Method and system for monitoring stability of a wellbore

Abstract

 The disclosure relates to a method for monitoring stability of a wellbore (20). The method comprises measuring parameters relative to the wellbore, and estimating during a time period an equivalent static density (ESD) indicator profile and/or an equivalent circulating density (ECD) indicator profile for the wellbore based on the measured parameters. The profile may include values at a plurality of depths for a given time of the time period. The method may also comprise selecting in the time period a time window during which a predetermined event occurred and determining based on the ESD or ECD indicator profile in the selected time window, a depth at which the event occurred.

Description

BACKGROUND

[0001] The disclosure relates to a method and system for monitoring hydraulic and/or mechanical stability of a wellbore.

[0002] During the life of the wellbore and in particular during drilling of a wellbore, it is common to monitor the wellbore stability in order to detect for instance kicks and losses which are undesirable events perturbing the drilling of the wellbore.

[0003] To avoid them, it is common to estimate indicators about density of a drilling fluid entering the wellbore and compare them with data relative to the formation called "mud window". These indicators are for instance an equivalent static density (ESD) of the drilling fluid and an equivalent circulating density (ECD) of the drilling fluid.

[0004] ECD and ESD are generally calculated at bit depth. When an event occurs in the wellbore, like a kick or a loss is detected, the ESD and/or ECD is compared at bit depth to the mud window and the event is reported at bit depth. That does not take into account the fact that the event may have occurred at a different depth in the wellbore.

SUMMARY

[0005] The disclosure relates to a method for monitoring stability of a wellbore. The method comprises measuring parameters relative to the wellbore, and estimating profile of an equivalent static density (ESD) indicator and/or of an equivalent circulating density (ECD) indicator for the wellbore based on the measured parameters. The profile includes values associated to a plurality of depths for a given time. The method may also comprise selecting in the time period a time window relative to an event, such as a time window during which a predetermined event occurred, and determining based on the ESD or ECD profile in the selected time window, the depth of the event.

[0006] Such a method may be performed during the drilling operation so as to take the appropriate actions after the depth of the event has been determined but it can also be performed at any time of the life of the well, for instance during cementing. Knowing the depth of the event may help choosing these actions more accurately and drilling more efficiently.

[0007] The indicator may be ECD or ESD and/or but also a static or dynamic pressure which may also be representative of ESD and ECD respectively.

[0008] The disclosure also relates to a system for monitoring stability of a wellbore, comprising sensors for measuring parameters relative to the wellbore, and a processor for estimating profile of an equivalent static density (ESD) indicator and/or of an equivalent circulating density (ECD) indicator for the wellbore based on the measured parameters. The profile may include values at a plurality of depths for a given time. The processor may also select a time window relative to a predetermined event and determine based on the ESD or ECD profile in the selected time window, the depth of the event.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Various aspects of this disclosure may be better understood upon reading the following detailed description and upon reference to the drawings in which:

FIG. 1 is a schematic drawing of a drilling assembly used for drilling a wellbore and comprising a system according to an embodiment of the disclosure,

FIG. 2 is a flowchart of a method for monitoring the stability of a wellbore according to an embodiment of the disclosure.

FIG. 3 is a flowchart of a method for estimating and ESD and ECD profile according to an embodiment of the disclosure.

FIG.4 is a plot of ECD and ESD profile according to an embodiment of the disclosure,

FIG. 5 is a flowchart of a method for determining a depth of an event according to an embodiment of the disclosure;

FIG.6 is a plot of ECD and ESD profile for event depth-flagging according to an embodiment of the disclosure.

FIG. 7 is a flowchart of a method for determining a depth of an event according to an embodiment of the disclosure.

DETAILED DESCRIPTION

[0010] One or more specific embodiments of the present disclosure will be described below. These described embodiments are examples of the presently disclosed techniques. Additionally, in an effort to provide a concise description of these embodiments, some features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions may be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would still be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.

[0011] When introducing elements of various embodiments of the present disclosure, the articles "a," "an," and "the" are intended to mean that there are one or more of the elements. The terms "comprising," "including," and "having" are intended to be inclusive and mean that there may be additional elements other than the listed elements. Additionally, it should be understood that references to "one embodiment" or "an embodiment" of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features.

[0012] The disclosure is in the field of monitoring the well stability. For instance, while drilling, it is indeed advised to remain in a predetermined pressure area (called "mud window") as regards the well so as to avoid the fluids from the formation to get out of the formation and, on the contrary, to avoid fracturing the formation or to avoid the mechanical breakage of the wellbore wall or the collapse of the wellbore. It may also be interesting to detect events that happen in the well and that indicate that the drilling is not performed in an optimal way.

[0013] In this context, the disclosure relates to a method for monitoring stability (hydraulic and/or mechanical) of a wellbore, wherein one or several parameters relative to the wellbore are measured and an equivalent static density (ESD) indicator profile and/or an equivalent circulating density (ECD) indicator profile for the wellbore are estimated based on the measured parameters, the profile including values for a plurality of depths at a given time of the time period. The profile therefore indicates the state of the well at any depth at each given time. The method also comprise, in particular when an event like a kick or a loss is detected, selecting a time window relative to the event, for instance a time window during which the event should have happened and/or before the time at which the event occurred. This window may be a predetermined time (or moment) or a plurality of predetermined times (or moments). On the basis of the ESD or ECD indicator profile in the selected time window, the method also enables to determine the depth at which the event occurred. This method may be implemented in real-time or almost in real-time, at the wellsite, during drilling or during other operations. It may help for instance to modify the drilling parameters right away in order to stay in the mud window and maintain the well equilibrium.

[0014] With the foregoing in mind, FIG. 1 illustrates a drilling system 10 that includes a control system 12 for monitoring the stability of a wellbore 20 drilled into a geological formation 14 drilled with a drill bit 16. In some cases, an underreamer 18 may also be used for drilling the formation. In the drilling system 10, a drilling rig 22 at surface 24 may rotate a drill string 26 having a bottom-hole assembly (BHA) 28 at its lower end.

[0015] As illustrated in FIG. 1, the BHA 28 includes the drill bit 16 and the underreamer 18. The drill bit 16 is located on the downhole end of the BHA and configured to drill or cut the geological formation about the bottom of the wellbore 20. The underreamer 18 is disposed above (e.g., away from the downhole end of the BHA) the drill bit 16.

[0016] As the BHA 28 is rotated, a drilling fluid pump 30 is used to pump drilling fluid 32, which may be referred to as "mud" or "drilling mud," downward through the center of the drill string 26 in the direction of the arrow to the drill bit 16. The drilling fluid 32, which is used to cool and lubricate the drill bit 16, exits the drill string 26 through the drill bit 16. The drilling fluid 32 then carries drill cuttings away from the bottom of the wellbore 20 as it flows back to the surface 24, as shown by the arrows through an annulus 33 between the drill string 26 and the formation 14. In addition, as the drilling fluid 23 flows through the annulus 33 between the drill string 26 and the formation 14, the drilling fluid 32 may begin to invade and mix with fluid stored in the formation, which may be referred to as formation fluid (e.g., natural gas, or oil, or a combination thereof). At the surface 24, return drilling fluid 32 is filtered and conveyed back to a mud pit 44 for reuse.

[0017] The BHA 28 may also include one or more downhole tools. The downhole tools may collect a variety of information relating to the geological formation 14 and/or the state of drilling of the well. For instance, a measurement-while-drilling (MWD) tool 40 may measure certain drilling parameters, such as the temperature, pressure, orientation of the drilling units (e.g., the drill bit 16 and the underreamer 18), angular speed of the drilling units, weight applied to the drilling units, torque generated by the drilling units, distance drilled per unit angular rotation (the depth-of-cut), rate of penetration, and so forth. Likewise, a logging-while-drilling (LWD) tool 42 may measure the physical properties of the geological formation 14, such as density, porosity, resistivity, lithology, and so forth.

[0018] The MWD tool 40 and/or the LWD tool 42 may collect a variety of data 46 that may be stored and processed in the BHA 28 or, as illustrated in FIG. 1, may be sent to the surface 24 for processing. The data 46 may be sent via a control and data acquisition system 48 to a data processing system 50 of the control system 12. The control and data acquisition system 48 may receive the data 46 in any suitable way. In one example, the control and data acquisition system 48 may transfer the data 46 via electrical signals pulsed through the geological formation 14 or via mud pulse telemetry using the drilling fluid 32. In another example, the data 46 may be retrieved directly from the MWD tool 40 and/or the LWD tool 42 upon return to the surface 24.

[0019] The drilling assembly also comprises a surface measurement system 52. The surface measurement system 52 may include any suitable device to measure physical and/or chemical properties, for instance relative to the drilling fluid, such as the density, flow rate and/or the temperature of the drilling fluid entering or exiting the wellbore. The surface measurement system 52 may also be directly coupled to an above-the-surface portion of the drilling rig 22 to measure certain drilling parameters, such as the temperature, pressure, orientation of the drilling units (e.g., the drill bit 16 and the underreamer 18), weight applied to the drilling units, torque generated by the drilling units, rotation velocity of the drilling units, distance drilled per unit angular rotation (the depth-of-cut), flow rates of drilling fluid pumps, and so forth. Data 54 collected by the surface measurement system 52 may be processed in the surface measurement system 52 and sent via the control and data acquisition system 48 to the data processing system 50, or may be sent via the control and data acquisition system 48 to the data processing system 50 directly. Likewise, the control and data acquisition system 48 may receive the data 54 in any suitable way.

[0020] The data processing system 50 may include a processor 56, memory 58, storage 60, a display 62, and/or a user input 64. The data processing system 50 may use the data 46, 54 to monitor stability of the wellbore and more particularly estimate depths of events occurring in the wellbore. More specifically, as will be discussed in greater detail below, the data processing system 50 may estimate an equivalent static density (ESD) profile and/or an equivalent circulating density (ECD) profile for the wellbore based on the measured parameter for a predetermined time period, the profile including values associated to respective depths for each time of the time period, select a time window relative to the event and determine based on the ESD or ECD profile in the selected time window, the depth of the event. To process the data 46, 54, the processor 56 may execute instructions stored in the memory 58 and/or storage 60. As such, the memory 58 and/or the storage 60 of the data processing system 50 may be any suitable article of manufacture that can store the instructions. The memory 46 and/or the storage 60 may be ROM memory, random-access memory (RAM), flash memory, an optical storage medium, or a hard disk drive, to name a few examples. The display 62 may be any suitable electronic display that can display logs and/or other information relative to monitoring the stability of a wellbore. The user input 64 may be any suitable device that can be used by a user to input instructions, parameters, boundary conditions, or the like for estimating ESD or ECD and/or the depth of the event. The user input 64 may include a mouse, a keyboard, a touchpad, a touch screen, a voice recognition system, or the like.

[0021] A example of a general method 100 of monitoring stability of a wellbore according to an embodiment of the disclosure is described in the flowchart of FIG.2. In this example, the method is performed during drilling. The method comprises measuring, as shown in box 102, at least a parameter relative to the drilling operation and/or the wellbore. Such a parameter may be a parameter of the drilling fluid, such as a density or a temperature of the drilling fluid, measured when the fluid enters the wellbore, when it exits, or downhole. Parameters relative to the drilling assembly, like the torque or the rotation velocity of the drill string 26 or the flow rate or the discharge pressure of a pump for injecting the drilling fluid into the drill string 26, or parameters relative to the formation, taken at surface or downhole, may also be measured.

[0022] The method also comprises estimating, as shown in box 104, during a time period, an equivalent static density (ESD) profile and/or an equivalent circulating density (ECD) profile for the wellbore, based on the measured parameters. The profile includes a plurality of values each associated to respective depth for a given time of the time period. The time period may be the whole duration of the drilling of a well or the duration of a portion of a drilling operation. The ECD and ESD profile give information about the well equilibrium. In this embodiment of the disclosure, they are estimated in real-time, for each depth and each time of the time period, as will be explained in greater details below. Of course, as shown on FIG.2, the measuring 102 and estimating 104 parts of the method are performed repetitively during the whole time period so that the ESD and/or ECD profile are obtained at a plurality of times representative of the whole time period. For instance if measurements are taken periodically (period being for instance a second), the ESD and/or ECD profile may be estimated at the same rate. The frequency of measurements and/or estimation may however be different from what has been described and also different from each other.

[0023] The method may also comprise verifying (box 106) if an anomaly has been detected by the surface 52 and/or the downhole 40, 42 measurement units. Such anomaly may be for instance a kick corresponding to an entry of formation fluid into the wellbore; a loss, corresponding to an entry of drilling fluid coming from the wellbore into the formation or a stuck pipe in view of differential sticking, corresponding to at least a portion of drill string that is unable to rotate or to be pulled due to an interaction with the formation surrounding the wellbore. The anomaly may be detected right away when the undesirable event occurs or after its occurrence, for instance when a gas peak, coming from the formation and indicating the occurrence of a formation fluid influx, or cavings, indicating a failure of the borehole wall, exiting the wellbore is detected. If an anomaly has been detected, the method comprises selecting (box 108) a time window during the time period at which the associated undesirable event has occurred. The time window may be a predetermined time (or moment) or may comprise several predetermined times (or moments), such as a predetermined time range, depending of what event has been detected and how. When the time window is selected, the method comprises estimating (box 110), based on the ESD or ECD profile in the selected time window, the depth in the wellbore at which the event most likely occurred, as will be described in more details below. Of course, as is also shown in the flowchart of FIG.2, the process of verifying if an anomaly has been detected (box 106) occurs repetitively in order to detect each anomaly occurring during the time period and/or drilling of the well.

[0024] FIG. 3 describes in more details a method 200 of estimation of the ESD and ECD profiles corresponding to box 104 of FIG.2. It is to be understood that this method is a method according to an embodiment of the disclosure and that several other methods for estimating ESD and/or ECD may be used. For instance, ESD and/or ECD may be determined at one location in the wellbore, for instance at bit, and may be considered as constant over the whole depth of the wellbore, or at least of the open-hole portion of the wellbore. ESD and/or ECD profiles may also be estimated taking into account some but not the totality of the parameters described below, for instance the density but not the temperature. The method according to FIG.2 may also include estimation of the ESD profile and not ECD profile or of the ECD profile and not ESD profile.

[0025] The method of estimation 200 represented of the flowchart of FIG.3 comprises measuring parameters of the drilling fluid such as the density ρ_x and temperature T_x of the drilling fluid entering the wellbore (box 202) and associating the parameters to an entry time t_inx of the drilling fluid into the wellbore and storing the measured parameters in association with the entry time t_inx (box 204). The method also comprises measuring parameters relative to the drilling assembly (box 206) and/or wellbore to determine parameters such as the volumes of the various sections of the wellbore (inside of the drillstring and annulus between the drillstring and the openhole and/or the casing). In view of these measurements, the method comprises projecting at a later time t_N or t_N+1 at which depth the drilling fluid entered into the wellbore at t_inx would be situated (box 208). Based on this projection, the method associates at each time t_N or t_N+1 an entry time of the drilling fluid into the wellbore to a depth of the wellbore. For instance, at time t_N, the drilling fluid entered into the wellbore at t_inx is situated in the wellbore at depth D_N (depth D_N at time t_N is associated with entry time t_in) while at time t_N+1, the drilling fluid entered into the wellbore at t_inx is situated in the wellbore at depth D_N+1 (depth D_N+1 at time t_N+1 is associated with entry time t_inx).

[0026] The method then comprises estimating the temperature profile for each time t_N or t_N+1 (box 210), meaning that each depth of the wellbore will be associated with a temperature for each time. Estimating of the temperature at a particular depth and time may be based on parameters measured at the surface and/or measured downhole. These parameters may comprise a temperature of the drilling fluid measured at the entry time associated with its depth at the time t_N of the determination, and for instance torque, rotation speed, flow rate of the drilling fluid and injection pressure. It may also be based on a heat exchange model of the wellbore. The estimation may be based on known models such as a Karstad model as disclosed in the following article ("Analysis of Temperature Measurements during Drilling", E. Karstad, 1997, SPE38603) hereby incorporated by reference.

[0027] The method also comprises estimating a static pressure profile (box 212) based on the density of the drilling fluid measured at surface for the entry time corresponding to each depth of the drilling fluid at time t_N of the estimation (as determined at box 208) and on the temperature (estimated at box 210) in the wellbore for this depth. The density of the drilling fluid at the particular time and depth is determined thanks to these elements and to a model of the drilling fluid linking for instance volume, pressure and temperature of the drilling fluid. Such a model may be a generic model or a specific model taking into account properties of the specific used drilling fluid. The static pressure profile is determined according to the following equation:


Wherein t_N is a particular time and D_N a particular depth, p the density of the drilling fluid, g the gravity constant, D_n a variable depth between the surface and the particular depth D_N.

[0028] The method may also comprise measuring or estimating friction pressure losses in the wellbore that may be measured and/or calculated by any method known in the art (box 214) and estimate (box 216) the dynamic pressure profile based on the same parameter than for static pressure with the following equation :


Wherein t_N is a particular time and D_N a particular depth, ρ the density of the density, g the gravity constant, D_n a variable depth between the surface and the particular depth D_N and ΔP the friction pressure losses between two depths.

[0029] The method also comprises (box 217) converting the static pressure profile into ESD profile and the dynamic pressure profile into ECD profile with the following equations.

[0030] The method may also comprise storing the measured values of the parameters (temperature, density, etc.) and optionally ECD and ESD profiles and/or static and dynamic pressure profiles for each time t_N at which they were estimated (box 218). Alternatively, ECD and ESD profiles may be calculated from the stored data if needed, for instance when asked by the user.

[0031] Once the ECD and/or ESD profiles are estimated, they may be displayed for a user to read and interpret them. For instance, FIG.4 shows a log of the ECD and ESD profile plotted in function of the depth. The ECD profile is shown at 220, the ESD profile is shown at 222 and the log also shows the pore pressure equivalent density 224 relative to the pressure of the formation, that is the limit under which a kick may occur, and the fracturation pressure equivalent density (also named in the following fracture equivalent density) 226, that is the limit above which a loss may occur - as the drilling operation may fracture the formation. The envelope 228 showing for the extremum (i.e. maximum and minimum) of the ESD/ECD over time may also been displayed, as well as the envelope 230 and 232 showing the uncertainties due to the estimation of the pore pressure and fracture equivalent density profiles 224 and 226. This enables a user to visualize precisely the mud window (delimited by the pore pressure and fracture equivalent density profiles and in which the ECD and ESD profiles should be) and the equilibrium of the well over time. This log may be displayed in real-time on display 62 or remotely from the well. A time slider 234 showing the state of the log at any previous time chosen by the user by sliding the bar is also displayed.. The method according to one embodiment of the disclosure may therefore comprise other operations just as :

• Calculating and displaying an extremum of a ESD and/or ECD profile over a time period at each depth,
• Estimating a pore pressure equivalent density and/or fracture equivalent density profile and optionally calculating and displaying the uncertainties of the pore pressure and/or the fracture equivalent density profile estimation at each depth.

[0032] Now that the ESD and ECD profiles have been estimated and optionally displayed, several methods of determining the depth at which an event occurred will be described in reference to FIG.5 and FIG.6.

[0033] The method 300 of FIG.5 comprises detecting an anomaly in the wellbore (box 302) and corresponding to an event, here a loss. The loss is determined thanks to measurements at a predetermined time, for instance a measurement comparing the flows in and out of the wellbore of the drilling fluid. Then, the method comprises selecting the time window relative to the event (box 304), corresponding here to the predetermined time at which the event occurred. The time window may comprise the predetermined time but also a time interval around the predetermined time. It extracts the ESD and/or ECD profiles at the predetermined time (box 306) from the memory 58 for instance. It then selects a depth window in which the event should have occurred (box 308) and determines the depth at which the event occurred on the basis of the ESD and/or ECD and fracture equivalent density profiles (box 310) at the predetermined time in the depth window. The depth window may for instance be a portion where the wellbore was open-holed or a portion of the wellbore where the event most likely happened, based on parameters such as lithology and/or permeability. Box 306 may also comprise extracting the measured parameters associated with predetermined time window and calculating the ECD for each time of the time window.

[0034] In a first embodiment, box 310 corresponds to determining an extremum of ECD or ESD over depth at the predetermined time, here, for detecting a loss, a maximum of ECD, and selecting the depth of the event as the one where the ECD is extreme, here maximal. In a second embodiment, fracture equivalent density and/or pore pressure equivalent density profiles are also extracted from the memory at box 308 and box 310 comprises estimating the difference between one of the ESD and ECD profile and one of the pore pressure and fracture equivalent density profile, here, when detecting a loss, between an ECD profile and the fracture equivalent density profile. It also comprises determining a extremum (in relative value) of the calculated difference over depth at the predetermined time, here maximum, and selecting the depth of the event as the one at which the difference is extreme, here maximal. Here, and in the following, minimum and maximum are calculated in relative value, i.e. taking into account negative values of the difference.

[0035] When detecting a kick at box 302, the selected depth corresponds, in the first embodiment, to the one associated with the minimum of the ESD profile and, in the second embodiment, to the one associated with the minimum difference between the ESD and pore pressure equivalent density profiles. When detecting a stuck pipe possibly due to excessive differential pressure at box 302, the selected depth corresponds, in the first embodiment, to the one associated with the maximum of the ESD and/or ECD profile and, in the second embodiment, to the one associated with the maximum difference between the ESD and/or ECD and pore pressure equivalent density profiles.

[0036] FIG.6 shows a log 320 of a wellbore parameters at a predetermined time at which an event (here, a loss) occurred and at the predetermined depth window (between 3000 and 3150 meters approx.). This log enables to associate the event with the depth at which it occurred. It shows the ECD profile 322 and ESD profile 324 estimated thanks to the method described in reference to FIG.2, as well as the pore pressure equivalent density profile 326 and the fracture equivalent density profile 328. For comparison purposes, the log also represents the ECD as measured by a MWD tool at bit 330. This curve does not represent the ECD at the predetermined time but the ECD at a predetermined depth, measured when the bit was at that particular depth.

[0037] At predetermined time, the bit is at depth 332 corresponding to 3130 meters approx. The estimated ECD and measured MWD at bit correspond which indicates the accuracy of the estimated ECD. As it can be seen on FIG.6, the event will however not be associated with the bit depth but at depth 324, corresponding to 3075 meters approx. because that is where the maximum difference between the ECD and fracture equivalent density pressure is. As can be seen, to be able to obtain such data, the MWD measurement 330 of ECD that does not give an entire profile for a predetermined time is not useful.

[0038] The estimation of the difference and selection of depth may be done by the user with the help of this chart. However, it may also be performed automatically by the processor 56 or any other appropriate processor.

[0039] The method 340 as referenced on FIG.7 is a method of estimating a depth of an event occurring in the wellbore according to another embodiment of the disclosure.

[0040] This method comprises detecting an anomaly (box 342), this event being measured at surface by the surface equipement 52. This anomaly may then be a gas peak detection, or a cavings detection. This method also comprises selecting a depth window at which the event associated with the anomaly may have occurred (box 344), for instance the open-holed portion of the wellbore, and calculating a lag time for the drilling fluid so as to obtain from the exit time of the gas peak or the cavings the time at which the drilling fluid was at each depth in the wellbore (box 346). The method then comprises selecting a time window (box 348), the time window comprising the times at which the drilling fluid (exiting the wellbore at the exit time at which the gas peak or cavings was detected) was in the depth window and extracting the data corresponding to that time and depth windows (box 349). The time window here comprises several predetermined times. When the depth and time selection has been made, the method comprises determining the depth and time of the event (box 350).

[0041] The determination may comprise selecting a criterion). For gas peak, this criterion may be ESD or difference between ESD and pore pressure equivalent density profile or ratio based thereof such as difference between ESD and pore pressure equivalent density over average difference or over pore pressure equivalent density value, etc. For cavings, this criterion may be ESD or difference between ESD or ECD and mechanical stability equivalent density profiles, indicating the pressure at which the well may break or collapse, or ratio based thereof such as difference between ESD or ECD and mechanical stability equivalent density over average difference or over mechanical stability equivalent density value, etc.

[0042] Once selected, the determination may comprise determining the extremum of the selected criterion over time and depth. It may be done by plotting over depth the extremum over time of this criterion at each depth for instance. Any other method may be appropriate.

[0043] When a gas peak is detected, the criterion may be in particular the minimum of ESD or the minimum of the difference between ESD or ECD and pore pressure equivalent density may be considered for determining the depth of the event. When cavings are detected, the criterion may be in particular the minimum of ESD or ECD or the minimum of the difference between ESD or ECD and borehole mechanical equivalent density may be considered for determining the depth of the event. More particularly, ESD may be used when the injection pumps are stopped while ECD may be used when the injection pumps of the well are working and drilling fluid is injected into the borehole.

[0044] When the depth has been determined, the method may comprise performing an action for correcting the drilling in order to stop the event (box 352). Such actions may be changing the drilling parameters such as increasing the mud weight, decreasing the flow rate; backreaming, etc.

[0045] In view of the entirety of the present disclosure, including the figures, a person skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same uses and/or achieving the same aspects introduced herein. A person skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions and alterations herein without departing from the spirit and scope of the present disclosure. For example, although the preceding description has been described herein with reference to particular means, materials and embodiments, it is not intended to be limited to the particulars disclosed herein; rather, it extends to functionally equivalent structures, methods, and uses, such as are within the scope of the appended claims.

[0046] The method and device according to the disclosure may for instance have other features, such as flagging a time at which an event is detected and/or the time and optionally depth at which the estimated ESD and/or ECD goes out of the mud window. Determination of depth of the event may be based on the flags (depths associated with the highest number of flags in the time window, etc.). The estimation of ESD and/or ECD may not be performed as disclosed above. Plus, part of the method may be performed remote from the rig, and/or after the drilling, for instance to analyze the wellbore.

[0047] Anyway, the disclosure may relate to method for monitoring stability of a wellbore, comprising:

• Measuring at least a parameter relative to the wellbore,
• Estimating during a time period an equivalent static density (ESD) indicator profile and/or an equivalent circulating density (ECD) indicator profile for the wellbore based on the measured parameters, the profile including a plurality of values associated to respective depths for a given time of said time period,
• Selecting in the time period a time window relative to a predetermined event,
• Determining based on the ESD or ECD indicator profile in the selected time window, a depth of the event.

[0048] The method may be performed during the drilling operations (drilling, tripping, etc.) and/or at any other moment of the life of the well, for instance during cementing.

[0049] The ESD or ECD indicator may be ECD or ESD itself or other variable representative of either ESD or ECD such as dynamic pressure (representative of ECD) or static pressure (representative of ESD)

[0050] The measured parameters may include density and/or temperature of a drilling fluid, a flowrate of the drilling fluid, etc.

[0051] The method may also comprise storing the ESD and/or ECD indicator profile corresponding to each time, and extracting the stored ESD and/or ECD indicator values for the selected time window. Alternatively, the measured values of the parameters may be stored and then extracted for the selected time window and the ESD and/or ECD indicator profiles may be estimated on the basis of the extracted values.

[0052] It may also comprise selecting a depth window in the plurality of depths. The selection of the depth window may be based on the geometry of the well (open-holed portion for instance) and/or on other parameters such as the lithology and/or permeability that may help determining where the event most probably occurred.

[0053] Determining the depth of the event may comprise determining a criterion relative to the ESD and/or ECD indicator and selecting a depth at which the value of the criterion in the time window is an extremum.

[0054] According to an embodiment, the criterion is the ESD and/or ECD value. Determining the depth of the event may comprise comparing the ESD or ECD values at different depths during the time window and selecting a depth at which the ESD or ECD value is an extremum. More particularly, the depth selected may be the one at which the ESD indicator value is a minimum or the ECD indicator value is a maximum.

[0055] According to another embodiment, the criterion is a difference between the ESD or ECD indicator value and an estimator value. Determining the depth of the event may comprise comparing the ESD or ECD indicator values at different depths with a value of an estimator at the corresponding depth and selecting the depth at which the difference between the ESD or ECD indicator value and the estimator value is extreme (the difference being in relative values, and not in absolute values, meaning it can take into account negative values). The estimator may be the pore pressure equivalent density and/or the fracture equivalent density and/or borehole stability equivalent density. The method may also calculate such estimators. The ECD indicator profile may be compared with the fracture equivalent density profile, while the ESD indicator profile may be compared with the pore pressure equivalent density profile. Both may be compared with the borehole stability equivalent density. During certain operation such aspulling-out operation, the ECD indicator values may be inferior to ESD indicator values at corresponding time and depth. In this case, ECD indicator may also be compared with pore pressure equivalent density. In particular, the minimum of ESD and ECD indicator value may be compared to the corresponding pore pressure equivalent density value and/or borehole stability equivalent density and the maximum of ESD or ECD indicator value may be compared to the corresponding fracture equivalent density value. Anyway, any of the ESD and ECD indicators may be compared with any of the estimators.

[0056] When the ESD or ECD indicator is not ESD or ECD itself, of course, the ECD or ESD indicator is compared to other estimators such as the pore pressure, the fracturation pressure and/or the borehole mechanical stability pressure when the ESD or ECD indicator is static or dynamic pressure. The estimator may be for instance any pore pressure indicator, fracturation pressure indicator or mechanical stability indicator.

[0057] The method may also comprise detecting the event. The event may be a kick, a loss or a stuck pipe. The time window may then comprise the predetermined time at which the event was detected. It may contain no other time than the predetermined time or a time interval surrounding the predetermined time.

[0058] The method may also comprise detecting a gas peak or cavings at surface, and calculating a lag time so as to determine the time window comprising several predetermined times (or moments) during which the event generating the gas peak or cavings has occurred. The time window may correspond to the time window calculated thanks to lag times or to a time interval surrounding the calculated time window.

[0059] The method may be triggered by the detection of an event and/or the detection of gas peaks or cavings arriving at surface.

[0060] If the time window comprises several times, the method may also comprise determining a time of the time window at which the event occurred, optionally simultaneously with the determination of the depth.

[0061] The method may also comprise plotting, for each depth, an extreme value of a criteria for this depth during the time window in particular if the time window comprises several times. The criteria may be one or several of the ones defined above (ESD minimum value, ECD maximal value, maximal difference - in relative value- between ECD and fracture equivalent density or minimal difference between ESD and pore pressure equivalent density or ESD/ECD and borehole mechanical stability equivalent density, etc.).

[0062] The method may also comprise calculating and displaying an extremum of a ESD and/or ECD indicator profile over a time period at each depth, for instance plotting minimum ESD and maximum ECD values over the time period or time window. Pore indicator, fracture indicator and borehole mechanical stability indicator values, as well as their uncertainty range, may also be calculated and displayed.

[0063] Estimating the ESD and/or ECD indicator profile may also comprise :

• Measuring at surface a density of a drilling fluid entering the wellbore and associating with an entry time,
• At each time, associating with each depth the entry time of the drilling fluid situated at this depth into the wellbore,
• At each time, estimating a temperature profile of the wellbore, on the basis of at least a temperature measurement,
• At each time, calculating the density of the drilling fluid for each depth in function of the measured drilling fluid density at corresponding entry time, and of the temperature at said depth.

[0064] Temperature may be determined based on the temperature of the drilling fluid measured when the drilling fluid is entering the wellbore and/or of measurements made downhole and/or modeling of the heat exchanges along the circulation path of the mud in the drill string and the wellbore. If temperature is measured when mud enters the wellbore, it may also be associated with an entry time.

[0065] Estimating the ECD profile may also comprise :

• At each time, estimating the friction pressure losses in the wellbore,
• At each time, calculating the density of the drilling fluid for each depth in function of the measured drilling fluid density at corresponding entry time, the temperature and pressure losses at said depth.

[0066] The method may also comprise taking appropriate action in function of the depth at which the event is associated, for instance stopping the drilling and/or modifying the drilling parameters, etc.

[0067] The method may be performed in real-time or almost real-time and at the wellsite. It may also be performed remotely and/or after the drilling of the well is over.

[0068] Examples of the method according to the disclosure may be the following :

[0069] Detecting a loss at a predetermined time; determining the maximum ECD value at the predetermined time or the maximum difference (in relative value) between the ECD value and the fracture equivalent density value, the depth at which the loss occurred being determined as the one at which the ECD value is the maximum or the difference between the ECD value and the fracture equivalent density value is maximum.

[0070] Detecting a kick at a predetermined time; determining the minimum ESD value at the predetermined time or the minimum difference between the ESD value and the pore pressure equivalent density value, the depth at which the loss occurred being determined as the one at which the ESD value is the minimum or the difference between the ESD value and the pore pressure equivalent density value is minimum.

[0071] Detecting a stuck pipe due to excessive differential pressure at a predetermined time; determining the maximum ESD or ECD value at the predetermined time or the maximum difference between the ESD or ECD value and the pore pressure equivalent density value, the depth at which the loss occurred being determined as the one at which the ESD or ECD value is the maximum or the difference between the ESD or ECD value and the pore pressure equivalent density value is maximum.

[0072] Detecting a gas peak at surface; calculating a lag time so as to determine the time window comprising several times during which the event generating the gas peak has occurred; determining the minimum ESD or ECD value or the minimum of difference between the ESD or ECD value and the pore pressure equivalent density value over time and depth, the time and depth at which the formation fluid influx generating the gas peak occurred are determined as time and depth at which the ESD or ECD value is the minimum or the difference between the ESD value and the pore pressure equivalent density value is minimum

[0073] Detecting cavings at surface; calculating a lag time so as to determine the time window comprising several times during which the event generating the cavings has occurred; determining the maximum ESD or ECD value or the minimum of difference between the ESD or ECD value and the borehole mechanical stability equivalent density value over time and depth, the time and depth at which the breakage of the wellbore generating the cavings occurred are determined as time and depth at which the ESD or ECD value is the minimum or the difference between the ESD or ECD value and the borehole mechanical stability equivalent density value is minimum.

[0074] The disclosure also relates to a method for estimating an ESD and/or ECD profile comprising :

• Measuring at surface a temperature and density of a drilling fluid entering the wellbore and associating with an entry time,
• At each time, associating with each depth the entry time of the drilling fluid situated at this depth into the wellbore,
• At each time, estimating a temperature profile of the wellbore, in function of the temperature measured at entry time associated with each depth, and/or of measurements made downhole and/or modeling of the heat exchanges along the circulation path of the mud in the drillstring and the wellbore,
• At each time, calculating the density of the drilling fluid for each depth in function of the measured drilling fluid density at corresponding entry time, and the temperature at said depth.

[0075] Estimating the ECD profile may also comprise :

• At each time, estimating the friction pressure losses in the wellbore,
• At each time, calculating the density of the drilling fluid for each depth in function of the measured drilling fluid density at corresponding entry time, the temperature and pressure losses at said depth.

[0076] The disclosure also relates to a system for monitoring stability of a wellbore, comprising:

• At least a sensor for measuring at least a parameter relative to the wellbore,
• A processor configured to :

  ∘ estimate during a time period a profile of an equivalent static density (ESD) indicator and/or an equivalent circulating density (ECD) indicator for the wellbore based on the measured parameters, the profile including plurality of values associated to respective depths for a given time of said time period,

  ∘ select in the time period a time window relative to a predetermined event

  ∘ determining based on the ESD or ECD indicator profile in the selected time window, the depth of the event.

Claims

1. A method for monitoring stability of a wellbore, comprising :

- measuring at least a parameter relative to the wellbore,

- estimating, during a time period, a profile of an equivalent static density (ESD) indicator and/or of an equivalent circulating density (ECD) indicator for the wellbore based on the measured parameters, the profile including plurality of values associated to respective depths for a given time of said time period,

- selecting in the time period a time window relative to a predetermined event,

- determining based on the ESD or ECD indicator profile in the selected time window, a depth of the event.

2. The method according to claim 1, comprising storing the ESD and/or ECD indicator profile corresponding to each time, and extracting the stored ESD and/or ECD indicator values for the selected time window.

3. The method according to claim 1, comprising storing measured values of the parameters, extracting the measured values for the selected time window and estimating the ESD and/or ECD indicator profiles on the basis of the extracted values

4. The method according to any of the preceding claims, wherein the measured parameters comprise density and/or temperature of a drilling fluid and/or a flowrate of the drilling fluid and/or an injection pressure of the drilling fluid, and/or torque and/or rotation speed of a drill string.

5. The method according to any of the preceding claims, comprising detecting the event, including one of the following :

- detecting a kick,

- detecting a loss,

- detecting a pipe differential sticking,

wherein the time window comprises the predetermined time at which the kick or loss or sticking was detected.

6. The method according to any of the preceding claims, comprising :

- detecting a gas peak or cavings arrival at surface,

- calculating a lag time so as to determine the time window comprising several predetermined times during which the event generating the gas peak or cavings may have occurred.

7. The method according to any preceding claim, wherein determining the depth of the event comprises determining a criterion relative to the ESD and/or ECD indicator profile and selecting a depth at which the value of the criterion in the time window is an extremum.

8. The method according to claim 7, comprising plotting, for each depth, an extremum value of the criterion for this depth during the time window.

9. The method of claim 7 or 8, wherein the criterion is the ESD and/or ECD indicator value.

10. The method according to claim 9, comprising :

- Selecting the depth at which the ESD indicator value is a minimum, and/or

- Selecting the depth at which the ECD indicator value is a maximum.

11. The method according to any of the claims 7 or 8, wherein determining the depth of the event comprises comparing the ESD or ECD indicator values at different depths with a value of an estimator at the corresponding depth, the criterion being the difference between the ESD or ECD indicator value and the estimator value.

12. The method according to claim 11, wherein the estimator is the pore pressure indicator, the fracturation pressure indicator or the borehole mechanical stability indicator.

13. The method according to claim 12, comprising :

- Selecting the depth at which the difference between the ESD indicator value and the pore pressure indicator value is minimal, and/or

- Selecting the depth at which the difference between the ECD indicator value and the fracturation pressure indicatorvalue is maximal, and/or

- Selecting the depth at which the difference between the ECD indicator value and the mechanical stability indicator is minimal

14. The method according to any of the preceding claims, wherein the estimating the ESD and/or ECD indicator profile comprise :

- measuring at surface a temperature and density of a drilling fluid entering the wellbore and associating with an entry time,

- at each time, associating with each depth the entry time of the drilling fluid situated at this depth into the wellbore,

- at each time, estimating a temperature profile of the wellbore, in function of the temperature measured at entry time associated with each depth,

- at each time, calculating the density of the drilling fluid for each depth in function of the measured drilling fluid density at corresponding entry time, and of the temperature at said depth.

15. A system for monitoring stability of a wellbore, comprising :

- at least a sensor for measuring at least a parameter relative to the wellbore,

- a processor configured to :

∘ estimate during a time period a profile of an equivalent static density (ESD) indicator and/or of an equivalent circulating density (ECD) indicator for the wellbore based on the measured parameters, the profile including plurality of values associated to respective depths for a given time of said time period,

∘ select in the time period a time window relative to a predetermined event

∘ determining based on the ESD or ECD indicator profile in the selected time window, a depth of the event.

Drawing