Method for determining lower limit of plugging thickness of complex lithologic layer based on pressure field simulation

Abstract

The invention discloses a method for determining the lower limit of the plugging thickness of a complex lithologic layer based on pressure field simulation, and the method comprises the steps: determining an up-down superposition relation and the total thickness of the two according to the lithologic longitudinal distribution characteristics of an overlying stratum in a set range; respectively obtaining formulas for preparing siltstone and shale through experimental fitting; integrally forming and preparing a roof rock sample; arranging a pressurizing device and pressure sensors on the bottom surface of the rock sample, and arranging a plurality of pressure sensors on the side surface of the rock sample at set intervals; pressurizing the rock sample by using a pressurizing device until the bottom pressure is the set pressure, and stabilizing the bottom pressure at the set pressure until the measured value of each pressure sensor is stable; according to the measured value of the pressure sensor and the up-and-down superposition relation of siltstone and shale, the lower limit of the top plate plugging thickness is determined; and similarly, determining the bottom plate plugging thickness lower limit. According to the method, the lower limit of the plugging thickness of the top and bottom plates of the shale reservoir can be reasonably quantified by simplifying lithology and pressure field simulation.

Description

Technical Field

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

[0002] my country's medium- and low-maturity oil shale has huge resource potential, and in-situ mining technology is increasingly gaining attention as an effective way to achieve its industrialized mining. In-situ mining technology is to directly heat the underground shale reservoir to cause it to crack underground, and the generated oil and gas is extracted through production wells.

[0003] In the same geological environment, the sealing strength of the caprock is highly correlated with the lithology. Caprocks of different lithologies are formed under various geological actions. According to the caprocks that have been explored so far, the lithologies are mainly gypsum, argillaceous rock and dense carbonate caprocks, and there are few examples of other lithologies being explored as caprocks.

[0004] Whether the overlying rock formation can be effectively sealed during the in-situ mining of shale reservoirs is one of the key factors to improve the recovery rate. The existing technology does not have an effective means to evaluate the roof sealing performance during the 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, and through a specific implementation method, provide a method for determining the lower limit of the sealing thickness of complex lithology layers based on pressure field simulation, which can reasonably quantify the lower limit of the sealing thickness of the shale reservoir top plate by simplifying the lithology and pressure field simulation methods.

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

[0007] According to the lithology vertical distribution characteristics of the overlying strata within the set range, determine the upper and lower superposition relationship of the siltstone layer and the mudstone layer, and the total thickness of the siltstone layer and the total thickness of the mudstone layer;

[0008] Based on the measured properties of multiple siltstone samples, a first formula for siltstone is prepared by experimental fitting, and 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 a plurality of first pressure sensors are arranged on the side surface of the roof rock sample at a first set interval. The roof rock sample is pressurized by the first pressurizing device until the measured value of the first pressure sensor at the bottom is a first set pressure, and is stabilized at this pressure until the measured value variation amplitude of each first pressure sensor is less than a set threshold value. 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, the 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] Taking the average values ​​of the grain size distribution and the average values ​​of the mineral composition of the siltstone layer as the current formula, preparing a siltstone sample, 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 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 values ​​of the corresponding parameters by thickness-weighted averaging;

[0016] The average values ​​of the particle size distribution and the mineral composition of the shale layer are used as the current formula to prepare a shale sample, and 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 according to 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 the upper side,

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

[0022] 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 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 plugging 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 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:

[0025] 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 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 plugging thickness; if not, determine the total thickness of the siltstone layer as the lower limit of the roof plugging 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 according to 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 where 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 plugging thickness.

[0030] In some embodiments, if the upper and lower overlapping relationship is that the siltstone layer is on the upper side, 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:

[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 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 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:

[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 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 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 shale layer, the bottom surface of the 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 mudstone formation, the method further includes executing the following step of determining the lower limit of the bottom plate plugging thickness:

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

[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 is the first set pressure, and stabilized at the pressure until the pressure change amplitude of each second pressure sensor is less than the set threshold, and the highest position where the bottom plate rock sample pressure is not higher than the third set pressure is determined according to the pressure measured by the second pressure sensor;

[0040] The lower limit of the bottom plate plugging 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 comprises:

[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 mudstone, the method further includes executing the following step of determining the lower limit of the bottom plate plugging thickness:

[0045] According to the lithology vertical distribution characteristics of the underlying rock layer within the set range, determine the upper and lower superposition relationship of the siltstone layer and the mudstone layer, and the total thickness of the siltstone layer and the total thickness of the mudstone layer;

[0046] preparing a third formula for siltstone by experimental fitting based on measured properties of a plurality of siltstone samples within the set range of the underlying rock formation, and preparing a fourth formula for shale by experimental fitting based on measured properties of a plurality of shale samples within the 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, the third formula and the fourth formula, 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 a plurality of second pressure sensors are arranged on the side surface 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 a first set pressure, and the pressure is stabilized until the measurement value variation amplitude of each second pressure sensor is less than a 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 strata.

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

[0050] (1) The method for determining the lower limit of the plugging thickness of a complex lithology layer based on pressure field simulation provided by an embodiment of the present invention is to prepare a roof rock sample by integral molding, and to pressurize the roof rock sample by 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, and the lower limit of the plugging 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 between the siltstone layer and the mudstone layer. The lower limit of the plugging thickness of the roof of the shale reservoir is determined based on the pressure field simulation by an experimental method, and the roof plugging conditions in the in-situ exploitation of the shale reservoir are specifically quantified, which can effectively guide production and improve the economic benefits of in-situ exploitation of shale oil and gas.

[0051] (2) The method for determining the lower limit of the plugging thickness of a complex lithology layer based on pressure field simulation provided by an embodiment of the present invention determines the upper and lower superposition relationship of the siltstone layer and the mudstone layer, and the total thickness of the siltstone layer and the total thickness of the mudstone layer according to the lithology 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 top plate 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, and reasonably simplifies the sample, so that the determination of the lower limit of the plugging thickness of the complex lithology top plate with interlayers of mudstone and siltstone can be realized.

[0052] (3) It is currently generally believed that only mudstone can effectively seal shale reservoirs during in-situ mining. The method for determining the lower limit of the sealing thickness of complex lithology layers based on pressure field simulation provided in the embodiment of the present invention suggests that if the overlying rock layer of the shale reservoir is an interlayer of siltstone and mudstone, 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, the prior art often only focuses on the sealing properties of the roof, while ignoring the sealing effect of the floor. For in-situ pressurized production of shale reservoirs, poor sealing performance of the floor will still lead to the loss of oil and gas, affecting the recovery rate. The method for determining the lower limit of the sealing thickness of a complex lithology layer 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, thereby ensuring the reliability of the determination of the sealing performance and further providing a guarantee for the improvement of the recovery rate.

[0054] Other features and advantages of the present invention will be described in the following description, and partly become apparent from the description, or understood by practicing the present invention. The purpose 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 lithology layer based on pressure field simulation in Embodiment 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 lithology layer based on pressure field simulation in Embodiment 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 lithology layer based on pressure field simulation in Embodiment 3 of the present invention;

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

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

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

[0063] The exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although the 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. On the contrary, 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 in the present invention are only for describing special embodiments and are not intended to limit the present invention. In addition, for the numerical range in the present invention, it should be understood that each intermediate value between the upper and lower limits of the scope is also specifically disclosed. Each smaller range between the intermediate value in any stated value or stated range and any other stated value or intermediate value in the described range is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded in the scope.

[0065] Unless otherwise indicated, all technical and scientific terms used herein have the same meanings 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 a 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" and the like are open terms, meaning including but not limited to.

[0067] The inventors found that during the in-situ mining of shale reservoirs, the source rock is heated to a certain pressure and begins to crack to generate hydrocarbons. 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 the simulation experiment of the pressure field. In addition, it is generally believed that only mud shale can produce a plugging effect during the in-situ mining of shale reservoirs, but the inventors found that the complex lithology of interlayers of 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] Embodiment 1

[0069] Embodiment 1 of the present invention provides a method for determining the lower limit of the plugging thickness of a complex lithology 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 between the siltstone layer and the mudstone layer, and the total thickness of the siltstone layer and the total thickness of the mudstone layer according to the lithology vertical distribution characteristics of the overlying strata within a set range.

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

[0072] According to empirical data, the plugging thickness of the roof during in-situ mining of shale reservoirs is usually within 300-400 cm, so the length (thickness) of the roof rock sample is set to be no less than 500 cm. If it is thicker, the experimental difficulty and cost will increase; if it is thinner, it will easily fall below the lower limit of the actual plugging thickness of the roof, resulting in experimental failure (unable to measure the required roof plugging thickness of shale in-situ). That is, the setting range of the overlying stratum is within the range of 500 cm in thickness, preferably, within the range of 500 cm in thickness from the position closest to the top surface 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, exceeding 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 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, and based on the measured properties of multiple shale samples, a second formula of shale is prepared by experimental fitting.

[0075] In some embodiments, fitting of 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 values ​​of the corresponding parameters by thickness-weighted averaging; preparing a siltstone sample using the average values ​​of the particle size distribution and the average values ​​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-size samples can be synthesized first, the porosity of the synthesized samples can be measured, and then 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 of a 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 values ​​of the corresponding parameters by thickness weighting; preparing mud shale samples using the average values ​​of the particle size distribution and the average values ​​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 ​​all meet the error threshold, thereby obtaining the second formula for preparing mud shale.

[0078] Optionally, in the process of fitting the first formula and the second formula, 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 measuring the measurement results of corresponding rock samples and averaging them by thickness weighted average.

[0079] Step S13: preparing a roof rock sample by an integrated molding method 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) may 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 side of the top plate rock sample is wrapped with sealing material, and 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, 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 a first set pressure, and stabilize at this pressure until the measurement value change amplitude of each first pressure sensor is less than a set threshold, and 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 by a fracture pressure experiment of the rock.

[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 sensor is 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 in the above manner. The first pressurizing device is used to pressurize the bottom of the roof rock sample, and the 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 it 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 sensor can be collected from bottom to top in sequence.

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

[0091] 1. Determine the lowest position where the roof rock sample pressure is not higher than the second set pressure based on the measurement value of the first pressure sensor, and determine the lower limit of the roof plugging 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 to the case where the fracture pressure difference between the siltstone layer and the mud shale layer is not large. The second set pressure is the average fracture pressure of the roof rock sample, which can be the average value of the fracture pressure thickness weighted average of the siltstone layer and the mud shale layer.

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

[0094] The first pressurizing device is used to pressurize the bottom of the roof 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) to make the bottom pressure of the roof rock sample constant at P0, and stabilize it 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 from bottom to top in sequence until the currently collected pressure is no greater than Px. The lowest position where the roof rock sample pressure 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 an equal proportion method based on the currently collected pressure and the previous collected pressure and the current position of the first pressure sensor and the position of the previous first pressure sensor.

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

[0097] 1. The relationship between the upper and lower superposition is that the siltstone layer is on the upper

[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 plugging 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 plugging thickness; if not, determine that the roof rock sample cannot be plugged.

[0100] 2. The relationship between the upper and lower superposition 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 plugging 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 plugging thickness; if not, determine the total thickness of the siltstone layer as the lower limit of the roof plugging thickness.

[0103] 2. According to the measurement value of the first pressure sensor and the overlapping relationship between the siltstone layer and the shale 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 plugging thickness.

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

[0105] According to the measured value of the first pressure sensor and the stacking relationship between the siltstone layer and the mudstone layer, the lowest position where the roof rock sample pressure meets the set pressure requirement is determined, including:

[0106] 1. The relationship between the upper and lower superposition is that the siltstone layer is on the upper

[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 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 top plate rock sample cannot be sealed.

[0109] 2. The relationship between the upper and lower superposition 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 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 shale layer, the bottom surface of the 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 for determining the lower limit of the plugging thickness of a complex lithology layer based on pressure field simulation provided in the first embodiment of the present invention is to prepare a roof rock sample by integral molding, and to pressurize the roof rock sample by 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, and the lower limit of the plugging 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 between the siltstone layer and the mudstone layer. The lower limit of the plugging thickness of the roof of the shale reservoir is determined based on the pressure field simulation by an experimental method, and the roof plugging conditions of the shale reservoir in situ process are specifically quantified, which can effectively guide production and improve the economic benefits of in-situ exploitation of shale oil and gas.

[0115] The method for determining the lower limit of the plugging thickness of a complex lithology layer based on pressure field simulation provided in Example 1 of the present invention determines the upper and lower superposition relationship of the siltstone layer and the mudstone layer, and the total thickness of the siltstone layer and the total thickness of the mudstone layer according to the lithology 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 top plate rock sample by integral molding. The prepared synthetic sample takes into account the properties of mudstone and siltstone, as well as the vertical superposition relationship between the two, and reasonably simplifies the sample, so that the determination of the lower limit of the plugging thickness of the complex lithology top plate with interlayers of mudstone and siltstone can be realized.

[0116] At present, it is generally believed that only mudstone can effectively seal the shale reservoir during in-situ mining. The method for determining the lower limit of the sealing thickness of complex lithology layers based on pressure field simulation provided in Example 1 of the present invention suggests that if the overlying rock layer of the shale reservoir is interlayered with siltstone and mudstone, 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.

[0117] Embodiment 2

[0118] Embodiment 2 of the present invention provides another method for determining the lower limit of the plugging thickness of a complex lithology 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 layer, directly execute 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, execute the following steps to determine the lower limit of the bottom plate plugging thickness; if not, determine that the underlying formation does not have a plugging property 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 between the pressure sensors arranged in the top plate rock sample and the pressure sensors arranged in the bottom plate rock sample.

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

[0124] The second pressurizing device can be the same as the first pressurizing device, and the "first" and "second" here are just 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 sensor is arranged axially on the side of the bottom plate rock sample. The second set interval of the arrangement 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 on the top is the first set pressure, and stabilize it 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 at which 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 a fracturing 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) to make the top pressure of the bottom plate rock sample constant at P0, and stabilize it 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 from top to bottom in sequence until the currently collected pressure is no greater than Px. The highest position where the pressure of the top plate rock sample is not 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 current position of the 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: Determine 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 the safety correction factor (greater than 1), and the product of the safety correction factor and the determined distance is used as the lower limit of the bottom plate plugging thickness of the shale reservoir.

[0132] Embodiment 3

[0133] Embodiment 3 of the present invention provides another method for determining the lower limit of the sealing thickness of complex lithology 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 suitable for 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 between the siltstone layer and the mudstone layer, and the total thickness of the siltstone layer and the total thickness of the mudstone layer according to the lithology vertical distribution characteristics of the underlying rock layer within a set range.

[0136] The setting range of the underlying rock formation can be within a thickness range of 500 cm, preferably, within a thickness range of 500 cm downward from the 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, prepare a third formula for siltstone through experimental fitting, and based on the measured properties of multiple shale samples within the set range of the underlying rock formation, prepare a fourth formula for shale through experimental fitting.

[0138] Step S33: preparing a bottom plate rock sample by an integrated molding method according to the stacking relationship of the siltstone layer and the mudstone layer of the underlying rock layer, the total thickness of the siltstone layer and the total thickness of the mudstone layer, the third formula and the fourth formula.

[0139] Step S34: arrange a second pressurizing device and a second pressure sensor on the top surface of the bottom plate rock sample, arrange multiple second pressure sensors on the side of the bottom plate rock sample at a second set interval, use the second pressurizing device 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 at this pressure until the measurement value change amplitude of each second pressure sensor is less than the set threshold value, and determine the lower limit of the sealing thickness of the bottom plate rock sample based on the measurement value of the second pressure sensor and the overlapping relationship of the siltstone layer and the mudstone layer of the underlying rock strata.

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

[0141] When studying the sealing properties of shale reservoirs, the prior art often only focuses on the sealing properties of the top plate, while ignoring the sealing effect of the bottom plate. For in-situ mining of shale reservoirs, poor bottom plate sealing performance will still lead to the loss of oil and gas, affecting the recovery rate. The method for determining the sealing thickness of complex lithology adjacent layers of shale reservoirs based on pressure simulation provided in the second and third embodiments of the present invention fully considers the sealing properties of the bottom plate, ensures the reliability of the determination of the sealing performance, and further provides a guarantee for the improvement of the recovery rate.

[0142] The above three embodiments can be applied separately; 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. The top and bottom plate thicknesses of the shale reservoir both meet the corresponding lower limits of the plugging thickness, and then it is determined that the plugging requirements are met.

[0143] Embodiment 4

[0144] Embodiment 4 of the present invention provides a specific application of a method for determining a lower limit of the top plate plugging thickness of a shale reservoir, and verifies its accuracy through numerical simulation, including the following steps:

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

[0146] 2) The particle size distribution, mineral composition, initial kerogen hydrocarbon production 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, according to the superposition relationship and thickness, a roof rock sample containing a homogeneous siltstone layer and a homogeneous shale layer is synthesized at one time (the thickness of the shale layer is equal to 400 cm, and the thickness of the siltstone layer is equal to 100 cm. The cumulative deviation of the above parameters from the design value is not more than 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 approximated to be 20 MPa.

[0152] 8) Wrap the top plate rock sample with a pressure-resistant steel sleeve with a pressure resistance of not less than 80Mpa, and then wrap it with rock wool as a heat-insulating material, and then place the top plate rock sample into 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 with an interval of 10 cm.

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

[0155] 11) After the bottom pressure of the roof rock sample is constant at P030 minutes (the measured values ​​of each sensor are stable), the roof rock sample pressure sensor data is continuously collected. The thickness from the position of the pressure Px to the bottom of the sample is calculated to be 160cm, which is greater than the thickness of the siltstone layer, that is, the lower limit of the roof plugging thickness.

[0156] 12) A numerical model consistent with the samples and experimental conditions was established. The shale was on top and 400 cm thick. Its TOC, porosity, Tx, Px, εx, and mineral composition were the same as those of the artificial samples used in the experiment. The siltstone was on the bottom and 100 cm thick. Its porosity, Px, εx, and mineral composition were the same as those of the artificial samples used in the experiment. The Px (fracture pressure) of the top mudstone sample was set to 22 MPa, and the Px (fracture pressure) of the top 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 in the whole simulation period, indicating that oil and gas were blocked at this location.

[0159] 15) Observation Figure 5 Oil saturation at medium pressure line 25Mpa (see Figure 6 During the whole 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. The comparison proves that the top plate with a thickness of 165 cm meets the plugging requirements. The error between the numerical simulation results and the experimental results (160 cm) is 3.1%, so 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 process 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 protection of the present disclosure. The attached method claims present the 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 above 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 the embodiments of the claimed subject matter require more features than are clearly stated in each claim. On the contrary, as reflected in the appended claims, the invention is in a state of having less than all the features of the disclosed individual embodiments. Therefore, the appended claims are 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 impossible to describe all possible combinations of components or methods for the purpose of describing the above embodiments, but it should be recognized by those skilled in the art that the various embodiments can 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. In addition, with respect to the term "comprising" used in the specification or claims, the word is covered in a manner similar to the term "including", just as "including," explained as a transitional word in the claims. In addition, any term "or" used in the specification of the claims is intended to represent "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 plugging thickness of a complex lithology layer based on pressure field simulation, wherein the complex lithology layer is an overlying stratum of a shale reservoir, and the complex lithology layer is an interlayer of siltstone and mudstone, characterized in that: The method comprises: According to the lithology vertical distribution characteristics of the overlying strata within the set range, determine the upper and lower superposition relationship of the siltstone layer and the mudstone layer, and the total thickness of the siltstone layer and the total thickness of the mudstone layer; Based on the measured properties of multiple siltstone samples, a first formula for siltstone is prepared by experimental fitting, and 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 a plurality of first pressure sensors are arranged on the side surface of the roof rock sample at a first set interval. The roof rock sample is pressurized by the first pressurizing device until the measured value of the first pressure sensor at the bottom is a first set pressure, and is stabilized at this pressure until the measured value variation amplitude of each first pressure sensor is less than a set threshold value. 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, characterized in that 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; Taking the average values ​​of the grain size distribution and the average values ​​of the mineral composition of the siltstone layer as the current formula, preparing a siltstone sample, 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 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 values ​​of the corresponding parameters by thickness-weighted averaging; The average values ​​of the particle size distribution and the mineral composition of the shale layer are used as the current formula to prepare a shale sample, and 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, characterized in that The first set pressure is the fracture pressure of the shale reservoir.

4. The method according to claim 1, characterized in that The step of determining the lower limit of the plugging thickness of the roof rock sample according to the measured 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, characterized in that If the upper and lower overlapping relationship is that the siltstone layer is on the upper side, determining the lower limit of the roof plugging thickness according to the upper and lower overlapping relationship and the lowest position includes: 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 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 plugging thickness; if not, determine that the roof rock sample cannot be sealed.

6. The method according to claim 4, characterized in that If the upper and lower overlapping relationship is that the siltstone layer is at the bottom, determining the lower limit of the roof plugging thickness according to the upper and lower overlapping relationship and the lowest position includes: 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 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 plugging thickness; if not, determine the total thickness of the siltstone layer as the lower limit of the roof plugging 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, characterized in that The step of determining the lower limit of the plugging thickness of the roof rock sample according to the measured 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 where 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 plugging thickness.

9. The method according to claim 8, characterized in that If the upper and lower overlapping relationship is that the siltstone layer is on the upper side, determining the lowest position where the roof rock sample pressure meets the set pressure requirement according to 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 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, characterized in that 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 according to 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 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 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 shale layer, the bottom surface of the 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, characterized in that If the underlying rock layer of the shale reservoir is a mudstone formation, the method further includes executing the following step of determining the lower limit of the bottom plate plugging thickness: A bottom plate rock sample is prepared by coring the underlying rock formation of the shale reservoir, a side of the bottom plate rock sample is wrapped with a sealing material, a second pressurizing device and a second pressure sensor are arranged on the top surface of the bottom plate rock sample, and a plurality of second pressure sensors are arranged on the side of the bottom plate rock sample at a second set interval; The bottom plate rock sample is pressurized by the second pressurizing device until the pressure measured by the second pressure sensor at the top is the first set pressure, and stabilized at the pressure until the pressure change amplitude of each second pressure sensor is less than the set threshold, and the highest position where the bottom plate rock sample pressure is not higher than the third set pressure is determined according to the pressure measured by the second pressure sensor; The lower limit of the bottom plate plugging 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, characterized in that If the underlying rock layer of the shale reservoir is an underlying siltstone layer, it also includes: 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, characterized in that: 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 interbedded siltstone and mudstone, the method further includes executing the following step of determining the lower limit of the bottom plate plugging thickness: According to the lithology vertical distribution characteristics of the underlying rock layer within the set range, determine the upper and lower superposition relationship of the siltstone layer and the mudstone layer, and the total thickness of the siltstone layer and the total thickness of the mudstone layer; preparing a third formula for siltstone by experimental fitting based on measured properties of a plurality of siltstone samples within the set range of the underlying rock formation, and preparing a fourth formula for shale by experimental fitting based on measured properties of a plurality of shale samples within the 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, the third formula and the fourth formula, 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 a plurality of second pressure sensors are arranged on the side surface 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 a first set pressure, and the pressure is stabilized until the measurement value variation amplitude of each second pressure sensor is less than a 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 strata.

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