Method for evaluating gas packing capacity of cement sheath

Abstract

The invention relates to the field of cement sheath detection, in particular to a method for evaluating the gas packing capacity of a cement sheath, which comprises the following steps of: testing the gas breakthrough pressure of the cement sheath with different heights and cementation characterization parameters by simulating the service temperature, pressure and gas well bore conditions of the cement sheath on site; and a cement sheath packing capacity expression is established through a multivariate linear fitting method according to an experimental result, so that the cement sheath packing capacities of different well sections are accurately evaluated, and the evaluation efficiency of the gas packing capacity of the cement sheath is improved.

Description

Technical Field

[0001] The present invention relates to the field of cement sheath detection, and in particular to a method for evaluating the gas isolation capability of a cement sheath. Background Art

[0002] After years of service, the quality of the wellbore gradually deteriorates, and there is a risk that high-pressure natural gas will leak along the cement sheath to non-target layers, or even leak to the surface along the cement sheath interface, posing a major safety hazard. For the evaluation of cement sheath isolation capacity, current research methods are mainly based on indoor simulation experiments.

[0003] Chinese patent publication number: CN109681190A, discloses a high-temperature and high-pressure gas well cement ring seal integrity evaluation system, which can maintain the cement ring according to the downhole working conditions of the high-temperature and high-pressure gas well, and evaluate the influence of temperature and pressure changes on cement ring seal failure in a simulated gas channeling manner.

[0004] However, the following problems still exist in the prior art: in the prior art, cement sheaths of different cementing qualities cannot be simulated, and no cement sheath sealing ability evaluation model is proposed. Therefore, it is impossible to quantitatively evaluate the cement sheath sealing ability under different cementing quality conditions, which affects the evaluation efficiency of the cement sheath gas sealing ability. Summary of the invention

[0005] To this end, the present invention provides a method for evaluating the gas isolation capacity of a cement ring, so as to overcome the problems in the prior art that cement rings with different cementing qualities cannot be simulated, a cement ring sealing capacity evaluation model is not proposed, and the cement ring sealing capacity under different cementing quality conditions cannot be quantitatively evaluated, which affects the evaluation efficiency of the gas isolation capacity of the cement ring.

[0006] To achieve the above object, the present invention provides a method for evaluating the gas isolation capability of a cement sheath, comprising:

[0007] Step S1, configuring a formation rock ring matching the wellbore structure of the gas well and a casing of corresponding size;

[0008] Step S2, pouring cement slurry into the annular space between the casing and the formation rock ring, obtaining a cement ring test piece at a predetermined time with a gas well temperature and a gas well pressure, and performing an acoustic amplitude test, including obtaining acoustic amplitude values ​​of different detection points at predetermined intervals to determine a gradient acoustic amplitude discrete amount, determining a gradient layout interval of the detection points, obtaining acoustic amplitude values ​​of different detection points again, and calculating a cementation characterization parameter;

[0009] Step S3, repeating step S2, obtaining bonding characterization parameters of cement ring test pieces at different heights, screening the cement ring test pieces based on the range of the bonding characterization parameters, and determining high-quality cement ring test pieces;

[0010] Step S4, placing the high-quality cement ring test piece in a closed space, injecting high-pressure nitrogen into the bottom of the high-quality cement ring test piece, and gradually increasing the pressure in a step-by-step manner until the cement ring interface is destroyed, recording the gas injection pressure at the destruction moment, and determining the gas injection pressure as the cement ring isolation capacity of the corresponding high-quality cement ring test piece;

[0011] Step S5, respectively substituting each group of experimental data into the cement sheath isolation capacity expression to solve the correlation coefficient for the gas well body structure, and obtaining the cement sheath isolation capacity expression for the target well section of the gas well body structure; the experimental data include cementation characterization parameters, cement sheath isolation capacity and height of high-quality cement sheath test pieces;

[0012] Step S6, substituting the determined correlation coefficient into the cement sheath isolation capability expression, and determining the cement sheath isolation capability for the target well section according to the cementation characteristic parameter of the target well section.

[0013] Furthermore, in step S2, the process of obtaining the sound amplitude values ​​of different detection points at predetermined intervals to determine the discrete amount of the gradient sound amplitude includes:

[0014] Acquire the sound amplitude value of each detection point, calculate the variance of each sound amplitude value, and determine the obtained variance as the gradient sound amplitude discrete amount;

[0015] Wherein, each of the detection points is arranged at the predetermined intervals perpendicular to the ground direction.

[0016] Furthermore, in step S2, the process of determining the gradient layout interval of the detection points includes:

[0017] Determine the gradient layout interval for a single cement sheath test piece based on the discrete amount of gradient acoustic amplitude;

[0018] The gradient arrangement interval determined based on the discrete amount of the gradient sound amplitude is negatively correlated with the discrete amount of the gradient sound amplitude.

[0019] Furthermore, in step S2, the process of calculating the bonding characterization parameter includes:

[0020] Solve the average value of each cementation characterization component to obtain the cementation characterization parameter;

[0021] The cementation characterization component is calculated according to formula (1):

[0022]

[0023] Where BI is the bonding characteristic component for a single detection point, A is the acoustic amplitude of the detection point, and A fp A is the casing sound amplitude at the detection point, g is the reference bonding sound amplitude of the cement sheath.

[0024] Furthermore, in step S3, the cement sheath test pieces are screened based on the range of the bonding characterization parameter, and the process of determining high-quality cement sheath test pieces includes:

[0025] If the bonding characterization parameters of a single mud ring test piece are within a predetermined bonding characterization range, the single mud ring test piece is determined to be a high-quality cement ring test piece.

[0026] Furthermore, in step S5, the correlation coefficient for the gas wellbore structure is solved by the cement sheath isolation capability expression (2):

[0027] P fc =a×C×h+b×h+d×C+e(2)

[0028] Where P fc is the cement sheath isolation capacity of a single high-quality cement sheath test piece, a is the comprehensive correlation coefficient, b is the height correlation coefficient, d is the bonding correlation coefficient, e is the capacity correlation coefficient, C is the bonding characterization parameter of a single high-quality cement sheath test piece, and h is the height of a single high-quality cement sheath test piece.

[0029] Furthermore, in step S4, the process of obtaining the cement sheath isolation capability expression of the target well section for the wellbore structure of the gas well includes:

[0030] Substituting the solved correlation coefficients into the cement sheath isolation capacity expression (2) to obtain the cement sheath isolation capacity expression for the target well section of the gas wellbore structure.

[0031] Furthermore, in step S5, the process of obtaining cement sheath bonding data of the target well section includes:

[0032] Obtaining the gradient layout intervals for different cement sheath test pieces determined in step S2, and solving for the mean value of the gradient layout intervals;

[0033] Setting detection points in the target well section with the corresponding gradient layout interval mean and detecting the acoustic amplitude at the corresponding detection points;

[0034] Solving cementation characterization parameters of several monitoring sections of the target well section based on the acoustic amplitude value of each detection point;

[0035] The monitoring section includes detection points.

[0036] Furthermore, adjacent monitoring sections that meet the screening conditions are determined as a single analysis section, and the cement sheath isolation capacity of the target well section is determined based on the cementation characterization parameters and length of each analysis section;

[0037] Wherein, the screening condition includes that the difference ratio of the bonding characterization parameters of adjacent monitoring sections is less than a predetermined difference ratio threshold.

[0038] Furthermore, in step S6, the cement sheath isolation capacity of the target well section is determined according to formula (3);

[0039]

[0040] Where: Pf is the cement sheath isolation capacity of the target well section, C i is the cementation characterization parameter of the i-th analysis section, i = 1, 2, 3, ... n, n is the total number of analysis sections, h i is the length of the i-th analysis segment.

[0041] Compared with the prior art, the present invention obtains several cement ring test pieces that meet the requirements of field gas wells, and obtains the specific parameters of each cement ring through experiments, including the cement sealing capacity of the corresponding bonding characterization parameters at different heights, and substitutes the obtained parameters into the cement ring sealing capacity expression to obtain the correlation coefficient for the field gas wells, thereby determining the cement ring sealing capacity expression for the field gas wells, evaluating the cement ring gas sealing capacity in a digital way, and improving the evaluation accuracy of the cement ring gas sealing capacity.

[0042] Furthermore, the gradient acoustic amplitude discrete quantity of the cement ring test piece is determined. The gradient acoustic amplitude discrete quantity characterizes the stability of the bonding of the cement ring test piece prepared according to the on-site gas wellbore structure. In the actual preparation process, different formation rock structures will have different effects on the bonding degree of the cement ring test piece. According to the bonding stability of the cement ring test piece, the gradient layout interval for a single cement ring test piece is determined to adjust the data acquisition amount for the cement ring test piece, so as to improve the accuracy of the bonding characterization parameters and further improve the evaluation efficiency of the gas isolation capacity of the cement ring.

[0043] Furthermore, the cementation complexity parameter in the target well section is determined. The cementation complexity parameter characterizes the cementation complexity of the cement sheath in the target well section. The cementation complexity is positively correlated with the number of target well section divisions. The data acquisition amount in the target well section is determined in a targeted manner, which effectively saves computing power. The cement sheath isolation capacity of the target well section is obtained according to formula (3) determined by the correlation coefficient for the target well section obtained through experiments. While accurately evaluating the cement sheath isolation capacity of the target well section in a digital manner, the evaluation efficiency of the cement sheath gas isolation capacity is further improved.

[0044] Furthermore, a closed space matching the gas wellbore structure is configured to obtain a cement ring test piece, and a cement ring of corresponding cementing quality is simulated. The cement ring isolation length and cementing quality are comprehensively considered to determine the cement ring isolation capacity expression in a targeted manner. Cementing quality is a key factor affecting the cement ring isolation capacity. In actual scenarios, a gas well is composed of cement rings of different lengths and cementation characterization parameters. The present invention conducts a targeted evaluation of the cement ring isolation capacity under different well conditions, effectively ensuring the safe and efficient development of oil and gas fields.

[0045] Furthermore, through simulation experiments, the downhole temperature and pressure conditions during on-site cementing were simulated, and the gas isolation capacity of the cement sheath under different heights and different cementing quality conditions was tested. The cement sheath isolation capacity expression was fitted based on the experimental results, and then combined with the bonding characterization parameters of the on-site gas wells, the cement sheath isolation capacity of different monitoring sections was determined, providing accurate guidance for the evaluation of the safe operation limits of gas wells.

[0046] Furthermore, the temperature, pressure, and gas wellbore conditions of the cement sheath in service were simulated, and the gas breakthrough pressure of the cement sheath at different heights and cementation characterization parameters was tested. Based on the experimental results, the cement sheath isolation capacity expression was established through the multivariate linear fitting method, so as to accurately evaluate the cement sheath isolation capacity in different well sections and provide important guidance for gas well safety evaluation. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 It is a flowchart of the steps of the method for evaluating the gas isolation capability of cement sheath according to an embodiment of the invention;

[0048] Figure 2 The present invention is a logical decision diagram for screening cement sheath test pieces based on the range of bonding characterization parameters according to an embodiment of the invention. DETAILED DESCRIPTION

[0049] In order to make the objects and advantages of the present invention more clearly understood, the present invention is further described below in conjunction with embodiments; it should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0050] The preferred embodiments of the present invention are described below with reference to the accompanying drawings. It should be understood by those skilled in the art that these embodiments are only used to explain the technical principles of the present invention and are not intended to limit the protection scope of the present invention.

[0051] It should be noted that, in the description of the present invention, terms such as "up", "down", "left", "right", "inside" and "outside" indicating directions or positional relationships are based on the directions or positional relationships shown in the drawings. This is merely for the convenience of description and does not indicate or imply that the device or element must have a specific orientation, be constructed and operated in a specific orientation. Therefore, it cannot be understood as a limitation on the present invention.

[0052] In addition, it should be noted that in the description of the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For those skilled in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0053] See also Figure 1 and Figure 2 As shown, they are respectively a flow chart of the steps of the method for evaluating the gas sealing capability of cement sheath according to an embodiment of the present invention, and a logic determination diagram for screening cement sheath test pieces based on the range of the bonding characterization parameter; the method for evaluating the gas sealing capability of cement sheath according to an embodiment of the present invention comprises:

[0054] Step S1, configuring a formation rock ring matching the wellbore structure of the gas well and a casing of corresponding size;

[0055] Step S2, pouring cement slurry into the annular space between the casing and the formation rock ring, obtaining a cement ring test piece at a predetermined time with a gas well temperature and a gas well pressure, and performing an acoustic amplitude test, including obtaining acoustic amplitude values ​​of different detection points at predetermined intervals to determine a gradient acoustic amplitude discrete amount, determining a gradient layout interval of the detection points, obtaining acoustic amplitude values ​​of different detection points again, and calculating a cementation characterization parameter;

[0056] Step S3, repeating step S2, obtaining bonding characterization parameters of cement ring test pieces at different heights, screening the cement ring test pieces based on the range of the bonding characterization parameters, and determining high-quality cement ring test pieces;

[0057] Step S4, placing the high-quality cement ring test piece in a closed space, injecting high-pressure nitrogen into the bottom of the high-quality cement ring test piece, and gradually increasing the pressure in a step-by-step manner until the cement ring interface is destroyed, recording the gas injection pressure at the destruction moment, and determining the gas injection pressure as the cement ring isolation capacity of the corresponding high-quality cement ring test piece;

[0058] Step S5, respectively substituting each group of experimental data into the cement sheath isolation capacity expression to solve the correlation coefficient for the gas well body structure, and then obtaining the cement sheath isolation capacity expression for the target well section of the gas well body structure; the experimental data include cementation characterization parameters, cement sheath isolation capacity and height of high-quality cement sheath test pieces;

[0059] Step S6, substituting the determined correlation coefficient into the cement sheath isolation capability expression, and determining the cement sheath isolation capability for the target well section according to the cementation characteristic parameter of the target well section.

[0060] Specifically, the preparation method of the formation rock ring is not limited. It can be understood that the purpose of preparing the formation rock ring is to simulate the formation structure of the target well section. Technical personnel in this field can collect soil materials from the formation where the target well section is located, and make the soil materials into an annular structure to obtain the formation rock ring.

[0061] Specifically, the structure of the closed space is not limited, and a temperature control device for adjusting the temperature and a pressure pump for adjusting the pressure in the closed space are arranged inside, which will not be elaborated here.

[0062] Specifically, there is no specific limitation on the device for detecting the sound amplitude, and it can be a sound amplitude detection instrument, which will not be described in detail here.

[0063] Specifically, several cement ring test pieces that meet the requirements of field gas wells are obtained, and the specific parameters of each cement ring are obtained through experiments, including the cement sealing capacity of the corresponding bonding characterization parameters at different heights. The obtained parameters are substituted into the cement ring sealing capacity expression to obtain the correlation coefficient for the field gas wells, thereby determining the cement ring sealing capacity expression for the field gas wells, and evaluating the gas sealing capacity of the cement ring in a digital way, thereby improving the evaluation accuracy of the cement ring gas sealing capacity.

[0064] Specifically, in step S2, the process of obtaining the sound amplitude values ​​of different detection points at predetermined intervals to determine the discrete amount of the gradient sound amplitude includes:

[0065] Acquire the sound amplitude value of each detection point, calculate the variance of each sound amplitude value, and determine the obtained variance as the gradient sound amplitude discrete amount;

[0066] Wherein, each of the detection points is arranged at the predetermined intervals perpendicular to the ground direction.

[0067] Specifically, in step S2, the process of determining the gradient layout interval of the detection points includes:

[0068] Determine the gradient layout interval for a single cement sheath test piece based on the discrete amount of gradient acoustic amplitude;

[0069] The gradient arrangement interval determined based on the discrete amount of the gradient sound amplitude is negatively correlated with the discrete amount of the gradient sound amplitude.

[0070] In this embodiment, optionally,

[0071] The gradient sound amplitude discrete quantity S is compared with the preset first discrete quantity S1 and the second preset discrete quantity S2,

[0072] If S≤S1, the gradient layout interval is determined as the first gradient layout interval L1, and L1=0.9L0;

[0073] If S1<S≤S2, the gradient layout interval is determined as the second gradient layout interval L2, and L2=0.8L0;

[0074] If S>S2, the gradient layout interval is determined to be the third gradient layout interval L3, and L3=0.7L0;

[0075] Wherein, L0 is the preset interval, 0.3m<L0<1.5m, S1=1.2S0, S2=1.5S0, and S0 is the average value of the discrete amount of the gradient sound amplitude of each cement sheath in history.

[0076] Specifically, the gradient acoustic amplitude discrete quantity of the cement ring test piece is determined. The gradient acoustic amplitude discrete quantity characterizes the stability of the bonding of the cement ring test piece prepared according to the on-site gas wellbore structure. In the actual preparation process, different formation rock structures will have different effects on the bonding degree of the cement ring test piece. According to the bonding stability of the cement ring test piece, the gradient layout interval for a single cement ring test piece is determined to adjust the data acquisition amount for the cement ring test piece, so as to improve the accuracy of the bonding characterization parameters and further improve the evaluation efficiency of the gas isolation capacity of the cement ring.

[0077] Specifically, in step S2, the process of calculating the bonding characterization parameter includes:

[0078] Solve the average value of each cementation characterization component to obtain the cementation characterization parameter;

[0079] The cementation characterization component is calculated according to formula (1):

[0080]

[0081] Where BI is the bonding characteristic component for a single detection point, A is the acoustic amplitude of the detection point, and A fp A is the casing sound amplitude at the detection point, g is the reference bonding sound amplitude of the cement sheath.

[0082] Specifically, it can be understood that the casing sound amplitude at the test point is the sound amplitude of the casing at the corresponding test point measured in advance before pouring cement; it can be understood that the reference bonding sound amplitude is determined based on the sound amplitude values ​​obtained at the test points of several cement ring test pieces, among which the proportion of different sound amplitude values ​​is determined, and the maximum proportion sound amplitude value is determined as the reference bonding sound amplitude.

[0083] Specifically, in step S3, the cement sheath test pieces are screened based on the range of the bonding characterization parameters, and the process of determining high-quality cement sheath test pieces includes:

[0084] If the bonding characterization parameters of a single mud ring test piece are within a predetermined bonding characterization range, the single mud ring test piece is determined to be a high-quality cement ring test piece.

[0085] Specifically, the predetermined bonding characterization range is [0.19, 0.82].

[0086] Specifically, in step S5, the correlation coefficient for the gas wellbore structure is solved by the cement sheath isolation capability expression (2):

[0087] P fc =a×C×h+b×h+d×C+e(2)

[0088] Where P fc is the cement sheath isolation capacity of a single high-quality cement sheath test piece, a is the comprehensive correlation coefficient, b is the height correlation coefficient, d is the bonding correlation coefficient, e is the capacity correlation coefficient, C is the bonding characterization parameter of a single high-quality cement sheath test piece, and h is the height of a single high-quality cement sheath test piece.

[0089] Specifically, in step S4, the process of obtaining the cement sheath isolation capability expression of the target well section for the wellbore structure of the gas well includes:

[0090] Substituting the solved correlation coefficients into the cement sheath isolation capacity expression (2) to obtain the cement sheath isolation capacity expression for the target well section of the gas wellbore structure.

[0091] Specifically, in step S5, the process of obtaining cement sheath bonding data of the target well section includes:

[0092] Obtaining the gradient layout intervals for different cement sheath test pieces determined in step S2, and solving for the mean value of the gradient layout intervals;

[0093] A detection point is set in the target well section with the corresponding gradient layout interval mean value and the acoustic amplitude is detected at the corresponding detection point. It can be understood that the spacing between each detection point is an integer multiple of the gradient layout interval mean value;

[0094] Solving cementation characterization parameters of several monitoring sections of the target well section based on the acoustic amplitude value of each detection point;

[0095] The monitoring section includes detection points. It can be understood that the length of the monitoring section can be the same as the interval between the detection points, and the detection points are located in the middle of the monitoring section.

[0096] Specifically, the adjacent monitoring sections that meet the screening conditions are determined as a single analysis section, and the cement sheath isolation capacity of the target well section is determined based on the cementation characterization parameters and length of each analysis section;

[0097] Wherein, the screening condition includes that the difference ratio of the bonding characterization parameters of adjacent monitoring sections is less than a predetermined difference ratio threshold.

[0098] Specifically, the process of obtaining the difference ratio of the bonding characterization parameters of adjacent monitoring sections is to calculate the absolute value of the difference of the bonding characterization parameters of adjacent monitoring sections, solve the ratio of the absolute value to the larger of the two bonding characterization parameters, and obtain the difference ratio of the bonding characterization parameters.

[0099] Specifically, the threshold value of the difference ratio of cementation characterization parameters is selected in the interval [0.2, 0.3].

[0100] Specifically, each adjacent monitoring section with the same cementation characterization parameter is determined as a single analysis section, and the cement sheath isolation capacity of the target well section is determined based on the cementation characterization parameter and length of each analysis section.

[0101] Specifically, in step S6, the cement sheath isolation capacity of the target well section is determined according to formula (3);

[0102]

[0103] Where: Pf is the cement sheath isolation capacity of the target well section, C i is the cementation characterization parameter of the i-th analysis section, i = 1, 2, 3, ... n, n is the total number of analysis sections, h i is the length of the i-th analysis segment.

[0104] Specifically, the cement sheath isolation capacity of the target well section is obtained according to formula (3) determined by the correlation coefficient for the target well section obtained through experiments. While accurately evaluating the cement sheath isolation capacity of the target well section in a digital way, the evaluation efficiency of the cement sheath gas isolation capacity is further improved.

[0105] Example 1

[0106] The casing is uniformly configured with an outer diameter of 139.7 mm, a formation rock ring with an outer diameter of 400 mm and an inner diameter of 215.9 mm.

[0107] The cement slurry was prepared using the G-grade cement used on site, and the cement slurry was filled in the annular space formed by the casing and the formation. The uniform pouring height was 1m, and then the cement slurry was cured for 48 hours according to the on-site temperature and pressure conditions. The thickness of the obtained cement ring test piece was 38.1mm.

[0108] Repeat the above process to prepare cement ring test pieces, perform acoustic amplitude detection on each cement ring test piece, determine the bonding characterization parameters, and determine the high-quality cement ring test pieces;

[0109] Among them, taking the bonding parameter C1=0.82 of the high-quality cement sheath test piece as an example, high-pressure nitrogen was injected into the bottom of the cement sheath using a closed space, and the pressure was gradually increased in a step-by-step manner until the cement sheath interface was broken by the gas pressure. The gas injection pressure at the moment of failure was recorded as 2.9MPa, and the cement sheath isolation capacity when the bonding parameter C1=0.82 was recorded as 2.9MPa;

[0110] Taking the bonding characterization parameter C2=0.59 of the high-quality cement sheath test piece as an example, high-pressure nitrogen was injected into the bottom of the cement sheath using a closed space, and the pressure was gradually increased in a step-by-step manner until the cement sheath interface was broken through by the gas pressure. The gas injection pressure at the moment of failure was recorded as 2.3 MPa, and the cement sheath isolation capacity when the bonding characterization parameter C2=0.59 was recorded as 2.3 MPa.

[0111] Taking the bonding characterization parameter C3=0.42 of the high-quality cement sheath test piece as an example, high-pressure nitrogen was injected into the bottom of the cement sheath using a closed space, and the pressure was gradually increased in a step-by-step manner until the cement sheath interface was broken through by the gas pressure. The gas injection pressure at the moment of failure was recorded as 1.6 MPa, and the cement sheath isolation capacity when the bonding characterization parameter C3=0.42 was recorded as 1.6 MPa.

[0112] Taking the bonding characterization parameter C4=0.19 of the high-quality cement sheath test piece as an example, high-pressure nitrogen was injected into the bottom of the cement sheath using a closed space, and the pressure was gradually increased in a step-by-step manner until the cement sheath interface was broken through by the gas pressure. The gas injection pressure at the moment of failure was recorded as 0.7 MPa, and the cement sheath isolation capacity when the bonding characterization parameter C4=0.19 was recorded as 0.7 MPa.

[0113] Cement ring test pieces with different casting heights were prepared, among which,

[0114] The bonding characteristic parameter of the cement sheath test piece with a casting height of 1m is 0.92, and the corresponding measured cement sheath sealing capacity is 3.7MPa;

[0115] The bonding characteristic parameter of the cement sheath test piece with a casting height of 0.6m is 0.82, and the corresponding measured cement sheath sealing capacity is 2.2MPa;

[0116] The bonding characterization parameter of the cement ring test piece with a casting height of 0.4m is 0.91, and the corresponding measured cement ring sealing capacity is 1.8MPa.

[0117] Based on the previous experimental results, the bonding characterization parameters of the high-quality cement ring test pieces with a casting height of 1m were 0.82, 0.59, 0.42, and 0.19, and the cement ring sealing capacities were 2.9MPa, 2.3MPa, 1.6MPa, and 0.7MPa, respectively; and the bonding characterization parameters of the high-quality cement ring test pieces with casting heights of 1.0m, 0.6m, and 0.4m were 0.92, 0.82, and 0.91, and the cement ring sealing capacities were 3.7MPa, 2.2MPa, and 1.8MPa, respectively.

[0118] Substituting the experimental data into the cement sheath isolation capacity expression (2),

[0119] It is found that a=2, b=0.003, d=1.82, e=0.025.

[0120] Substituting the solved correlation coefficients into the cement sheath isolation capacity expression (2), the cement sheath isolation capacity expression for the target well section of the gas wellbore structure is obtained as follows:

[0121] P fc =2C×h+0.003h+1.82C+0.025;

[0122] Determine the cementation characterization parameters of different analysis sections of the target well section. The relevant data are shown in Table 1. The cement sheath isolation capacity of the target well section is determined according to formula (3), where:

[0123]

[0124] In the formula: Ci is the bonding characterization parameter of the i-th analysis section, which are 0.5, 0.8, 0.5, and 0.3 respectively; hi is the length of the i-th analysis section, which are 2m, 4m, 3m, and 2m respectively; n is the total number of analysis sections, which is divided into 4 sections this time.

[0125] Table 1 Cementation characterization parameters corresponding to the detection points at different depths in the target well section (905m~915m)

[0126]

[0127]

[0128] So far, the technical solutions of the present invention have been described in conjunction with the preferred embodiments shown in the accompanying drawings. However, it is easy for those skilled in the art to understand that the protection scope of the present invention is obviously not limited to these specific embodiments. Without departing from the principle of the present invention, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after these changes or substitutions will fall within the protection scope of the present invention.

Claims

1. A method for evaluating the gas isolation capability of cement sheath, characterized in that: include: Step S1, configuring a formation rock ring matching the wellbore structure of the gas well and a casing of corresponding size; Step S2, pouring cement slurry into the annular space between the casing and the formation rock ring, obtaining a cement ring test piece at a predetermined time with the gas well temperature and gas well pressure, and performing acoustic amplitude detection, including obtaining the acoustic amplitude values ​​of different detection points at predetermined intervals to determine the gradient acoustic amplitude discrete amount, determining the gradient layout interval of the detection points, obtaining the acoustic amplitude values ​​of different detection points again, and calculating the cementation characterization parameter; the process of obtaining the acoustic amplitude values ​​of different detection points at predetermined intervals to determine the gradient acoustic amplitude discrete amount includes obtaining the acoustic amplitude values ​​of each detection point, calculating the variance of each acoustic amplitude value, and determining the obtained variance as the gradient acoustic amplitude discrete amount; Step S3, repeating step S2, obtaining bonding characterization parameters of cement ring test pieces at different heights, screening the cement ring test pieces based on the range of the bonding characterization parameters, and determining high-quality cement ring test pieces; Step S4, placing the high-quality cement ring test piece in a closed space, injecting high-pressure nitrogen into the bottom of the high-quality cement ring test piece, and gradually increasing the pressure in a step-by-step manner until the cement ring interface is destroyed, recording the gas injection pressure at the destruction moment, and determining the gas injection pressure as the cement ring isolation capacity of the corresponding high-quality cement ring test piece; Step S5, respectively substituting each group of experimental data into the cement sheath isolation capacity expression to solve the correlation coefficient for the gas well body structure, and obtaining the cement sheath isolation capacity expression for the target well section of the gas well body structure; the experimental data include cementation characterization parameters, cement sheath isolation capacity and height of high-quality cement sheath test pieces; Step S6, substituting the determined correlation coefficient into the cement sheath isolation capability expression, and determining the cement sheath isolation capability for the target well section according to the cementation characteristic parameter of the target well section.

2. The method for evaluating gas isolation capability of cement sheath according to claim 1, characterized in that: The detection points are arranged perpendicular to the ground.

3. The method for evaluating cement sheath gas isolation capability according to claim 2, characterized in that: The detection points are arranged at the predetermined intervals.

4. The method for evaluating cement sheath gas isolation capability according to claim 3, characterized in that: In step S2, the process of determining the gradient layout interval of the detection points includes: Determine the gradient layout interval for a single cement sheath test piece based on the discrete amount of gradient acoustic amplitude; The gradient arrangement interval determined based on the discrete amount of the gradient sound amplitude is negatively correlated with the discrete amount of the gradient sound amplitude.

5. The method for evaluating cement sheath gas isolation capability according to claim 4, characterized in that: In step S2, the process of calculating the bonding characterization parameter includes: The average value of each cementation characterization component is solved to obtain the cementation characterization parameter.

6. The method for evaluating cement sheath gas isolation capability according to claim 5, characterized in that: In step S2, the bonding characteristic component is calculated according to formula (1): Where BI is the bonding characteristic component for a single detection point, A is the acoustic amplitude of the detection point, and A fp A is the casing sound amplitude at the detection point, g is the reference bonding sound amplitude of the cement sheath.

7. The method for evaluating cement sheath gas isolation capability according to claim 6, characterized in that: In step S3, the cement sheath test pieces are screened based on the range of the bonding characterization parameter, and the process of determining high-quality cement sheath test pieces includes: If the bonding characterization parameters of a single mud ring test piece are within a predetermined bonding characterization range, the single mud ring test piece is determined to be a high-quality cement ring test piece.

8. The method for evaluating gas isolation capability of cement sheath according to claim 7, characterized in that: In step S5, the correlation coefficient for the gas wellbore structure is solved by the cement sheath isolation capability expression (2): P fc =a×C×h+b×h+d×C+e(2) Where P fc is the cement sheath isolation capacity of a single high-quality cement sheath test piece, a is the comprehensive correlation coefficient, b is the height correlation coefficient, d is the bonding correlation coefficient, e is the capacity correlation coefficient, C is the bonding characterization parameter of a single high-quality cement sheath test piece, and h is the height of a single high-quality cement sheath test piece.

9. The method for evaluating cement sheath gas isolation capability according to claim 8, characterized in that: In step S4, the process of obtaining the cement sheath isolation capability expression of the target well section for the wellbore structure of the gas well includes: Substituting the solved correlation coefficients into the cement sheath isolation capacity expression (2) to obtain the cement sheath isolation capacity expression for the target well section of the gas wellbore structure.

10. The method for evaluating gas isolation capability of cement sheath according to claim 9, characterized in that: In step S5, the process of obtaining cement sheath bonding data of the target well section includes: Obtaining the gradient layout intervals for different cement sheath test pieces determined in step S2, and solving for the mean value of the gradient layout intervals; Setting detection points in the target well section with the corresponding gradient layout interval mean and detecting the acoustic amplitude at the corresponding detection points; The cementation characterization parameters of several monitoring sections of the target well section are solved based on the acoustic amplitude value of each detection point.

11. The method for evaluating cement sheath gas isolation capability according to claim 10, characterized in that: The monitoring segment includes detection points.

12. The method for evaluating gas isolation capability of cement sheath according to claim 11, characterized in that: The adjacent monitoring sections that meet the screening conditions are determined as a single analysis section, and the cement sheath isolation capacity of the target well section is determined based on the cementation characterization parameters and length of each analysis section.

13. The method for evaluating gas isolation capability of cement sheath according to claim 12, characterized in that: The screening condition includes that the difference ratio of the bonding characterization parameters of adjacent monitoring sections is less than a predetermined difference ratio threshold.

14. The method for evaluating gas isolation capability of cement sheath according to claim 13, characterized in that: In step S6, the cement sheath isolation capacity of the target well section is determined according to formula (3); Where: Pf is the cement sheath isolation capacity of the target well section, C i is the cementation characterization parameter of the i-th analysis section, i = 1, 2, 3, ... n, n is the total number of analysis sections, h i is the length of the i-th analysis segment.

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