AN IMPROVED METHOD FOR DETERMINING THE INTEGRITY STATUS OF OIL AND GAS WELLS

Abstract

 Method for assessing the status of a well (W) or of a similar borehole extending from a surface (S) downward into a subsurface, preferably for assessing the integrity status of an oil and gas well, comprising:
- collecting/sampling (101) of subsurface fluids exiting from the subsurface of said well or of a nearby well,
- determining (120) the subsurface origin of said subsurface exiting fluids by using the composition features of released fluids, in particular of released gasses, that are released following the application of a mechanical action, preferably a crushing action, on subsurface rock samples having known subsurface origins in said well or in a nearby well.

Description

FIELD OF INVENTION

[0001] The present invention relates to the technical field of oil and gas wells. The method according to the invention is configured and intended to determine the integrity status of a well, in particular the integrity status of an oil and gas well.

STATE OF THE ART - PRIOR ART

[0002] Accumulations of oil and gas that are appropriate for economic exploitation typically occur at depths of hundreds to several thousand meters below ground level or seabed. Boreholes drilled to explore for and to extract oil and gas must penetrate the overlying sediments to reach the target petroleum reservoirs.

[0003] Typical well construction practice uses steel casing to isolate and stabilize the well bore. Each cylindrical steel casing is run into the hole and is surrounded by mud M and cement CM to be stabilized in place. Several casing strings are needed to reach deep targets, resulting in concentric annuli of decreasing diameter with depth (see figure 1).

[0004] A typical construction of a well W comprises a well head WH protruding above the surface S and a subsurface structure SS inserted in a borehole drilled below the surface and provided with nested casings W1, W2 and W3, and also with an innermost production tube P.

[0005] In particular, the first casing to be installed is the surface casing W1, then - depending on the geology and well design - an intermediate casing W2 may be installed at a certain depth below the surface casing. The deepest casing, that is the so-called production casing W3, is perforated within the reservoir to expose to surface the hydrocarbon-bearing sediments and the fluids that are produced within the production tubing P placed in the well. In particular, once the production casing W3 is installed and cemented, the innermost production tube P for transporting the hydrocarbons H to the surface is inserted inside the production casing W3.

[0006] The well W comprises a section G, also defined as gas chamber, wherein gas accumulates due to geological formations or drilling conditions. This gas can be a mixture of hydrocarbons (e.g., natural gas), other gasses present in the subsurface, or gasses and liquids introduced by the operator.

[0007] The annular space between the production casing W3 and the production tube P defines the annulus A, the annular space between the intermediate casing W2 and the production casing W3 defines the annulus B, while the annular space between the surface casing W1 and the intermediate casing W2 defines the annulus C.

[0008] At the top of the wellbore there is the assembly of the well head WH comprising the casing head CH and the tubing head TH for supporting and sealing the casings and the tube respectively. The assembly of the well head WH also includes ports, respectively M-A, M-B and M-C, wherein each of them is fluidically connected with a corresponding annulus A, B and C for monitoring the pressure in said annulus and for sampling fluid present.

[0009] High subsurface pressures are controlled using packers and valves, which are intended to contain the fluids within the production tubing as it flows from the reservoir to the well head at the surface.

[0010] Under normal operating conditions the pressure within the various casing strings should be low and stable. Some annulus pressure build-up may occur due to thermal expansion, but this can be reduced back to atmospheric pressure by bleeding gas or fluid through a needle valve on the well head. Annulus pressure may also build-up when there is a loss of well integrity that results in the leakage of fluids across the engineered barriers. This is known as "sustained casing pressure" (SCP) or "sustained annulus pressure" (SAP) within the petroleum industry and can - in the worst case - lead to well failure and resulting threat to human life and environmental damage.

[0011] Loss of sealing integrity within oil and gas wells and build-up of sustained annulus pressure can have many different causes and often occurs due to cracks, corrosion and mechanical failure of casing strings, production tubing, downhole packers and valves used in well construction. Incomplete bonding of cement between casing and penetrated rock formations and/or cracks within cement can also act as fluid conduits into well annuli.

[0012] Leakage within the production tubing, packers or subsurface valves within the well may permit the escape of production fluids into the surrounding casing annuli. Alternatively, an incomplete bond between casing, cement and rock formation may permit fluid migration from external sources into the casing annuli. For example, as shown in figure 2, fluids from the oil or gas reservoir OG, from an aquifer layer AL, from a coal layer CL or from a non-reservoir gas layer NL could enter into the production tube and/or into the surrounding casing annuli.

[0013] It is common for wells to have multiple intervals of hydrocarbon-bearing rocks from either thermogenic or biogenic origin behind casing.

[0014] Downhole tools utilizing a variety of physical sensors can be used to identify the quality of cement bond to casing, the presence of corrosion in casing or production tubing, and the likely occurrence of internal leakage within the well bore. These techniques require tools to be lowered into the well, and production to be suspended. Existing approaches have limited ability to identify the sources of externally sourced fluids within the annuli.

OBJECTS OF THE INVENTION

[0015] The object of the invention is to propose a method for determining the status of a well, in particular for determining the integrity status of a well, which overcomes, at least in part, the drawbacks of traditional solutions.

[0016] Another object of the invention is to propose a method that allows to determine the origin of the fluids exiting from the subsurface of a well.

[0017] Another object of the invention is to propose a method that allows to determine the origin of the fluids exiting at the well head and coming from an annulus of the subsurface structure of the same well.

[0018] Another object of the invention is to propose a method that allows to identify the origin of gas responsible for casing annulus pressure build-up.

[0019] Another object of the invention is to propose a method that allows to identify the possible fluid flow paths into the well head.

[0020] Another object of the invention is to propose a method that allows to discover the origin of gas responsible for casing annulus pressure build-up and the possible flow paths into the well head without intervention or loss of production.

[0021] Another object of the invention is to propose a method that is of easy, quick and low-cost implementation.

[0022] Another purpose of the invention is to propose a method that is precise, reliable and accurate.

[0023] Another object of the invention is to propose a method that has an alternative and/or improved characterization, both in constructive and functional terms, with respect to the traditional ones.

[0024] Another object of the invention is to propose a method that is alternative to the traditional ones.

[0025] Another object of the invention is to propose a method which meets high functional standards, and at the same time has an affordable cost, thus allowing the possibility of its utilization on a large scale.

SUMMARY OF THE INVENTION

[0026] All these objects, both alone and in any combination thereof, and others which will result from the following description are achieved, according to the invention, with a method with the features of claim 1.

[0027] The present invention relates to a method for assessing the status of a well or of a similar borehole extending from a surface S downward into a subsurface, in particular for assessing the integrity status of an oil and gas well, comprising:

• collecting/sampling of subsurface fluids, in particular of subsurface gasses, exiting from a subsurface of said well or of a nearby well,
• determining the subsurface origin of said exiting fluids by using the composition features of released fluids, in particular of released gasses, that are released following to the application of a mechanical action, preferably a crushing action, on subsurface rock samples of said well or of a nearby well having known subsurface origins in the said well or in nearby wells.

[0028] Advantageously, in the method according to the invention the subsurface origin of subsurface exiting fluids so determined is used for determining the integrity status of the well, in particular for identifying the origin within the well of fluids leakage and/or the fluids potential migration pathways.

[0029] Advantageously, the method according to the invention provides information that can be used to determine the subsurface origin of gasses that accumulate within oil and gas annuli caused by incomplete isolation of hydrocarbon-bearing strata.

[0030] Advantageously, the method according to the invention allows to identify the origin of gas responsible for sustained annular pressure or sustained casing pressure in oil and gas wells.

[0031] Advantageously, the method according to the invention allows to identify the subsurface origin of non-reservoir gas exiting from the subsurface of a well on the basis of a detailed molecular composition and/or isotopic data of gasses released from the mechanical crushing of samples of drill cuttings, side wall cores or core samples collected at known depth origins in the same or nearby well.

[0032] Preferably, the released fluids, in particular of released gasses, are released by applying a mechanical crushing to the subsurface rock samples.

[0033] Preferably, the subsurface rock samples comprise drill cuttings, side wall core chips and/or conventional core chips.

[0034] For example, the rock samples obtained from all drill cuttings, side wall core chips and/or conventional core chips (even obtained at different known depths) are placed into a sealed vessel and are mechanically crushed so as to liberate the entrained gas, thus obtaining said released gasses.

[0035] Preferably, said subsurface exiting fluids, in particular of the subsurface exiting gasses, are collected/sampled at a well head, protruding from the surface, of the same well or of a nearby well
Preferably, the subsurface exiting fluids exiting from the subsurface structure are collected at or above a corresponding surface level.

[0036] Preferably, the composition features of the subsurface exiting fluids are compared with the composition features of the released fluids, in particular of released gasses, that are released following mechanical action applied to the subsurface rock samples from the well or nearby wells.

[0037] Preferably, the fluids exiting from the subsurface are collected at a well head of an oil and gas well, wherein said well comprises said well head protruding above the surface and a subsurface structure inserted in a borehole drilled below the surface and provided with nested casings and an innermost tube. In particular, the subsurface structure comprises a plurality of annuli that are defined by an outer casing and inner casing nested into the outer casing, and/or defined between the innermost casing for transporting the hydrocarbons H to the surface. More in detail, the casings and the innermost tube of the subsurface structure extend also in the well head and/or are in fluid communication with the well head. Ideally, each annulus of the subsurface structure is fluidically connected with a corresponding region of the well head so as to collect fluid sample of each annulus in correspondence of the well head. Ideally, the inside of the innermost tube is fluidically connected with a corresponding region of the well head so as to collect at the well head a corresponding sample of the fluid flowing through said tube.

[0038] Preferably, the fluids exiting from the subsurface and that are collected in correspondence of the well head are responsible for pressure build-up in one of the annuli of the subsurface structure.

[0039] Preferably, the fluids exiting from the subsurface are collected in correspondence of the well head and comprise fluids exiting from at least one annulus and from the innermost tube for transporting the hydrocarbons to the surface.

[0040] Ideally, the subsurface exiting fluids are collected in correspondence of the well head for each annulus showing a corresponding pressure build up.

[0041] Ideally, the subsurface exiting fluids are collected during routine monitoring operations in correspondence of the well head and for each annulus.

[0042] Preferably, the collected/sampled fluids exiting from the subsurface structure comprises gasses coming from the innermost production tube and/or gasses coming from at least one annulus of the subsurface structure.

[0043] Ideally, the collected/sampled fluids exiting from the subsurface structure comprises gasses coming from at least one annulus of the subsurface structure wherein a pressure build-up is sensed.

[0044] Ideally, said fluids exiting from the subsurface structure are collected at the well head when a pressure build-up in one of the annuli of the subsurface structure is sensed or during routine monitoring operations.

[0045] Preferably, the composition features comprise respectively the molecular composition of the subsurface exiting gasses and the molecular composition of the released gasses.

[0046] Preferably, the composition features comprise respectively the isotopic ratio data of the subsurface exiting gasses and the isotopic ratio data of the released gasses.

[0047] Ideally, the composition features of the subsurface exiting gasses and of the released gasses comprise the respective carbon isotope ratios of C1-C5 hydrocarbons.

[0048] Ideally, the composition features of the subsurface exiting gasses and of the released gasses comprise the respective hydrogen isotope ratio of methane.

[0049] Preferably, said method is further characterized in that the composition features of exiting fluids are firstly analyzed to determine whether the exiting fluids are derived from an internal producing region of the well or from an external non-producing region of the well, that is whether the exiting fluids come from the innermost production tube or from the external casings of the subsurface structure.

[0050] Preferably, said analysis of the exiting fluids to determine whether the corresponding fluids are derived from the producing region of the well or from an external/non-producing region of the well is based on the molecular composition, the carbon isotope ratios of C1-C5 hydrocarbons and the hydrogen isotope ratio of methane.

[0051] Preferably, said method comprises:

• a preliminary step wherein the composition features of exiting fluids are analyzed to determine whether such exiting fluids are derived from a first region or from a second region, and
• if the exiting fluids result to be from the second/external region, the subsurface origin of said exiting fluids is determined by using the composition features of released fluids, in particular of released gasses, that are mechanically released from subsurface rock samples having known subsurface origins.

[0052] Ideally, said first region is a producing/reservoir region and, in particular, it is a region defined by the innermost/production tube of which a corresponding first exiting gas sample is collected.

[0053] Ideally, said second region is a region not corresponding to the innermost/production tube (P). Ideally, said second region is a non-producing/non-reservoir region and, in particular, it is region that surrounds externally the annulus of which a corresponding second exiting gas is collected.

[0054] Advantageously, the method according to the invention allows to determine whether gas causing annular pressure build-up sensed at the well head is originated via an internal leakage from the innermost/production tube of the well, or if such pressure build-up is originated from influx of gas from an external region and - if external - the subsurface origin of such exiting gasses is identified on the basis of the composition features (in particular the molecular composition and isotopic data) of gasses released by the mechanical crushing of subsurface rock samples (for example samples of drill cuttings, side wall cores or core samples).

[0055] Preferably, the preliminary step for determining whether the exiting fluids are derived from said first region or from said second region comprises a comparison between the composition features of the fluids exiting from the innermost tube of the well wherein said innermost tube is the production tube for transporting the hydrocarbons to the surface, and of the fluids exiting from a pressurised annulus of the well, in particular from the annulus wherein a pressure build-up is sensed. Advantageously this allows to determine whether gas causing annular pressure build-up originates in the producing formation via internal leakage within the production zone of the well bore or if the pressure originates from influx of gas from an external (non-producing) zone.

[0056] More preferably, the preliminary step for determining whether the exiting fluids are derived from said first region or from said second region comprises:

• the sampling of the gas exiting at the well head from the innermost tube of the well wherein said innermost tube is the production tube for transporting the hydrocarbons to the surface, so as to get at least one first exiting gas sample representing the gas of the production stream,
• the sampling of the gas exiting at the well head from a pressurised annulus of the well, in particular from the annulus wherein a pressure build-up is sensed, and/or from a non-pressurized annulus, in particular during routine monitoring operations, so as to get at least one second exiting gas sample representing the gas from the pressurised annulus of the well,
• the preliminary analysis of said at least one first exiting gas sample and of said at least one second exiting gas sample,
• the comparison/matching of the results data of said analysis for the first exiting gas sample with the results data of said analysis for the second exiting gas sample so as to determine the degree of similarity between the first exiting gas and the second exiting gas,
• determining whether the second exiting gas is derived from said first region or from said second region based on the degree of similarity so determined, and in particular if the first exiting gas and the second exiting gas are similar then the second exiting gas is considered as derived from said first region (i.e. from the innermost/production tube) while if the first exiting gas and the second exiting gas are not similar then the second exiting gas is considered as derived from said second region (i.e. from the external of the annulus from which the second exiting gas was collected).

[0057] Advantageously, the method according to the invention allows to determine whether gas causing annular pressure build-up originates in the producing formation via internal leakage within the well bore, or if the pressure originates from influx of gas from an external (non-producing/non-reservoir) zone and - if external - it allows to determine the subsurface origin of non-reservoir gas identified on the basis of a molecular composition and/or isotopic data from the mechanical crushing of samples of drill cuttings, side wall cores or core samples collected by the same or nearby well.

[0058] Preferably, for each rock sample of a plurality of rock samples collected at different known origins/depths of the same or nearby well, the corresponding gasses released from the crushing of the rock sample are analyzed so as to define/build a profile of the composition features of the released gasses for each different known subsurface origin/depth.

[0059] More preferably, the method comprises an analysis of the gas released from the crushing of the rock sample, ideally for each rock sample coming from a plurality of different known depths/origin in the same or nearby well.

[0060] Preferably, the method comprises a comparison of results data of the analysis of the released gas with the results data of the analysis of the second exiting gas sample so as to determine the subsurface origin of the second exiting gas.

[0061] Preferably, said method is characterized in that the composition features of the released fluids from subsurface rock samples is associated to the subsurface origin of the corresponding rock samples, so as to define/build a reference profile (for example organized as a look-up table, a log or other organized dataset wherein to each input is associated the related output) comprising for each subsurface origin of the rock sample the corresponding composition features of the fluids released from said rock sample.

[0062] Preferably, said method being characterized in that the composition features of the exiting fluids are compared with - and in particular are searched within - the composition features of the reference profile as defined in order to determine the corresponding subsurface origin of the exiting fluids. In particular, the subsurface origin of the exiting fluids is determined as the subsurface origin of the released fluids having the same or similar composition features.

[0063] Preferably, said subsurface origin comprises data about or representative of the subsurface depth within the same or nearby wells.

[0064] Preferably, said subsurface origin comprises data about or representative of a specific zone within the subsurface structure of the well and/or of each annulus defined between two nested casings of the subsurface structure and/or between the innermost casing and the innermost tube for transporting the hydrocarbons to the surface.

[0065] Preferably, the analysis of exiting gas samples and/or the analysis of released gas is/are performed via gas chromatography to assess molecular abundance of hydrogen, nitrogen, oxygen, carbon dioxide, hydrogen sulphide, methane, ethane, propane, isobutane, normal butane, neopentane, isopentane, normal pentane, summed hexane plus heavier hydrocarbons, and unsaturated hydrocarbons.

[0066] Preferably, the analysis of exiting gas samples and/or the analysis of released gas is/are performed via gas chromatography-isotope ratio mass spectrometry (GC-IRMS) to assess the ratio of 13C to 12C in methane, ethane, propane, isobutane, normal butane, isopentane, normal pentane and carbon dioxide.

[0067] Preferably, the analysis of exiting gas samples and/or the analysis of released gas is/are performed via GC-IRMS to assess the ratio of deuterium (2H) to protium (1H) in methane and H2-.

[0068] Preferably, the analysis of exiting gas samples and/or the analysis of released gas is/are performed via GC-IRMS to assess the ratio of 14N to 15N in N2.

[0069] Ideally, the analysis of exiting gas samples and/or the analysis of released gas is/are performed:

• via gas chromatography to assess molecular abundance of hydrogen, nitrogen, oxygen, carbon dioxide, hydrogen sulphide, methane, ethane, propane, isobutane, normal butane, neopentane, isopentane, normal pentane, summed hexane plus heavier hydrocarbons, and unsaturated hydrocarbons;
• via gas chromatography-isotope ratio mass spectrometry (GC-IRMS) to assess the ratio of 13C to 12C in methane, ethane, propane, isobutane, normal butane, isopentane, normal pentane and carbon dioxide;
• via GC-IRMS to assess the ratio of deuterium (2H) to protium (1H) in methane and H2-.

[0070] Advantageously, the method according to the invention may be used to identify the subsurface origin of gasses leaking from plugged and abandoned wells.

[0071] Advantageously, the method according to the invention may be used to identify the subsurface origin of gasses exiting at the surface in areas where drilling activities have occurred.

[0072] Advantageously, the method according to the invention may be used to identify subsurface origin of gasses leaking from geothermal wells.

[0073] Advantageously, the method according to the invention may be used to identify subsurface origin of hydrocarbon gasses liberated from carbon dioxide storage zone(s) that are detected at surface or within monitoring wells.

[0074] The skilled person will appreciate that all preferred or optional features of the invention described with reference to only some aspects or embodiments of the invention may be applied to all aspects of the invention.

[0075] It will be appreciated that optional features applicable to one aspect of the invention can be used in any combination, and in any number. Moreover, they can also be used with any of the other aspects of the invention in any combination and in any number. This includes, but is not limited to, the dependent claims from any claim being used as dependent claims for any other claim in the claims of this application.

DESCRIPTION OF THE FIGURES

[0076] The present invention is hereinafter further clarified in some of its preferred embodiments shown for purely exemplifying and non-limiting purposes with reference to the accompanying drawings, in which:

Figure 1

shows a cross section view of a typical oil or gas production well showing the typical structure of casing and annuli in an oil or gas production well,

Figure 2

shows a cross section view of a well bore showing examples of potential fluid flow paths responsible for the build-up of sustained annuli pressure or sustained casing pressure,

Figure 3

shows a diagram of the steps in the method according to the invention,

Figure 4A and 4B

compares normalized molecular composition (Fig. 4A) and carbon isotope character (Fig. 4B) of C1-C5 hydrocarbons in production gas and annulus gas from a first producing well, wherein the similarity of the two gasses indicates that the annulus gas is derived from the producing reservoir,

Figure 5A and 5B

compares normalized molecular composition (Fig. 5A) and carbon isotope character (Fig. 5B) of C1-C5 hydrocarbons in production gas (solid line) and annulus gas (dashed line) from a producing well, wherein the differences between the two gasses indicates that the annulus gas is not derived from the producing reservoir and has a source from a different subsurface zone penetrated by the well,

Figure 6

shows a general schematic view of the apparatus used to releases gas from rock samples,

Figure 7

shows an example of a released gas character log generated by the crusher analysis technology that can be used to identify potential origin intervals for gas influx into the annuli of the well,

Figure 8

shows an example of a comparison Carbon isotope ratios and Hydrogen isotope ratios of methane for the gasses G1 exiting at the surface from a leaking, abandoned and buried well head, and for the gas CG1 and CG2 released by the mechanical crushing of two rock samples, respectively A and B having different known subsurface depth origins, of an abandoned well.

DETAILED DESCRIPTION OF THE INVENTION AND OF ITS PREFERRED AND EXEMPLARY EMBODIMENTS

[0077] The method according to the invention may be used for assessing the integrity status of an oil and gas well W as the one illustrated in Fig. 1 and described hereinabove.

[0078] In particular, the well W comprises a well head WH protruding above the surface S and a subsurface structure SS inserted in a borehole drilled in the soil below the surface and provided with nested casings W1, W2 and W3, and also with an innermost production tube P. The subsurface structure SS comprises a plurality of concentric annuli A, B, C of decreasing diameter with depth. More in detail, the subsurface structure SS comprises a plurality of annuli A, B, C that are defined between an outer casing and a corresponding inner casing nested into the outer casing (respectively between W1 and W2 for the annulus C, and between W2 and W3 for the annulus B), and between the innermost casing W3 and an innermost production tube P (for the annulus A); said annuli A, B, C being in fluid communication with the well head WH.

[0079] In particular, the method according to the invention allows to identify the subsurface origin of gasses accumulating in one or more of the annuli A, B and C of the well.

[0080] The method according to the invention for assessing the integrity status of a well W or of a similar borehole extending from a surface S downward into the subsurface, in particular for assessing the integrity status of an oil and gas well, comprising:

• collecting/sampling 101 of the fluids, in particular of the gasses, exiting at a well head WH of said well W or of a nearby well and coming from a subsurface structure SS of said well or of a nearby well,
• determining 120 the subsurface origin of said exiting fluids by using the composition features of released fluids, in particular of released gasses, that are released following to the application of a mechanical action, preferably a crushing action, on subsurface rock samples having known subsurface origins in said well or in a nearby well.

[0081] In one possible embodiment of the present invention, the method determines the subsurface origin of all exiting gasses collected at the wellhead by using the composition features of released fluids, in particular of released gasses, that are released following the application of a mechanical action, preferably a crushing action, on subsurface rock samples having known subsurface origins in said well or in a nearby well.

[0082] In one possible embodiment of the present invention, the method is configured to assess the integrity status of a well W by determining always and only the subsurface origin of all exiting gasses collected at the wellhead by using the composition features of released fluids, in particular of released gasses, that are released following to the application of a mechanical action, preferably a crushing action, on subsurface rock samples having known subsurface origins.

[0083] In one possible embodiment of the present invention, the method is configured to assess the integrity status of a well W by identifying the origin of fluids responsible for annular pressure build-up in oil and gas wells W and it comprises the steps of:

• sampling/collecting 101A a first gas exiting from the production tubing,
• sampling/collecting 101B a second gas exiting from any annuli of the well and in particular the annulus of wells showing pressure build-up (Sustained Annulus Pressure or Sustained Casing Pressure),
• preliminary analysis 102 of the molecular composition and isotopic ratios of the said two exiting gas samples,
• preliminary comparison/matching 103 of the molecular composition, the carbon isotope ratios of C1-C5 hydrocarbons and the hydrogen isotope ratio of methane of said two exiting gas samples so collected to determine whether the second gas sample exiting from the annulus is derived from a producing reservoir (first region) or from a non-producing interval (second region),
• analysis 121 of the molecular composition and isotopic ratios of gasses released from a mechanical action, preferably a crushing action, on rock samples (drill cuttings, side wall core chips, conventional core chips) having known origins,
• comparison/matching 122 of the molecular composition, the carbon isotope ratios of C1-C5 hydrocarbons and the hydrogen isotope ratio of methane between said second gas sample exiting from the annulus so collected and the released gasses so analyzed,
• determining 120 the subsurface origin of said exiting fluids in view of said comparison/matching 122.

[0084] Preferably, said method is configured so as to define/build from a profile/log with all the analyses 121 released gasses at different known origins, wherein in particular to each analysis 121 of the released gasses is associated with the corresponding known origin of the rock sample from which the corresponding gasses have been released.

[0085] More in detail, in a possible embodiment of the present invention, the method comprises the following steps of:

a) sampling 101A a first gas exiting from the production tubing of the well into a cylinder at the well head;

b) sampling 101B a second gas exiting from the pressurised annulus into a cylinder at the well head;

c) preliminary analysing 102 the two gas samples, respectively a first preliminary analysis for the first gas and a second preliminary analysis for the second gas, via:

c1: gas chromatography to assess molecular abundance of hydrogen, nitrogen, oxygen, carbon dioxide, hydrogen sulphide, methane, ethane, propane, isobutane, normal butane, neopentane, isopentane, normal pentane, summed hexane plus heavier hydrocarbons, and unsaturated hydrocarbons;

c2: via gas chromatography-isotope ratio mass spectrometry (GC-IRMS) to assess the ratio of 13C to 12C in methane, ethane, propane, isobutane, normal butane, isopentane, normal pentane and carbon dioxide;

c3: via GC-IRMS to assess the ratio of deuterium (2H) to protium (1H) in methane and H2;

c4: via GC-IRMS to assess the ratio of 14N to 15N in N2;

d) preliminary comparison 103 of the gas analysis results to determine the degree of similarity between the second gas sampled from the annulus and the first gas sampled from the production stream;

e) placing a sample of rock from drill cuttings into a sealed vessel and mechanically crushing said rock sample to release/liberate entrained gas;

f) analysis 122 of released gas for molecular and isotopic composition (as "c" above)-;

f1: gas chromatography to assess molecular abundance of hydrogen, nitrogen, oxygen, carbon dioxide, hydrogen sulphide, methane, ethane, propane, isobutane, normal butane, neopentane, isopentane, normal pentane, summed hexane plus heavier hydrocarbons, and unsaturated hydrocarbons;

f2: via gas chromatography-isotope ratio mass spectrometry (GC-IRMS) to assess the ratio of 13C to 12C in methane, ethane, propane, isobutane, normal butane, isopentane, normal pentane and carbon dioxide-;

f3: via GC-IRMS to assess the ratio of deuterium (2H) to protium (1H) in methane and H2;

g) repeat the steps e) and f) for all available drill cuttings samples collected at different known depths so as to define/build a corresponding profile wherein the results of the analysis of each released gas are associated to the corresponding known origin/depth;

h) comparison 122 between the second preliminary analysis results made on the second gas sample exiting from the annulus and the profile of the released gasses so defined,

i) determining 120 the subsurface origin of said second gas sample exiting from the annulus in view of said comparison 122.

[0086] Preferably, the steps f) - i) are performed if there is no similarity between the second gas sampled from the annulus and the first gas sampled from the production stream, while if there is similarity between the second gas exiting/sampled from the annulus and the first gas sampled from the production stream, then it is determined that the second gas exiting/sampled comes from the production stream, that is the source of the first gas sample.

[0087] In particular, the first gas exiting sample may be representative of the production from the well. Preferably, the sampling 101A of production gas at the surface may be accomplished by attaching a steel cylinder to a sampling point on the flow line at the well head, or on a production separator vessel. Preferably, the flow lines and sampling vessels are adequately purged of extraneous gasses before collecting the production sample for analysis, so as to be ensure that the gas sample is representative of the production from the well.

[0088] In particular, the second exiting gas sample is representative of the gas exiting from the annulus of the well. Preferably, the sampling 101B of annulus gas may be achieved by attaching an evacuated steel cylinder to the annulus bleed valve at the well head and opening the annulus bleed valve.

[0089] Steel cylinders containing the pressurized production and annulus gas samples may be transferred to an analytical laboratory for the preliminary analyses 102 of their respective molecular and isotopic composition.

[0090] Preferably, molecular compositional analysis is achieved by gas chromatography. For example, the gas sample cylinders are sub-sampled via a pressure reducing valve attached to the outlet of the cylinder and a known volume of gas is injected into a gas chromatograph. Gas chromatographic analysis is carried out using both flame ionization detection (FID) and thermal conductivity detection (TCD) to allow the detection and quantification of hydrogen, nitrogen, oxygen, carbon dioxide, hydrogen sulphide, methane, ethane, propane, isobutane, normal butane, isopentane, normal pentane and summed hexane plus heavier hydrocarbons within the natural gas mixture.

[0091] Additional analyses may be carried out on gas sub-samples using a gas chromatograph attached to an isotope ratio mass spectrometer (GC-IRMS) to measure the ratio of 13C to 12C in methane, ethane, propane, isobutane, normal butane, isopentane, normal pentane and carbon dioxide and the ratio of 2H to 1H in methane. Results may be reported using delta notation (δ13C and δD) referenced to Vienna Pee Dee Belemnite (VPDB) and Vienna Standard Mean Ocean Water (VSMOW) for carbon and hydrogen respectively.

[0092] Example results of the results of the preliminary gas analyses are shown in figure 4, which compares the molecular composition and carbon isotope values of C1-C5 hydrocarbons in samples of production gas and annulus gas from a producing well. In this case there is a clear match between the analysis results of the two gasses, which indicates that the annulus gas is likely to originate from the producing reservoir and that there is a loss of sealing integrity within the well which allows fluid migration into the annulus.

[0093] Figure 5A and 5B shows an example where there is not a good match between the compositional and carbon isotope data of the production and the annulus gas. In this case, the data can be used to prove that the annulus gas does not originate in the producing reservoir but from a non-producing region.

[0094] In the method according to the invention, the analysis 121 of gas released from archived samples of rock material collected at surface during the drilling of the well or nearby wells are used to identify a non-reservoir origin of the annulus gas. Preferably, cuttings are washed to remove drilling mud and air dried prior to storage. Preferably, they are typically collected at about 10metres intervals and so offer the opportunity to construct "logs" of gas characteristics throughout the drilled section. Ideally, cuttings from the same production well may be analyzed, but samples from nearby wells in the same field can be used if such samples are not available. Core chip samples and side wall core samples may also be used for the analysis.

[0095] The released gas from the rock sample may be collected as follows. A sample of washed and dried rock 10 is placed into a gas tight receptacle 11 provided with a mechanical crushing device 12. The receptacle 11 is connected by means of a flow line 13 to an analytical instrument 16 comprising a gas chromatograph 14 with a detector 15 (for example an IRMS) for gas composition and isotope analysis of liberated gas components. The rock sample 10 is crushed to release entrained gas, which is then transferred to the analytical instrument 16 via the flow line 13 (see Figure 6).

[0096] Preferably, the analysis 121 of the mechanically released gas is carried out in the same way as the preliminary analysis 102 of the production and annulus gas samples.

[0097] Preferably, the results of the analysis 121 may be plotted against depth in the well (see Figure 7) and, ideally, compared to geological and petrophysical logs to aid interpretation.

[0098] Figure 8 shows an example of a match between the exiting gas G1 collected at the surface for an abandoned well with the gasses CG1 and CG2 released from subsurface rock samples having known subsurface origins, respectively at the upper depth 1 and at the lower depth 2. The matching of the composition features of G1 with the ones of CG1 pinpoints that the origin of the gas leaking G1 is from to the upper depth 1. In particular, the depth interval 1 associated to the released gasses CG1 having the closest isotopic and compositional match to the exiting gas G1 can be assigned as the origin of the fluid influx into the well and thus, advantageously, the fluid flow paths can be identified to design remediation efforts.

[0099] The method according to the invention may be also used to identify the subsurface origin of gasses emissions/leakage from plugged and/or abandoned wells, from geothermal wells, from carbon dioxide storage zone(s) and/or from areas where drilling activities have occurred (both in marine and terrestrial environments).

[0100] The present invention has been illustrated and described in some of its preferred embodiments, but it is understood that executive variants can be applied to them in practice, without however departing from the scope of protection of the present patent for industrial invention.

[0101] In relation to the detailed description of the different embodiments of the invention, it will be understood that one or more technical features of one embodiment can be used in combination with one or more technical features of any other embodiment where the transferred use of the one or more technical features would be immediately apparent to a person of ordinary skill in the art to carry out a similar function in a similar way on the other embodiment.

[0102] In the preceding discussion of the invention, unless stated to the contrary, the disclosure of alternative values for the upper or lower limit of the permitted range of a parameter, coupled with an indication that one of the values is more highly preferred than the other, is to be construed as an implied statement that each intermediate value of the parameter, lying between the more preferred and the less preferred of the alternatives, is itself preferred to the less preferred value and also to each value lying between the less preferred value and the intermediate value.

[0103] The features disclosed in the foregoing description or the following drawings, expressed in their specific forms or in terms of a means for performing a disclosed function, or a method or a process of attaining the disclosed result, as appropriate, may separately, or in any combination of such features be utilized for realizing the invention in diverse forms thereof.

Claims

1. Method for assessing the status of a well (W) or of a similar borehole extending from a surface (S) downward into a subsurface, preferably for assessing the integrity status of an oil and gas well, comprising:

- collecting/sampling (101) of subsurface fluids exiting from the subsurface of said well or of a nearby well,

- determining (120) the subsurface origin of said subsurface exiting fluids by using the composition features of released fluids, in particular of released gasses, that are released following the application of a mechanical action, preferably a crushing action, on subsurface rock samples having known subsurface origins in said well or in a nearby well.

2. Method according to claim 1, characterized in that the subsurface rock samples comprise drill cuttings, side wall core chips and/or conventional core chips.

3. Method according to one or more of the previous claims, characterized in that the subsurface exiting fluids, in particular of the subsurface exiting gasses, are collected/sampled at a well head (WH), of said well or of a nearby well, protruding from the surface (S).

4. Method according to one or more of the previous claims, characterized in that:

- said subsurface exiting fluids exit from a subsurface structure (SS) of the well (W) comprising a plurality of annuli (A, B, C) that are defined by an outer casing and a corresponding inner casing nested into the outer casing, and between the innermost casing (W3) and an innermost production tube (P), said annuli (A, B, C) being in fluid communication with the well head (WH),

- said fluids exiting from the subsurface structure (SS) are collected at the well head (WH) when a pressure build-up in one of the annuli of the subsurface structure is sensed and/or during routine monitoring operations.

5. Method according to one or more of the previous claims, characterized in that:

- said subsurface exiting fluids exit from a subsurface structure (SS) of the well (W) comprising a plurality of annuli (A, B, C) that are defined by an outer casing and a corresponding inner casing nested into the outer casing, and between the innermost casing (W3) and an innermost production tube (P), said annuli (A, B, C) being in fluid communication with the well head (WH),

- the collected/sampled fluids exiting from the subsurface structure (SS) comprises gasses coming from at least one annulus of the subsurface structure (SS) wherein a pressure build-up is sensed and/or during routine monitoring operations

6. Method according to one or more of the previous claims, characterized in that the collected/sampled fluids exiting from the subsurface structure (SS) comprises gasses coming from the innermost production tube (P).

7. Method according to one or more of the previous claims, characterized in that:

- said subsurface exiting fluids exit from a subsurface structure (SS) of the well (W) comprising a plurality of annuli (A, B, C) that are defined by an outer casing and a corresponding inner casing nested into the outer casing, and between the innermost casing (W3) and an innermost production tube (P), said annuli (A, B, C) being in fluid communication with the well head (WH),

- the collected/sampled fluids exiting from the subsurface structure (SS) comprises gasses coming from the innermost production tube (P) and also gasses coming from at least one annulus of the subsurface structure (SS)

8. Method according to one or more of the previous claims, characterized in that the composition features of said exiting fluids are compared with the composition features of the released fluids.

9. Method according to one or more of the previous claims, characterized in that the composition features comprise molecular composition.

10. Method according to one or more of the previous claims, characterized in that the composition features comprise isotopic data.

11. Method according to one or more of the previous claims, characterized in that the composition features comprise carbon isotope ratios of C1-C5 hydrocarbons and/or hydrogen isotope ratio of methane.

12. Method according to one or more of the previous claims, characterized in that said method comprises:

- a preliminary step wherein the composition features of exiting fluids are analyzed to determine whether such exiting fluids are derived from a first region or from a second region, and

- if the exiting fluids result to be from the second/external region, the subsurface origin of exiting fluids is determined by using the composition features of released fluids, in particular of released gasses, that are mechanically released from subsurface rock samples having known subsurface origins.

13. Method according to one or more of the previous claims, characterized in that it comprises:

- sampling (101A) of a first gas exiting at the well head (WH) from an innermost tube (P) of the well wherein said innermost tube is the production tube for transporting the hydrocarbons to the surface, so as to get at least one first exiting gas sample representing the gas of the production stream,

- sampling (101B) of a second gas exiting at the head assembly from a pressurised annulus of the well, in particular from the annulus wherein a pressure build-up is sensed, and/or from a non-pressurized annulus, in particular during routine monitoring operations, so as to get at least one second exiting gas sample representing the gas from the annulus of the well,

- the preliminary analysis (102) of said at least one first exiting gas sample and of said at least one second exiting gas sample,

- the preliminary comparing/matching (103) the results data of said analysis for the first exiting gas sample with the results data of said analysis for the second exiting gas sample so as to determine the degree of similarity between the first exiting gas and the second exiting gas,

- determining, in view of said preliminary comparing/matching, whether the second exiting gas is derived from said first region corresponding to the innermost/production tube (P) or from said second region not corresponding to the innermost/production tube (P)

- if the exiting fluids result to be from the second/external region, the subsurface origin of said second exiting fluid is determined by using the composition features of released fluids, in particular of released gasses, that are mechanically released from subsurface rock samples having known subsurface origins.

14. Method according to one or more of the previous claims, characterized in that it comprises a comparison/matching of results data of the analysis (122) of the released gas with the results data of the preliminary analysis of the second exiting gas sample so as to determine the subsurface origin of the second exiting gas.

15. Method according to one or more of the previous claims, characterized in that for each rock sample of a plurality of rock samples collected at different known origins/depths of the same or nearby well, the corresponding gasses released from the crushing of the rock sample are analyzed so as to define/build a profile of the composition features of the released gasses for each different known subsurface origin/depth.

Drawing