A method for evaluating the gas sealing ability of a cementing ring

Abstract

The present application relates to the field of cement ring detection, and more particularly to a cement ring gas sealing capacity evaluation method, which simulates the temperature, pressure and gas well bore conditions of the field cement ring service, tests the cement ring gas breakthrough pressure of different heights and cementation representation parameters, and establishes a cement ring sealing capacity expression according to the experimental results through a multiple linear fitting method, so as to accurately evaluate the cement ring sealing capacity of different well sections and improve the evaluation efficiency of the cement ring gas sealing capacity.

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 sealing capability of a cement sheath. Background Art

[0002] After years of service, gas wellbore quality gradually deteriorates, creating the risk of high-pressure natural gas leaking along the cement sheath to non-target formations or even to the surface along the cement sheath interface, posing a significant safety hazard. Current research methods for evaluating the isolation capacity of cement sheaths primarily rely on laboratory simulation experiments.

[0003] Chinese patent publication number: CN109681190A discloses a high-temperature and high-pressure gas well cement sheath seal integrity evaluation system. The system can maintain the cement sheath according to the downhole operating conditions of the high-temperature and high-pressure gas well and evaluate the influence of temperature and pressure changes on cement sheath seal failure by simulating gas channeling.

[0004] However, the existing technology still has the following problems: it cannot simulate cement sheaths of different cementing qualities, and no cement sheath sealing ability evaluation model has been 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 isolation ability. Summary of the Invention

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

[0006] To achieve the above objectives, 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 that matches 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, curing for a predetermined time at the gas well temperature and gas well pressure, obtaining a cement ring test piece, and performing an acoustic amplitude test, including obtaining acoustic amplitude values ​​at different test points at predetermined intervals to determine a gradient acoustic amplitude discrete amount, determining a gradient layout interval of the test points, obtaining acoustic amplitude values ​​at different test points again, and calculating a cementation characterization parameter;

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

[0010] Step S4: placing the high-quality cement sheath test piece in a closed space, injecting high-pressure nitrogen into the bottom of the high-quality cement sheath test piece, and gradually increasing the pressure in a stepwise manner until the cement sheath interface is destroyed. The injection pressure at the moment of destruction is recorded, and the injection pressure is determined as the cement sheath isolation capacity of the corresponding high-quality cement sheath test piece;

[0011] Step S5: Substituting each set of experimental data into the cement sheath isolation capacity expression to solve the correlation coefficient for the gas wellbore structure, and obtaining the cement sheath isolation capacity expression for the target wellbore section of the gas wellbore structure; the experimental data includes cementation characterization parameters, cement sheath isolation capacity, and the height of the high-quality cement sheath test piece;

[0012] Step S6: Substitute the determined correlation coefficient into the cement sheath isolation capability expression, and determine the cement sheath isolation capability for the target well section based on the cementation characteristic parameters 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] Obtaining the sound amplitude value of each detection point, calculating the variance of each sound amplitude value, and determining the obtained variance as the gradient sound amplitude discrete quantity;

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

[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 characteristic parameters 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 characteristic parameters, 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 for the target well section of the gas wellbore structure includes:

[0030] Substituting the solved correlation coefficients into the cement sheath isolation capability expression (2) we can obtain the cement sheath isolation capability 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] Determining cementation characterization parameters of several monitoring sections of the target well section based on the acoustic amplitude of each detection point;

[0035] The monitoring section includes detection points.

[0036] Furthermore, adjacent monitoring sections that meet the screening criteria are determined as single analysis sections, 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] 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 existing technology, the present invention obtains several cement sheath test pieces that meet the requirements of field gas wells, and obtains the specific parameters of each cement sheath through experiments, including the cement sealing capacity of the corresponding bonding characterization parameters at different heights. The obtained parameters are substituted into the cement sheath sealing capacity expression to obtain the correlation coefficient for the field gas well, thereby determining the cement sheath sealing capacity expression for the field gas well, and digitally evaluating the cement sheath gas sealing capacity, thereby improving the evaluation accuracy of the cement sheath gas sealing capacity.

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

[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 sections. 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 sheath test piece, and a cement sheath of corresponding cementing quality is simulated. The cement sheath isolation length and cementing quality are comprehensively considered to determine the cement sheath isolation capacity expression in a targeted manner. Cementing quality is a key factor affecting the cement sheath isolation capacity. In actual scenarios, a gas well is composed of cement sheaths of different lengths and cementation characterization parameters. The present invention conducts a targeted evaluation of the cement sheath isolation capacity under different well conditions, effectively ensuring the safe and efficient development of oil and gas fields.

[0045] Furthermore, through simulation experiments to simulate the downhole temperature and pressure conditions during on-site cementing, the gas isolation capacity of the cement sheath was tested under different heights and different cementing quality conditions. Based on the experimental results, the cement sheath isolation capacity expression was fitted, 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 on-site cement sheath 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, an expression for the cement sheath sealing capacity was established through the multivariate linear fitting method, thereby accurately evaluating the cement sheath sealing capacity in different well sections and providing important guidance for gas well safety evaluation. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 This 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 This is a logic 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 merely used to explain the present invention and are not intended 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 scope of protection 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 accompanying drawings. This is only 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] Moreover, it needs to be explained that, in the description of the present application, unless explicitly specified and limited, the terms "mount", "connect", "connection" should be understood broadly, for example, it can be fixed connection, or detachable connection, or integrally connected; it can be mechanical connection, or electrical connection; it can be directly connected, or indirectly connected through intermediate medium, or the communication inside two elements. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0053] Please refer to Figure 1 and Figure 2 respectively, the step flow chart of the cement sheath gas sealing capacity evaluation method of the embodiment of the present application, and the logical decision diagram for screening the cement sheath experimental piece based on the range of the cementation representation parameter; the cement sheath gas sealing capacity evaluation method of the embodiment of the present application comprises:

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

[0055] Step S2, pouring cement slurry into the annular space between the casing and the formation rock ring, curing for a predetermined time at the temperature and pressure of the gas well to obtain a cement sheath experimental piece and perform amplitude detection, including obtaining the amplitude values of different detection points at a predetermined interval to determine the gradient amplitude discrete quantity, determining the gradient layout interval of the detection points, again obtaining the amplitude values of different detection points, and calculating the cementation representation parameter;

[0056] Step S3, repeating the step S2 to obtain the cementation representation parameters of several cement sheath experimental pieces with different heights, screening the cement sheath experimental piece based on the range of the cementation representation parameter, and determining a high-quality cement sheath experimental piece;

[0057] Step S4, placing the high-quality cement sheath experimental piece in a closed space, injecting high-pressure nitrogen at the bottom of the high-quality cement sheath experimental piece, gradually increasing the pressure in steps until the cement sheath interface is damaged, recording the injection pressure at the moment of damage, and determining the injection pressure as the cement sheath sealing capacity of the corresponding high-quality cement sheath experimental piece;

[0058] Step S5, substituting each group of experimental data into the cement sheath sealing capacity expression to solve the correlation coefficient for the wellbore structure of the gas well, and then obtaining the cement sheath sealing capacity expression for the target well section of the wellbore structure of the gas well; the experimental data includes the cementation representation parameter, the cement sheath sealing capacity, and the height of the high-quality cement sheath experimental piece;

[0059] Step S6, substituting the determined correlation coefficient into the cement sheath sealing capacity expression, and determining the cement sheath sealing capacity for the target well section according to the cementation representation 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. Those skilled in the art can collect soil materials from the formation where the target well section is located, and make the soil materials into a ring structure to obtain the formation rock ring.

[0061] Specifically, the structure of the enclosed space is not limited, and a temperature control device for adjusting the temperature and a pressure pump for adjusting the pressure in the enclosed space are set inside, which will not be described in detail.

[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 sheath test pieces that meet the requirements of field gas wells were obtained, and the specific parameters of each cement sheath were obtained through experiments, including the cement sealing capacity of the corresponding bonding characterization parameters at different heights. The obtained parameters were substituted into the cement sheath sealing capacity expression to obtain the correlation coefficient for the field gas wells, thereby determining the cement sheath sealing capacity expression for the field gas wells, and evaluating the cement sheath gas sealing capacity in a digital way, thereby improving the evaluation accuracy of the cement sheath 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] Obtaining the sound amplitude value of each detection point, calculating the variance of each sound amplitude value, and determining the obtained variance as the gradient sound amplitude discrete quantity;

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

[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] Compare the gradient sound amplitude discrete value S with the preset first discrete value S1 and the second preset discrete value S2,

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

[0073] If S1<S≤S2, the gradient layout interval is determined to be 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] Among them, 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 sheath test piece is determined. The gradient acoustic amplitude discrete quantity represents the stability of the bonding of the cement sheath test piece prepared according to the on-site gas wellbore structure. During the actual preparation process, different formation rock structures will have different effects on the bonding degree of the cement sheath test piece. According to the bonding stability of the cement sheath test piece, the gradient layout interval for a single cement sheath test piece is determined, thereby adjusting the data acquisition amount for the cement sheath test piece to improve the accuracy of the bonding characterization parameters and further improve the efficiency of evaluating the gas sealing capacity of the cement sheath.

[0077] Specifically, in step S2, the process of calculating the bonding characteristic parameters 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 detection point is the sound amplitude of the casing at the corresponding detection point measured in advance before pouring cement; it can be understood that the reference bonding sound amplitude is determined based on the sound amplitude obtained at the detection points of several cement ring test pieces, among which the proportion of different sound amplitudes is determined, and the maximum proportion sound amplitude is determined as the reference bonding sound amplitude.

[0083] Specifically, in step S3, the process of screening cement sheath test pieces based on the range of the bonding characteristic parameters to determine 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 cementation 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 gas wellbore structure includes:

[0090] Substituting the solved correlation coefficients into the cement sheath isolation capability expression (2) we can obtain the cement sheath isolation capability 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] Detection points are set in the target well section with the corresponding gradient layout interval mean value and the acoustic amplitude is detected at the corresponding detection points. It can be understood that the spacing between each detection point is an integer multiple of the gradient layout interval mean value;

[0094] Determining cementation characterization parameters of several monitoring sections of the target well section based on the acoustic amplitude 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, adjacent monitoring sections that meet the screening criteria are identified as single analysis sections, and the cement sheath isolation capacity of the target well section is determined based on the cement characterization parameters and length of each analysis section;

[0097] 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 between 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 cementation characterization parameter difference ratio threshold is selected within the interval [0.2, 0.3].

[0100] Specifically, each adjacent monitoring section with the same cementation characterization parameters 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 parameters 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 manner, the evaluation efficiency of the cement sheath gas isolation capacity is further improved.

[0105] Example 1

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

[0107] The cement slurry was prepared using Grade G cement used on site. The annular space formed by the casing and the formation was filled with cement slurry. The pouring height was uniformly set at 1m. The cement slurry was then cured for 48 hours according to the on-site temperature and pressure conditions. The resulting cement ring test piece had a thickness of 38.1mm.

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

[0109] Taking the bonding parameter C1=0.82 of a 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 2.9 MPa, and the cement sheath isolation capacity when the bonding parameter C1=0.82 was recorded as 2.9 MPa.

[0110] Taking the bonding characterization parameter C2=0.59 of a 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 sealing 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 a 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 sealing 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 a 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 sealing capacity when the bonding characterization parameter C4=0.19 was recorded as 0.7 MPa.

[0113] Prepare cement ring test pieces with different casting heights,

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

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

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

[0117] Based on the results of previous experiments, the bonding characterization parameters of high-quality cement ring test pieces with a casting height of 1m were 0.82, 0.59, 0.42, and 0.19, respectively, and the cement ring sealing capacities were 2.9MPa, 2.3MPa, 1.6MPa, and 0.7MPa, respectively; and the bonding characterization parameters of 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, respectively, 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 capability expression (2), the cement sheath isolation capability 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] Where: Ci is the cementation 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] Thus far, the technical solutions of the present invention have been described in conjunction with the preferred embodiments shown in the accompanying drawings. However, it will be readily understood by those skilled in the art that the scope of protection of the present invention is obviously not limited to these specific embodiments. Without departing from the principles of the present invention, those skilled in the art may make equivalent changes or substitutions to the relevant technical features, and the technical solutions after such changes or substitutions will fall within the scope of protection of the present invention.

Claims

1. A method for evaluating the gas isolation capability of a cement sheath, characterized in that: include: Step S1, configuring a formation rock ring that matches 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, curing for a predetermined time at the gas well temperature and gas well pressure to obtain a cement ring test piece and perform acoustic amplitude testing, including obtaining acoustic amplitude values ​​of different detection points at predetermined intervals to determine a gradient acoustic amplitude discrete amount, determining the gradient layout interval of the detection points, obtaining acoustic amplitude values ​​of different detection points again, and calculating a cementation characterization parameter; the process of obtaining acoustic amplitude values ​​of different detection points at predetermined intervals to determine the gradient acoustic amplitude discrete amount includes obtaining the acoustic amplitude value 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 to obtain bonding characterization parameters of several cement sheath test pieces at different heights, screening the cement sheath test pieces based on the range of the bonding characterization parameters, and determining high-quality cement sheath test pieces; Step S4: placing the high-quality cement sheath test piece in a closed space, injecting high-pressure nitrogen into the bottom of the high-quality cement sheath test piece, and gradually increasing the pressure in a stepwise manner until the cement sheath interface is destroyed. The injection pressure at the moment of destruction is recorded, and the injection pressure is determined as the cement sheath isolation capacity of the corresponding high-quality cement sheath test piece; Step S5: Substituting each set of experimental data into the cement sheath isolation capacity expression to solve the correlation coefficient for the gas wellbore structure, and obtaining the cement sheath isolation capacity expression for the target wellbore section of the gas wellbore structure; the experimental data includes cementation characterization parameters, cement sheath isolation capacity, and the height of the high-quality cement sheath test piece; Step S6: Substitute the determined correlation coefficient into the cement sheath isolation capability expression, and determine the cement sheath isolation capability for the target well section based on the cementation characteristic parameters of the target well section.

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

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

4. The method for evaluating cement sheath gas isolation capability according to claim 3, wherein: 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 characteristic parameters 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 the gas isolation capability of cement sheath according to claim 6, characterized in that: In step S3, the process of screening cement sheath test pieces based on the range of the bonding characteristic parameters and 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 cement sheath gas isolation capability 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 gas wellbore structure includes: Substituting the solved correlation coefficients into the cement sheath isolation capability expression (2) we can obtain the cement sheath isolation capability expression for the target well section of the gas wellbore structure.

10. The method for evaluating cement sheath gas isolation capability 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, wherein: The monitoring section includes detection points.

12. The method for evaluating cement sheath gas isolation capability according to claim 11, wherein: The adjacent monitoring sections that meet the screening conditions are determined as single analysis sections, 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 the gas isolation capability of cement sheath according to claim 12, wherein: The screening condition includes that the difference ratio of the bonding characteristic parameters of adjacent monitoring sections is less than a predetermined difference ratio threshold.

14. The method for evaluating cement sheath gas isolation capability according to claim 13, wherein: 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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