Method for evaluating cement sheath through pulsed neutron full-spectrum logging

Abstract

The invention relates to the technical field of oil well and gas well logging, in particular to a method for evaluating a cement sheath through pulsed neutron full-spectrum logging. The method comprises the following steps: arranging two detectors with different distances from a neutron generator, namely a near-end detector and a far-end detector, respectively acquiring near-end and far-end non-elastic calcium yield and captured calcium yield logging curves according to the two detectors at different positions, and identifying a cement sheath density change region; identifying whether the cement sheath is filled with water and gas and whether the cement sheath is deformed and expanded in the identified cement sheath density change area; interpreting the filling rate of water filling and / or gas filling in the cement sheath; and explaining the density of the cement sheath by using a GRAT logging curve. According to the method for evaluating the cement sheath through pulse neutron full-spectrum logging, on one hand, information is provided for shaft integrity, and on the other hand, a foundation can be laid for correcting the influence of set cement on stratum oil / gas saturation interpretation.

Description

Technical Field

[0001] The present invention relates to the technical field of well logging for oil wells and gas wells, and particularly relates to a method for evaluating a cement sheath by pulsed neutron full-spectrum logging. Background Art

[0002] During the oilfield development process, the cement sheath plays a crucial role. It not only supports and protects the casing but also prevents crossflow between layers, avoiding the waste of oil and gas resources due to underground crossflow into other formations or safety accidents caused by crossflow to the ground. Pulsed neutron full-spectrum logging can obtain multiple logging curves such as C / O, Cl / H, Σ (imaging), etc. in one logging run, interpret the remaining oil / gas saturation of the formation, and judge the water flooding level. During the logging interpretation process, the absence of the cement sheath will also affect the interpretation result of the remaining oil saturation.

[0003] The oil well cement sheath is generally obtained by mixing G-class cement with water to obtain cement stone, and the main components are CaO, SiO 2 , Al 2 O 3 etc., and the density is usually 1.95 g / cm 3 . However, when the ratio of G-class cement to water changes during the mixing of cement stone, the density will change, and during the logging process, the filling degree of the annular space of the cement sheath. The unfilled space of the cement sheath may be replaced by gas or water, which will affect the pulsed neutron full-spectrum logging. Therefore, it is necessary to evaluate the density of the cement sheath and the filling rate of the cement stone. On the one hand, it can correct the influence of the cement stone on the interpretation of the formation oil / gas saturation, and on the other hand, it can also provide information for wellbore integrity.

[0004] Chinese Patent No. 202211542803.0 discloses a method for inverting the azimuth gamma density of the cement sheath in horizontal wells. This technology is based on fractal theory, image processing technology, and the structural characteristics and failure mechanisms of the casing-cement sheath-formation composite body. Using the correlation between the fractal dimension of the interface morphology of the blank group of the casing-cement sheath and the fractal dimension of the microscopic pore morphology of the cement sheath body and other mechanical properties, the integrity of the blank group of the cement sheath is quantitatively evaluated. Using the relationship between the relevant parameters of the conditional control group after the action of alternating load and the macroscopic mechanical properties of the cement sheath, the integrity of the control group of the cement sheath is quantitatively evaluated.

[0005] In actual application and research and development, it is found that the existing technology can judge the integrity of the cement sheath and interpret the density of the cement sheath through logging tools, but it cannot judge whether there is water / gas filling the cement sheath in the case of the absence of the cement sheath, which further affects the accuracy of the interpretation result of the remaining oil saturation. Summary of the Invention

[0006] (I) Technical Problems to be Solved

[0007] The present invention provides a method for evaluating a cement sheath by pulsed neutron full-spectrum logging, so as to overcome the defect that the existing technology cannot determine whether there is water / gas filling the cement sheath in the case of cement sheath loss, resulting in affecting the interpretation result of the remaining oil saturation.

[0008] (II) Technical solution

[0009] To solve the above problems, the present invention provides a method for evaluating a cement sheath by pulsed neutron full-spectrum logging, including:

[0010] Set two detectors at different distances from the neutron generator, namely a proximal detector and a distal detector, and respectively obtain the near and far non-elastic calcium yield and capture calcium yield logging curves according to the detectors at two different positions to identify the area where the density of the cement sheath changes;

[0011] Identify whether the cement sheath is filled with water or gas and whether the cement sheath is deformed and enlarged in the identified area where the density of the cement sheath changes;

[0012] Interpret the filling rate of water and / or gas filling in the cement sheath;

[0013] Use the GRAT logging curve to interpret the density of the cement sheath.

[0014] Preferably, the area where the density of the cement sheath changes is manifested as: the logging curve of the decrease in the density of the cement sheath shows that both the near and far non-elastic calcium yield curves decrease, and the decrease amplitude of the non-elastic calcium yield curve of the distal detector is greater after their superposition; the increase amplitudes are basically the same after the superposition of the near and far capture calcium yield curves.

[0015] Preferably, obtain the near and far non-elastic oxygen yield, non-elastic carbon yield and Cl / H ratio of chlorine and hydrogen through the proximal and distal detectors respectively, and compare them:

[0016] When the cement sheath is filled with water, it is manifested as: the non-elastic oxygen yield curves of the proximal and distal detectors increase, and the increase amplitude of the non-elastic oxygen yield of the proximal detector is greater after their superposition; the Cl / H curves of the proximal and distal detectors both decrease, and the decrease amplitude of the Cl / H curve of the distal detector is greater after their superposition;

[0017] When the cement sheath is filled with gas, it is manifested as: the non-elastic carbon yield curves of the proximal and distal detectors increase, and the increase amplitude of the non-elastic carbon yield curve of the proximal detector is greater after their superposition; the Cl / H curves of the proximal and distal detectors both increase, and the increase amplitude of the Cl / H curve of the distal detector is greater after their superposition;

[0018] When the cement sheath is deformed and causes enlargement, the non-elastic carbon yield curve, non-elastic oxygen yield curve, and the capture Cl / H curves of the proximal and distal detectors all decrease.

[0019] Preferably, the specific interpretation of the filling rate in the cement sheath includes:

[0020] When the cement sheath is filled with water, the increment of the inelastic oxygen yield curve caused by the water in the cement sheath is used to explain the filling rate of the cement sheath;

[0021] and / or

[0022] When the cement sheath is filled with gas, the increment of the inelastic carbon yield curve caused by the water in the cement sheath is used to explain the filling rate of the cement sheath.

[0023] Preferably, when the cement sheath is filled with water, the filling rate of the cement sheath is expressed as the water filling rate, and the water filling rate D 水 According to formulas (1) and (2), it can be obtained that:

[0024] D 水 =(ΔO - a) / b

[0025] ΔO = O 测井 - O 基线

[0026] In the formula: ΔO is the change in oxygen yield caused by the water in the cement sheath; a and b are constants; O 测井 is the oxygen yield logging value; O 基线 is the oxygen yield logging value of the undamaged part of the cement sheath.

[0027] Preferably, when the cement sheath is filled with gas, the filling rate of the cement sheath is expressed as the gas filling rate, and the water filling rate D 气 According to formulas (1) and (2), it can be obtained that:

[0028] D 气 =(ΔC - a) / b

[0029] ΔC = C 测井 - C 基线

[0030] In the formula: ΔC is the change in carbon yield caused by the water in the cement sheath; a and b are constants; C 测井 is the carbon yield logging value; C 基线 is the carbon yield logging value of the undamaged part of the cement sheath.

[0031] Preferably, the method for interpreting the density of the cement sheath by using the GRAT logging curve specifically includes:

[0032] ρ=(a - GRAT) / b, where a and b are constants,

[0033] ρ is the density of the cement sheath;

[0034] where GRAT is a logging curve, and the method for extracting its logging curve is

[0035]

[0036] Wherein, FINC is the inelastic count of the far detector; FCAPC is the capture count of the far detector; e is a constant; NINC is the inelastic count of the near detector.

[0037] (III) Beneficial effects

[0038] The present invention provides a method for evaluating a cement sheath by pulsed neutron full-spectrum logging. By using logging curves such as inelastic calcium yield and capture calcium yield of different detectors, the area where the density of the cement sheath changes is identified. By using logging curves such as oxygen yield and carbon yield of different detectors, the filling of water or gas in the cement sheath or the deformation of the cement sheath is identified, and the filling rate and density of the cement stone are interpreted. On the one hand, it provides information for wellbore integrity, and on the other hand, it lays a foundation for correcting the influence of the cement stone on the interpretation of formation oil / gas saturation. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 is a flow chart of the method for evaluating a cement sheath by pulsed neutron full-spectrum logging according to an embodiment of the present invention;

[0040] Figure 2 is a schematic diagram for distinguishing density changes in three regions: wellbore, cement sheath, and formation. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0041] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0042] The present invention provides a method for evaluating a cement sheath by pulsed neutron full-spectrum logging. By using pulsed neutron full-spectrum logging data, the area where the density of the cement sheath changes is identified, the filling of water or gas in the cement sheath or the deformation of the cement sheath is identified, and the filling rate and density of the cement stone are interpreted. This requires that the pulsed neutron logging tool has more than 2 detectors. The detector at a position relatively close to the pulsed neutron logging tool is defined as the proximal detector, and the detector at a position relatively far from the pulsed neutron logging tool is defined as the distal detector. The near and far inelastic energy spectra, capture energy spectra, and time spectra are obtained through the proximal and distal detectors respectively, and logging curves such as inelastic Ca yield, inelastic O yield, inelastic C yield, capture Ca yield, Cl / H, GRAT, etc. are extracted from the energy spectra.

[0043] As Figure 1 shown, the present invention provides a method for evaluating a cement sheath by pulsed neutron full-spectrum logging, which specifically includes:

[0044] Step S1: Set two detectors at different distances from the neutron generator, namely the proximal detector and the distal detector. Obtain the non-elastic calcium yield and capture calcium yield logging curves for the near and far ends respectively according to the detectors at the two different positions, and identify the area where the density of the cement sheath changes.

[0045] In specific applications, the density changes in three regions, namely the borehole, the cement sheath, and the formation, can be distinguished respectively. Refer to Figure 2 , and compare the non-elastic calcium yields and capture calcium yields obtained by the near and far detectors respectively. The specific differences are as follows:

[0046] When the density of the borehole decreases, the logging curves show that both the near and far non-elastic calcium yield curves decrease, and after superimposing, the decrease amplitude of the non-elastic calcium yield curve of the far detector is greater. While both the near and far capture calcium yield curves increase, and after superimposing, the increase amplitude of the capture calcium yield curve of the near detector is greater;

[0047] When the density of the cement sheath decreases, the logging curves show that both the near and far non-elastic calcium yield curves decrease, and after superimposing, the decrease amplitude of the non-elastic calcium yield curve of the far detector is greater. While after superimposing the near and far capture calcium yield curves, the increase amplitudes are basically the same;

[0048] When the density of the formation decreases, the logging curves show that the near non-elastic calcium yield curve is not affected by the density, while the far non-elastic calcium yield curve decreases. And both the near and far capture calcium yield curves increase, and after superimposing, the increase amplitude of the capture calcium yield curve of the far detector is greater.

[0049] Step S2: Identify whether the cement sheath is filled with water or gas and whether the cement sheath is deformed and enlarged in the identified area where the density of the cement sheath changes.

[0050] Specifically include:

[0051] When the cement sheath is filled with water, it is manifested as: the non-elastic oxygen yield curves of the near and far detectors increase, and after superimposing, the increase amplitude of the non-elastic oxygen yield of the near detector is greater; both the near and far detector Cl / H curves decrease, and after superimposing, the decrease amplitude of the Cl / H curve of the far detector is greater;

[0052] When the cement sheath is filled with gas, it is manifested as the non-elastic carbon yield curves of the near and far detectors increase, and after superimposing, the increase amplitude of the non-elastic carbon yield curve of the near detector is greater. Both the near and far detector Cl / H curves increase, and after superimposing, the increase amplitude of the Cl / H curve of the far detector is greater;

[0053] When the cement sheath is deformed and causes enlargement, the non-elastic carbon yield curve, the non-elastic oxygen yield curve, and the near and far detector capture Cl / H curves all decrease.

[0054] Step S3: Interpret the filling rate of water and / or gas in the cement sheath.

[0055] Specifically, when the cement sheath is filled with water, the increment of the inelastic oxygen yield curve of the near detector caused by the water in the cement sheath is used to explain the filling rate of the cement sheath; when the cement sheath is filled with gas, the increment of the inelastic carbon yield curve of the near detector caused by the water in the cement sheath is used to explain the filling rate of the cement sheath. Taking the case where the cement sheath is filled with water as an example, the filling rate of the cement sheath is expressed as the water filling rate, and the interpretation model reference is as follows:

[0056] D 水 =(ΔO - a) / b

[0057] ΔO = O 测井 - O 基线

[0058] Where: ΔO is the change in oxygen yield caused by the water in the cement sheath; a and b are constants; O 测井 is the oxygen yield logging value; O 基线 is the oxygen yield logging value of the undamaged part of the cement sheath.

[0059] In specific applications, there are certain differences in the constants of different instruments. For example, when using a certain existing logging instrument to interpret the filling rate of the water cement sheath, the following formula can be used:

[0060] D 水 =(ΔO - 6.69E - 3) / 0.15

[0061] When the cement sheath is filled with gas, the filling rate of the cement sheath is expressed as the gas filling rate, and the calculation model of the water filling rate D 气 is as follows:

[0062] D 气 =(ΔC - a) / b

[0063] ΔC = C 测井 - C 基线

[0064] Where: ΔC is the change in carbon yield caused by the water in the cement sheath; a and b are constants;

[0065] C 测井 is the carbon yield logging value; C 基线 is the carbon yield logging value of the undamaged part of the cement sheath.

[0066] Step S4. Use the GRAT logging curve to interpret the density of the cement sheath.

[0067] Specifically, the calculation formula for interpreting the density of the cement sheath using the GRAT logging curve is:

[0068] ρ=(a - GRAT) / b, where a and b are constants,

[0069] ρ is the density of the cement sheath;

[0070] Among them, GRAT is a logging curve, and the method for extracting its logging curve is

[0071]

[0072] Among them, FINC is the inelastic count of the far detector; FCAPC is the capture count of the far detector; e is a constant; NINC is the inelastic count of the near detector.

[0073] In practical applications, the constants of different instruments are different. For the instruments used in combination in this embodiment, the following formula is obtained:

[0074] ρ = (0.2334 - GRAT) / 0.015.

[0075] A method for evaluating a cement sheath by pulsed neutron full-spectrum logging provided by the present invention uses logging curves such as the inelastic Ca yield and capture Ca yield of different detectors to identify the regions where the density of the cement sheath changes, uses logging curves such as the O yield and C yield of different detectors to identify the filling of water or gas in the cement sheath or the deformation of the cement sheath, and interprets the filling rate and density of the cement stone. On the one hand, the purpose of the invention can also provide information for wellbore integrity, and on the other hand, it can lay a foundation for correcting the influence of the cement stone on the interpretation of formation oil / gas saturation.

[0076] The above embodiments are only used to illustrate the present invention and are not intended to limit the present invention. Those of ordinary skill in the relevant technical fields can also make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, all equivalent technical solutions also belong to the scope of the present invention, and the patent protection scope of the present invention shall be defined by the claims.

Claims

1. A method for evaluating a cement sheath by pulsed neutron full-spectrum logging, characterized in that, it includes: Two detectors at different distances from the neutron generator are set, namely a proximal detector and a distal detector. According to the detectors at two different positions, near and far inelastic calcium yield and capture calcium yield logging curves are obtained respectively to identify the area where the density of the cement sheath changes; In the identified area where the density of the cement sheath changes, identify whether the cement sheath is filled with water or gas and whether the cement sheath is deformed and enlarged in diameter; Interpret the filling rate of water and / or gas filling in the cement sheath; Use the GRAT logging curve to interpret the density of the cement sheath.

2. The method for evaluating a cement sheath by pulsed neutron full-spectrum logging according to claim 1, characterized in that, it includes: The area where the density of the cement sheath changes is manifested as: the logging curve of the decrease in the density of the cement sheath shows that both the near and far inelastic calcium yield curves decrease, and after the two are superimposed, the decrease amplitude of the inelastic calcium yield curve of the distal detector is greater; After the near and far capture calcium yield curves are superimposed, the increase amplitude is basically the same.

3. The method for evaluating a cement sheath by pulsed neutron full-spectrum logging according to claim 1, characterized in that, Near and far inelastic oxygen yield, inelastic carbon yield and Cl / H are obtained respectively by the near and far detectors and compared: When the cement sheath is filled with water, it is manifested as: the inelastic oxygen yield curves of the near and far detectors increase, and after the two are superimposed, the increase amplitude of the inelastic oxygen yield of the proximal detector is greater; The Cl / H curves of the near and far detectors both decrease, and after the two are superimposed, the decrease amplitude of the Cl / H curve of the distal detector is greater; When the cement sheath is filled with gas, it is manifested as that the inelastic carbon yield curves of the near and far detectors increase, and after the two are superimposed, the increase amplitude of the inelastic carbon yield curve of the proximal detector is greater, the Cl / H curves of the near and far detectors both increase, and after the two are superimposed, the increase amplitude of the Cl / H curve of the distal detector is greater; When the cement sheath is deformed and causes an enlarged diameter, the inelastic carbon yield curve, the inelastic oxygen yield curve, and the capture Cl / H curves of the near and far detectors all decrease.

4. The method for evaluating a cement sheath by pulsed neutron full-spectrum logging according to claim 1, characterized in that, The specific interpretation of the filling rate in the cement sheath includes: When the cement sheath is filled with water, the increment of the inelastic oxygen yield curve caused by the water in the cement sheath is used to interpret the filling rate of the cement sheath; and / or When the cement sheath is filled with gas, the increment of the inelastic carbon yield curve caused by the water in the cement sheath is used to interpret the filling rate of the cement sheath.

5. The method for evaluating a cement sheath by pulsed neutron full-spectrum logging according to claim 4, characterized in that, When the cement sheath is filled with water, the filling rate of the cement sheath is expressed as the water filling rate, and the water filling rate is D 水 It can be obtained from Formula (1) and Formula (2) that: D 水 = (ΔO - a) / b ΔO=O 测井 -O 基线 Where: ΔO is the change in oxygen yield caused by water in the cement sheath; a and b are constants; O 测井 is the oxygen yield logging value; O 基线 is the oxygen yield logging value of the undamaged part of the cement sheath.

6. The method for evaluating a cement sheath by pulsed neutron full-spectrum logging according to claim 4, characterized in that, When the cement sheath is filled with gas, the filling rate of the cement sheath is expressed as the gas filling rate, and the water filling rate is D 气 It can be obtained from Formula (1) and Formula (2) that: D 气 = (ΔC - a) / b ΔC = C 测井 - C 基线 Where: ΔC is the change in carbon yield caused by water in the cement sheath; a and b are constants; C 测井 is the logging value of carbon yield; C 基线 is the logging value of carbon yield of the undamaged part of the cement sheath.

7. The method for evaluating a cement sheath by pulsed neutron full-spectrum logging according to claim 4, characterized in that, The specific interpretation of the density of the cement sheath by using the GRAT logging curve includes: ρ = (a - GRAT) / b, where a and b are constants, ρ is the density of the cement sheath; wherein, GRAT is a logging curve, and its logging curve extraction method is wherein, FINC is the inelastic count of the distal detector; FCAPC is the capture count of the distal detector; e is a constant; NINC is the inelastic count of the proximal detector.

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