Method and device for testing starting pressure gradient of weakly cemented argillaceous siltstone

Abstract

The invention discloses a method and a device for testing the starting pressure gradient of weakly cemented argillaceous siltstone. The method comprises the following steps: putting the weakly cemented argillaceous siltstone into a core holder, vacuumizing the core holder and the weakly cemented argillaceous siltstone in the core holder, and saturating formation water; setting initial confining pressure, and pressurizing the core holder for multiple times until no liquid is discharged from the outlet end of the core holder; carrying out displacement tests on the displacement fluid according to the sequentially increasing speed of the pump speed, timing each displacement test, and recording the displacement pressure of the core holder, the confining pressure change and the displacement fluid capacity; converting into variable pressure displacement, performing a displacement test on the change of the core holder according to a displacement pressure regulation and control process, and recording the displacement pressure, the confining pressure change and the displacement fluid capacity of the core holder; sequentially increasing the confining pressure, and repeating the displacement test until no displacement fluid exists at the outlet end of the core holder; the invention designs a method for testing the water phase starting pressure gradient of the sandstone core under the conditions of normal temperature and normal pressure.

Description

Technical Field

[0001] The present invention relates to the technical field of sandstone displacement driving pressure, and particularly relates to a method and device for testing the starting pressure gradient of weakly cemented argillaceous siltstone. Background Technique

[0002] There is a phenomenon of starting pressure gradient during the seepage process of natural gas hydrate in low-permeability loose argillaceous siltstone. Below this pressure gradient, natural gas hydrate does not flow. Due to different formation seepage conditions and injection fluid properties, the starting pressure gradients required for the flow of natural gas hydrate are different. With the development of natural gas hydrate, the starting pressure gradient has an inestimable impact on development, and it is a comprehensive manifestation of abnormal fluid boundary layer properties and fluid plasticity.

[0003] The methods for studying the starting pressure gradient mainly involve at most the indoor physical experiment simulation method and the numerical experiment method. Among them, the indoor physical experiment simulation method is the most intuitive and currently relatively recognized effective method. Specifically, the method of measuring the relationship between pressure difference and flow rate under a steady flow state is adopted. That is, after saturating the core or sand-packed tube with crude oil, the steady-state flow rate under a certain pressure difference is measured, and then the starting pressure gradient is obtained by using mathematical methods. Its basic principle is to measure the flow rate of the fluid through the core after stabilizing at different displacement pressure differences, draw the relationship diagram between the flow rate and the pressure gradient, and obtain the starting pressure gradient by regressing the curve. The method for measuring the starting pressure gradient in the laboratory still adopts the conventional method of measuring the relationship between the steady-state pressure difference and the flow rate, that is, after saturating the core with crude oil, under a series of pressure differences, record the steady-state flow rate at each point.

[0004] The commonly used indoor physical experiment simulation method and numerical experiment method both use oil-phase displacement test operations. The oil phase affects the calculation of core permeability, and there are technical problems of large calculation errors. Summary of the Invention[[ID=I7]]

[0005] The purpose of the present invention is to provide a method and device for testing the starting pressure gradient of weakly cemented argillaceous siltstone, so as to solve the technical problems in the prior art that the oil-phase displacement test operation is used, the oil phase affects the calculation of core permeability, and there are large calculation errors.

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

[0007] A device for testing the starting pressure gradient of weakly cemented argillaceous siltstone includes a core holder for clamping the weakly cemented argillaceous siltstone. A six-way valve is provided at the inlet end of the core holder. The six-way valve is respectively used to apply displacement pressure, confining pressure and displacement aqueous phase to the core holder. The outlet end of the core holder is connected to a graduated cylinder, and the graduated cylinder measures the volume of the liquid overflowing from the weakly cemented argillaceous siltstone through a liquid level sensor inside it;

[0008] Pressure sensors are provided at both the inlet end and the outlet end 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] A pressure sensor is also provided at the outlet end of the six-way valve. The pressure sensor at the outlet end of the six-way valve is used to measure the displacement pressure and confining pressure injected into the core holder.

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

[0011] In addition, the present invention also provides a test method for a weak-cemented argillaceous siltstone startup pressure gradient test device, including the following steps:

[0012] Step 100: Place the weak-cemented argillaceous siltstone into the core holder, connect the test device, evacuate and saturate the core holder and the weak-cemented argillaceous siltstone inside with formation water.

[0013] Step 200: Set the initial confining pressure, open the inlet end and the outlet end of the core holder, and perform multiple displacement pressurizations on the core holder until the pressure inside the core holder is stable and no liquid flows out of the outlet end of the core holder.

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

[0016] Step 500: Gradually increase the confining pressure in sequence, repeat the above steps 200 - step 400 until no displacement fluid overflows from the outlet end of the core holder, and stop the test.

[0017] As a preferred solution of the present invention, in step 100, after saturating the weak-cemented argillaceous siltstone with formation water, the reservoir core of the weak-cemented argillaceous siltstone presents a semi-fluidized state.

[0018] As a preferred embodiment of the present invention, in step 200, after applying a displacement pressure to the core holder, the reservoir core of the semi-fluidized weakly cemented argillaceous siltstone deforms, and a phenomenon of confining pressure drop occurs;

[0019] By applying displacement pressure to the core holder multiple times until no more liquid flows out of the core holder after the pressure stabilizes.

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

[0021] Apply a confining pressure with an initial value of 2 MPa to the core holder;

[0022] Set the initial pump speed of the displacement aqueous phase to 0.005 ml / min, start the displacement and time it, and record the confining pressure inside the core holder throughout the process;

[0023] After the displacement test is stable according to the initial pump speed, change the pump speed to 0.015 ml / min, 0.05 ml / min, 0.1 ml / min, 0.3 ml / min, and 0.6 ml / min respectively in sequence, and then count the displacement fluid at the outlet end of the core holder again, and record the displacement time and the confining pressure inside the core holder in real time.

[0024] As a preferred embodiment of the present invention, in step 400, after the displacement pressure at the inlet end of the core holder reaches the upper limit of the safe injection pressure, adjust the displacement test to variable-pressure displacement, and change the displacement pressure in sequence according to 1.6 MPa - 2.2 MPa - 1.6 MPa;

[0025] Count the displacement fluid at the outlet end of the core holder during each displacement test, and record the displacement time, confining pressure, and displacement pressure difference inside the core holder in real time;

[0026] Among them, 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 specifically does not exceed 80% of the confining pressure.

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

[0028] Create a core confining pressure change diagram based on the displacement time of the core holder and the confining pressure inside the core holder recorded in real time according to the said step 300 and step 400.

[0029] Based on the displacement time and confining pressure corresponding to the variable-speed displacement tests at 0.005 ml / min, 0.015 ml / min, 0.05 ml / min, 0.1 ml / min, 0.3 ml / min, 0.6 ml / min, and the variable-pressure displacement test of 1.6 MPa - 2.2 MPa - 1.6 MPa, determine the time point when the confining pressure stops decreasing and starts to rise again, and take this time point as the true liquid production point.

[0030] As a preferred solution of the present invention, based on multiple displacement tests after the true liquid production point according to the said step 300 and step 400, successively obtain the displacement pressures corresponding to the inlet and outlet ends of the core holder, and calculate the displacement pressure differences corresponding to each displacement test in the said step 300 and step 400.

[0031] Determine the equivalent permeability corresponding to multiple displacement tests after the true liquid production point in the said step 300 and step 400, and create a relationship diagram between displacement pressure difference and permeability to corroborate the true liquid production point.

[0032] As a preferred solution of the present invention, in step 500, successively increase the confining pressure to 4 MPa, 8 MPa, and 12 MPa respectively, and after each increase in confining pressure, perform steps 200 - 400 until no displacement fluid overflows from the outlet end of the core holder, then stop the test.

[0033] The present invention has the following beneficial effects compared with the prior art:

[0034] The present invention designs a test method and device for the aqueous phase starting pressure gradient of low-permeability loose argillaceous siltstone cores under normal temperature and pressure conditions. By creating test conditions with different confining pressures, the aqueous phase starting pressure gradients of cores corresponding to different confining pressures can be obtained, thereby facilitating the analysis of the variation law of the aqueous phase starting pressure gradient of low-permeability loose argillaceous siltstone with core permeability. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only exemplary, and for those of ordinary skill in the art, without creative efforts, other implementation drawings can be obtained according to the provided drawings.

[0036] Figure 1 It is a schematic diagram of the overall structure of the starting pressure gradient test device of the embodiment of the present invention;

[0037] Figure 2 Schematic flow diagram of the starting pressure gradient test method according to an embodiment of the present invention;

[0038] Figure 3 Core confining pressure variation diagram according to an embodiment of the present invention;

[0039] Figure 4 Relationship diagram between displacement pressure difference and permeability according to an embodiment of the present invention;

[0040] The reference numerals in the figure are respectively represented as follows:

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

[0042] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0043] Embodiment 1

[0044] As Figure 1 shown, the present invention provides a starting pressure gradient test device for weakly cemented argillaceous siltstone, including a core holder 1 for clamping weakly cemented argillaceous siltstone. A six-way valve 2 is provided at the inlet end of the core holder 1. The six-way valve 2 is respectively used to apply displacement pressure, confining pressure and displacement aqueous 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 liquid volume overflowed from the weakly cemented argillaceous siltstone through the liquid level sensor inside it.

[0045] Pressure sensors 5 are provided at both the inlet end and the outlet end 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, low-permeability loose argillaceous siltstone core samples are used for the core. The argillaceous siltstone natural gas hydrate reservoir shows the characteristics of weak cementation. After the hydrate decomposes, the reservoir core presents a semi-fluidized state.

[0048] The core holder 1 is specifically a large-diameter thermoplastic tube. The large-diameter thermoplastic tube is fixed outside the core. After the core holder 1 is placed in an oven and uniformly heated and shaped, the core holder 1 becomes a cylindrical hose that can deform with the core under pressure.

[0049] After filling the core sample into the core holder 1, water-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 there is no muddy leakage at both ends.

[0050] At least two piston containers 6 are connected to the inlet end of the six-way valve 2. 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 vacuum into the core holder 1 through the piston container 6, and the high-pressure precision injection pump 4 also injects displacement aqueous phase into the core holder 1 through the piston container 6.

[0051] The testing method of the above-mentioned weak-cemented argillaceous siltstone starting pressure gradient testing device is as Figure 2 shown, and includes the following steps:

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

[0053] Before step 100, connect the testing device and configure standard brine to test the sealing performance of the testing device.

[0054] After saturating the weak-cemented argillaceous siltstone with formation water, the reservoir core of the weak-cemented argillaceous siltstone presents a semi-fluidized state.

[0055] Step 200: Set the initial confining pressure, open the inlet end and the outlet end of the core holder, and displace and pressurize the core holder multiple times until the pressure inside the core holder is stable and no liquid flows out from the outlet end of the core holder.

[0056] In step 200, after applying displacement pressure to the core holder, the reservoir core of the semi-fluidized weak-cemented argillaceous siltstone deforms, and the confining pressure drops. By displacing and pressurizing the core holder multiple times until the pressure is stable and the core holder no longer discharges liquid.

[0057] That is, in this embodiment, since the reservoir core after the hydrate decomposes presents a semi-fluidized state after being saturated with formation water and is extremely easy to deform under pressure, when the core holder is pressurized at this time, the core is squeezed and deformed, and the confining pressure will continuously drop.

[0058] Combine with the starting pressure gradient test device for weakly cemented argillaceous siltstone. Use one of the piston containers 6 to apply a confining pressure with an initial value of 2 MPa to the core holder, and then use the other piston container 6 to apply a displacement pressure to the core holder until the reservoir core of the semi-fluid plastic weakly cemented argillaceous siltstone is compacted and no more liquid flows out of the core holder after the pressure stabilizes.

[0059] Step 300: Conduct a displacement test on the core holder, and conduct the displacement test with the displacement fluid at an increasing pump speed in sequence. 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.

[0060] Step 400: After the inlet end of the core holder reaches the upper limit of the safe injection pressure, switch to variable-pressure displacement. Conduct a displacement test on the core holder according to the displacement pressure regulation 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.

[0061] Step 500: Gradually increase the confining pressure in sequence, and repeat the above steps 200 - step 400 until no more displacement fluid overflows from the outlet end of the core holder, then stop the test.

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

[0063] Apply a confining pressure with an initial value of 2 MPa to the core holder;

[0064] Set the initial pump speed of the displacement aqueous phase to 0.005 ml / min, start the displacement and time it, and record the confining pressure inside the core holder throughout the process;

[0065] After the displacement test is stable according to the initial pump speed, change the pump speed to 0.015 ml / min, 0.05 ml / min, 0.1 ml / min, 0.3 ml / min, and 0.6 ml / min respectively in sequence. Then, count the displacement fluid at the outlet end of the core holder again, and record the displacement time and the confining pressure inside the core holder in real time.

[0066] In step 400, when the 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 reaches the upper limit of the safe injection pressure, specifically, the displacement pressure at the inlet end of the core holder does not exceed 80% of the set confining pressure inside the core holder, adjust the displacement test to variable-pressure displacement, and change the displacement pressure in sequence as 1.6 MPa - 2.2 MPa - 1.6 MPa.

[0067] Count the displacement fluid at the outlet end of the core holder during each displacement test, and record the displacement time, confining pressure, and displacement pressure difference inside the core holder in real time.

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

[0069] Based on the displacement time of the core holder and the confining pressure inside the core holder recorded in real time in step 300 and step 400, create a core confining pressure change diagram.

[0070] Based on the displacement time and confining pressure corresponding to the variable-speed displacement tests of 0.005 ml / min, 0.015 ml / min, 0.05 ml / min, 0.1 ml / min, 0.3 ml / min, 0.6 ml / min, and the variable-pressure displacement test of 1.6 MPa - 2.2 MPa - 1.6 MPa, determine the time point when the confining pressure stops decreasing and then increases, and take this time point as the true liquid production point.

[0071] Based on multiple displacement tests after the true liquid production point in step 300 and step 400, sequentially obtain the displacement pressures corresponding to the inlet and outlet ends of the core holder, and calculate the displacement pressure differences corresponding to each displacement test in step 300 and step 400.

[0072] Determine the equivalent permeability corresponding to multiple displacement tests after the true liquid production point in step 300 and step 400, and create a relationship diagram between displacement pressure difference and permeability to corroborate the true liquid production point.

[0073] In step 500, increase the confining pressure in sequence to 4 MPa, 8 MPa, and 12 MPa respectively. Specifically, use it. And after each increase in confining pressure, re-perform the displacement test according to steps 200 - 400 until no liquid is produced at the outlet end of the core holder. At this time, stop the pressurization work of the high-pressure precision injection pump, take out the weakly cemented argillaceous siltstone for drying and weighing.

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

[0075] Example 2

[0076] After obtaining the low-permeability loose argillaceous siltstone core samples provided by the Guangzhou Marine Geological Survey, number the core samples as: BC06B, BC08B.

[0077] The total salinity of the formation water used for saturating the core samples is 4500 mg / L, and the ion composition is shown in Table 1.

[0078] Table 1 Formation water ion composition

[0079]

[0080] Fix a large-diameter thermoplastic pipe outside a 2.5-cm-diameter and 26-cm-long core. After uniformly heating and shaping it in an oven, it becomes a cylindrical hose with an inner diameter of 2.5 cm and a length of 26 cm that can deform with the core under pressure. Fill the cylindrical hose with rock samples and place permeable stones at both ends. During the filling process, the core is fully compacted. This sand-filled mold can ensure that the core does not leak during the displacement process and there is no muddy leakage at both ends. A total of 13 core samples are prepared in the experiment. Among them, the core parameters of the 13 core samples are shown in Table 2 below.

[0081] Table 2 Experimental core parameter table

[0082]

[0083]

[0084] According to the above test method of the weak cemented argillaceous siltstone startup pressure gradient test device, displacement tests are carried out on each core in the order of pre-displacement variable speed and post-displacement variable pressure. The displacement test is carried out at an initial confining pressure of 2 MPa, and the displacement speed is sequentially changed at 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, and then the displacement pressure is changed in the order of 1.6 MPa - 2.2 Mpa - 1.6 MPa to determine the change in the displacement fluid volume under this displacement test.

[0085] Then, displacement tests are carried out at confining pressures of 4 MPa and 8 MPa to determine the change in the displacement fluid volume under each displacement test respectively.

[0086] Specifically, for the core BC08B-1-2, this group of cores is in the horizontal displacement mode. The confining pressure is set at 2 MPa. During the displacement process, two injection methods of displacement variable speed and displacement variable pressure are used to measure the startup pressure gradient. The change in the confining pressure of the BC08B-1-2 core during the displacement process is as Figure 3 shown.

[0087] It can be seen from the confining pressure change diagram of the BC08B-1-2 core that at the initial stage of displacement, affected by the continuous deformation of the core, the confining pressure of the core continuously decreases. As the displacement progresses, the decreasing speed of the confining pressure of the core gradually decreases. When the displacement reaches 10113 s, the confining pressure stops decreasing and reverses to increase, indicating that due to continuous injection at the inlet end, the inlet end pressure continuously rises and then forms an effective displacement.

[0088] The injected fluid flows in the core, so that the pressure can be transmitted in the core, causing reverse deformation of the core, and thus the confining pressure continuously rises. According to this experimental phenomenon and the corresponding principle, the point at which the confining pressure stops decreasing and then rises is taken as the starting point for establishing effective displacement in the core. At this time, the fluid discharged at the outlet end is considered to be the output of the actual injected fluid (the liquid discharged at the outlet end before this point is the liquid discharged due to the deformation of the core), that is, this point is the true liquid discharge point, and the pressure gradient corresponding to the previous moment of this point is the lower limit of the starting pressure gradient.

[0089] During the displacement process, the relationship between the displacement pressure difference of the BC08B-1-2 core and the equivalent permeability of the displacement fluid is as Figure 4 shown. Since there is energy accumulation at the inlet end of the core before the establishment of effective displacement, the equivalent permeability of fluid seepage is significantly larger in the initial stage after the injected fluid breaks through. As the displacement pressure difference increases, the equivalent permeability rapidly decreases and gradually stabilizes at about 0.014 md.

[0090] After the displacement pressure difference > 1.2 MPa, the equivalent permeability of the core shows a stepped jump to 0.025 md, reflecting the occurrence of channel deformation during the core displacement process, resulting in improved permeability. Therefore, the moment before the true liquid discharge point is taken as the starting analysis point.

[0091] The above embodiments are only exemplary embodiments of the present application and are not used to limit the present application. The protection scope of the present application is defined by the claims. Those skilled in the art can make various modifications or equivalent substitutions within the essence and protection scope of the present application, and such modifications or equivalent substitutions should also be regarded as falling within the protection scope of the present application.

Claims

1. A starting pressure gradient test device for weakly cemented argillaceous siltstone, characterized in that it includes a core holder (1) for clamping the weakly cemented argillaceous siltstone. A six-way valve (2) is provided at the inlet end of the core holder (1). The six-way valve (2) is respectively used to apply displacement pressure, confining pressure and displacement aqueous phase to the core holder (1). A graduated cylinder (3) is connected to the outlet end of the core holder (1). The graduated cylinder (3) measures the volume of the liquid overflowing from the weakly cemented argillaceous siltstone through a liquid level sensor inside it; pressure sensors (5) are provided at both the inlet end and the outlet end 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); 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).

2. The starting pressure gradient test device for weakly cemented argillaceous siltstone according to claim 1, characterized in that at least two piston containers (6) are connected to the inlet end of the six-way valve (2). 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 through the piston container (6) into the core holder (1), and the high-pressure precision injection pump (4) also injects the displacement aqueous phase into the core holder (1) through the piston container (6).

3. A testing method for a starting pressure gradient testing device of weakly cemented argillaceous siltstone according to any one of claims 1-2, characterized in that It includes the following steps: Step 100: Place the weakly cemented argillaceous siltstone into the core holder, connect the test device, evacuate and saturate the core holder and the weakly cemented argillaceous siltstone inside it with formation water; Step 200: Set the initial confining pressure, open the inlet end and the outlet end of the core holder, displace and pressurize the core holder multiple times until the pressure inside the core holder is stable and no liquid flows out from the outlet end of the core holder; Step 300: Conduct a displacement test on the core holder, and conduct the displacement test with the displacement fluid at an increasing pump speed in sequence. Time each displacement test, and record the changes in the displacement pressure, confining pressure and the volume of the displacement fluid corresponding to the inlet and outlet ends of the core holder; Step 400: After the inlet end of the core holder reaches the upper limit of the safe injection pressure, switch to pressure displacement, conduct a displacement test on the core holder according to the displacement pressure regulation process, time each displacement test, and record the changes in the displacement pressure, confining pressure and the volume of the displacement fluid corresponding to the inlet and outlet ends of the core holder; Step 500: Gradually increase the confining pressure in sequence, repeat the above steps 200 - step 400 until no displacement fluid overflows from the outlet end of the core holder, and stop the test.

4. The test method of the starting pressure gradient test device for weakly cemented argillaceous siltstone according to claim 3, characterized in that In the step 100, after the weakly cemented argillaceous siltstone is saturated with formation water, the reservoir core of the weakly cemented argillaceous siltstone presents a semi-fluidized state.

5. The test method of a test device for starting pressure gradient of weakly cemented argillaceous siltstone according to claim 4 or 3, characterized in that In the step 200, after applying a displacement pressure to the core holder, the reservoir core of the semi-fluidized weakly cemented argillaceous siltstone deforms, and a phenomenon of confining pressure drop occurs; By displacing and pressurizing the core holder multiple times until no liquid flows out of the core holder after the pressure becomes stable.

6. The test method of a test device for starting pressure gradient of weakly cemented argillaceous siltstone according to claim 3, characterized in that In the step 300, the implementation method of the displacement test on the core holder is as follows: Apply a confining pressure with an initial value of 2 MPa to the core holder; Set the initial pump speed of the displacement aqueous phase to 0.005 ml / min, start the displacement and time, and record the confining pressure in the core holder throughout the process; After the displacement test is stable according to the initial pump speed, change the pump speed to 0.015 ml / min, 0.05 ml / min, 0.1 ml / min, 0.3 ml / min, and 0.6 ml / min in sequence respectively, and then count the displacement fluid at the outlet end of the core holder again, and record the displacement time and the confining pressure in the core holder in real time.

7. The test method of a test device for starting pressure gradient of weakly cemented argillaceous siltstone according to claim 6, characterized in that In the step 400, after the pressure at the inlet end of the core holder reaches the upper limit of the safe injection pressure, adjust the displacement test to variable-pressure displacement, and change the displacement pressure in sequence according to 1.6 MPa - 2.2 MPa - 1.6 MPa; Count the displacement fluid at the outlet end of the core holder during each displacement test, and record the displacement time, confining pressure and displacement pressure difference in the core holder in real time; Among them, in the 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 specifically does not exceed 80% of the confining pressure.

8. The test method and device for starting pressure gradient of weakly cemented argillaceous siltstone according to claim 7, characterized in that Based on the displacement time of the core holder recorded in real time in the step 300, determine the displacement process of the variable-speed displacement test on the core holder, and based on the displacement time of the core holder recorded in real time in the step 400, determine the displacement process of the variable-pressure displacement test on the core holder; Based on the displacement time of the core holder recorded in real time in the step 300 and the step 400 and the confining pressure in the core holder, create a core confining pressure change diagram; Based on the displacement tests with variable 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 variable-pressure displacement test of 1.6 MPa - 2.2 MPa - 1.6 MPa, determine the time point when the confining pressure stops decreasing and then rises, and use this time point as the true liquid output point.

9. The method and device for testing the starting pressure gradient of weakly cemented argillaceous siltstone according to claim 8, wherein Based on the multiple displacement tests after the true liquid output point in the steps 300 and 400, sequentially obtain the displacement pressures corresponding to the inlet and outlet ends of the core holder, and calculate the displacement pressure differences corresponding to the steps 300 and 400 in each displacement test; Determine the equivalent permeability corresponding to the multiple displacement tests after the true liquid output point in the steps 300 and 400, create a relationship diagram between the displacement pressure difference and the permeability to corroborate the true liquid output point.

10. The method and device for testing the starting pressure gradient of weakly cemented argillaceous siltstone according to claim 3, wherein In step 500, increase the confining pressure sequentially to 4 MPa, 8 MPa, and 12 MPa respectively, and after each increase in the confining pressure, perform steps 200 - 400 until no displacement fluid overflows from the outlet end of the core holder, and then stop the test.

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