A method for determining the lower limit of plugging thickness in complex lithologic layers based on pressure field simulation

Abstract

The present invention discloses a method for determining the lower limit of the sealing thickness of a complex lithologic layer based on pressure field simulation, comprising: determining the upper and lower superposition relationship and the total thickness of the two according to the lithologic vertical distribution characteristics of the overlying strata within a set range; obtaining the formula for preparing siltstone and mud shale respectively through experimental fitting; preparing a top plate rock sample by integral molding; arranging a pressurizing device and a pressure sensor on the bottom surface of the rock sample, and arranging multiple pressure sensors on the side of the rock sample at set intervals; pressurizing the rock sample with the pressurizing device to a set pressure at the bottom, and stabilizing it at this pressure until the measured values ​​of each pressure sensor are stable; determining the lower limit of the top plate sealing thickness according to the measured values ​​of the pressure sensors and the upper and lower superposition relationship of siltstone and mud shale; and similarly determining the lower limit of the bottom plate sealing thickness. This method can reasonably quantify the lower limit of the top and bottom plate sealing thickness of a shale reservoir by simplifying the lithologic and pressure field simulation methods.

Description

Technical Field

[0001] The present invention relates to the field of petroleum geology and shale reservoir mining technology, and in particular to a method for determining the lower limit of the sealing thickness of a complex lithology layer based on pressure field simulation. Background Art

[0002] my country's medium- to low-maturity oil shale reserves hold enormous potential, and in-situ mining technology, as an effective approach to industrializing its production, is gaining increasing attention. In-situ mining involves directly heating the shale reservoir underground, causing it to crack underground. The resulting oil and gas is then extracted through production wells.

[0003] Within the same geological environment, the sealing strength of the caprock is highly correlated with its lithology. Various geological processes form caprocks of varying lithologies. Based on the caprocks that have been explored to date, gypsum, argillaceous rock, and dense carbonate are the predominant lithologies, with few examples of other lithologies being explored as caprocks.

[0004] Whether the overburden can effectively seal shale reservoirs during in-situ mining is one of the key factors in improving oil recovery. However, there is no effective means to evaluate the roof sealing performance during in-situ mining of shale reservoirs. Summary of the Invention

[0005] In order to at least partially solve the above-mentioned technical problems existing in the prior art, the inventors have made the present invention. Through specific implementation methods, a method for determining the lower limit of the sealing thickness of complex lithologic layers based on pressure field simulation is provided. The method can reasonably quantify the lower limit of the sealing thickness of the shale reservoir roof by simplifying the lithologic and pressure field simulation methods.

[0006] An embodiment of the present invention provides a method for determining a lower limit of a plugging thickness of a complex lithologic layer based on pressure field simulation, wherein the complex lithologic layer is an overlying stratum of a shale reservoir, and the complex lithologic layer is an interbedded layer of siltstone and mud shale. The method includes:

[0007] Determine the vertical superposition relationship of the siltstone layer and the mud shale layer, and the total thickness of the siltstone layer and the total thickness of the mud shale layer based on the lithologic vertical distribution characteristics of the overlying strata within a set range;

[0008] Based on the measured properties of multiple siltstone samples, a first formula for siltstone is prepared by experimental fitting; based on the measured properties of multiple shale samples, a second formula for shale is prepared by experimental fitting;

[0009] According to the upper and lower overlapping relationship, the total thickness of the siltstone layer and the total thickness of the shale layer, the first formula and the second formula, a roof rock sample is prepared by an integrated molding method;

[0010] A first pressurizing device and a first pressure sensor are arranged on the bottom surface of the roof rock sample, and multiple first pressure sensors are arranged on the side of the roof rock sample at first set intervals. The first pressurizing device is used to pressurize the roof rock sample until the measured value of the first pressure sensor at the bottom is a first set pressure, and stabilizes at this pressure until the change amplitude of the measured value of each first pressure sensor is less than a set threshold. The lower limit of the sealing thickness of the roof rock sample is determined based on the measured value of the first pressure sensor and the upper and lower overlapping relationship.

[0011] In some embodiments, preparing a first formula of siltstone by experimental fitting based on measured properties of a plurality of siltstone samples comprises:

[0012] Measuring the particle size distribution, mineral composition, fracture pressure, minimum starting pressure, and porosity of rock samples of each siltstone layer within the set range of the overlying stratum, and obtaining the average value of the corresponding parameters by thickness-weighted averaging;

[0013] Using the average values ​​of the grain size distribution and the average values ​​of the mineral composition of the siltstone layer as the current formula, a siltstone sample is prepared, and the current formula is adjusted until the errors between the porosity, fracture pressure, and minimum starting pressure of the currently prepared siltstone sample and the corresponding average values ​​meet the error threshold, thereby obtaining a first formula for preparing siltstone;

[0014] Accordingly, the second formula for preparing shale by experimental fitting based on the measured properties of the plurality of shale samples includes:

[0015] Measuring the particle size distribution, mineral composition, TOC, porosity, and fracture pressure of rock samples of each shale layer within the set range, and obtaining the average value of the corresponding parameters by thickness-weighted averaging;

[0016] A shale sample is prepared using the average values ​​of the particle size distribution and the average values ​​of the mineral composition of the shale layer as the current formula. The current formula is adjusted until the errors between the TOC, porosity, and fracture pressure of the currently prepared shale sample and the corresponding average values ​​meet the error threshold, thereby obtaining a second formula for preparing shale.

[0017] In some embodiments, the first set pressure is the fracture pressure of the shale reservoir.

[0018] In some embodiments, determining the lower limit of the roof rock sample plugging thickness based on the measurement value of the first pressure sensor and the upper and lower overlapping relationship includes:

[0019] The lowest position where the roof rock sample pressure is not higher than the second set pressure is determined according to the measurement value of the first pressure sensor, and the lower limit of the roof plugging thickness is determined according to the upper and lower overlapping relationship and the lowest position.

[0020] In some embodiments, if the upper and lower stacking relationship is that the siltstone layer is on top,

[0021] The determining of the lower limit of the top plate sealing thickness according to the upper and lower overlapping relationship and the lowest position includes:

[0022] If the lowest position is located in the mud shale layer of the roof rock sample, the distance from the lowest position to the bottom surface of the roof rock sample is determined as the lower limit of the roof plugging thickness;

[0023] If the lowest position is located in the siltstone layer of the roof rock sample, determine whether the minimum starting pressure of the siltstone layer is greater than the shale reservoir fracture pressure; if so, determine the distance from the lowest position to the bottom surface of the roof rock sample as the lower limit of the roof sealing thickness; if not, determine that the roof rock sample cannot be sealed.

[0024] In some embodiments, if the upper and lower overlapping relationship is that the siltstone layer is at the bottom, determining the lowest position at which the roof rock sample pressure meets the set pressure requirement based on the measurement value of the first pressure sensor and the upper and lower overlapping relationship includes:

[0025] If the lowest position is located in the mud shale layer of the roof rock sample, the distance from the lowest position to the bottom surface of the roof rock sample is determined as the lower limit of the roof plugging thickness;

[0026] If the lowest position is located in the siltstone layer of the roof rock sample, determine whether the minimum starting pressure of the siltstone layer is greater than the shale reservoir fracture pressure; if so, determine the distance from the lowest position to the bottom surface of the roof rock sample as the lower limit of the roof sealing thickness; if not, determine the total thickness of the siltstone layer as the lower limit of the roof sealing thickness.

[0027] In some embodiments, the second set pressure is an average fracture pressure of the roof rock sample.

[0028] In some embodiments, determining the lower limit of the roof rock sample plugging thickness based on the measurement value of the first pressure sensor and the upper and lower overlapping relationship includes:

[0029] According to the measurement value of the first pressure sensor and the upper and lower overlapping relationship, the lowest position of the roof rock sample at which the pressure meets the set pressure requirement is determined, and the distance from the lowest position to the bottom surface of the roof rock sample is determined as the lower limit of the roof sealing thickness.

[0030] In some embodiments, if the upper and lower overlapping relationship is that the siltstone layer is on top, determining the lowest position at which the roof rock sample pressure meets the set pressure requirement based on the measurement value of the first pressure sensor and the upper and lower overlapping relationship includes:

[0031] If the pressure in the shale layer drops to the fracture pressure of the shale layer, the position where the pressure in the shale layer is equal to the fracture pressure of the shale layer is determined as the lowest position that meets the set pressure requirement;

[0032] If the pressure in the siltstone layer drops to the fracture pressure of the siltstone layer, determine whether the minimum starting pressure of the siltstone layer is greater than the fracture pressure of the shale reservoir; if so, determine the position where the pressure in the siltstone layer is the fracture pressure of the siltstone layer as the lowest position that meets the set pressure requirement; if not, determine that the roof rock sample cannot be sealed.

[0033] In some embodiments, if the upper and lower overlapping relationship is that the siltstone layer is at the bottom, determining the lowest position at which the roof rock sample pressure meets the set pressure requirement based on the measurement value of the first pressure sensor and the upper and lower overlapping relationship includes:

[0034] If the pressure in the siltstone layer drops to the fracture pressure of the siltstone layer, determine whether the minimum starting pressure of the siltstone layer is greater than the fracture pressure of the shale reservoir; if so, determine the position in the siltstone layer where the pressure is at the fracture pressure of the siltstone layer as the lowest position that meets the set pressure requirement; if not, determine the bottom surface of the shale layer as the lowest position that meets the set pressure requirement;

[0035] If the maximum pressure in the siltstone layer is greater than the fracture pressure of the siltstone layer but less than the fracture pressure of the mud shale layer, the bottom surface of the mud shale layer is determined as the lowest position that meets the set pressure requirement;

[0036] If the maximum pressure in the shale layer drops to the fracture pressure of the shale layer, the position where the pressure in the shale layer is equal to the fracture pressure of the shale layer is determined as the lowest position that meets the set pressure requirement.

[0037] In some embodiments, if the underlying rock formation of the shale reservoir is a mud shale formation, the method further includes performing the following step of determining the lower limit of the bottom plate plugging thickness:

[0038] A bottom plate rock sample is prepared using a coring section of an underlying rock formation of the shale reservoir, a side surface of the bottom plate rock sample is wrapped with a sealing material, a second pressurizing device and a second pressure sensor are arranged on a top surface of the bottom plate rock sample, and a plurality of second pressure sensors are arranged on the side surface of the bottom plate rock sample at second set intervals;

[0039] The bottom plate rock sample is pressurized by the second pressurizing device until the pressure measured by the second pressure sensor at the top reaches the first set pressure, and is stabilized at this pressure until the amplitude of the pressure change measured by each second pressure sensor is less than a set threshold, and the maximum position at which the bottom plate rock sample pressure is not higher than the third set pressure is determined based on the pressure measured by the second pressure sensor;

[0040] The lower limit of the bottom plate sealing thickness of the shale reservoir is determined according to the distance between the position and the top surface.

[0041] In some embodiments, if the underlying rock layer of the shale reservoir is an underlying siltstone layer, the method further includes:

[0042] Determining whether the shale reservoir fracture pressure is less than the minimum starting pressure of the underlying siltstone layer;

[0043] If so, execute the step of determining the lower limit of the bottom plate sealing thickness.

[0044] In some embodiments, if the underlying rock layer of the shale reservoir is an interbedded layer of siltstone and mud shale, the method further includes performing the following step of determining the lower limit of the bottom plate plugging thickness:

[0045] Determine the vertical superposition relationship of the siltstone layer and the mud shale layer, and the total thickness of the siltstone layer and the total thickness of the mud shale layer based on the lithologic vertical distribution characteristics of the underlying rock layer within a set range;

[0046] preparing a third formulation for siltstone by experimental fitting based on measured properties of multiple siltstone samples within a set range of the underlying rock formation, and preparing a fourth formulation for shale by experimental fitting based on measured properties of multiple shale samples within a set range of the underlying rock formation;

[0047] According to the upper and lower overlapping relationship of the siltstone layer and the mud shale layer of the underlying rock layer, the total thickness of the siltstone layer and the total thickness of the mud shale layer, and the third and fourth formulas, a bottom plate rock sample is prepared by an integrated molding method;

[0048] A second pressurizing device and a second pressure sensor are arranged on the top surface of the bottom plate rock sample, and multiple second pressure sensors are arranged on the side of the bottom plate rock sample at second set intervals. The second pressurizing device is used to pressurize the bottom plate rock sample until the measured value of the second pressure sensor on the top is a first set pressure, and stabilizes at this pressure until the change amplitude of the measured value of each second pressure sensor is less than a set threshold. The lower limit of the sealing thickness of the bottom plate rock sample is determined based on the measured value of the second pressure sensor and the overlapping relationship between the siltstone layer and the mud shale layer of the underlying rock stratum.

[0049] The beneficial effects of the above technical solutions provided by the embodiments of the present invention include at least:

[0050] (1) The method provided in the embodiment of the present invention for determining the lower limit of the plugging thickness of a complex lithologic layer based on pressure field simulation is to prepare a roof rock sample by integral molding, and to pressurize the roof rock sample using a first pressurizing device so that the measured pressure of the first pressure sensor at the bottom is stabilized at a first set pressure until the measured values ​​of each first pressure sensor are stable. The lower limit of the plugging thickness of the roof rock sample is determined based on the measured values ​​of the first pressure sensor and the relationship between the upper and lower superposition of the siltstone layer and the mudstone layer. The lower limit of the plugging thickness of the roof of a shale reservoir is determined based on the pressure field simulation by an experimental method, and the roof plugging conditions during the in-situ mining of a shale reservoir are specifically quantified, which can effectively guide production and improve the economic benefits of in-situ mining of shale oil and gas.

[0051] (2) The method provided in the embodiment of the present invention for determining the lower limit of the sealing thickness of a complex lithologic layer based on pressure field simulation determines the vertical superposition relationship of the siltstone layer and the mudstone layer, as well as the total thickness of the siltstone layer and the total thickness of the mudstone layer, based on the lithologic vertical distribution characteristics of the overlying strata within a set range; obtains the formula for preparing siltstone and mudstone respectively through experimental fitting; and prepares the roof rock sample by integral molding. The prepared synthetic sample takes into account both the properties of mudstone and siltstone and the vertical superposition relationship between the two, rationally simplifies the sample, and thus realizes the determination of the lower limit of the sealing thickness of the complex lithologic roof with interbedded mudstone and siltstone.

[0052] (3) It is currently generally believed that only mud shale can effectively seal shale reservoirs during in-situ mining. The method provided in the embodiment of the present invention for determining the lower limit of the sealing thickness of complex lithologic layers based on pressure field simulation suggests that if the overlying rock layer of the shale reservoir is an interbedded layer of siltstone and mud shale, it may also produce a sealing effect during the in-situ mining of the shale reservoir, thereby broadening the exploration prospects of shale oil and gas.

[0053] (4) When studying the sealing properties of shale reservoirs, existing technologies often focus only on the sealing properties of the roof, while ignoring the sealing function of the floor. However, for in-situ pressurized production of shale reservoirs, poor floor sealing performance will still lead to oil and gas loss, affecting the recovery rate. The method for determining the lower limit of the sealing thickness of complex lithologic layers based on pressure field simulation provided by the embodiment of the present invention not only considers the sealing properties of the roof, but also fully considers the sealing properties of the floor, thus ensuring the reliability of the determination of the sealing performance and further providing a guarantee for improving the recovery rate.

[0054] Other features and advantages of the present invention will be described in the following description, and in part will become apparent from the description, or will be understood by practicing the present invention. The purposes and other advantages of the present invention can be realized and obtained by the structures particularly pointed out in the written description, claims, and drawings.

[0055] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] The accompanying drawings are used to provide a further understanding of the present invention and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention and do not constitute a limitation of the present invention. In the accompanying drawings:

[0057] Figure 1 This is a flow chart of a method for determining the lower limit of the plugging thickness of a complex lithologic layer based on pressure field simulation in Example 1 of the present invention;

[0058] Figure 2 This is a flow chart of a method for determining the lower limit of the plugging thickness of a complex lithologic layer based on pressure field simulation in Example 2 of the present invention;

[0059] Figure 3 This is a flow chart of a method for determining the lower limit of the plugging thickness of a complex lithologic layer based on pressure field simulation in Example 3 of the present invention;

[0060] Figure 4 This is a schematic diagram of the axial cross-section of the roof rock sample in Example 4 of the present invention;

[0061] Figure 5 This is a pressure field simulation diagram in Example 4 of the present invention;

[0062] Figure 6 for Figure 5 Oil saturation change diagram at the middle pressure line of 20MPa and 25MPa. DETAILED DESCRIPTION

[0063] Exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although exemplary embodiments of the present disclosure are shown in the accompanying drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. Rather, these embodiments are provided to enable a more thorough understanding of the present disclosure and to fully convey the scope of the present disclosure to those skilled in the art.

[0064] It should be understood that the terms described herein are intended only to describe particular embodiments and are not intended to limit the present invention. In addition, for numerical ranges herein, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Each smaller range between any intermediate value within a stated value or stated range and any other stated value or intermediate value within the stated range is also encompassed by the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded within the scope.

[0065] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the invention belongs. Although the present invention describes only preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials related to the documents. In the event of any conflict with any incorporated document, the content of this specification shall prevail.

[0066] In the description of the present invention, it should be noted that the terms “include”, “including”, “have”, “contain”, etc. are all open terms, which mean including but not limited to.

[0067] The inventors discovered that during the in-situ mining of shale reservoirs, the source rock begins to crack and generate hydrocarbons after being heated to a certain pressure. The flow of hydrocarbon substances causes the top and bottom plates of the shale reservoir to be affected by fluid pressure. Therefore, the lower limit of the plugging thickness of the top and bottom plates can be reasonably determined through pressure field simulation experiments. In addition, it is generally believed that only mud shale can produce a plugging effect during the in-situ mining of shale reservoirs. The inventors discovered that the complex lithology of interbedded mud shale and siltstone may also produce a plugging effect during the in-situ mining of shale reservoirs. The embodiment of the present invention provides a method for determining the lower limit of the plugging thickness of complex lithology layers based on pressure field simulation, which can reasonably quantify the lower limit of the plugging thickness of the top plate of the shale reservoir by simplifying the lithology and pressure field simulation methods.

[0068] Example 1

[0069] The first embodiment of the present invention provides a method for determining the lower limit of the sealing thickness of a complex lithologic layer based on pressure field simulation, referring to Figure 1 As shown, the following steps are included:

[0070] Step S11: Determine the vertical superposition relationship of the siltstone layer and the mud shale layer, and the total thickness of the siltstone layer and the total thickness of the mud shale layer according to the vertical distribution characteristics of the lithology of the overlying strata within a set range.

[0071] The complex lithologic layer is the overlying stratum of the shale reservoir, which is composed of interbedded siltstone and mud shale.

[0072] Empirical data indicates that the roof seal thickness during in-situ mining of shale reservoirs is typically within 300-400 centimeters. Therefore, the length (thickness) of the roof rock sample should be no less than 500 cm. Any thicker increases experimental difficulty and cost; any thinner will easily fall below the actual lower limit of the roof seal thickness, resulting in experimental failure (unable to measure the required in-situ roof seal thickness for shale). This means that the overlying stratum should be within a 500 cm thickness range, preferably within 500 cm from the location closest to the top of the shale reservoir upward.

[0073] The set range is divided into an upper and a lower part by an averaging method. If the cumulative thickness of the lower shale layer exceeds the first set ratio (for example, more than 60%), the superposition relationship is determined to be the shale layer at the bottom and the siltstone layer at the top; if the cumulative thickness of the lower shale layer is lower than the second set ratio (for example, lower than 40%), the superposition relationship is determined to be the shale layer at the top and the siltstone layer at the bottom; if the cumulative thickness of the lower shale layer is within the set ratio range (for example, 40% to 60%), that is, the ratios of the two are equivalent, then the lower part within the set range is further divided into an upper and a lower part by an averaging method, and the above method is repeated to determine the upper and lower superposition relationship of the siltstone layer and the shale layer.

[0074] Step S12: Based on the measured properties of multiple siltstone samples, a first formula of siltstone is prepared by experimental fitting; based on the measured properties of multiple shale samples, a second formula of shale is prepared by experimental fitting.

[0075] In some embodiments, fitting a first formula for preparing siltstone may include: measuring the particle size distribution, mineral composition, porosity, fracture pressure, and minimum starting pressure of rock samples of each siltstone layer within a set range of the overlying stratum, and obtaining the average value of the corresponding parameters by thickness-weighted averaging; preparing a siltstone sample using the average value of the particle size distribution and the average value of the mineral composition of the siltstone layer as the current formula, and adjusting the current formula until the errors between the porosity, fracture pressure, and minimum starting pressure of the currently prepared siltstone sample and the corresponding average values ​​meet an error threshold, thereby obtaining the first formula for preparing siltstone.

[0076] Small-sized samples can be synthesized first, the porosity of the synthesized samples can be measured, and the results can be calibrated with the measurement results of natural samples. Then, the current particle size distribution and mineral composition can be adjusted and optimized, and new artificial samples can be synthesized. Finally, the error between the measurement results of the synthetic samples and the natural samples does not exceed the error threshold (which can be set to 15%).

[0077] In some embodiments, fitting the second formula for preparing mud shale may include: measuring the particle size distribution, mineral composition, TOC, porosity and fracture pressure of rock samples of each mud shale layer within a set range of the overlying stratum, and obtaining the average value of the corresponding parameters by thickness weighting; preparing the mud shale sample with the average value of the particle size distribution and the average value of the mineral composition of the mud shale layer as the current formula, and adjusting the current formula until the errors between the TOC, porosity and fracture pressure of the currently prepared mud shale sample and the corresponding average values ​​meet the error threshold, thereby obtaining the second formula for preparing mud shale.

[0078] Optionally, in the process of fitting the first and second formulas, the parameters used for calibration may include not only the above parameters but also other parameters. Correspondingly, the measurement results of natural samples of these parameters are obtained by thickness-weighted averaging the measurement results of the corresponding rock samples.

[0079] Step S13: preparing a roof rock sample by integral molding according to the upper and lower overlapping relationship, the total thickness of the siltstone layer and the total thickness of the shale layer, the first formula and the second formula.

[0080] Roof rock samples of the same cross-sectional area (eg cylindrical with a diameter of 5 cm) can be prepared.

[0081] Taking the above setting range of 500 cm thickness as an example, the thickness of the roof rock sample is also 500 cm.

[0082] The sides of the top plate rock sample are wrapped with sealing material. The sealing property ensures that the fluid applied from the bottom of the rock sample cannot enter between the rock sample and the thermal insulation sealing material during the fluid pressurization process. The pressure resistance of the sealing material is required to be not less than 80MPa.

[0083] Step S14: Arrange a first pressurizing device and a first pressure sensor on the bottom surface of the roof rock sample, and arrange multiple first pressure sensors on the side of the roof rock sample at a first set interval. Use the first pressurizing device to pressurize the roof rock sample until the measurement value of the first pressure sensor at the bottom is the first set pressure, and stabilize it at this pressure until the change amplitude of the measurement value of each first pressure sensor is less than the set threshold value. Determine the lower limit of the sealing thickness of the roof rock sample based on the measurement value of the first pressure sensor and the upper and lower overlapping relationship.

[0084] In the embodiment of the present application, from bottom to top corresponds to from bottom to top.

[0085] In some embodiments, the first pressurizing device is a fluid pressurizing device.

[0086] In some embodiments, the first set pressure is the fracture pressure of the shale reservoir, which can be determined through a rock fracture pressure experiment.

[0087] The fracture pressure of shale reservoirs is usually around 50 MPa, and the fracture pressure of overlying rock formations is usually around 20 MPa.

[0088] The first pressure sensors are arranged axially on the side of the roof rock sample, and the first set interval of the arrangement is set according to the measurement accuracy requirement, usually at an interval of 10 cm.

[0089] The fracture pressure P0 of the shale reservoir is determined using the aforementioned method. A first pressure-increasing device is used to pressurize the bottom of the roof sample using fluid boosting at a rate not exceeding 300 kPa / day (the specific value is flexibly set based on on-site construction conditions). The pressure at the bottom of the roof sample is maintained at a constant P0 for at least 30 minutes, until the fluctuation in the measured values ​​of each sensor is less than the set threshold. Once the pressure stabilizes, the measured values ​​of each first pressure sensor are collected. Because the pressure of the roof sample gradually decreases from bottom to top, the measured values ​​of the first pressure sensors can be collected sequentially from bottom to top.

[0090] In some embodiments, the lower limit of the roof rock sample plugging thickness is determined based on the measurement value of the first pressure sensor and the vertical superposition relationship between the siltstone layer and the mudstone layer, which may include the following two situations:

[0091] 1. Determine the lowest position where the roof rock sample pressure is no higher than the second set pressure based on the measurement value of the first pressure sensor, and determine the lower limit of the roof sealing thickness based on the upper and lower overlapping relationship and the lowest position of the siltstone layer and the shale layer.

[0092] This situation is applicable when the fracture pressure difference between the siltstone layer and the mud shale layer is not much. The second set pressure is the average fracture pressure of the roof rock sample, which can be the average value of the fracture pressure of the siltstone layer and the mud shale layer weighted by thickness.

[0093] The first set pressure, ie, the shale reservoir fracture pressure P0, and the second set pressure, ie, the overlying siltstone layer fracture pressure Px, are determined in the above manner.

[0094] The bottom of the roof rock sample is pressurized by using a first pressurizing device, and fluid is used for pressurization. The pressurization speed does not exceed 300KPa / day (the specific value is flexibly set according to the on-site construction conditions), so that the pressure at the bottom of the roof rock sample is constant at P0 and is stable for at least 30 minutes until the change amplitude of the measured value of each sensor is less than the set threshold. That is, after the pressure is stable, the measured values ​​of each first pressure sensor are collected. Since the pressure of the roof rock sample gradually decreases from bottom to top, the measured values ​​of the first pressure sensors can be collected in sequence from bottom to top until the currently collected pressure is no greater than Px. The lowest position where the pressure of the roof rock sample is no higher than Px is determined according to the position of the corresponding sensor.

[0095] If the currently collected pressure is the second set pressure Px, the position of the corresponding first pressure sensor is directly determined as the lowest position where the roof rock sample pressure is not higher than Px; if the currently collected pressure begins to be less than Px (the first pressure less than Px), the lowest position where the roof rock sample pressure is not higher than Px is determined by the proportional method based on the currently collected pressure and the previous collected pressure and the position of the current first pressure sensor and the position of the previous first pressure sensor.

[0096] The lower limit of the roof sealing thickness is determined based on the superposition relationship and the lowest position of the siltstone layer and the mud shale layer, which can include the following two situations:

[0097] 1. The superposition relationship is that the siltstone layer is on top.

[0098] (1) If the lowest position is located in the mudstone layer of the roof rock sample, the distance from the lowest position to the bottom surface of the roof rock sample is determined as the lower limit of the roof sealing thickness.

[0099] (2) If the lowest position is located in the siltstone layer of the roof rock sample, determine whether the minimum starting pressure of the siltstone layer is greater than the fracture pressure of the shale reservoir; if so, determine the distance from the lowest position to the bottom surface of the roof rock sample as the lower limit of the roof sealing thickness; if not, determine that the roof rock sample cannot be sealed.

[0100] 2. The superposition relationship is that the siltstone layer is at the bottom.

[0101] (1) If the lowest position is located in the mudstone layer of the roof rock sample, the distance from the lowest position to the bottom surface of the roof rock sample is determined as the lower limit of the roof sealing thickness.

[0102] (2) If the lowest position is located in the siltstone layer of the roof rock sample, determine whether the minimum starting pressure of the siltstone layer is greater than the fracture pressure of the shale reservoir; if so, determine the distance from the lowest position to the bottom surface of the roof rock sample as the lower limit of the roof sealing thickness; if not, determine the total thickness of the siltstone layer as the lower limit of the roof sealing thickness.

[0103] 2. Based on the measurement value of the first pressure sensor and the overlapping relationship between the siltstone layer and the mudstone layer, determine the lowest position where the roof rock sample pressure meets the set pressure requirement, and determine the distance from the lowest position to the bottom surface of the roof rock sample as the lower limit of the roof sealing thickness.

[0104] This situation is applicable to situations where the fracture pressure difference between the siltstone layer and the mud shale layer is large (usually the fracture pressure of siltstone is less than that of mud shale) and the lower limit of the sealing thickness is determined with high accuracy.

[0105] Based on the measurement value of the first pressure sensor and the superposition relationship between the siltstone layer and the mudstone layer, determine the lowest position where the roof rock sample pressure meets the set pressure requirement. Depending on the superposition relationship, the lowest position includes:

[0106] 1. The superposition relationship is that the siltstone layer is on top.

[0107] (1) If the pressure in the shale layer drops to the fracture pressure of the shale layer, the position where the pressure in the shale layer is equal to the fracture pressure of the shale layer is determined as the lowest position that meets the set pressure requirement;

[0108] (2) If the pressure in the siltstone layer drops to the fracture pressure of the siltstone layer, determine whether the minimum starting pressure of the siltstone layer is greater than the fracture pressure of the shale reservoir; if so, determine the position where the pressure in the siltstone layer is the fracture pressure of the siltstone layer as the lowest position that meets the set pressure requirement; if not, determine that the roof rock sample cannot be sealed.

[0109] 2. The superposition relationship is that the siltstone layer is at the bottom.

[0110] (1) If the pressure in the siltstone layer drops to the fracture pressure of the siltstone layer, determine whether the minimum starting pressure of the siltstone layer is greater than the fracture pressure of the shale reservoir; if so, determine the position where the pressure in the siltstone layer is equal to the fracture pressure of the siltstone layer as the lowest position that meets the set pressure requirement; if not, determine the bottom surface of the shale layer as the lowest position that meets the set pressure requirement;

[0111] (2) If the maximum pressure in the siltstone layer is greater than the fracture pressure of the siltstone layer but less than the fracture pressure of the mud shale layer, the bottom surface of the mud shale layer is determined as the lowest position that meets the set pressure requirement;

[0112] (3) If the maximum pressure in the shale layer drops to the fracture pressure of the shale layer, the position where the pressure in the shale layer is equal to the fracture pressure of the shale layer is determined as the lowest position that meets the set pressure requirement.

[0113] In some embodiments, other factors affecting the plugging performance may be further considered to determine a safety correction factor (greater than 1), and the safety correction factor may be used to correct the lower limit of the roof plugging thickness determined above.

[0114] The method provided in Example 1 of the present invention for determining the lower limit of the plugging thickness of a complex lithologic layer based on pressure field simulation comprises: preparing a roof rock sample by integral molding; pressurizing the roof rock sample using a first pressurizing device so that the measured pressure of the first pressure sensor at the bottom stabilizes at a first set pressure; and determining the lower limit of the plugging thickness of the roof rock sample based on the measured values ​​of the first pressure sensors and the relationship between the upper and lower superposition of the siltstone layer and the mudstone layer. The lower limit of the roof plugging thickness of a shale reservoir is determined by experimental methods based on pressure field simulation, and the roof plugging conditions during the in-situ process of the shale reservoir are specifically quantified, which can effectively guide production and improve the economic benefits of in-situ shale oil and gas extraction.

[0115] The method provided in Example 1 of the present invention for determining the lower limit of the sealing thickness of complex lithologic layers based on pressure field simulation determines the vertical superposition relationship of siltstone and mudstone layers, as well as the total thickness of the siltstone and mudstone layers, based on the lithologic vertical distribution characteristics of the overlying strata within a set range. Formulas for preparing siltstone and mudstone are obtained through experimental fitting, respectively. A roof rock sample is then prepared by integral molding. The prepared synthetic sample takes into account both the properties of mudstone and siltstone, as well as their vertical superposition relationship, rationally simplifying the sample and thereby enabling the determination of the lower limit of the sealing thickness of a complex lithologic roof interbedded with mudstone and siltstone.

[0116] It is generally believed that only mud shale can effectively seal shale reservoirs during in-situ mining. The method provided in Example 1 of the present invention for determining the lower limit of the sealing thickness of complex lithologic layers based on pressure field simulation suggests that if the overlying rock layer of the shale reservoir is an interbedded layer of siltstone and mud shale, a sealing effect may also be produced during the in-situ mining of the shale reservoir, thereby broadening the exploration prospects of shale oil and gas.

[0117] Example 2

[0118] The second embodiment of the present invention provides another method for determining the lower limit of the plugging thickness of a complex lithologic layer based on pressure field simulation, specifically a method for determining the lower limit of the bottom plate plugging thickness of a shale reservoir.

[0119] If the underlying rock layer of the shale reservoir is a mudstone formation, directly perform the following steps to determine the lower limit of the bottom plate plugging thickness; if the underlying rock layer of the shale reservoir is an underlying siltstone layer, determine whether the shale reservoir fracture pressure is less than the minimum starting pressure of the underlying siltstone layer; if so, perform the following steps to determine the lower limit of the bottom plate plugging thickness; if not, determine that the underlying formation does not have sealing properties during the in-situ mining of the shale reservoir.

[0120] Specifically, refer to Figure 2 As shown, the following steps are included:

[0121] Step S21: Prepare a bottom plate rock sample using the coring section of the underlying rock formation of the shale reservoir, wrap the side of the bottom plate rock sample with a sealing material, arrange a second pressurizing device and a second pressure sensor on the top surface of the bottom plate rock sample, and arrange multiple second pressure sensors on the side of the bottom plate rock sample at a second set interval.

[0122] The second pressure sensor may be the same pressure sensor as the first pressure sensor, and the "first" and "second" here are only used to distinguish whether the pressure sensor is set in the top plate rock sample or the bottom plate rock sample.

[0123] A bottom rock sample of the same cross-sectional area (e.g., cylindrical with a diameter of 5 cm) is prepared from a cored section of the underlying stratum of the shale reservoir. The length (thickness) of the bottom rock sample is not less than 500 cm.

[0124] The second pressurizing device can be the same as the first pressurizing device. The "first" and "second" here are only used to distinguish whether the pressurizing device is set on the bottom surface of the top plate rock sample or the top surface of the bottom plate rock sample.

[0125] The second pressure sensors are arranged axially on the side of the bottom plate rock sample. The second set interval is set according to the measurement accuracy requirement and can be consistent with the first set interval, usually 10 cm.

[0126] Step S22: Use the second pressurizing device to pressurize the bottom plate rock sample until the measured pressure of the second pressure sensor at the top is the first set pressure, and stabilize at this pressure until the measured pressure change amplitude of each second pressure sensor is less than the set threshold value, and determine the highest position where the bottom plate rock sample pressure is not higher than the third set pressure based on the measured pressure of the second pressure sensor.

[0127] In some embodiments, the third set pressure is the fracture pressure of the underlying formation.

[0128] Taking the third set pressure as Px as an example, the second pressurizing device is used to pressurize the top of the bottom plate rock sample, and fluid is used for pressurization. The pressurization speed does not exceed 300KPa / day (the specific value is flexibly set according to the on-site construction conditions), so that the pressure at the top of the bottom plate rock sample is constant at P0 and stabilized for at least 30 minutes until the change amplitude of the measured value of each sensor is less than the set threshold. That is, after the pressure is stable, the measured values ​​of each second pressure sensor are collected. Since the pressure of the bottom plate rock sample gradually decreases from top to bottom, the measured values ​​of the second pressure sensors can be collected in sequence from top to bottom until the currently collected pressure is no greater than Px. The highest position where the pressure of the top plate rock sample is no higher than Px is determined according to the position of the corresponding sensor.

[0129] If the currently collected pressure is Px, the position of the corresponding second pressure sensor can be directly determined as the position where the bottom plate rock sample pressure begins to drop to Px; if the currently collected pressure begins to be less than Px (the first pressure less than Px), based on the currently collected pressure and the previous collected pressure and the position of the current second pressure sensor and the position of the previous second pressure sensor, the highest position where the bottom plate rock sample pressure is not higher than Px is determined by the proportional method.

[0130] Step S23: determining the lower limit of the bottom plate plugging thickness of the shale reservoir according to the distance from the position to the top surface.

[0131] The distance from this position to the top surface can be directly determined as the lower limit of the bottom plate plugging thickness of the shale reservoir; other factors affecting the plugging performance can also be further considered to determine a safety correction factor (greater than 1), and the product of the safety correction factor and the determined distance can be used as the lower limit of the bottom plate plugging thickness of the shale reservoir.

[0132] Example 3

[0133] Embodiment 3 of the present invention provides another method for determining the lower limit of the sealing thickness of complex lithologic layers based on pressure field simulation, specifically a method for determining the lower limit of the bottom plate sealing thickness of a shale reservoir, which is applicable to the case where the underlying rock layer of the shale reservoir is an interlayer of siltstone and mudstone. The method for determining the lower limit of the top plate sealing thickness of the shale reservoir in the above embodiment 1 is still applicable here.

[0134] Specifically, refer to Figure 3 As shown, the following steps are included:

[0135] Step S31: Determine the vertical superposition relationship of the siltstone layer and the mud shale layer, and the total thickness of the siltstone layer and the total thickness of the mud shale layer based on the lithologic vertical distribution characteristics of the underlying rock layer within a set range.

[0136] The setting range of the underlying rock layer can be within a thickness range of 500 cm, preferably, within a thickness range of 500 cm downward from a position closest to the top surface of the shale reservoir.

[0137] Step S32: Based on the measured properties of multiple siltstone samples within the set range of the underlying rock formation, a third formula for siltstone is prepared by experimental fitting, and based on the measured properties of multiple shale samples within the set range of the underlying rock formation, a fourth formula for shale is prepared by experimental fitting.

[0138] Step S33: preparing a bottom plate rock sample by integral molding according to the upper and lower overlapping relationship of the siltstone layer and the mud shale layer of the underlying rock strata, the total thickness of the siltstone layer and the total thickness of the mud shale layer, the third formula and the fourth formula.

[0139] Step S34: a second pressurizing device and a second pressure sensor are arranged on the top surface of the bottom plate rock sample, and multiple second pressure sensors are arranged on the side of the bottom plate rock sample at a second set interval. The second pressurizing device is used to pressurize the bottom plate rock sample until the measurement value of the second pressure sensor at the top is the first set pressure, and stabilize it at this pressure until the measurement value change amplitude of each second pressure sensor is less than the set threshold value. The lower limit of the sealing thickness of the bottom plate rock sample is determined based on the measurement value of the second pressure sensor and the overlapping relationship between the siltstone layer and the mudstone layer of the underlying rock layer.

[0140] The execution process of step S34 can refer to the execution process of step S14, which will not be repeated here.

[0141] Existing techniques for studying the sealing properties of shale reservoirs often focus solely on the sealing properties of the roof, while ignoring the sealing function of the floor. However, for in-situ mining of shale reservoirs, poor floor sealing performance can still lead to oil and gas loss, impacting recovery rates. The methods for determining the sealing thickness of complex lithologic adjacent layers in shale reservoirs based on pressure simulation, provided in Examples 2 and 3 of the present invention, fully consider the sealing properties of the floor, ensuring the reliability of the sealing performance determination and further guaranteeing improved recovery rates.

[0142] The above three embodiments can be applied individually; embodiment one and embodiment two can also be applied in combination, or embodiment one and embodiment three can be applied in combination, to determine both the lower limit of the top plate plugging thickness of the shale reservoir and the lower limit of the bottom plate plugging thickness of the shale reservoir. Only when the top and bottom plate thicknesses of the shale reservoir both meet the corresponding lower limits of the plugging thickness can it be determined that the plugging requirements are met.

[0143] Example 4

[0144] A fourth embodiment of the present invention provides a specific application of a method for determining a lower limit of the roof plugging thickness of a shale reservoir, and verifies its accuracy through numerical simulation, including the following steps:

[0145] 1) Count the total thickness of the siltstone layer and the total thickness of the shale layer in the overlying strata within the set range.

[0146] 2) The particle size distribution, mineral composition, initial kerogen hydrocarbon generation temperature Tx, TOC, porosity Φ, fracture pressure Px, and critical damage strain value εx of different shale layer samples were measured respectively, and the average values ​​of the above parameters of the shale layer were calculated using the thickness-weighted average method.

[0147] 3) The particle size distribution, mineral composition, porosity Φ, fracture pressure Px, and critical damage strain value εx of different siltstone layer samples were measured respectively, and the average values ​​of the above parameters of the siltstone layer were calculated using the thickness weighted average method.

[0148] 4) According to the geological conditions, the artificial samples in this experiment were determined to have a siltstone layer at the bottom and a shale layer at the top.

[0149] 5) According to the averaged parameter results, based on the superposition relationship and thickness, a roof rock sample containing a homogeneous siltstone layer and a homogeneous mudstone layer is synthesized at one time (the mudstone layer thickness is equal to 400 cm, the siltstone layer thickness is equal to 100 cm, and the cumulative deviation of the above parameters from the design value does not exceed 15%, and the sample preparation is qualified. The roof rock sample is a cylinder with a diameter of 5 cm, see Figure 4 The figure shows a schematic cross-sectional view of the roof rock sample along the axial direction.

[0150] 6) Determine the Po (fracture pressure) of the reservoir rock sample at 50 MPa.

[0151] 7) The Px (fracture pressure) of the roof mudstone sample was measured to be 22 MPa, and the Px (fracture pressure) of the siltstone sample was 20 MPa. The fracture pressure of the entire roof rock sample was approximately 20 MPa.

[0152] 8) Wrap the top rock sample with a pressure-resistant steel sleeve with a pressure resistance of not less than 80 MPa, and then wrap it with rock wool as an insulating material. Place the top rock sample in the experimental device. The sealing ensures that the fluid applied at the bottom of the sample cannot enter between the sample and the steel sleeve.

[0153] 9) Starting from the bottom of the roof rock sample, several pressure sensors are arranged along the axial direction of the roof rock sample at intervals of 10 cm.

[0154] 10) Fluid pressurization is used on the bottom of the roof rock sample at a rate of 300 kPa / day and a peak pressure of 50 MPa (Po).

[0155] 11) After the bottom pressure of the roof sample remains constant at P0 for 30 minutes (when each sensor's measurement value reaches stability), continuously collect data from the roof sample's pressure sensors. The thickness from the location of pressure Px to the bottom of the sample is calculated to be 160 cm, which is greater than the thickness of the siltstone layer and is the lower limit of the roof seal thickness.

[0156] 12) A numerical model consistent with the samples and experimental conditions was constructed. The shale layer was located at the top, with a thickness of 400 cm. Its TOC, porosity, Tx, Px, εx, and mineral composition were identical to those of the artificial samples used in the experiment. The siltstone layer was located at the bottom, with a thickness of 100 cm. Its porosity, Px, εx, and mineral composition were identical to those of the artificial samples used in the experiment. The Px (fracture pressure) of the roof shale sample was set to 22 MPa, and the Px (fracture pressure) of the roof siltstone sample was set to 20 MPa.

[0157] 13) Keep the pressure at the bottom of the roof rock sample constant at P0 (50 MPa), complete the numerical simulation of the pressure field, and wait for the grid pressure to stabilize.

[0158] 14) See Figure 5 The figure shows the simulated pressure field of the roof rock sample. Figure 5 Oil saturation of the Px pressure line (20Mpa) (see Figure 6 There was no change during the entire simulation period, indicating that oil and gas were blocked at this location.

[0159] 15) Observation Figure 5 Oil saturation at the medium pressure line 25Mpa (see Figure 6 During the entire simulation period, there are obvious changes, indicating that the oil and gas are not blocked at this location.

[0160] 16) The thickness of the Px pressure line is 165 cm. Comparison shows that a roof thickness of 165 cm meets the plugging requirements. The error between the numerical simulation results and the experimental results (160 cm) is 3.1%, thus the numerical simulation verifies the reliability of the experimental results.

[0161] It should be understood that the specific order or hierarchy of steps in the disclosed processes is an example of an exemplary method. Based on design preferences, it should be understood that the specific order or hierarchy of steps in the process can be rearranged without departing from the scope of the present disclosure. The accompanying method claims present elements of the various steps in an exemplary order and are not intended to be limited to the specific order or hierarchy described.

[0162] In the foregoing detailed description, various features are grouped together in a single embodiment to simplify the disclosure. This method of disclosure should not be interpreted as reflecting an intention that embodiments of the claimed subject matter require more features than are expressly recited in each claim. On the contrary, as reflected in the appended claims, the invention comprises less than all the features of any individual disclosed embodiment. The appended claims are therefore hereby expressly incorporated into the detailed description, with each claim standing on its own as a separate preferred embodiment of the invention.

[0163] The above description includes examples of one or more embodiments. Of course, it is not possible to describe all possible combinations of parts or methods for the purpose of describing the above embodiments, but it will be appreciated by those skilled in the art that the various embodiments may be further combined and arranged. Therefore, the embodiments described herein are intended to cover all such changes, modifications and variations that fall within the scope of protection of the appended claims. Furthermore, to the extent that the term "comprising" is used in the specification or claims, the word is intended to be encompassed in a manner similar to the term "including," as explained in terms of "including," used as a transitional word in the claims. Furthermore, any term "or" used in the specification of the claims is intended to mean a "non-exclusive or." The terms "first," "second," etc. are used for descriptive purposes and are not to be understood as indicating or implying relative importance.

Claims

1. A method for determining the lower limit of the sealing thickness of a complex lithologic layer based on pressure field simulation, wherein the complex lithologic layer is the overlying stratum of a shale reservoir, and the complex lithologic layer is an interbedded layer of siltstone and mud shale, characterized in that: The method comprises: Determine the vertical superposition relationship of the siltstone layer and the mud shale layer, and the total thickness of the siltstone layer and the total thickness of the mud shale layer based on the lithologic vertical distribution characteristics of the overlying strata within a set range; Based on the measured properties of multiple siltstone samples, a first formula for siltstone is prepared by experimental fitting; based on the measured properties of multiple shale samples, a second formula for shale is prepared by experimental fitting; According to the upper and lower overlapping relationship, the total thickness of the siltstone layer and the total thickness of the shale layer, the first formula and the second formula, a roof rock sample is prepared by an integrated molding method; A first pressurizing device and a first pressure sensor are arranged on the bottom surface of the roof rock sample, and multiple first pressure sensors are arranged on the side of the roof rock sample at first set intervals. The first pressurizing device is used to pressurize the roof rock sample until the measured value of the first pressure sensor at the bottom is a first set pressure, and stabilizes at this pressure until the change amplitude of the measured value of each first pressure sensor is less than a set threshold. The lower limit of the sealing thickness of the roof rock sample is determined based on the measured value of the first pressure sensor and the upper and lower overlapping relationship.

2. The method according to claim 1, wherein The method of preparing a first formula of siltstone by experimental fitting based on measured properties of a plurality of siltstone samples comprises: Measuring the particle size distribution, mineral composition, fracture pressure, minimum starting pressure, and porosity of rock samples of each siltstone layer within the set range of the overlying stratum, and obtaining the average value of the corresponding parameters by thickness-weighted averaging; Using the average values ​​of the grain size distribution and the average values ​​of the mineral composition of the siltstone layer as the current formula, a siltstone sample is prepared, and the current formula is adjusted until the errors between the porosity, fracture pressure, and minimum starting pressure of the currently prepared siltstone sample and the corresponding average values ​​meet the error threshold, thereby obtaining a first formula for preparing siltstone; Accordingly, the second formula for preparing shale by experimental fitting based on the measured properties of the plurality of shale samples includes: Measuring the particle size distribution, mineral composition, TOC, porosity, and fracture pressure of rock samples of each shale layer within the set range, and obtaining the average value of the corresponding parameters by thickness-weighted averaging; A shale sample is prepared using the average values ​​of the particle size distribution and the average values ​​of the mineral composition of the shale layer as the current formula. The current formula is adjusted until the errors between the TOC, porosity, and fracture pressure of the currently prepared shale sample and the corresponding average values ​​meet the error threshold, thereby obtaining a second formula for preparing shale.

3. The method according to claim 1, wherein The first set pressure is the fracture pressure of the shale reservoir.

4. The method according to claim 1, wherein The step of determining the lower limit of the roof rock sample plugging thickness based on the measurement value of the first pressure sensor and the upper and lower overlapping relationship includes: The lowest position where the roof rock sample pressure is not higher than the second set pressure is determined according to the measurement value of the first pressure sensor, and the lower limit of the roof plugging thickness is determined according to the upper and lower overlapping relationship and the lowest position.

5. The method according to claim 4, wherein If the upper and lower overlapping relationship is that the siltstone layer is on top, determining the lower limit of the roof sealing thickness according to the upper and lower overlapping relationship and the lowest position includes: If the lowest position is located in the mud shale layer of the roof rock sample, the distance from the lowest position to the bottom surface of the roof rock sample is determined as the lower limit of the roof plugging thickness; If the lowest position is located in the siltstone layer of the roof rock sample, determine whether the minimum starting pressure of the siltstone layer is greater than the shale reservoir fracture pressure; if so, determine the distance from the lowest position to the bottom surface of the roof rock sample as the lower limit of the roof sealing thickness; if not, determine that the roof rock sample cannot be sealed.

6. The method according to claim 4, wherein If the upper and lower overlapping relationship is that the siltstone layer is at the bottom, determining the lower limit of the roof sealing thickness according to the upper and lower overlapping relationship and the lowest position includes: If the lowest position is located in the mud shale layer of the roof rock sample, the distance from the lowest position to the bottom surface of the roof rock sample is determined as the lower limit of the roof plugging thickness; If the lowest position is located in the siltstone layer of the roof rock sample, determine whether the minimum starting pressure of the siltstone layer is greater than the shale reservoir fracture pressure; if so, determine the distance from the lowest position to the bottom surface of the roof rock sample as the lower limit of the roof sealing thickness; if not, determine the total thickness of the siltstone layer as the lower limit of the roof sealing thickness.

7. The method according to any one of claims 4 to 6, characterized in that: The second set pressure is the average fracture pressure of the roof rock sample.

8. The method according to claim 1, wherein The step of determining the lower limit of the roof rock sample plugging thickness based on the measurement value of the first pressure sensor and the upper and lower overlapping relationship includes: According to the measurement value of the first pressure sensor and the upper and lower overlapping relationship, the lowest position of the roof rock sample at which the pressure meets the set pressure requirement is determined, and the distance from the lowest position to the bottom surface of the roof rock sample is determined as the lower limit of the roof sealing thickness.

9. The method according to claim 8, wherein If the upper and lower overlapping relationship is that the siltstone layer is on top, determining the lowest position where the roof rock sample pressure meets the set pressure requirement based on the measurement value of the first pressure sensor and the upper and lower overlapping relationship includes: If the pressure in the shale layer drops to the fracture pressure of the shale layer, the position where the pressure in the shale layer is equal to the fracture pressure of the shale layer is determined as the lowest position that meets the set pressure requirement; If the pressure in the siltstone layer drops to the fracture pressure of the siltstone layer, determine whether the minimum starting pressure of the siltstone layer is greater than the fracture pressure of the shale reservoir; if so, determine the position where the pressure in the siltstone layer is the fracture pressure of the siltstone layer as the lowest position that meets the set pressure requirement; if not, determine that the roof rock sample cannot be sealed.

10. The method according to claim 8, wherein If the upper and lower overlapping relationship is that the siltstone layer is at the bottom, determining the lowest position where the roof rock sample pressure meets the set pressure requirement based on the measurement value of the first pressure sensor and the upper and lower overlapping relationship includes: If the pressure in the siltstone layer drops to the fracture pressure of the siltstone layer, determine whether the minimum starting pressure of the siltstone layer is greater than the fracture pressure of the shale reservoir; if so, determine the position in the siltstone layer where the pressure is at the fracture pressure of the siltstone layer as the lowest position that meets the set pressure requirement; if not, determine the bottom surface of the shale layer as the lowest position that meets the set pressure requirement; If the maximum pressure in the siltstone layer is greater than the fracture pressure of the siltstone layer but less than the fracture pressure of the mud shale layer, the bottom surface of the mud shale layer is determined as the lowest position that meets the set pressure requirement; If the maximum pressure in the shale layer drops to the fracture pressure of the shale layer, the position where the pressure in the shale layer is equal to the fracture pressure of the shale layer is determined as the lowest position that meets the set pressure requirement.

11. The method according to claim 1, wherein If the underlying rock layer of the shale reservoir is a mud shale formation, the method further includes performing the following steps of determining the lower limit of the bottom plate plugging thickness: A bottom plate rock sample is prepared using a coring section of an underlying rock formation of the shale reservoir, a side surface of the bottom plate rock sample is wrapped with a sealing material, a second pressurizing device and a second pressure sensor are arranged on a top surface of the bottom plate rock sample, and a plurality of second pressure sensors are arranged on the side surface of the bottom plate rock sample at second set intervals; The bottom plate rock sample is pressurized by the second pressurizing device until the pressure measured by the second pressure sensor at the top reaches the first set pressure, and is stabilized at this pressure until the amplitude of the pressure change measured by each second pressure sensor is less than a set threshold, and the maximum position at which the bottom plate rock sample pressure is not higher than the third set pressure is determined based on the pressure measured by the second pressure sensor; The lower limit of the bottom plate sealing thickness of the shale reservoir is determined according to the distance between the position and the top surface.

12. The method according to claim 11, wherein If the underlying rock layer of the shale reservoir is an underlying siltstone layer, the following also applies: Determining whether the shale reservoir fracture pressure is less than the minimum starting pressure of the underlying siltstone layer; If so, execute the step of determining the lower limit of the bottom plate sealing thickness.

13. The method according to claim 11 or 12, wherein: The third set pressure is the fracture pressure of the underlying rock formation.

14. The method according to claim 1, wherein If the underlying rock layer of the shale reservoir is an interbed of siltstone and mud shale, the method further includes performing the following step of determining the lower limit of the bottom plate plugging thickness: Determine the vertical superposition relationship of the siltstone layer and the mud shale layer, and the total thickness of the siltstone layer and the total thickness of the mud shale layer based on the lithologic vertical distribution characteristics of the underlying rock layer within a set range; preparing a third formulation for siltstone by experimental fitting based on measured properties of multiple siltstone samples within a set range of the underlying rock formation, and preparing a fourth formulation for shale by experimental fitting based on measured properties of multiple shale samples within a set range of the underlying rock formation; According to the upper and lower overlapping relationship of the siltstone layer and the mud shale layer of the underlying rock layer, the total thickness of the siltstone layer and the total thickness of the mud shale layer, and the third and fourth formulas, a bottom plate rock sample is prepared by an integrated molding method; A second pressurizing device and a second pressure sensor are arranged on the top surface of the bottom plate rock sample, and multiple second pressure sensors are arranged on the side of the bottom plate rock sample at second set intervals. The second pressurizing device is used to pressurize the bottom plate rock sample until the measured value of the second pressure sensor on the top is a first set pressure, and stabilizes at this pressure until the change amplitude of the measured value of each second pressure sensor is less than a set threshold. The lower limit of the sealing thickness of the bottom plate rock sample is determined based on the measured value of the second pressure sensor and the overlapping relationship between the siltstone layer and the mud shale layer of the underlying rock stratum.

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