A fourth system shallow gas cap layer sealing mechanics parameter analysis correction method and system
Patent Information
- Application Number
- CN202611467952.3
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2026-09-21
- Publication Date
- 2026-10-27
AI Technical Summary
[0003]现有试验方法在处理分析含有机质的第四系浅层气黏土盖层沉积物时，普遍忽略土骨架在初次受压后会发生不可恢复土体结构调整对后续气体成藏封闭性量测的影响，导致多次加卸压循环试验中测得的孔隙气压与气体饱和度关系出现闭环不闭合的偏差，且该偏差随卸压后试样静置时长呈现无法预测的漂移，这种历史依赖的记忆效应使得基于单一加卸压循环或短期测试获得的盖层土体封闭力学参数难以真实反映盖层沉积物在长期复杂应力历史下的实际封闭能力，从而降低了对第四系浅层气盖层封闭性分析和评价的准确性
[0046]1.通过记录至少两次循环加卸压试验中孔隙气压与气体饱和度的对应关系，并将相邻两次循环的加卸压曲线外推至气体饱和度的初始状态以获取闭环不闭合的偏差量，将土骨架结构调整引发的记忆效应转化为可量化的偏差指标，使原本被忽略的不可恢复变形对浅层气封闭能力的影响得以被测量和标定，设置多个不同静置时长并重复测量偏差量，将偏差量随静置时长的漂移规律从看似无序的变化中提取为偏差量随静置时长的变化序列，克服了短期单一测试无法反映记忆效应时间演化特征的缺陷。
Smart Images
Figure CN122982879A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of sealing performance testing technology for Quaternary cap deposits, and more specifically, to a method and system for analyzing and correcting the sealing mechanical parameters of shallow Quaternary gas caps. Background Technology
[0002] In the construction of bridges, tunnels, and other engineering projects spanning rivers and lakes, the presence of shallow Quaternary gas strata often triggers engineering disasters. The core of shallow gas accumulation lies in the intrusion, migration, accumulation, and caprock sealing processes of gas within sediment pores. Currently, the main methods for analyzing the mechanisms of Quaternary shallow gas accumulation include indoor gas injection simulation experiments, gas intrusion model experiments, and numerical calculations based on porous media seepage theory. Experiments typically employ cyclic pressurization and depressurization methods, measuring the gas saturation relationship under different pore pressures to infer the reservoir's gas storage capacity and the caprock's gas sealing threshold. Existing analyses often assume the soil skeleton is elastic or elastoplastic, believing that the pressure and saturation relationship during pressurization and depressurization follows a reversible or monotonically lagging pattern, and neglecting the influence of time factors on soil structure adjustment.
[0003] Existing experimental methods, when analyzing Quaternary shallow gas-bearing clay cap sediments containing organic matter, generally neglect the impact of irreversible soil structural adjustments after the initial compression on subsequent gas accumulation and sealing measurements. This leads to a non-closed-loop deviation in the relationship between pore pressure and gas saturation measured in multiple pressurization and depressurization cycles. Furthermore, this deviation exhibits unpredictable drift with the duration of sample settling after depressurization. This history-dependent memory effect makes it difficult for cap soil sealing mechanical parameters obtained based on a single pressurization and depressurization cycle or short-term tests to accurately reflect the actual sealing capacity of cap sediments under long-term complex stress histories, thereby reducing the accuracy of the sealing analysis and evaluation of Quaternary shallow gas caps. Summary of the Invention
[0004] In order to overcome the above-mentioned defects of the prior art, the present invention provides a method and system for analyzing and correcting the mechanical parameters of the Quaternary shallow gas cap layer closure, so as to solve the problems mentioned in the background art.
[0005] To achieve the above objectives, the present invention provides the following technical solution:
[0006] A method for analyzing and correcting the mechanical parameters of a Quaternary shallow gas cap, comprising the following steps:
[0007] S1: Perform at least two cyclic pressurization and depressurization tests on Quaternary caprock sediment samples and record the relationship between pore gas pressure and gas saturation in each cycle;
[0008] S2: Extrapolate the pressurization and depressurization curves of two adjacent cycles to the initial state of gas saturation, calculate the difference in intercepts of the two pressurization and depressurization curves on the pore pressure coordinate axis, and use the difference in intercepts as the deviation of the closed loop not being closed.
[0009] S3: After depressurization, set multiple different resting times, measure the breakthrough pressure when pressurizing again, and repeat steps S1 to S2 to obtain the sequence of deviation changes with resting time.
[0010] S4: Convert the settling time in the change sequence into a logarithmic value, determine the linear relationship between the deviation and the logarithmic value of the settling time in the logarithmic coordinate system, and extrapolate the linear relationship to the deviation corresponding to the logarithmic value of the settling time of the next order of magnitude as the prediction memory value.
[0011] S5: Use the predicted memory value to correct the relationship between pore pressure and gas saturation measured in the first cycle, and output the corrected caprock sealing mechanical parameters.
[0012] Furthermore, at least two cyclic pressurization and depressurization tests were conducted on Quaternary caprock sediment samples, and the relationship between pore gas pressure and gas saturation was recorded in each cycle, including:
[0013] Quaternary cap sediment samples were installed in a triaxial pressure chamber, and initial confining pressure and initial counter pressure were applied to simulate in-situ stress conditions.
[0014] The pore pressure at one end of the sample was gradually increased by a set pressure increment. After the pore pressure stabilized at each level, the amount of pore water displaced by the gas was measured and the gas saturation was calculated. The relationship between pore pressure and gas saturation during the pressurization process was recorded.
[0015] When the pore pressure rises to the preset upper limit, the pore pressure is gradually reduced by the set pressure reduction. After the pore pressure stabilizes at each level, the gas saturation is measured again, and the correspondence between pore pressure and gas saturation during the depressurization process is recorded.
[0016] After completing one pressurization and depressurization cycle, the process of gradually increasing and decreasing the pore pressure is repeated to obtain the corresponding relationship between pore pressure and gas saturation during pressurization in at least two cycles, and the corresponding relationship between pore pressure and gas saturation during depressurization in at least two cycles.
[0017] Furthermore, the criterion for stable pore pressure is that the change in pore pressure readings for three consecutive times does not exceed 1‰ of the applied initial back pressure; the conversion of gas saturation is based on the difference between the initial pore volume and the cumulative amount of discharged pore water, divided by the initial pore volume.
[0018] Furthermore, the pressurization and depressurization curves of two adjacent cycles are extrapolated to the initial state of gas saturation, and the intercept difference between the two pressurization and depressurization curves on the pore pressure coordinate axis is calculated. The intercept difference is used as the deviation of closed-loop non-closure, including:
[0019] Extract the pressurization / unpressurization curves for the first cycle and the second cycle from the corresponding relationships between pore pressure and gas saturation during pressurization and depressurization during depressurization in at least two cycles.
[0020] The pressure loading and unloading curve corresponding to the first cycle is extrapolated along the direction of decreasing gas saturation to the initial state of gas saturation, and the intercept of the pressure loading and unloading curve corresponding to the first cycle on the pore pressure coordinate axis is obtained. The pressure loading and unloading curve corresponding to the second cycle is extrapolated along the direction of decreasing gas saturation to the initial state of gas saturation, and the intercept of the pressure loading and unloading curve corresponding to the second cycle on the pore pressure coordinate axis is obtained.
[0021] Calculate the difference between the intercept of the pressure loading / unloading curve corresponding to the first cycle on the pore pressure coordinate axis and the intercept of the pressure loading / unloading curve corresponding to the second cycle on the pore pressure coordinate axis, and use the difference as the deviation of the closed loop not closing.
[0022] Furthermore, the method of extrapolating the pressure loading and unloading curve to the initial state of gas saturation is to linearly extend the data point corresponding to the lowest gas saturation in the pressure loading and unloading curve in the direction of decreasing gas saturation until it intersects the vertical axis corresponding to the initial state value of gas saturation.
[0023] Furthermore, after depressurization, multiple different settling times were set, and the breakthrough pressure upon repressurization was measured. Steps S1 to S2 were repeated to obtain the sequence of deviation changes with settling time, including:
[0024] After completing one pressurization and depressurization cycle and removing the pore gas pressure to the initial value, the Quaternary caprock sediment sample was kept in the depressurized state for the first settling time.
[0025] After the first settling period, the pore pressure was gradually increased by a set pressure increment. The pore pressure when the gas began to enter the Quaternary caprock sediment sample was recorded as the breakthrough pressure. At least two more cycles of pressurization and depressurization tests were completed, and the correspondence between pore pressure and gas saturation during the pressurization process and the correspondence between pore pressure and gas saturation during the depressurization process were recorded in each cycle.
[0026] Based on the correspondence between pore pressure and gas saturation during the pressurization process and the correspondence between pore pressure and gas saturation during the depressurization process obtained under the first settling time, the deviation of closed loop non-closure corresponding to the first settling time is obtained by calculating the difference between the intercept of the pressurization / depressurization curve corresponding to the first cycle on the pore pressure coordinate axis and the intercept of the pressurization / depressurization curve corresponding to the second cycle on the pore pressure coordinate axis.
[0027] Replace the Quaternary caprock sediment sample, perform another pressure-relief cycle on the replaced sample and then depressurize it. Maintain a second settling time different from the first settling time under depressurization. Repeat the breakthrough pressure measurement, record at least two cycles of pressure-relief test and calculate the deviation to obtain the deviation of the closed loop not closing for the second settling time.
[0028] The process of obtaining the deviation amount of closed loop non-closure is repeated for at least two different resting times. All resting times and their corresponding deviation amounts of closed loop non-closure are collected to form a sequence of deviation amount changes with resting time.
[0029] Furthermore, the settling time in the variation sequence is converted into a logarithmic value. A linear relationship between the deviation and the logarithmic value of the settling time is determined in a logarithmic coordinate system. The deviation corresponding to the logarithmic value of the settling time at the next order of magnitude is extrapolated to this linear relationship and used as the prediction memory value, including:
[0030] Extract all resting times and the corresponding deviations that are not closed from the sequence of deviation changes with resting time;
[0031] Calculate the logarithm of the total resting time;
[0032] Using the logarithm of the settling time as the x-axis and the deviation of the non-closed loop as the y-axis, a straight line is determined in the logarithmic coordinate system such that the data points are distributed along the straight line, which serves as the linear relationship between the deviation and the logarithm of the settling time.
[0033] Extrapolate the straight line along the horizontal axis to the logarithm of the next order of magnitude of resting time, read the corresponding deviation of the closed loop not closing, and use the read deviation of the closed loop not closing as the prediction memory value.
[0034] Furthermore, the relationship between pore pressure and gas saturation measured in the first cycle is corrected using predicted memory values, and the corrected caprock closure mechanical parameters are output, including:
[0035] The predicted memory value is subtracted from each pore pressure value in the correspondence between pore pressure and gas saturation during the first cycle of pressurization to obtain the corrected correspondence between pore pressure and gas saturation during the first cycle of pressurization.
[0036] The predicted memory value is subtracted from each pore pressure value in the correspondence between pore pressure and gas saturation in the first cycle of depressurization to obtain the corrected correspondence between pore pressure and gas saturation in the first cycle of depressurization.
[0037] In the corrected first cycle pressurization process, the pore pressure and gas saturation correspondence are determined by taking the pore pressure corresponding to the first increase in gas saturation from the initial state as the corrected caprock breakthrough pressure, and the corrected caprock sealing mechanical parameters are output.
[0038] Furthermore, the revised caprock sealing mechanical parameters also include the pore pressure corresponding to the gas saturation when the gas saturation falls back to the initial state during the revised first cycle of depressurization as the residual sealing pressure.
[0039] On the other hand, the present invention provides a system for analyzing and correcting the mechanical parameters of Quaternary shallow gas cap closure, comprising the following modules:
[0040] The cyclic pressurization module is used to perform at least two cyclic pressurization tests on Quaternary caprock sediment samples and record the relationship between pore gas pressure and gas saturation in each cycle.
[0041] The deviation calculation module is used to extrapolate the pressurization and depressurization curves of two adjacent cycles to the initial state of gas saturation, calculate the difference in intercepts of the two pressurization and depressurization curves on the pore pressure coordinate axis, and use the difference in intercepts as the deviation of the closed loop not being closed.
[0042] The sequence acquisition module is used to set multiple different resting times after depressurization, measure the breakthrough pressure when pressurization is applied again, and repeat the execution of the depressurization module and the deviation calculation module to obtain the sequence of deviation changes with resting time.
[0043] The predictive memory value module is used to convert the settling time in the change sequence into a logarithmic value, determine the linear relationship between the deviation and the logarithmic value of the settling time in the logarithmic coordinate system, and extrapolate the linear relationship to the deviation corresponding to the logarithmic value of the settling time of the next order of magnitude as the predictive memory value.
[0044] The correction output module is used to correct the relationship between pore pressure and gas saturation measured in the first cycle using the predicted memory value, and outputs the corrected caprock closure mechanical parameters.
[0045] Compared with the prior art, the present invention has the following beneficial effects:
[0046] 1. By recording the correspondence between pore gas pressure and gas saturation in at least two cycles of pressurization and depressurization tests, and extrapolating the pressurization and depressurization curves of two adjacent cycles to the initial state of gas saturation, the deviation amount of closed-loop non-closure is obtained. The memory effect caused by soil skeleton structure adjustment is transformed into a quantifiable deviation index, so that the influence of previously ignored irreversible deformation on shallow gas sealing capacity can be measured and calibrated. Multiple different resting times are set and the deviation amount is repeatedly measured. The drift law of deviation amount with resting time is extracted from the seemingly disordered change into a sequence of deviation amount change with resting time, which overcomes the defect that short-term single test cannot reflect the time evolution characteristics of memory effect.
[0047] 2. The settling time is converted into a logarithmic value, and a linear relationship between the deviation and the logarithmic value of the settling time is established in a logarithmic coordinate system. The predicted memory value is obtained by extrapolating to the next order of magnitude of settling time. The relationship between pore gas pressure and gas saturation measured in the first cycle is corrected using the predicted memory value, and the corrected caprock sealing mechanical parameters are output. This makes the final caprock breakthrough pressure deduct the offset caused by the cumulative adjustment of the soil skeleton structure over time, thus more realistically reflecting the actual gas intrusion and sealing capacity of Quaternary caprock sediments after experiencing a complex stress history. Attached Figure Description
[0048] Figure 1 This is a flowchart of a method for analyzing and correcting the mechanical parameters of a Quaternary shallow gas cap layer according to the present invention;
[0049] Figure 2 This is a schematic diagram of the structure of a Quaternary shallow gas cap closure mechanical parameter analysis and correction system according to the present invention. Detailed Implementation
[0050] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.
[0051] Example 1: Figure 1 This invention provides a method for analyzing and correcting the mechanical parameters of Quaternary shallow gas cap closure, which includes the following steps:
[0052] S1: Perform at least two cyclic pressurization and depressurization tests on Quaternary caprock sediment samples and record the relationship between pore gas pressure and gas saturation in each cycle;
[0053] S2: Extrapolate the pressurization and depressurization curves of two adjacent cycles to the initial state of gas saturation, calculate the difference in intercepts of the two pressurization and depressurization curves on the pore pressure coordinate axis, and use the difference in intercepts as the deviation of the closed loop not being closed.
[0054] S3: After depressurization, set multiple different resting times, measure the breakthrough pressure when pressurizing again, and repeat steps S1 to S2 to obtain the sequence of deviation changes with resting time.
[0055] S4: Convert the settling time in the change sequence into a logarithmic value, determine the linear relationship between the deviation and the logarithmic value of the settling time in the logarithmic coordinate system, and extrapolate the linear relationship to the deviation corresponding to the logarithmic value of the settling time of the next order of magnitude as the prediction memory value.
[0056] S5: Use the predicted memory value to correct the relationship between pore pressure and gas saturation measured in the first cycle, and output the corrected caprock sealing mechanical parameters.
[0057] In the specific implementation of S1, a undisturbed Quaternary caprock sediment sample is used. After opening the sampling tube in the chamber, it is quickly shaped into a cylinder with a diameter of 50 mm and a height of 100 mm. The sample is then encapsulated in a rubber membrane and installed on the base of the triaxial pressure chamber. The triaxial pressure chamber is pre-filled with degassed water. After installation, any residual air bubbles in the pressure chamber are discharged. The stress conditions of the Quaternary caprock sediment sample in the in-situ strata are simulated by applying initial confining pressure and initial back pressure. The initial confining pressure is determined based on the effective stress of the overlying strata corresponding to the sampling depth of the Quaternary caprock sediment sample. For example, when the sampling depth is 30 m, the initial confining pressure can be set to 600 kPa. The magnitude of the initial back pressure is referenced to the pore water pressure of the sample under in-situ conditions, for example, it can be set to 200 kPa. During the application of the initial confining pressure and initial back pressure, the sample reaches an equilibrium state by monitoring the sample drainage or pore pressure dissipation. When the change in pore pressure readings for three consecutive times does not exceed 1‰ of the applied initial back pressure, the in-situ stress condition simulation is considered complete.
[0058] After simulating in-situ stress conditions, the initial confining pressure and initial back pressure are kept constant. Pore pressure is applied to one end of the sample via a gas pressure controller at the top of the triaxial pressure chamber. The pore pressure is gradually increased in preset pressure increments. These preset pressure increments are determined based on the estimated breakthrough pressure of Quaternary caprock sediment samples. The estimated breakthrough pressure is obtained through pre-experimental or empirical estimations of samples from the same stratum. The preset pressure increment is 5% to 10% of the estimated breakthrough pressure; for example, if the estimated breakthrough pressure is 500 kPa, the preset pressure increment can be set to 25 kPa. After each increment of pore pressure, the pressure is maintained constant, and the pore pressure reading and the amount of pore water discharged are continuously monitored. The pore pressure readings are monitored using a high-precision pressure sensor installed between the gas pressure controller and the sample, with sampling intervals not exceeding 30 seconds. The criterion for stable pore pressure is that the change in pore pressure readings over three consecutive readings does not exceed 1‰ of the applied initial back pressure. In other words, when the absolute value of the difference between three consecutive readings is less than the initial back pressure multiplied by 0.001, the pore pressure at that level is considered stable. After the pore pressure stabilizes, the amount of pore water displaced by gas at that pore pressure level is recorded. The amount of pore water displaced is measured using a high-precision measuring tube or a precision balance connected to the sample drainage end. The minimum graduation of the high-precision measuring tube should not exceed 0.1 mL, and the resolution of the precision balance should not be less than 0.01 g. The cumulative amount of pore water displaced since the initial application of pore pressure is recorded as the cumulative displaced pore water volume. The conversion of gas saturation is based on the difference between the initial pore volume and the cumulative amount of water discharged from the pores, divided by the initial pore volume. The initial pore volume is measured after the initial confining pressure and initial back pressure are applied, but before any pore gas pressure is applied. The measurement method involves weighing the saturated sample and calculating the initial pore volume based on the total mass and particle density of the sample. The particle density is pre-determined using the hydrostatic bottle method. The converted gas saturation at each pore pressure level and the corresponding pore pressure at that level form a set of correspondences between pore pressure and gas saturation during the pressurization process. These correspondences are recorded level by level to form a sequence of relationships between pore pressure and gas saturation during the pressurization process.
[0059] Once the pore pressure reaches a preset upper limit, it is gradually reduced. The preset upper limit is determined based on empirical pressures where the gas saturation in the Quaternary caprock sediment sample approaches 1, or the pressure at which significant gas penetration occurs. For example, the pore pressure corresponding to a gas saturation of 0.9 or higher, or a significant increase in drainage rate, is used as the preset upper limit. The pore pressure is reduced gradually by a set pressure decrease, with the set pressure decrease and the set pressure increase being the same value to ensure symmetry between the pressure stages during depressurization and pressurization. After each level of pore pressure reduction, the pore pressure is maintained constant, and pore pressure readings are continuously monitored until the stability criterion is met: the change in pore pressure readings for three consecutive times does not exceed 1‰ of the applied initial back pressure. After the pore pressure stabilizes, the cumulative discharged pore water volume is measured again. At this point, due to the reduced pore pressure, the discharged pore water volume may show a negative increase. The current gas saturation is recalculated based on the actual measured cumulative discharged pore water volume, using the same conversion method as during the pressurization process. The pore pressure and its corresponding gas saturation at each stage of depressurization are recorded as the correspondence between pore pressure and gas saturation during the depressurization process. This step-by-step recording forms a sequence of correspondences between pore pressure and gas saturation during the depressurization process.
[0060] The entire process of pressurizing to a preset upper limit and then depressurizing until the pore pressure equals the initial value constitutes one pressurization-depressurization cycle. After completing one pressurization-depressurization cycle, without removing the Quaternary caprock sediment sample, the next pressurization-depressurization cycle is immediately repeated. The process of gradually increasing and decreasing the pore pressure is repeated, with the operation method exactly the same as the first cycle, and the pressure increment, pressure decrement, stability criterion, and gas saturation conversion method are all kept consistent. Through repeated operation, at least two cycles' corresponding sequences of pore pressure and gas saturation during the pressurization process and at least two cycles' corresponding sequences of pore pressure and gas saturation during the depressurization process are obtained. The corresponding sequences for each cycle are recorded independently for subsequent extraction of the pressurization-depressurization curves corresponding to the first and second cycles.
[0061] In a specific implementation of S2, the pressurization / unpressurization curves for the first cycle and the second cycle are extracted from the corresponding sequences of pore pressure and gas saturation during the pressurization process of at least two cycles and during the depressurization process of at least two cycles, respectively. The pressurization / unpressurization curve for the first cycle is composed of all data points from the corresponding sequences of pore pressure and gas saturation during the pressurization and depressurization processes of the first and second cycles. The pressurization / unpressurization curve for the second cycle is composed of all data points from the corresponding sequences of pore pressure and gas saturation during the pressurization and depressurization processes of the second and second cycles. The pressurization and depressurization curves are plotted with pore pressure on the ordinate and gas saturation on the abscissa. The pore pressure on the ordinate is measured in kilopascals, and the gas saturation on the abscissa is a dimensionless quantity, ranging from the initial state value of gas saturation to the highest gas saturation value reached during the pressurization process. The initial state of gas saturation refers to the gas saturation value corresponding to the pores inside the Quaternary caprock sediment sample being saturated with water before the application of pore pressure; this value is generally zero.
[0062] The pressurization / unloading curve corresponding to the first cycle is extrapolated along the direction of decreasing gas saturation to the initial state of gas saturation, yielding the intercept of the pressurization / unloading curve on the pore pressure coordinate axis. The direction of decreasing gas saturation refers to extending from the existing data points in the pressurization / unloading curve towards the direction of decreasing gas saturation. Specifically, the extrapolation is performed by determining the data point corresponding to the lowest gas saturation in the pressurization / unloading curve corresponding to the first cycle. This lowest gas saturation data point is the data point with the smallest gas saturation value in the sequence of pore pressure and gas saturation during the first cycle of pressurization. This lowest gas saturation data point is then connected to the point on the vertical axis corresponding to the initial gas saturation value by a linear extension. The slope of the linear extension is jointly determined by the data point corresponding to the lowest gas saturation and the data point with the second lowest gas saturation during the decompression process adjacent to the data point corresponding to the lowest gas saturation. The slope of the line connecting the data point corresponding to the lowest gas saturation and the data point with the second lowest gas saturation is taken as the slope of the linear extension. The physical meaning of this slope is the approximately linear rate of change of pore pressure with gas saturation at the end of decompression, and its dimensions are the same as pore pressure. The vertical axis corresponding to the initial state value of gas saturation refers to the pore pressure coordinate axis perpendicular to the horizontal axis, where the gas saturation is equal to zero. The pore pressure value corresponding to the intersection of the linear extension line and the pore pressure coordinate axis is the intercept of the decompression curve corresponding to the first cycle on the pore pressure coordinate axis. This intercept represents the pore pressure value extrapolated from the decompression test data of the first cycle under the ideal fully saturated state of zero gas saturation.
[0063] The pressure loading / unloading curve for the second cycle is extrapolated along the decreasing gas saturation direction to the initial state of gas saturation, yielding the intercept of the pressure loading / unloading curve for the second cycle on the pore pressure coordinate axis. The extrapolation method is the same as that for the pressure loading / unloading curve for the first cycle. The data point corresponding to the lowest gas saturation is determined in the pressure loading / unloading curve for the second cycle. This data point is the one with the smallest gas saturation value in the sequence of pore pressure and gas saturation during the pressure unloading process of the second cycle. The data point corresponding to the lowest gas saturation in the pressure loading / unloading curve for the second cycle is linearly extended in the decreasing gas saturation direction. The slope of this linear extension is determined by the data point corresponding to the lowest gas saturation and the next lowest gas saturation data point during the pressure unloading process adjacent to it. The slope of the line connecting the data point corresponding to the lowest gas saturation and the next lowest gas saturation data point is used as the slope of the linear extension. The linear extension line intersects the pore pressure coordinate axis where the gas saturation on the horizontal axis equals zero. The pore pressure value corresponding to the intersection point is the intercept of the pressure loading / unloading curve for the second cycle on the pore pressure coordinate axis. The intercept of the pressure loading / unloading curve for the first cycle on the pore pressure coordinate axis is obtained in exactly the same way as the intercept of the pressure loading / unloading curve for the second cycle, ensuring the comparability of the basic data for subsequent intercept difference calculations.
[0064] The difference between the intercepts of the pressure loading / unloading curves for the first cycle and the second cycle on the pore pressure coordinate axis is calculated, and this difference is taken as the deviation of the closed-loop non-closure. The deviation of the closed-loop non-closure is calculated as: ΔP = |P1 - P2|; where ΔP represents the deviation of the closed-loop non-closure, in kPa; P1 represents the intercept of the pressure loading / unloading curve for the first cycle on the pore pressure coordinate axis, in kPa; and P2 represents the intercept of the pressure loading / unloading curve for the second cycle on the pore pressure coordinate axis, in kPa. The deviation of the closed-loop non-closure reflects the degree of deviation of the pore pressure corresponding to the initial state of gas saturation of the pressure loading / unloading curves in the two cycles. Its physical meaning lies in the irreversible structural adjustment of the soil skeleton in the Quaternary cap sediment sample after being compressed in the first cycle, which leads to a difference in gas intrusion behavior between the second and first cycles. This difference prevents the pressure loading / unloading curves from closing at the initial state of gas saturation, resulting in the phenomenon of closed-loop non-closure.
[0065] In the specific implementation of S3, after completing one pressurization-depressurization cycle and removing the pore gas pressure to its initial value, the Quaternary caprock sediment sample is kept in the depressurized state for a first settling time. One pressurization-depressurization cycle refers to the complete process starting from applying initial confining pressure and initial back pressure to simulate in-situ stress conditions, gradually increasing the pore gas pressure to a preset upper limit value with a set pressure increment, and then gradually decreasing the pore gas pressure to its initial value with a set pressure decrease. The termination point of this process is when the pore gas pressure returns to its initial value after depressurization. The initial value of the pore gas pressure refers to the gas pressure inside the pores of the sample before any external gas pressure is applied, i.e., the set value of the initial back pressure. Maintaining the depressurized state means that after the pore gas pressure returns to its initial value, the pressurization path of the gas pressure controller is closed, maintaining the initial confining pressure and initial back pressure unchanged, so that there is no net inflow or outflow of gas inside the Quaternary caprock sediment sample. The first settling time is determined by obtaining the estimated memory effect development timescale of Quaternary cap sediment samples through preliminary tests of samples from the same stratum. For example, a set of preliminary tests is conducted with several different settling times and the corresponding deviation of the closed loop not closing is measured. The settling time at which the deviation of the closed loop not closing begins to show a significant change is used as a reference value for the estimated memory effect development timescale. Based on this, the first settling time is selected, for example, set to 30 minutes.
[0066] After the initial settling period, the pore pressure was gradually increased in increments of a set pressure. The pore pressure at which gas began to enter the Quaternary caprock sediment sample was recorded as the breakthrough pressure. At least two more cycles of pressurization and depressurization were performed, and the correlation between pore pressure and gas saturation during pressurization and depressurization was recorded for each cycle. The method of gradually increasing the pore pressure in increments of a set pressure was the same as the pressurization method in a single pressurization and depressurization cycle, and the set pressure increment was consistent with the set pressure increment used in a single pressurization and depressurization cycle. After each pore pressure stabilized, the change in the cumulative discharged pore water volume was observed. When the cumulative discharged pore water volume first showed a measurable increase, it was determined that gas had begun to enter the Quaternary caprock sediment sample, and the corresponding pore pressure value was recorded as the breakthrough pressure. A measurable increase refers to the increment of the cumulative discharged pore water volume exceeding the minimum resolvable water volume change corresponding to the smallest graduation of the high-precision measuring tube or the resolution of the precision balance. After recording the breakthrough pressure, the pore gas pressure is gradually increased by a set pressure increment until the preset upper limit value is reached. Then, the pore gas pressure is gradually decreased by a set pressure decrement until the initial value is reached, completing the first round of pressurization and depressurization cycles. The second round of pressurization and depressurization cycles is immediately repeated without removing the Quaternary caprock sediment sample. The corresponding sequences of pore gas pressure and gas saturation during pressurization and depressurization during depressurization are obtained for at least two cycles under the first settling time. Completing at least two cycles of pressurization and depressurization means performing both complete cycles, not just one cycle or a partial cycle after recording the breakthrough pressure.
[0067] Based on the correspondence between pore pressure and gas saturation during the pressurization process and the correspondence between pore pressure and gas saturation during the depressurization process obtained under the first settling time, the deviation of closed-loop non-closure corresponding to the first settling time is obtained by calculating the difference between the intercepts of the pressurization / depressurization curves corresponding to the first cycle and the intercepts of the pressurization / depressurization curves corresponding to the second cycle on the pore pressure coordinate axis. From the correspondence sequences of pore pressure and gas saturation during the pressurization process and the correspondence sequences of pore pressure and gas saturation during the depressurization process of at least two cycles obtained under the first settling time, the pressurization / depressurization curves corresponding to the first cycle and the second cycle are extracted. The pressure loading / unloading curve corresponding to the first cycle is extrapolated along the direction of decreasing gas saturation to the initial state of gas saturation, thus obtaining the intercept of the pressure loading / unloading curve corresponding to the first cycle on the pore pressure coordinate axis. The pressure loading / unloading curve corresponding to the second cycle is extrapolated along the direction of decreasing gas saturation to the initial state of gas saturation, thus obtaining the intercept of the pressure loading / unloading curve corresponding to the second cycle on the pore pressure coordinate axis.
[0068] Replace the Quaternary caprock sediment sample. Perform another pressurization-depressurization cycle on the replaced Quaternary caprock sediment sample and then depressurize it. Maintain a second settling time, different from the first settling time, under depressurization. Repeat the breakthrough pressure measurement, record at least two pressurization-depressurization cycles, and calculate the deviation to obtain the deviation corresponding to the second settling time. Replacing the Quaternary caprock sediment sample means taking an undisturbed sample from the same sampling layer with the same initial physical properties as the sample used in the first settling time test. These same initial physical properties include the same initial void ratio, the same initial water content, and the same particle size distribution, ensuring that the test results under different settling times only reflect the influence of the settling time and not the interference of differences in the sample itself. Install and initially balance the replaced Quaternary caprock sediment sample according to the operation of applying initial confining pressure and initial back pressure to simulate in-situ stress conditions. After completing one pressurization-depressurization cycle, depressurize the pore pressure to the initial value and maintain the second settling time under depressurization. The value of the second settling time differs from that of the first settling time, and the principle for determining the value of the second settling time is the same as that for the first settling time. Both are obtained through pre-experiments on samples from the same stratigraphic layer based on the estimated timescale of the memory effect development. For example, if the first settling time is 30 minutes, the second settling time can be set to 60 minutes. After the second settling time, the pore pressure is gradually increased by a set pressure increment. The pore pressure at which the gas begins to enter the replaced Quaternary cap sediment sample is recorded as the breakthrough pressure under the second settling time. At least two more cycles of pressurization and depressurization tests are performed, and the correspondence between pore pressure and gas saturation is recorded during the pressurization and depressurization processes. The deviation of the closed loop not closing corresponding to the second settling time is obtained by calculating the difference between the intercept of the pressurization and depressurization curve corresponding to the first cycle on the pore pressure coordinate axis and the intercept of the pressurization and depressurization curve corresponding to the second cycle on the pore pressure coordinate axis.
[0069] The process of obtaining the deviation amount of closed-loop non-closure is repeated for at least two different settling times. All settling times and their corresponding deviation amounts of closed-loop non-closure are collected to form a sequence of deviation changes with settling time. At least three different settling times are used to ensure sufficient data points for determining the linear relationship between the deviation amount and the logarithm of the settling time in a logarithmic coordinate system. The different settling times are arranged in ascending order of time, and the interval between adjacent settling times can be increased proportionally or by a predetermined time step, for example, set to 30 minutes, 60 minutes, 120 minutes, and 240 minutes respectively. For each different settling time, a new Quaternary caprock sediment sample is used to independently complete at least two cycles of pressurization and depressurization to maintain the settling time and measure the breakthrough pressure. The entire process of recording and calculating the deviation amount is then performed to obtain the deviation amount of closed-loop non-closure corresponding to each settling time. All settling times and their corresponding deviations from open-loop circuits are aggregated in a one-to-one correspondence. The unit of settling time is minutes, and the unit of deviation from open-loop circuits is kilopascals. The aggregated data constitute a sequence of deviation changes with settling time. Each data pair in the sequence consists of a settling time and the corresponding deviation from open-loop circuits for that settling time. The total number of data pairs equals the number of different settling times used.
[0070] In a specific implementation of S4, all resting times and corresponding non-closed-loop deviations are extracted from the deviation variation sequence with resting time. The deviation variation sequence with resting time is obtained from the aforementioned steps. The variation sequence contains multiple data pairs, each consisting of a resting time and the corresponding non-closed-loop deviation. The extraction operation involves listing the values of all resting times and all non-closed-loop deviations in the variation sequence, forming a resting time sequence and a non-closed-loop deviation sequence. The two sequences are of equal length and correspond one-to-one. The extracted resting time retains its original unit of minutes, and the extracted non-closed-loop deviation retains its original unit of kilopascals. For example, if a sequence of deviation changes with resting time contains 4 data pairs, then 4 resting time values can be extracted, such as 30, 60, 120, and 240, and 4 corresponding deviation values for non-closed loops, such as 3.2, 4.8, 6.5, and 8.1.
[0071] Calculate the logarithm of all settling times. The logarithm of settling times is calculated by taking the common logarithm (base 10) for each settling time value in the extracted settling time series. The logarithm of settling times is dimensionless. The physical basis for converting settling times from a linear to a logarithmic scale is that the rate of soil skeleton structure adjustment in Quaternary cap sediment samples gradually decreases with time, and the magnitude of structural adjustment on a logarithmic scale is approximately linearly related to the logarithm of time. For example, the logarithm corresponding to a settling time of 30 is log... 10 30≈1.48, the common logarithmic value with base 10 corresponding to a resting time of 60 is log. 10 60≈1.78, the common logarithmic value with base 10 corresponding to a resting time of 120 is log. 10 120≈2.08, the logarithm corresponding to a resting time of 240 is log 10 240 ≈ 2.38. The calculation results of the logarithmic values are retained to two decimal places or the number of decimal places is determined according to the measurement accuracy to ensure the reliability of the subsequent determination of linear relationships.
[0072] Using the logarithm of the resting time as the x-axis and the deviation of the open-loop system as the y-axis, a straight line is determined in the logarithmic coordinate system, such that the data points are distributed along the line, representing the linear relationship between the deviation and the logarithm of the resting time. A straight line is fitted to the coordinate points formed by the logarithms of all resting times and the corresponding deviations of the open-loop system. The fitting method uses the least squares method, i.e., finding a straight line that minimizes the sum of the squares of the perpendicular distances from all coordinate points to the line. The expression of the fitted straight line is: ΔP = k × lg(t) + b; where ΔP represents the deviation of the open-loop system (in kPa), lg(t) represents the logarithm of the resting time (dimensionless), k represents the slope (in kPa), which physically represents the rate of change of the deviation of the open-loop system with the logarithm of the resting time, and b represents the intercept (in kPa), which physically represents the theoretical value of the deviation of the open-loop system when the logarithm of the resting time is zero. The degree to which each data point is distributed along a straight line is evaluated using the linear correlation coefficient. The closer the absolute value of the linear correlation coefficient is to 1, the more significant the linear relationship between the deviation and the logarithm of the settling time. When the absolute value of the linear correlation coefficient is lower than the preset correlation threshold, the linear relationship is considered insignificant, and the reasons for the dispersion of the data points need to be checked. For example, check whether there are abnormal experimental operations or excessive differences in samples under a certain settling time. After excluding abnormal data points, the data should be refitted.
[0073] The preset correlation threshold is set as follows: calculate the average absolute percentage error between each data point and the fitted line, and use twice the average absolute percentage error as the reference value for the preset correlation threshold. For example, when the average absolute percentage error is 0.025, the preset correlation threshold is 0.95.
[0074] Extrapolate the straight line along the horizontal axis to the logarithm of the next order of magnitude of resting time, and read the corresponding deviation of the closed-loop non-closure. Use this deviation as the prediction memory value. The horizontal axis direction refers to the direction in which the logarithm of the resting time increases. Extrapolation means extending the straight line in the direction of increasing horizontal axis to a region exceeding the maximum value of the logarithm of the existing resting time, based on the linear expression fitted to the existing data points. The next order of magnitude of resting time refers to the resting time value that is one order of magnitude larger than the largest resting time value in the variation sequence. The calculation of the next order of magnitude of resting time is: multiply the largest resting time value in the variation sequence by 10. For example, when the largest resting time value in the variation sequence is 240 minutes, the next order of magnitude of resting time is 240 × 10 = 2400 minutes. The calculation of the logarithm of the next order of magnitude of resting time is: substitute the next order of magnitude of resting time value into a common logarithmic operation with base 10, such as log... 10 2400 ≈ 3.38. Substituting the logarithm of the next order of magnitude of settling time into the fitted linear expression ΔP = k × lg(t) + b, the predicted memory value is calculated. The formula for the predicted memory value is: ΔPyc = k × lg(tnext) + b; where ΔPyc represents the predicted memory value in kPa, lg(tnext) represents the logarithm of the next order of magnitude of settling time (dimensionless), k represents the slope of the fitted line in kPa, and b represents the intercept of the fitted line in kPa. The predicted memory value represents the level of deviation that may be achieved when the soil skeleton structure in the Quaternary cap sediment sample has been sufficiently adjusted, at a timescale much larger than the set settling time in the experiment, before the closed loop is closed.
[0075] In a specific implementation of S5, the predicted memory value is subtracted from each pore pressure value in the correspondence between pore pressure and gas saturation during the first cyclic pressurization process to obtain the corrected correspondence between pore pressure and gas saturation during the first cyclic pressurization process. The correspondence between pore pressure and gas saturation during the first cyclic pressurization process is a sequence of correspondences formed by recording each pore pressure value and its corresponding gas saturation value during the pressurization stage of the first pressurization and depressurization test on the Quaternary caprock sediment sample. The predicted memory value is the non-closed-loop deviation value read by extrapolating the straight line along the horizontal axis to the logarithm of the next order of magnitude of the resting time after determining the linear relationship between the deviation and the logarithm of the resting time in a logarithmic coordinate system. The unit of the predicted memory value is kilopascals. The operation of subtracting the predicted memory value from each pore pressure value is a point-by-point correction. The correction method is as follows: for each data point in the correspondence sequence between pore pressure and gas saturation during the first cycle of pressurization, the pore pressure value at that data point is replaced with the difference between the original pore pressure value and the predicted memory value, while the gas saturation value at that data point remains unchanged. The corrected pore pressure value is calculated as: Pxz = Pyc - ΔPyc, where Pxz represents the corrected pore pressure value in kPa, Pyc represents the original pore pressure value in the correspondence sequence between pore pressure and gas saturation during the first cycle of pressurization in kPa, and ΔPyc represents the predicted memory value in kPa. The corrected correspondence between pore pressure and gas saturation during the first cycle of pressurization is a correspondence sequence composed of the corrected pore pressure value and the original gas saturation value, with the same number of data points in the corrected sequence as before the correction. For example, in the correspondence between pore pressure and gas saturation during the first cycle of pressurization, the original pore pressure value for a certain data point is 200 kPa, the gas saturation value is 0.15, and the predicted memory value is 12 kPa. The corrected pore pressure value is 200 - 12 = 188 kPa, while the original gas saturation value of 0.15 remains unchanged. The corrected data point corresponds to a corrected pore pressure value of 188 kPa and a gas saturation value of 0.15. The physical basis for this correction operation is that the predicted memory value characterizes the level of memory effect corresponding to the full development of the soil skeleton's structural adjustment after a long period of settling in Quaternary cap sediment samples. Subtracting the predicted memory value from the original pore pressure value can eliminate the offset effect of soil skeleton structural adjustment on the pore pressure measurement value, making the corrected correspondence closer to the actual gas intrusion and sealing capacity of the Quaternary cap under complex stress history.
[0076] The corrected correspondence between pore pressure and gas saturation during the first depressurization cycle is obtained by subtracting the predicted memory value from each pore pressure value in the correspondence between pore pressure and gas saturation during the first depressurization cycle. This correspondence is a sequence of pore pressure values and their corresponding gas saturation values recorded step-by-step during the depressurization phase of the first depressurization test on Quaternary caprock sediment samples. The method of subtracting the predicted memory value from each pore pressure value is the same as when correcting the correspondence between pore pressure and gas saturation during the first pressurization cycle; that is, for each data point in the correspondence sequence, the pore pressure value at that data point is replaced with the difference between the original pore pressure value and the predicted memory value, while the gas saturation value remains unchanged. The corrected pore pressure value is calculated as: Pxzx = Pycx - ΔPyc, where Pxzx represents the pore pressure value in the corrected correspondence between pore pressure and gas saturation during the first cycle of depressurization (in kPa), Pycx represents the original pore pressure value in the same correspondence (in kPa), and ΔPyc represents the predicted memory value (in kPa). The corrected correspondence between pore pressure and gas saturation during the first cycle of depressurization is a sequence of the corrected pore pressure values and the original gas saturation values. The number of data points in the corrected sequence is the same as before the correction, and the pore pressure value at each data point is shifted by the magnitude corresponding to the predicted memory value in a decreasing direction. The physical basis for the correction operation is consistent with that for correcting the correspondence between pore pressure and gas saturation during the first cycle of pressurization, both involving eliminating the offset of pore pressure measurement values caused by soil skeleton structure adjustment by subtracting the predicted memory value.
[0077] In the corrected first-cycle pressurization process, the pore pressure and gas saturation relationship are correlated. The pore pressure corresponding to the first increase in gas saturation from its initial state is determined as the corrected caprock breakthrough pressure, and the corrected caprock sealing mechanical parameters are output. The initial state of gas saturation refers to the gas saturation value corresponding to the pores inside the Quaternary caprock sediment sample being saturated with water before the application of pore pressure; this value is generally zero. The method for determining the first increase in gas saturation from its initial state is as follows: in the corrected first-cycle pressurization process, starting from the first data point, the gas saturation value of each data point is checked point by point. When a data point is found where the gas saturation value is greater than zero for the first time, the corrected pore pressure value corresponding to that data point is determined as the corrected caprock sealing mechanical parameter. The unit of the corrected caprock sealing mechanical parameter is kPa. The corrected caprock sealing mechanical parameter is output by recording and presenting the values of the corrected caprock sealing mechanical parameter in the form of a data file or chart.
[0078] The revised caprock closure mechanical parameters also include the residual closure pressure, defined as the pore pressure at which the gas saturation returns to its initial state during the revised first-cycle depressurization process, within the correspondence between pore pressure and gas saturation. The initial state of gas saturation refers to the state at which the gas saturation gradually decreases as the pore pressure decreases during the revised first-cycle depressurization process, reaching zero. The residual closure pressure is determined by examining the gas saturation values point by point in the revised first-cycle depressurization process, starting from the last data point in the depressurization stage and proceeding backwards. The data point where the gas saturation value first equals zero is identified, and the revised pore pressure value corresponding to that data point is determined as the residual closure pressure. The unit of residual closure pressure is kPa. The residual sealing pressure characterizes the corrected pore gas pressure of a Quaternary caprock sediment sample after one complete pressurization-depressurization cycle, when the gas in the pores is completely displaced or redissolved by water and the sample returns to a water-saturated state. The corrected caprock breakthrough pressure and the residual sealing pressure are used together as the output of the corrected caprock sealing mechanical parameters.
[0079] Example 2: Figure 2 A schematic diagram of a system for analyzing and correcting the mechanical parameters of a Quaternary shallow gas cap layer closure according to the present invention is provided. This system includes the following modules:
[0080] The cyclic pressurization module is used to perform at least two cyclic pressurization tests on Quaternary caprock sediment samples and record the relationship between pore gas pressure and gas saturation in each cycle.
[0081] The deviation calculation module is used to extrapolate the pressure loading and unloading curves of two adjacent cycles to the initial state of gas saturation, calculate the difference in intercepts of the two pressure loading and unloading curves on the pore pressure coordinate axis, and use the difference in intercepts as the deviation of the closed loop not being closed.
[0082] The sequence acquisition module is used to set multiple different resting times after depressurization, measure the breakthrough pressure when pressurization is applied again, and repeat the execution of the depressurization module and the deviation calculation module to obtain the sequence of deviation changes with resting time.
[0083] The predictive memory value module is used to convert the settling time in the change sequence into a logarithmic value, determine the linear relationship between the deviation and the logarithmic value of the settling time in the logarithmic coordinate system, and extrapolate the linear relationship to the deviation corresponding to the logarithmic value of the settling time of the next order of magnitude as the predictive memory value.
[0084] The correction output module is used to correct the relationship between pore pressure and gas saturation measured in the first cycle using the predicted memory value, and outputs the corrected caprock closure mechanical parameters.
[0085] All calculations involved in the embodiments are dimensionless numerical calculations, and the preset parameters and thresholds in the calculations are set by those skilled in the art according to the actual situation.
[0086] The above embodiments can be implemented, in whole or in part, by software, hardware, firmware, or any other combination thereof. When implemented using software, the above embodiments can be implemented, in whole or in part, in the form of a computer program product.
[0087] Those skilled in the art will recognize that the modules and algorithm steps of the various examples described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are implemented in hardware or software depends on the specific application and inventive constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of this application.
Claims
1. A method for analyzing and correcting the mechanical parameters of a Quaternary shallow gas cap layer, characterized in that, Includes the following steps: S1: Perform at least two cyclic pressurization and depressurization tests on Quaternary caprock sediment samples and record the relationship between pore gas pressure and gas saturation in each cycle; S2: Extrapolate the pressurization and depressurization curves of two adjacent cycles to the initial state of gas saturation, calculate the difference in intercepts of the two pressurization and depressurization curves on the pore pressure coordinate axis, and use the difference in intercepts as the deviation of the closed loop not being closed. S3: After depressurization, set multiple different resting times, measure the breakthrough pressure when pressurizing again, and repeat steps S1 to S2 to obtain the sequence of deviation changes with resting time. S4: Convert the settling time in the change sequence into a logarithmic value, determine the linear relationship between the deviation and the logarithmic value of the settling time in the logarithmic coordinate system, and extrapolate the linear relationship to the deviation corresponding to the logarithmic value of the settling time of the next order of magnitude as the prediction memory value. S5: Use the predicted memory value to correct the relationship between pore pressure and gas saturation measured in the first cycle, and output the corrected caprock sealing mechanical parameters.
2. The method for analyzing and correcting the mechanical parameters of a Quaternary shallow gas cap layer as described in claim 1, characterized in that, At least two cyclic pressurization and depressurization tests were conducted on Quaternary caprock sediment samples, and the relationship between pore gas pressure and gas saturation was recorded in each cycle, including: Quaternary cap sediment samples were installed in a triaxial pressure chamber, and initial confining pressure and initial counter pressure were applied to simulate in-situ stress conditions. The pore pressure at one end of the sample was gradually increased by a set pressure increment. After the pore pressure stabilized at each level, the amount of pore water displaced by the gas was measured and the gas saturation was calculated. The relationship between pore pressure and gas saturation during the pressurization process was recorded. When the pore pressure rises to the preset upper limit, the pore pressure is gradually reduced by the set pressure reduction. After the pore pressure stabilizes at each level, the gas saturation is measured again, and the correspondence between pore pressure and gas saturation during the depressurization process is recorded. After completing one pressurization and depressurization cycle, the process of gradually increasing and decreasing the pore pressure is repeated to obtain the corresponding relationship between pore pressure and gas saturation during pressurization in at least two cycles, and the corresponding relationship between pore pressure and gas saturation during depressurization in at least two cycles.
3. The method for analyzing and correcting the mechanical parameters of a Quaternary shallow gas cap layer according to claim 2, characterized in that, The criterion for stable pore pressure is that the change in pore pressure readings for three consecutive times does not exceed 1‰ of the applied initial back pressure; the conversion of gas saturation is based on the difference between the initial pore volume and the cumulative amount of pore water discharged, divided by the initial pore volume.
4. The method for analyzing and correcting the mechanical parameters of a Quaternary shallow gas cap layer as described in claim 1, characterized in that, Extrapolate the pressurization and depressurization curves of two adjacent cycles to the initial state of gas saturation, calculate the difference in intercepts of the two pressurization and depressurization curves on the pore pressure coordinate axis, and use the intercept difference as the deviation of closed-loop non-closure, including: Extract the pressurization / unpressurization curves for the first cycle and the second cycle from the corresponding relationships between pore pressure and gas saturation during pressurization and depressurization during depressurization in at least two cycles. The pressure loading and unloading curve corresponding to the first cycle is extrapolated along the direction of decreasing gas saturation to the initial state of gas saturation, and the intercept of the pressure loading and unloading curve corresponding to the first cycle on the pore pressure coordinate axis is obtained. The pressure loading and unloading curve corresponding to the second cycle is extrapolated along the direction of decreasing gas saturation to the initial state of gas saturation, and the intercept of the pressure loading and unloading curve corresponding to the second cycle on the pore pressure coordinate axis is obtained. Calculate the difference between the intercept of the pressure loading / unloading curve corresponding to the first cycle on the pore pressure coordinate axis and the intercept of the pressure loading / unloading curve corresponding to the second cycle on the pore pressure coordinate axis, and use the difference as the deviation of the closed loop not closing.
5. The method for analyzing and correcting the mechanical parameters of a Quaternary shallow gas cap layer according to claim 4, characterized in that, The method of extrapolating the pressure loading and unloading curve to the initial state of gas saturation involves linearly extending the data point corresponding to the lowest gas saturation in the pressure loading and unloading curve in the direction of decreasing gas saturation until it intersects the vertical axis corresponding to the initial state value of gas saturation.
6. The method for analyzing and correcting the mechanical parameters of a Quaternary shallow gas cap layer according to claim 1, characterized in that, After depressurization, multiple different settling times were set, and the breakthrough pressure upon repressurization was measured for each time. Steps S1 to S2 were repeated to obtain the sequence of deviation changes with settling time, including: After completing one pressurization and depressurization cycle and removing the pore gas pressure to the initial value, the Quaternary caprock sediment sample was kept in the depressurized state for the first settling time. After the first settling period, the pore pressure was gradually increased by a set pressure increment. The pore pressure when the gas began to enter the Quaternary caprock sediment sample was recorded as the breakthrough pressure. At least two more cycles of pressurization and depressurization tests were completed, and the correspondence between pore pressure and gas saturation during the pressurization process and the correspondence between pore pressure and gas saturation during the depressurization process were recorded in each cycle. Based on the correspondence between pore pressure and gas saturation during the pressurization process and the correspondence between pore pressure and gas saturation during the depressurization process obtained under the first settling time, the deviation of closed loop non-closure corresponding to the first settling time is obtained by calculating the difference between the intercept of the pressurization / depressurization curve corresponding to the first cycle on the pore pressure coordinate axis and the intercept of the pressurization / depressurization curve corresponding to the second cycle on the pore pressure coordinate axis. Replace the Quaternary caprock sediment sample, perform another pressurization and depressurization cycle on the replaced Quaternary caprock sediment sample, and depressurize it. Maintain a second settling time different from the first settling time under depressurization. Repeat the breakthrough pressure measurement, record at least two pressurization and depressurization cycles, and calculate the deviation to obtain the deviation of the closed loop not closing corresponding to the second settling time. The process of obtaining the deviation amount of closed loop non-closure is repeated for at least two different resting times. All resting times and their corresponding deviation amounts of closed loop non-closure are collected to form a sequence of deviation amount changes with resting time.
7. The method for analyzing and correcting the mechanical parameters of a Quaternary shallow gas cap layer as described in claim 1, characterized in that, The settling time in the variation sequence is converted into a logarithmic value. A linear relationship between the deviation and the logarithmic value of the settling time is determined in a logarithmic coordinate system. This linear relationship is extrapolated to the logarithmic value of the settling time at the next order of magnitude, and the resulting deviation is used as the prediction memory value. Extract all resting times and the corresponding deviations that are not closed from the sequence of deviation changes with resting time; Calculate the logarithm of the total resting time; Using the logarithm of the settling time as the x-axis and the deviation of the non-closed loop as the y-axis, a straight line is determined in the logarithmic coordinate system such that the data points are distributed along the straight line, which serves as the linear relationship between the deviation and the logarithm of the settling time. Extrapolate the straight line along the horizontal axis to the logarithm of the next order of magnitude of the resting time, read the corresponding deviation of the closed loop not closing, and use the read deviation of the closed loop not closing as the prediction memory value.
8. The method for analyzing and correcting the mechanical parameters of a Quaternary shallow gas cap layer according to claim 1, characterized in that, The relationship between pore pressure and gas saturation measured in the first cycle is corrected using predicted memory values, and the corrected caprock closure mechanical parameters are output, including: The predicted memory value is subtracted from each pore pressure value in the correspondence between pore pressure and gas saturation during the first cycle of pressurization to obtain the corrected correspondence between pore pressure and gas saturation during the first cycle of pressurization. The predicted memory value is subtracted from each pore pressure value in the correspondence between pore pressure and gas saturation in the first cycle of depressurization to obtain the corrected correspondence between pore pressure and gas saturation in the first cycle of depressurization. In the corrected first cycle pressurization process, the pore pressure and gas saturation correspondence are determined by taking the pore pressure corresponding to the first increase in gas saturation from the initial state as the corrected caprock closure mechanical parameter, and the corrected caprock closure mechanical parameter is output.
9. The method for analyzing and correcting the mechanical parameters of a Quaternary shallow gas cap layer according to claim 8, characterized in that, The revised caprock closure mechanical parameters also include the pore pressure corresponding to the gas saturation when the gas saturation falls back to its initial state during the revised first cycle of depressurization, which is used as the residual closure pressure.
10. A system for analyzing and correcting the mechanical parameters of a Quaternary shallow gas cap layer, used to implement the method for analyzing and correcting the mechanical parameters of a Quaternary shallow gas cap layer as described in any one of claims 1-9, characterized in that, Includes the following modules: The cyclic pressurization module is used to perform at least two cyclic pressurization tests on Quaternary caprock sediment samples and record the relationship between pore gas pressure and gas saturation in each cycle. The deviation calculation module is used to extrapolate the pressure loading and unloading curves of two adjacent cycles to the initial state of gas saturation, calculate the difference in intercepts of the two pressure loading and unloading curves on the pore pressure coordinate axis, and use the difference in intercepts as the deviation of the closed loop not being closed. The sequence acquisition module is used to set multiple different resting times after depressurization, measure the breakthrough pressure when pressurization is applied again, and repeat the execution of the depressurization module and the deviation calculation module to obtain the sequence of deviation changes with resting time. The predictive memory value module is used to convert the settling time in the change sequence into a logarithmic value, determine the linear relationship between the deviation and the logarithmic value of the settling time in the logarithmic coordinate system, and extrapolate the linear relationship to the deviation corresponding to the logarithmic value of the settling time of the next order of magnitude as the predictive memory value. The correction output module is used to correct the relationship between pore pressure and gas saturation measured in the first cycle using the predicted memory value, and outputs the corrected caprock closure mechanical parameters.