pressure in the flowline to stabilize; and

(e) repeating steps (a) to (d) until a difference between the stabilized pressure in the flowline and the first pressure in the flowline is substantially smaller than the predetermined pressure drop;

(f) determining an initial estimate of the formation parameters from an analysis of one or more of the pressures.

(g) designing pretest criteria for performing a second pretest based on the initial estimate of the formation parameters;

(h) performing a pretest of the formation according to the designed pretest criteria whereby a refined estimate of the formation parameters is determined.

17. The method of claim 16, wherein the selected movement of the pretest piston is based on a prescribed change in a property in the flowline, wherein the property is one of reduction of pressure in the flowline, rate of pressure change in the flowline, an incremental volume extracted in the pretest chamber, a rate of change of the volume of the pretest chamber and combinations thereof

18. The method of claim 16, wherein the predetermined pressure drop is less than a difference between a pressure and a formation pressure.

19. The method of claim 16, further comprising:

(i) repeating steps (a) to (d) an additional time to obtain a new stabilized pressure in the flowline, the new stabilized pressure is used as an initial estimate of a formation pressure in the designing pretest criteria.

20. The method of claim 16, wherein the moving the pretest piston in a controlled manner is based on a selected rate of volume increase in the flowline, the selected rate of volume increase being based on a calculation that takes into account a mud compressibility.

21. The method of claim 16, wherein the moving the pretest piston in a controlled manner is based on a selected rate of pressure drop in the flowline, the selected rate of pressure drop being based on a calculation that takes into account a mud compressibility.

22. A method for estimating a formation pressure using a formation tester disposed in a wellbore penetrating a formation, the method comprising:

(a) measuring a first pressure in a flowline that is in fluid communication with the subterranean formation;

(b) moving a pretest piston in a controlled manner in a pretest chamber to create a predetermined pressure drop in the flowline;

(c) stopping the pretest piston after a selected movement of the pretest piston;

(d) allowing the pressure in the flowline to stabilize;

(e) repeating steps (a) to (d) until a difference between the stabilized pressure in the flowline and the first pressure in the flowline is substantially smaller than the predetermined pressure drop; and

(f) determining the formation pressure based on a final stabilized pressure in the flowline.

23. The method of claim 22, wherein the selected movement of the pretest piston is based on a prescribed change in a property in the flowline, wherein the property is a volume or a pressure.

24. The method of claim 22, wherein the predetermined pressure drop is less than a difference between a mud hydraulic pressure and a formation pressure.

25. The method of claim 22, further comprising:

(g) repeating steps (a) to (d) an additional time before the determining the formation pressure.

26. The method of claim 22, wherein the moving the pretest piston in a controlled manner is based on a selected rate of volume increase in the flowline, the selected rate of volume increase being based on a calculation that takes into account a mud compressibility.

27. The method of claim 22, wherein the moving the pretest piston in a controlled manner is based on a selected rate of pressure drop in the flowline, the selected rate of pressure drop being based on a calculation that takes into account a mud compressibility.

28. A method of determining mud compressibility using a downhole tool positioned in a wellbore adjacent a subterranean formation, the method comprising:

capturing wellbore fluid in the formation tester, the wellbore fluid in fluid communication with a pretest chamber having a movable piston therein;

selectively moving the piston in the pretest chamber to alter the volume of captured fluid in the downhole tool;

measuring the pressure of the captured fluid; and

estimating mud compressibility from the measured pressure.

29. The method of claim 28, further comprising determining one of the wellbore pressure, the wellbore temperature and combinations thereof.

30. The method of claim 29, wherein the step of determining is performed at a desired depth.

31. The method of claim 30, further comprising using the mud compressibility to determine downhole parameters at the desired depth.

32. The method of claim 28, further comprising using the mud compressibility to determine downhole parameters.

33. The method of claim 28, further comprising comparing the mud compressibility with an estimated mud compressibility determined from wellbore parameters.

34. The method of claim 33, wherein the wellbore parameters are one of the mud density, mud pressure, mud temperature and combinations thereof.

35. The method of claim 28, wherein the step of capturing is performed by sealingly engaging the downhole tool with an impermeable surface in the wellbore such that wellbore fluid is trapped therein.

36. The method of claim 28, wherein the impermeable surface is casing lining the wellbore.

37. The method of claim 28, wherein the movement of the piston creates one of a compression of the fluid, a decompression of the fluid and combinations thereof.

38. The method of claim 28, further comprising adjusting the estimated mud compressibility using a correction factor.

39. The method of claim 28, wherein the mud compressibility is determined by extrapolating a compressibility value determined at a different temperature or a different pressure.

Drawing