A weakly cemented argillaceous silt rock threshold pressure gradient test method and device

Abstract

The application discloses a kind of weak cementation argillaceous siltstone starting pressure gradient test method and device, including weak cementation argillaceous siltstone into core holder, vacuum and formation water saturation are extracted to core holder and weak cementation argillaceous siltstone in core holder;Set initial confining pressure, pressurize to core holder multiple times, until the outlet end of core holder no longer liquid outlet;And displacement fluid is sequentially increased in accordance with pump speed rate displacement test, time is counted for each displacement test, and the displacement pressure of core holder, confining pressure change and displacement fluid capacity are recorded;Change to variable pressure displacement, change according to displacement pressure control process displacement test to core holder, and the displacement pressure of core holder, confining pressure change and displacement fluid capacity are recorded;Confining pressure is sequentially adjusted, and displacement test is repeated, until the outlet end of core holder has no displacement fluid;The application designs sandstone core water phase starting pressure gradient test method under normal temperature and pressure conditions.

Description

Technical Field

[0001] This invention relates to the field of sandstone displacement driving pressure technology, specifically to a method and apparatus for testing the starting pressure gradient of weakly cemented argillaceous siltstone. Background Technology

[0002] During the seepage of natural gas hydrates in low-permeability, loose argillaceous siltstone, there exists a starting pressure gradient. Below this pressure gradient, the natural gas hydrates do not flow. Due to differences in formation seepage conditions and the properties of the injected fluid, the starting pressure gradient required for the flow of natural gas hydrates varies. As natural gas hydrates are developed, the starting pressure gradient has a significant impact on development. It is a comprehensive manifestation of abnormal fluid boundary layer properties and fluid plasticity.

[0003] The most commonly used methods for studying the initiation pressure gradient are indoor physical simulation and numerical experiments. Among these, indoor physical simulation is the most intuitive and currently widely recognized as an effective method. Specifically, it employs the conventional method of measuring the relationship between pressure difference and flow rate under steady-state flow conditions. This involves saturating a core or sand-filled pipe with crude oil, measuring the steady-state flow rate under a certain pressure difference, and then using mathematical methods to calculate the initiation pressure gradient. The basic principle is to measure the flow rate of the fluid through the core after different displacement pressure differences have stabilized, plot the relationship between flow rate and pressure gradient, and use regression curves to determine the initiation pressure gradient. The laboratory method for measuring the initiation pressure gradient also uses the conventional method of measuring the relationship between steady-state pressure difference and flow rate. This involves saturating a core with crude oil and recording the steady-state flow rate at each point under a series of pressure differences.

[0004] Commonly used indoor physical simulation and numerical experimental methods both use oil phase displacement tests. The calculation of the influence of the oil phase on core permeability has a technical problem of large calculation errors. Summary of the Invention

[0005] The purpose of this invention is to provide a method and apparatus for testing the starting pressure gradient of weakly cemented argillaceous siltstone, in order to solve the technical problem of large calculation errors in the calculation of core permeability caused by the influence of the oil phase on the oil phase displacement test operation in the prior art.

[0006] To solve the above-mentioned technical problems, the present invention specifically provides the following technical solution:

[0007] A pressure gradient testing device for weakly cemented argillaceous siltstone includes a core holder for holding the weakly cemented argillaceous siltstone. The inlet end of the core holder is equipped with a six-way valve, which is used to apply displacement pressure, confining pressure, and displacement of water phase to the core holder. The outlet end of the core holder is connected to a measuring cylinder, which measures the volume of liquid overflowing from the weakly cemented argillaceous siltstone through an internal liquid level sensor.

[0008] Pressure sensors are provided at both the inlet and outlet ends of the core holder. The pressure sensor at the inlet end of the core holder is used to measure the displacement pressure and confining pressure inside the core holder.

[0009] The outlet end of the six-way valve is also equipped with a pressure sensor, which is used to measure the displacement pressure and confining pressure injected into the core holder.

[0010] As a preferred embodiment of the present invention, the inlet end of the six-way valve is connected to at least two piston containers, and the other end of the piston containers is connected to a high-pressure precision injection pump through a valve. The high-pressure precision injection pump injects pressure or draws a vacuum into the core holder through the piston containers, and the high-pressure precision injection pump also injects a displacement water phase into the core holder through the piston containers.

[0011] In addition, the present invention also provides a testing method for a starting pressure gradient testing device for weakly cemented argillaceous siltstone, comprising the following steps:

[0012] Step 100: Place the weakly cemented argillaceous siltstone into the core holder, connect the testing device, and vacuum and saturate the core holder and the weakly cemented argillaceous siltstone inside the core holder with formation water.

[0013] Step 200: Set the initial confining pressure, open the inlet and outlet ends of the core holder, and repeatedly displace and pressurize the core holder until the pressure inside the core holder is stable and no more liquid is discharged from the outlet end of the core holder.

[0014] Step 300: Conduct a displacement test on the core holder, and conduct the displacement test with the displacement fluid increasing in sequence according to the pump speed. Time each displacement test and record the displacement pressure, confining pressure change and displacement fluid volume at the inlet and outlet of the core holder.

[0015] Step 400: After the inlet end of the core holder reaches the upper limit of the safe injection pressure, switch to pressure displacement. Perform displacement test on the core holder according to the displacement pressure control process. Time each displacement test and record the displacement pressure, confining pressure change and displacement fluid volume corresponding to the inlet and outlet ends of the core holder.

[0016] Step 500: Increase the confining pressure sequentially, and repeat steps 200-400 above until the outlet end of the core holder no longer overflows with displacement fluid, then stop the test.

[0017] In a preferred embodiment of the present invention, after the weakly cemented argillaceous siltstone is saturated with formation water in step 100, the reservoir core of the weakly cemented argillaceous siltstone exhibits a semi-fluid state.

[0018] In a preferred embodiment of the present invention, in step 200, after applying displacement pressure to the core holder, the semi-liquid reservoir core of the weakly cemented argillaceous siltstone undergoes deformation, resulting in a decrease in confining pressure.

[0019] By repeatedly displacing and pressurizing the core holder until the pressure stabilizes, the core holder stops discharging liquid.

[0020] As a preferred embodiment of the present invention, the method for performing a displacement test on the core holder in step 300 is as follows:

[0021] An initial confining pressure of 2 MPa is applied to the core holder;

[0022] The initial pump speed for displacing the aqueous phase was set to 0.005 ml / min. Displacement was started and timing was initiated. The confining pressure inside the core holder was recorded throughout the process.

[0023] After the displacement test at the initial pump speed stabilized, the pump speed was changed sequentially to 0.015 ml / min, 0.05 ml / min, 0.1 ml / min, 0.3 ml / min, and 0.6 ml / min. The displacement fluid at the outlet of the core holder was counted again, and the displacement time and confining pressure inside the core holder were recorded in real time.

[0024] As a preferred embodiment of the present invention, in step 400, after the pressure at the inlet end of the core holder reaches the upper limit of the safe injection pressure, the displacement test is adjusted to variable pressure displacement, and the displacement pressure is changed sequentially according to 1.6MPa-2.2MPa-1.6MPa respectively.

[0025] The displacement fluid at the outlet of the core holder during each displacement test was statistically analyzed, and the displacement time, confining pressure, and displacement pressure difference inside the core holder were recorded in real time.

[0026] In step 400, when the displacement pressure at the inlet end of the core holder reaches the upper limit of the safe injection pressure, the displacement pressure at the inlet end of the core holder shall not exceed 80% of the confining pressure.

[0027] As a preferred embodiment of the present invention, the displacement process of the core holder for variable speed displacement test is determined based on the displacement time of the core holder recorded in real time in step 300, and the displacement process of the core holder for variable pressure displacement test is determined based on the displacement time of the core holder recorded in real time in step 400.

[0028] Based on the displacement time of the core holder and the confining pressure inside the core holder recorded in real time in steps 300 and 400, a core confining pressure variation diagram is created.

[0029] Based on the displacement tests at varying speeds of 0.005 ml / min, 0.015 ml / min, 0.05 ml / min, 0.1 ml / min, 0.3 ml / min, and 0.6 ml / min, as well as the displacement times and confining pressures corresponding to the pressure variations from 1.6 MPa to 2.2 MPa to 1.6 MPa, the time points corresponding to the cessation of confining pressure decrease and subsequent increase are determined, and these time points are taken as the actual liquid discharge points.

[0030] As a preferred embodiment of the present invention, based on the multiple displacement tests after the actual liquid outlet point in steps 300 and 400, the displacement pressure corresponding to the inlet and outlet ends of the core holder is obtained sequentially, and the displacement pressure difference corresponding to each displacement test in steps 300 and 400 is calculated.

[0031] Determine the equivalent permeability corresponding to multiple displacement tests after the actual outlet point in steps 300 and 400, and create a graph showing the relationship between displacement pressure difference and permeability to corroborate the actual outlet point.

[0032] In a preferred embodiment of the present invention, in step 500, the confining pressure is increased sequentially by 4MPa, 8MPa, and 12MPa respectively, and after each increase in confining pressure, steps 200-400 are followed until the outlet end of the core holder no longer overflows with displacement fluid, at which point the test is stopped.

[0033] Compared with the prior art, the present invention has the following advantages:

[0034] This invention designs a method and apparatus for testing the water phase initiation pressure gradient of low-permeability loose argillaceous siltstone cores under normal temperature and pressure conditions. By creating test conditions with different confining pressures, the water phase initiation pressure gradient of the core corresponding to different confining pressures can be obtained, which facilitates the analysis of the variation law of water phase initiation pressure gradient of low-permeability loose argillaceous siltstone with core permeability. Attached Figure Description

[0035] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely exemplary, and those skilled in the art can derive other embodiments based on the provided drawings without creative effort.

[0036] Figure 1 This is a schematic diagram of the overall structure of the starting pressure gradient testing device according to an embodiment of the present invention;

[0037] Figure 2 This is a schematic flowchart of the pressure gradient test initiation method according to an embodiment of the present invention;

[0038] Figure 3 This is a diagram showing the variation of confining pressure in the core sample according to an embodiment of the present invention.

[0039] Figure 4 This is a graph showing the relationship between displacement pressure difference and permeability in an embodiment of the present invention.

[0040] The labels in the diagram represent the following:

[0041] 1-Core holder; 2-Six-way valve; 3-Measuring cylinder; 4-High-pressure precision injection pump; 5-Pressure sensor; 6-Piston container. Detailed Implementation

[0042] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0043] Example 1

[0044] like Figure 1 As shown, the present invention provides a starting pressure gradient testing device for weakly cemented argillaceous siltstone, including a core holder 1 for clamping the weakly cemented argillaceous siltstone. The inlet end of the core holder 1 is provided with a six-way valve 2, which is used to apply displacement pressure, confining pressure and displacement of water phase to the core holder 1 respectively. The outlet end of the core holder 1 is connected to a measuring cylinder 3, which measures the volume of liquid overflowing from the weakly cemented argillaceous siltstone through a liquid level sensor inside the measuring cylinder 3.

[0045] Pressure sensors 5 are provided at both the inlet and outlet ends of the core holder 1. The pressure sensor 5 at the inlet end of the core holder 1 is used to measure the displacement pressure and confining pressure inside the core holder 1.

[0046] A pressure sensor 5 is also provided at the outlet end of the six-way valve 2. The pressure sensor 5 at the outlet end of the six-way valve 2 is used to measure the displacement pressure and confining pressure injected into the core holder 1.

[0047] In this embodiment, the core sample used is a low-permeability, loose argillaceous siltstone. The argillaceous siltstone natural gas hydrate reservoir exhibits weak cementation characteristics, and the reservoir core after hydrate decomposition exhibits a semi-fluid plastic state.

[0048] The core holder 1 is specifically a large-diameter thermoplastic tube. The large-diameter thermoplastic tube is fixed to the outside of the core. After the core holder 1 is placed in an oven, it is heated and shaped evenly, so that the core holder 1 becomes a cylindrical flexible tube that can deform with the core after being pressurized.

[0049] After the rock sample is filled into the core holder 1, permeable stones are placed at both ends of the cylindrical hose. During the filling process, the core is fully compacted. This sand filling mold can ensure that the core does not leak during the displacement process and that there is no mud leakage at both ends.

[0050] The inlet end of the six-way valve 2 is connected to at least two piston containers 6. The other end of the piston containers 6 is connected to a high-pressure precision injection pump 4 through a valve. The high-pressure precision injection pump 4 injects pressure or draws a vacuum into the core holder 1 through the piston containers 6. The high-pressure precision injection pump 4 also injects a displacement water phase into the core holder 1 through the piston containers 6.

[0051] The test method for the above-mentioned starting pressure gradient test device for weakly cemented silty mudstone is as follows: Figure 2 As shown, it includes the following steps:

[0052] Step 100: Place the weakly cemented argillaceous siltstone into the core holder, connect the testing device, and vacuum and saturate the core holder and the weakly cemented argillaceous siltstone inside the core holder with formation water.

[0053] Before step 100, the testing device is turned on and a standard saline solution is prepared to perform a sealing test on the testing device.

[0054] After saturating formation water, the reservoir core of the weakly cemented argillaceous siltstone exhibits a semi-fluid state.

[0055] Step 200: Set the initial confining pressure, open the inlet and outlet ends of the core holder, and repeatedly displace and pressurize the core holder until the pressure inside the core holder is stable and no more liquid is discharged from the outlet end of the core holder.

[0056] In step 200, after applying displacement pressure to the core holder, the semi-liquid, weakly cemented argillaceous siltstone reservoir core deforms, causing a decrease in confining pressure. By repeatedly displacing and pressurizing the core holder until the pressure stabilizes, the core holder stops releasing liquid.

[0057] In this embodiment, after the core is saturated with formation water, the reservoir core after the hydrate decomposes is semi-liquid and plastic, and is easily deformed after pressure is applied. Therefore, when pressure is applied to the core holder, the core is squeezed and deformed, and the confining pressure will continue to decrease.

[0058] Combined with the pressure gradient test device for weakly cemented argillaceous siltstone, one piston container 6 was used to apply an initial confining pressure of 2 MPa to the core holder, and then the other piston container 6 was used to apply a displacement pressure to the core holder until the semi-liquid, weakly cemented argillaceous siltstone reservoir core was compacted and the core holder stopped leaking liquid after the pressure stabilized.

[0059] Step 300: Conduct a displacement test on the core holder, and increase the displacement fluid rate sequentially according to the pump speed. Time each displacement test and record the displacement pressure, confining pressure changes, and displacement fluid volume at the inlet and outlet of the core holder.

[0060] Step 400: After the inlet end of the core holder reaches the upper limit of the safe injection pressure, switch to pressure displacement. Perform displacement tests on the core holder according to the displacement pressure control process. Timing is recorded for each displacement test, and the displacement pressure, confining pressure changes, and displacement fluid volume corresponding to the inlet and outlet ends of the core holder are recorded.

[0061] Step 500: Increase the confining pressure sequentially, and repeat steps 200-400 above until the core holder no longer overflows with displacement fluid, then stop the test.

[0062] In step 300, the method for conducting a displacement test on the core holder is as follows:

[0063] An initial confining pressure of 2 MPa was applied to the core holder;

[0064] The initial pump speed for displacing the aqueous phase was set to 0.005 ml / min. Displacement was started and timing was initiated. The confining pressure inside the core holder was recorded throughout the process.

[0065] After the displacement test at the initial pump speed stabilized, the pump speed was changed sequentially to 0.015 ml / min, 0.05 ml / min, 0.1 ml / min, 0.3 ml / min, and 0.6 ml / min. The displacement fluid at the outlet of the core holder was counted again, and the displacement time and confining pressure inside the core holder were recorded in real time.

[0066] In step 400, after the pressure at the inlet end of the core holder reaches the upper limit of the safe injection pressure, when the displacement pressure at the inlet end of the core holder reaches the upper limit of the safe injection pressure, that is, when the displacement pressure at the inlet end of the core holder does not exceed 80% of the set confining pressure inside the core holder, the displacement test is adjusted to variable pressure displacement, and the displacement pressure is changed sequentially according to 1.6MPa-2.2MPa-1.6MPa.

[0067] The displacement fluid at the outlet of the core holder during each displacement test was statistically analyzed, and the displacement time, confining pressure, and displacement pressure difference inside the core holder were recorded in real time.

[0068] Based on the displacement time of the core holder recorded in real time in step 300, the displacement process of the variable speed displacement test on the core holder is determined. Based on the displacement time of the core holder recorded in real time in step 400, the displacement process of the variable pressure displacement test on the core holder is determined.

[0069] Based on the displacement time of the core holder and the confining pressure inside the core holder recorded in real time in steps 300 and 400, a core confining pressure variation map is created.

[0070] Based on the displacement tests at varying speeds of 0.005 ml / min, 0.015 ml / min, 0.05 ml / min, 0.1 ml / min, 0.3 ml / min, and 0.6 ml / min, as well as the displacement times and confining pressures corresponding to the pressure variations from 1.6 MPa to 2.2 MPa to 1.6 MPa, the time points corresponding to the cessation of confining pressure decrease and subsequent increase are determined, and these time points are taken as the actual liquid discharge points.

[0071] Based on the multiple displacement tests conducted after the actual liquid outlet point in steps 300 and 400, the displacement pressures corresponding to the inlet and outlet ends of the core holder are obtained sequentially, and the displacement pressure difference corresponding to each displacement test in steps 300 and 400 is calculated.

[0072] Determine the equivalent permeability corresponding to multiple displacement tests after the actual outlet point in steps 300 and 400, and create a graph showing the relationship between displacement pressure difference and permeability to corroborate the actual outlet point.

[0073] In step 500, the confining pressure is increased sequentially by 4MPa, 8MPa, and 12MPa. After each increase in confining pressure, the displacement test is repeated according to steps 200-400 until the outlet end of the core holder no longer produces liquid. At this point, the pressurization of the high-pressure precision injection pump is stopped, and the weakly cemented argillaceous siltstone is taken out, dried, and weighed.

[0074] This embodiment designs a method and device for testing the water phase initiation pressure gradient of low-permeability loose argillaceous siltstone cores under normal temperature and pressure conditions. By creating test conditions with different confining pressures, the water phase initiation pressure gradient of the cores corresponding to different confining pressures can be obtained, and then the variation law of water phase initiation pressure gradient of low-permeability loose argillaceous siltstone with core permeability can be analyzed.

[0075] Example 2

[0076] In this embodiment, after obtaining low-permeability loose argillaceous siltstone samples provided by the Guangzhou Marine Geological Survey, the samples were numbered BC06B and BC08B, respectively.

[0077] The total mineralization of the formation water used in the saturated rock samples was 4500 mg / L, and the ionic composition is shown in Table 1.

[0078] Table 1 Ionic composition of formation water

[0079]

[0080] A large-diameter thermoplastic tube was fixed to the outside of a 2.5cm diameter rock core. After being uniformly heated and shaped in an oven, it became a cylindrical flexible tube with an inner diameter of 2.5cm and a length of 26cm, which could deform with the rock core under pressure. Rock samples were filled into the cylindrical flexible tube, and permeable stones were placed at both ends. The rock core was fully compacted during the filling process. This sand-filling mold ensured that the rock core would not leak during displacement and that there was no mud leakage at both ends. A total of 13 rock core samples were prepared for the experiment. The core parameters of the 13 rock core samples are shown in Table 2 below.

[0081] Table 2. Experimental Core Parameters

[0082]

[0083]

[0084] Following the testing method of the aforementioned pressure gradient test device for weakly cemented argillaceous siltstone, displacement tests were conducted on each core sample using a combination of variable displacement rate and variable displacement pressure. The initial confining pressure was 2 MPa, and the displacement rate was changed sequentially to 0.005 ml / min, 0.015 ml / min, 0.05 ml / min, 0.1 ml / min, 0.3 ml / min, and 0.6 ml / min. Then, the displacement pressure was changed to 1.6 MPa-2.2 MPa-1.6 MPa for sequential pressure variation, and the change in displacement fluid volume under this displacement test was determined.

[0085] Then, displacement tests were conducted at confining pressures of 4 MPa and 8 MPa to determine the change in displacement fluid volume for each displacement test.

[0086] Specifically, for core BC08B-1-2, horizontal displacement was performed using a long core displacement method, with a confining pressure of 2 MPa. During displacement, two injection methods—variable displacement rate and variable displacement pressure—were used to measure the initiation pressure gradient. The changes in confining pressure of core BC08B-1-2 during displacement are as follows: Figure 3 As shown.

[0087] As shown in the confining pressure variation diagram of core BC08B-1-2, the confining pressure of the core continued to decrease during the initial stage of displacement due to the continuous deformation of the core. As displacement progressed, the rate of decrease in confining pressure gradually slowed down. At 10113s, the confining pressure stopped decreasing and started to rise again, reflecting that effective displacement was achieved after the continuous injection at the inlet end and the continuous rise in inlet pressure.

[0088] The injected fluid flows through the core, allowing pressure to propagate within it and causing reverse deformation, thus leading to a continuous increase in confining pressure. Based on this experimental phenomenon and its corresponding principle, the point where the confining pressure stops decreasing and begins to increase is taken as the starting point for establishing effective displacement of the core. The fluid discharged from the outlet at this point is considered to be the actual output of the injected fluid (the fluid discharged from the outlet before this point is due to core deformation), meaning this point is the true fluid discharge point, and the pressure gradient corresponding to the moment before this point is the lower limit of the starting pressure gradient.

[0089] The relationship between the displacement pressure difference and the equivalent permeability of the displacement fluid in the BC08B-1-2 core during the displacement process is as follows: Figure 4 As shown, due to the energy accumulation at the core inlet before effective displacement is established, the equivalent permeability of the fluid flow is significantly higher in the initial stage after the injected fluid breakthrough. As the displacement pressure difference increases, the equivalent permeability decreases rapidly and gradually stabilizes at around 0.014 md.

[0090] After the displacement pressure difference was greater than 1.2 MPa, the equivalent permeability of the core showed a stepwise jump, reaching 0.025 md, which reflects the channel deformation during the core displacement process, resulting in improved permeability. Therefore, the moment before the actual effluent point was taken as the starting point for analysis.

[0091] The above embodiments are merely exemplary embodiments of this application and are not intended to limit this application. The scope of protection of this application is defined by the claims. Those skilled in the art can make various modifications or equivalent substitutions to this application within its substance and scope of protection, and such modifications or equivalent substitutions should also be considered to fall within the scope of protection of this application.

Claims

1. A device for testing the starting pressure gradient of weakly cemented argillaceous siltstone, characterized in that, The invention includes a core holder (1) for holding weakly cemented argillaceous siltstone. The inlet end of the core holder (1) is provided with a six-way valve (2). The six-way valve (2) is used to apply displacement pressure, confining pressure and displacement of water phase to the core holder (1). The outlet end of the core holder (1) is connected to a measuring cylinder (3). The measuring cylinder (3) measures the volume of liquid overflowing from the weakly cemented argillaceous siltstone through a liquid level sensor inside it. The core holder (1) is equipped with pressure sensors (5) at both the inlet and outlet ends. The pressure sensor (5) at the inlet end of the core holder (1) is used to measure the displacement pressure and confining pressure inside the core holder (1). The outlet end of the six-way valve (2) is also equipped with a pressure sensor (5). The pressure sensor (5) at the outlet end of the six-way valve (2) is used to measure the displacement pressure and confining pressure injected into the core holder (1). The core holder (1) is a thermoplastic tube. The core holder (1) is fixed to the outside of the core. The core holder (1) is placed in an oven and heated and shaped evenly, so that the core holder (1) becomes a cylindrical hose that can deform with the core after being pressurized. After the rock sample is filled into the core holder (1), permeable stones are placed at both ends of the core holder (1). During the filling process, the core is fully compacted to ensure that the core does not leak during the displacement process and that there is no mud leakage at both ends.

2. The starting pressure gradient testing device for weakly cemented silty mudstone according to claim 1, characterized in that, The inlet end of the six-way valve (2) is connected to at least two piston containers (6), and the other end of the piston container (6) is connected to a high-pressure precision injection pump (4) through a valve. The high-pressure precision injection pump (4) injects pressure or evacuates the core holder (1) through the piston container (6), and the high-pressure precision injection pump (4) also injects a displacement water phase into the core holder (1) through the piston container (6).

3. A test method for a starting pressure gradient test device for weakly cemented silty mudstone according to any one of claims 1-2, characterized in that, Includes the following steps: Step 100: Place the weakly cemented argillaceous siltstone into the core holder, connect the testing device, and vacuum and saturate the core holder and the weakly cemented argillaceous siltstone inside the core holder with formation water. In step 100, after the weakly cemented argillaceous siltstone is saturated with formation water, the reservoir core of the weakly cemented argillaceous siltstone exhibits a semi-fluid state. Step 200: Set the initial confining pressure, open the inlet and outlet ends of the core holder, and repeatedly displace and pressurize the core holder until the pressure inside the core holder is stable and no more liquid is discharged from the outlet end of the core holder. In step 200, after applying displacement pressure to the core holder, the semi-liquid reservoir core of the weakly cemented argillaceous siltstone undergoes deformation, resulting in a decrease in confining pressure. By repeatedly displacing and pressurizing the core holder until the pressure stabilizes, the core holder stops discharging liquid. Step 300: Conduct a displacement test on the core holder, and conduct the displacement test with the displacement fluid increasing in sequence according to the pump speed. Time each displacement test and record the displacement pressure, confining pressure change and displacement fluid volume at the inlet and outlet of the core holder. In step 300, the method for conducting a displacement test on the core holder is as follows: An initial confining pressure is applied to the core holder; Set the initial pump speed for displacing the aqueous phase, start the displacement and start timing, and record the confining pressure inside the core holder throughout the process; After the displacement test at the initial pump speed stabilizes, the pump speed is changed sequentially, and the displacement fluid at the outlet of the core holder is counted again. The displacement time and the confining pressure inside the core holder are recorded in real time. Step 400: After the inlet end of the core holder reaches the upper limit of the safe injection pressure, switch to pressure displacement. Perform displacement test on the core holder according to the displacement pressure control process. Time each displacement test and record the displacement pressure, confining pressure change and displacement fluid volume corresponding to the inlet and outlet ends of the core holder. In step 400, after the pressure at the inlet end of the core holder reaches the upper limit of the safe injection pressure, the displacement test is adjusted to variable pressure displacement, and the displacement pressure is changed sequentially according to 1.6MPa-2.2MPa-1.6MPa. The displacement fluid at the outlet of the core holder during each displacement test is statistically analyzed, and the displacement time, confining pressure, and displacement pressure difference inside the core holder are recorded in real time. The time point corresponding to the cessation of confining pressure and subsequent increase is determined, and this time point is taken as the actual liquid outlet point. In step 400, when the displacement pressure at the inlet end of the core holder reaches the upper limit of the safe injection pressure, the displacement pressure at the inlet end of the core holder shall not exceed 80% of the confining pressure. Step 500: Increase the confining pressure sequentially, and repeat steps 200-400 above until the outlet end of the core holder no longer overflows with displacement fluid, then stop the test.

4. The method for testing the initiation pressure gradient of weakly cemented argillaceous siltstone according to claim 3, characterized in that, Based on the displacement time of the core holder recorded in real time in step 300, the displacement process of the variable speed displacement test on the core holder is determined; based on the displacement time of the core holder recorded in real time in step 400, the displacement process of the variable pressure displacement test on the core holder is determined. Based on the displacement time of the core holder and the confining pressure inside the core holder recorded in real time in steps 300 and 400, a core confining pressure variation diagram is created. Based on the displacement tests at varying speeds of 0.005 ml / min, 0.015 ml / min, 0.05 ml / min, 0.1 ml / min, 0.3 ml / min, and 0.6 ml / min, as well as the displacement time and confining pressure corresponding to the pressure variation tests from 1.6 MPa to 2.2 MPa to 1.6 MPa.

5. The method for testing the starting pressure gradient of weakly cemented argillaceous siltstone according to claim 4, characterized in that, Based on the multiple displacement tests after the actual liquid outlet point in steps 300 and 400, the displacement pressure corresponding to the inlet and outlet ends of the core holder is obtained sequentially, and the displacement pressure difference corresponding to each displacement test in steps 300 and 400 is calculated. Determine the equivalent permeability corresponding to multiple displacement tests after the actual outlet point in steps 300 and 400, and create a graph showing the relationship between displacement pressure difference and permeability to corroborate the actual outlet point.

6. The method for testing the initiation pressure gradient of weakly cemented argillaceous siltstone according to claim 3, characterized in that, In step 500, the confining pressure is increased sequentially by 4MPa, 8MPa, and 12MPa, and after each increase in confining pressure, steps 200-400 are followed until the outlet end of the core holder no longer overflows with displacement fluid, at which point the test is stopped.

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