Pressure head assembly for filter ring partition double seepage test under true triaxial stress and its use method

Abstract

The present invention discloses a filter ring partitioned double seepage test pressure head assembly under true triaxial stress and a method of using the pressure head, which includes a pair of pressure heads and a prefabricated test piece. The pressure head includes a filter ring, a groove, a fracturing hole seepage pipe and a crack seepage pipe. A filter ring is installed outside the front end pipe mouth of the fracturing hole seepage pipe for partitioning; a plurality of grooves are arranged around the fracturing hole seepage pipe on the front end face of the pressure head, and the grooves are connected to the crack seepage pipe, so that the fluid on the front end face of the lower pressure head can be converged and the fluid can be evenly distributed on the front end face of the upper pressure head; the filter ring on the lower pressure head can be replaced with different diameters according to the isotope determination results; the prefabricated test piece is set as a cube. Through the filter ring partitioning, the influence of the fluid flow in the fracturing hole and the radial cracks generated around it during the fracturing process on the test piece permeability determination is greatly reduced; the use of a cubic prefabricated test piece to apply complex stress conditions in a true three-dimensional situation can better simulate the real geological conditions and stress environment of the rock mass.

Description

Technical Field

[0001] The invention relates to the field of rock mechanics and engineering technology, and in particular to a filter ring partition double seepage test pressure head assembly under true triaxial stress and a use method thereof. Background Art

[0002] With the accelerated transformation of energy consumption structure, China's unconventional natural gas (shale gas, coalbed methane and tight sandstone gas, etc.) exploration and development has achieved rapid development, especially shale gas has made important breakthroughs, and the reserve and production have increased significantly, becoming an important source of natural gas reserve and production increase in China. However, it still faces difficulties and challenges at the technical level, and is weak in increasing reserves and production. At present, hydraulic fracturing technology is a very important technical means for unconventional natural gas production, but it also comes with how to evaluate the effect of fracturing rock reservoirs and how to adjust the operation plan before and after fracturing. There is currently a lack of quantitative testing methods for the changes in reservoir rock permeability before and after hydraulic fracturing. Therefore, it is of great significance to conduct in-depth research on the changes in permeability of rock reservoirs before and after the action of fluid fracturing on technical means.

[0003] The permeability test of rocks before and after fluid fracturing is of great significance to engineering practice and industrial safety. Most of the existing permeability test systems use fluid injection into the fracturing holes to fracture the prefabricated specimens to form a seepage channel, and then inject fluid through the fracturing holes again to calculate the flow rate to obtain the permeability. However, the prefabricated specimens processed for fluid fracturing often have problems affected by their fluid fracturing holes, such as the influence of the fluid flow in the hole before fracturing on the permeability and the radial cracks generated around the hole after fracturing, which hinders the one-dimensional seepage flow of the fluid parallel to the hole, resulting in inaccurate permeability calculation. However, the existence of fluid fracturing holes is a necessary condition for fluid fracturing and cannot be removed. On the one hand, there are currently two main methods for rock permeability testing: transient method and steady-state method, and they are carried out using different equipment. If it is necessary to measure the permeability of the same sample under two methods, it is necessary to unload the stress applied to the sample, and then use the test equipment of another permeability measurement method to carry out. This test method cannot measure the permeability of the same sample under two methods in the same test process, because the unloading of the surrounding stress during the permeability measurement process will cause the original pore structure of the sample to change, resulting in inaccurate results when using another permeability test method for the second time. On the other hand, the experimental equipment currently used to study the permeability of fluids before and after fracturing is rarely based on true triaxial stress conditions, and most of them are uniaxial or conventional triaxial stress conditions. Summary of the invention

[0004] In view of the defects of the above-mentioned prior art, the technical problem to be solved by the present invention is to provide a filter ring partitioned dual seepage test pressure head assembly and a method of use which can eliminate the influence of fracturing holes under true triaxial stress and can perform transient and steady-state permeability tests.

[0005] In order to solve the above technical problems, the present invention adopts the following technical scheme: a filter ring partitioned double seepage test pressure head assembly under true triaxial stress, comprising a pair of pressure heads and a prefabricated test piece, the prefabricated test piece is installed between the upper and lower pressure heads, wherein the pressure head is divided into an upper pressure head and a lower pressure head, the pressure head comprises a filter ring, a groove, a fracturing hole seepage pipe and a crack seepage pipe, the fracturing hole seepage pipe is vertically arranged in the pressure head and coaxial with the center of the pressure head, the front end is exposed and a filter ring is installed outside the pipe mouth for partitioning, the rear end is placed in the middle and rear part of the pressure head and is provided with a branch pipe connected to the outside of the side wall of the pressure head, and the branch pipe is connected to the permeability test system; the area of ​​the prefabricated test piece between the two filter rings is the fracturing hole seepage area, and the area outside the two filter rings is the crack seepage area; the crack seepage pipe is arranged between the outer wall of the pressure head and Between the fracturing hole seepage pipes, the front end is exposed, and the rear end is placed in the middle and rear part of the pressure head and is provided with a branch pipe connected to the outside of the pressure head side wall, and the branch pipe is connected to the permeability testing system; the front end surface of the pressure head is provided with a plurality of grooves around the fracturing hole seepage pipe, and the grooves are connected to the crack seepage pipe. The fracturing hole seepage pipe and the crack seepage pipe arranged on the upper pressure head are both seepage inlet pipes, and the fracturing hole seepage pipe and the crack seepage pipe arranged on the lower pressure head are both seepage outlet pipes, so that the fluid on the front end surface of the lower pressure head can not only converge to the crack seepage pipe, but also make the fluid flowing out of the crack seepage pipe evenly distributed on the front end surface of the upper pressure head; the filter ring on the lower pressure head is replaced with different diameters according to the fracturing conditions; the prefabricated specimen is arranged as a cube, and holes are vertically arranged in the upper middle part of the prefabricated specimen, and the holes are connected to the fracturing hole seepage pipe.

[0006] The diameter of the filter ring of the lower pressure head is determined by isotope determination, and the filter ring of the selected diameter is installed on the front end surface of the lower pressure head, including the following steps:

[0007] Step (i): After fracturing the prefabricated specimen, different isotopes of the same fluid are injected into the fracturing hole seepage pipe and the fracture seepage pipe of the upper pressure head to perform a seepage test;

[0008] Step (ii): detecting the content of non-corresponding isotopic fluid in the seepage pipe of the fracturing hole or the seepage pipe of the fracture at the lower pressure head, thereby determining the seepage effect of the seepage zone of the fracturing hole on the seepage zone of the fracture;

[0009] Step (iii): when the non-corresponding isotope content of the fracture seepage pipe is large, radial cracks around the fracturing hole exist in the fracture seepage zone, thereby increasing the radius of the filter ring, that is, reducing the range of the fracture seepage zone; the expansion of the filter ring radius is determined according to the non-corresponding isotope content of the fracturing hole seepage pipe. When the fracture seepage pipe does not contain the non-corresponding isotope content, the influence of the fracturing hole on the fracture seepage zone is eliminated, and the diameter of the filter ring is determined at the same time, and the filter ring of the selected diameter is installed;

[0010] Preferably, the pressure head has a narrow front end surface and a wide rear end surface, and the top is square to match the cross-sectional dimensions of the prefabricated specimen, so that the prefabricated specimen can be better stressed during the fracturing process.

[0011] Preferably, the middle and rear part of the pressure head is set as a cylinder, and four cut surfaces are provided according to the rectangular edge of the top of the pressure head as transition surfaces with the middle and rear cylinder, so that the front end surface of the pressure head is narrow and the rear end surface is wide, and the transition surface is easy to process.

[0012] Preferably, the groove is formed by a plurality of annular grooves coaxial with the fracturing hole seepage pipe, and a plurality of linear grooves uniformly distributed in an annular direction with the fracturing hole seepage pipe as the center.

[0013] Preferably, the fracturing hole seepage pipe and the crack seepage pipe are connected to the permeability testing system via a first bolt and a second bolt respectively.

[0014] Preferably, the rear end surface of the pressure head is marked with a cross line for positioning the pressure head when it is installed in the permeability testing system.

[0015] A method for using a filter ring partition double seepage test pressure head assembly under true triaxial stress, using a filter ring partition double seepage test pressure head assembly under true triaxial stress, a permeability test system used in conjunction with the pressure head is a true triaxial fluid-solid coupling experimental device seepage system, which has constant volume and constant pressure conditions, and includes the following steps:

[0016] Step 1: Making a prefabricated specimen, processing the original rock material and processing it into a prefabricated specimen with a hole in the center;

[0017] Step 2: Install two pressure heads symmetrically on the permeability test system, and install the prefabricated test piece between the two pressure heads. The holes in the prefabricated test piece are connected with the fracturing hole seepage pipe of the upper pressure head to form a fracturing channel.

[0018] Step 3: Install the pressure head, connect the fracturing hole seepage pipe and the fracture seepage pipe set on the upper pressure head to the seepage inlet pipe of the permeability test system, and connect the fracturing hole seepage pipe and the fracture seepage pipe set on the lower pressure head to the seepage outlet pipe of the permeability test system;

[0019] Step 4: Without unloading the prefabricated specimen during the whole process, the permeability of the prefabricated specimen before fracturing is tested by transient method to obtain the original permeability;

[0020] Step 5: Perform fluid fracturing treatment on the prefabricated specimen, open the fracturing hole seepage pipe of the upper pressure head and close its fracture seepage pipe at the same time, spray out through the fracturing hole seepage pipe of the upper pressure head under the constraint of the filter ring, and inject into the interior of the prefabricated specimen, generate fracturing cracks, and form a seepage channel; the fracturing fluid flows through the prefabricated specimen and reaches the top of the lower pressure head, the fluid in the fracturing hole seepage area flows out from the fracturing hole seepage pipe, and the fluid in the fracture seepage area flows out through the converging grooves to the fracture seepage pipe of the lower pressure head; after fracturing, inject the proppant to fill;

[0021] Step 6: Determine the diameter of the filter ring of the lower pressure head by isotope determination, and install the filter ring of the selected diameter to the front end surface of the lower pressure head, including the following steps:

[0022] Step (i): After fracturing the prefabricated specimen, different isotopes of the same fluid are injected into the fracturing hole seepage pipe and the fracture seepage pipe of the upper pressure head to perform a seepage test;

[0023] Step (ii): detecting the content of non-corresponding isotopic fluid in the seepage pipe of the fracturing hole or the seepage pipe of the fracture at the lower pressure head, thereby determining the seepage effect of the seepage zone of the fracturing hole on the seepage zone of the fracture;

[0024] Step (iii): when the non-corresponding isotope content of the fracture seepage pipe is large, radial cracks around the fracturing hole exist in the fracture seepage zone, thereby increasing the radius of the filter ring, that is, reducing the range of the fracture seepage zone; the expansion of the filter ring radius is determined according to the non-corresponding isotope content of the fracturing hole seepage pipe. When the fracture seepage pipe does not contain the non-corresponding isotope content, the influence of the fracturing hole on the fracture seepage zone is eliminated, and the diameter of the filter ring is determined at the same time, and the filter ring of the selected diameter is installed;

[0025] Step 7: Without unloading the prefabricated specimen during the whole process, the permeability of the prefabricated specimen after fracturing is measured by the steady-state method. The pressure heads on both sides open the fracture seepage tubes and close the fracture hole seepage tubes. The fluid without proppant enters from the fracture seepage tube of the upper pressure head and is evenly distributed on the surface of the specimen through the grooves. It flows through the fracture seepage channel in the fracture seepage zone, passes through the entire prefabricated specimen, reaches the top surface of the lower pressure head, and then flows out of the fracture seepage tube of the lower pressure head through the convergence of the grooves. After the seepage flow rate stabilizes, the seepage data is collected.

[0026] Preferably, the hole depth is 55 mm.

[0027] Preferably, in step six, the isotopes used in the isotope determination method are C12 and C14.

[0028] Preferably, a sealing gasket is provided in the filter ring provided on the upper pressure head to isolate the fluid from flowing outward, so that the fluid can be accurately injected into the hole during the fracturing process.

[0029] Beneficial effects of the present invention:

[0030] 1) The filter ring partition system divides the pressure head into the fracturing hole seepage zone and the fracture seepage zone, which greatly reduces the influence of the fluid flow in the fracturing hole and the radial cracks generated around it during the fracturing process on the permeability measurement of the specimen;

[0031] 2) The pressure head is combined with the permeability test system to realize the application of the integrated cross-scale permeability test method of transient method and steady-state method to measure permeability, and the in-situ cross-scale test of the permeability of rock specimens before and after fluid fracturing is completed, which not only improves the accuracy of the experimental results, but also analyzes the difference in seepage of rock specimens before and after fluid fracturing;

[0032] 3) The use of cubic prefabricated specimens can better simulate the actual geological conditions and stress environment of the rock mass under true triaxial stress conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is a structural schematic diagram of the present invention;

[0034] Figure 2 Schematic diagram of the structure of the upper pressure head;

[0035] Figure 3 is a perspective structural diagram of the upper pressure head;

[0036] Figure 4 for Figure 3 Right view of;

[0037] Figure 5 for Figure 3 Left view of

[0038] Figure 6 is a bottom view of the upper pressure head;

[0039] In the figure: 1-pressure head, 1a-upper pressure head, 1b-lower pressure head, 2-filter ring, 3-groove, 4-fracturing hole seepage pipe, 5-crack seepage pipe, 6-first bolt, 7-second bolt, 8-prefabricated specimen, 9-hole, 10-fracturing hole seepage area, 11-crack seepage area. DETAILED DESCRIPTION

[0040] In order to better understand the improvements made by the present invention relative to the prior art, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments.

[0041] like Figure 1As shown in FIG6 , a filter ring partition double seepage test pressure head assembly under true triaxial stress is mainly composed of a pair of pressure heads 1 and a prefabricated specimen 8, wherein the prefabricated specimen 8 is installed between the upper and lower pressure heads 1, and the pressure head 1 is divided into an upper pressure head 1a and a lower pressure head 1b. At the same time, the pressure head 1 has the end facing the prefabricated specimen 8 as the front section, and the end away from the prefabricated specimen 8 as the rear section. The rear end surface of the pressure head 1 is marked with a cross line for positioning the pressure head 1 when it is installed in the permeability test system.

[0042] The prefabricated specimen 8 is set as a cube with a square cross section, and a hole 9 is vertically set in the upper middle part of the prefabricated specimen 8. The pressure head 1 has a narrow front face and a wide rear face, and the top matches the cross-sectional dimensions of the prefabricated specimen 8 and is a square, so that the prefabricated specimen 8 can better withstand force during the fracturing process. Among them, the middle and rear part of the pressure head 1 is set as a cylinder, and it is best to set 4 sections according to the top rectangular edge of the pressure head 1 as a transition surface with the middle and rear cylinder, which is convenient for processing and manufacturing.

[0043] The pressure head 1 is mainly composed of a filter ring 2, a groove 3, a fracturing hole seepage pipe 4 and a crack seepage pipe 5. The fracturing hole seepage pipe 4 is vertically arranged in the pressure head 1 and is coaxial with the center of the pressure head 1. The front end is exposed and a filter ring 2 is installed outside the pipe mouth for partitioning. The rear end is placed in the middle and rear part of the pressure head 1 and is provided with a branch pipe connected to the outside of the side wall of the pressure head 1. The branch pipe is connected to the permeability test system, wherein the filter ring 2 on the lower pressure head 1b is replaced with different diameters according to the fracturing situation. The area between the two filter rings 2 of the prefabricated specimen 8 is the fracturing hole seepage area 10, and the area outside the two filter rings 2 is the crack seepage area 11. The hole 9 on the prefabricated specimen 8 is connected to the fracturing hole seepage pipe 4 to form a fracturing channel. The crack seepage pipe 5 is arranged between the outer wall of the pressure head 1 and the fracturing hole seepage pipe 4, the front end is exposed, the rear end is placed in the middle and rear part of the pressure head 1 and is provided with a branch pipe connected to the outside of the side wall of the pressure head 1, and the branch pipe is connected to the permeability test system. The fracturing hole seepage pipe 4 and the crack seepage pipe 5 are connected to the permeability testing system through a first bolt 6 and a second bolt 7 respectively.

[0044] The front end face of the pressure head 1 is provided with a plurality of grooves 3 around the fracturing hole seepage pipe 4, and the grooves 3 are connected to the fracture seepage pipe 5. The grooves 3 are preferably formed by a plurality of annular grooves coaxial with the fracturing hole seepage pipe 4, and a plurality of linear grooves uniformly distributed in an annular direction with the fracturing hole seepage pipe 4 as the center. The fracturing hole seepage pipe 4 and the fracture seepage pipe 5 provided on the upper side pressure head 1a are both seepage inlet pipes, and the fracturing hole seepage pipe 4 and the fracture seepage pipe 5 provided on the lower side pressure head 1b are both seepage outlet pipes, so that the fluid on the front end face of the lower side pressure head 1b can not only converge to the fracture seepage pipe 5, but also make the fluid flowing out through the fracture seepage pipe 5 uniformly distributed on the front end face of the upper side pressure head 1a.

[0045] A method for using a filter ring partition double seepage test pressure head assembly under true triaxial stress. According to the filter ring partition double seepage test pressure head assembly under true triaxial stress, the permeability test system used in conjunction with the pressure head 1 is a true triaxial fluid-solid coupling experimental device seepage system, and a multifunctional true triaxial fluid-solid coupling test system with a publication number of CN102735548A is used, which has constant volume and constant pressure conditions. The use steps are as follows:

[0046] Step 1: Make a prefabricated specimen 8. Process the original rock material and process it into a prefabricated specimen 8 with a hole 9 in the center. The depth of the hole 9 is 55 mm.

[0047] Step 2: Install two pressure heads 1 symmetrically on the permeability test system, and install the prefabricated test piece 8 between the two pressure heads 1. The hole 9 in the prefabricated test piece 8 is connected with the fracturing hole seepage pipe 4 of the upper pressure head 1a to form a fracturing channel.

[0048] Step 3: Install the pressure head 1, connect the fracturing hole seepage pipe 4 and the crack seepage pipe 5 arranged on the upper pressure head 1a to the seepage inlet pipe of the permeability test system, and connect the fracturing hole seepage pipe 4 and the crack seepage pipe 5 arranged on the lower pressure head 1b to the seepage outlet pipe of the permeability test system.

[0049] Step 4: Without depressurizing the prefabricated specimen 8 during the whole process, the permeability of the prefabricated specimen 8 before fracturing is tested by a transient method to obtain the original permeability.

[0050] Step 5: Perform fluid fracturing on the prefabricated specimen 8, open the fracturing hole seepage pipe 4 of the upper pressure head 1a and close its fracture seepage pipe 5, the fluid is ejected through the fracturing hole seepage pipe 4 of the upper pressure head 1a under the constraint of the filter ring 2, and injected into the prefabricated specimen 8, generating fracturing cracks and forming a seepage channel, wherein the filter ring 2 provided in the upper pressure head 1a is provided with a sealing gasket to isolate the fluid from flowing outward, so that the fluid is accurately injected into the hole 9 during the fracturing process. The fluid after fracturing flows through the prefabricated specimen 8 and reaches the top of the lower pressure head 1b, the fluid in the fracturing hole seepage area 10 flows out from the fracturing hole seepage pipe 4, and the fluid in the fracture seepage area 11 flows out from the fracture seepage pipe 5 of the lower pressure head 1b through the convergence of the groove 3. After fracturing, the proppant is filled and injected, and the proppant is a fluid such as mortar and epoxy resin to fill the cracks.

[0051] Step 6: Determine the diameter of the filter ring 2 of the lower pressure head 1b by isotope determination, and install the filter ring 2 of the selected diameter to the front end surface of the lower pressure head 1b, including the following steps:

[0052] Step (i): After fracturing the prefabricated specimen 8, different isotopes of the same fluid are injected into the fracturing hole seepage pipe 4 and the fracture seepage pipe 5 of the upper pressure head 1a to perform a seepage test. The isotopes are preferably C12 and C14.

[0053] Step (ii): Calculate the content of the non-corresponding isotope fluid in the fracturing hole seepage pipe 4 or the fracture seepage pipe 5 of the lower pressure head 1b, so as to determine the seepage effect of the fracturing hole seepage zone 10 on the fracture seepage zone 11.

[0054] Step (iii): If the non-isotopic fluid content is large, it means that the radial cracks around the fracturing hole have reached the fracture seepage zone 11, and the fracturing hole has a great influence on the seepage of the fracture seepage zone 11, thereby increasing the radius of the filter ring 2, that is, expanding the range of the fracturing hole seepage zone 10, until it contains no non-isotopic fluid content. At this time, the influence of the fracturing hole on the fracture seepage zone is eliminated, and at the same time, the diameter of the filter ring 2 is determined, and the filter ring 2 of the selected diameter is installed.

[0055] Step 7: Under the condition that the prefabricated specimen 8 is not depressurized during the whole process, the permeability of the prefabricated specimen 8 after fracturing is measured by steady-state method, and the pressure heads 1 on both sides open the fracture seepage pipe 5 and close the fracture hole seepage pipe 4. The fluid without proppant enters from the fracture seepage pipe 5 of the upper pressure head 1a and is evenly distributed on the surface of the specimen through the groove 3, and flows from the fracture seepage channel of the fracture seepage area 11, flows through the entire prefabricated specimen 8 to the top surface of the lower pressure head 1b, and then flows out of the fracture seepage pipe 5 of the lower pressure head 1b through the convergence of the groove. After the seepage flow rate is stable, the seepage data is collected.

[0056] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A method for using a filter ring partition double seepage test pressure head assembly under true triaxial stress, characterized in that: A filter ring partition double seepage test pressure head assembly under true triaxial stress is used, the pressure head assembly comprising a pair of pressure heads (1) and a prefabricated test piece (8), the prefabricated test piece (8) being installed between the upper and lower pressure heads (1), wherein the pressure head (1) is divided into an upper pressure head (1a) and a lower pressure head (1b), characterized in that: the pressure head (1) comprises a filter ring (2), a groove (3), a fracturing hole seepage pipe (4) and a crack seepage pipe (5), the fracturing hole seepage pipe (4) being vertically arranged in the pressure head (1) and being coaxial with the center of the pressure head (1), and the front end The prefabricated test piece (8) is exposed and a filter ring (2) is installed outside the pipe mouth for partitioning, the rear end is placed in the middle and rear part of the pressure head (1) and is provided with a branch pipe connected to the outside of the side wall of the pressure head (1), and the branch pipe is connected to the permeability testing system; the area between the two filter rings (2) of the prefabricated test piece (8) is the fracturing hole seepage area (10), and the area outside the two filter rings (2) is the fracture seepage area (11); the fracture seepage pipe (5) is arranged between the outer wall of the pressure head (1) and the fracturing hole seepage pipe (4), the front end is exposed, the rear end is placed in the middle and rear part of the pressure head (1) and is provided with a branch pipe connected to the outside of the side wall of the pressure head (1), and the branch pipe is connected to the permeability testing system. A branch pipe extending to the outside of the side wall of the pressure head (1), the branch pipe being connected to the permeability test system; a plurality of grooves (3) are provided on the front end surface of the pressure head (1) around the fracture hole seepage pipe (4), the grooves (3) being connected to the fracture seepage pipe (5), the fracture hole seepage pipe (4) and the fracture seepage pipe (5) provided on the upper pressure head (1a) are both seepage inlet pipes, and the fracture hole seepage pipe (4) and the fracture seepage pipe (5) provided on the lower pressure head (1b) are both seepage outlet pipes, so that the fluid on the front end surface of the lower pressure head (1b) can be converged to the fracture seepage pipe ( 5), and the fluid flowing out of the fracture seepage pipe (5) can be evenly distributed on the front end surface of the upper pressure head (1a); the filter ring (2) on the lower pressure head (1b) is replaced with different diameters according to the fracturing situation; the prefabricated test piece (8) is set as a cube, and a hole (9) is vertically provided in the upper part of the prefabricated test piece (8), and the hole (9) is connected to the fracture hole seepage pipe (4). The permeability test system used in conjunction with the pressure head (1) is a true triaxial fluid-solid coupling experimental device seepage system, which has constant volume and constant pressure conditions, and includes the following steps: Step 1: making a prefabricated specimen (8), processing the original rock material and processing it into a prefabricated specimen (8) with a hole (9) in the center; Step 2: two pressure heads (1) are symmetrically mounted on the permeability test system, and a prefabricated test piece (8) is mounted between the two pressure heads (1), and the hole (9) in the prefabricated test piece (8) is connected to the fracturing hole seepage pipe (4) of the upper pressure head (1a) to form a fracturing channel; Step 3: Install the pressure head (1), connect the fracturing hole seepage pipe (4) and the fracture seepage pipe (5) provided on the upper pressure head (1a) to the seepage inlet pipe of the permeability test system, and connect the fracturing hole seepage pipe (4) and the fracture seepage pipe (5) provided on the lower pressure head (1b) to the seepage outlet pipe of the permeability test system; Step 4: Without depressurizing the prefabricated specimen (8) during the whole process, the permeability of the prefabricated specimen (8) before fracturing is tested using a transient method to obtain an original permeability; Step 5: Perform fluid fracturing treatment on the prefabricated specimen (8), open the fracturing hole seepage pipe (4) of the upper pressure head (1a) and close its fracture seepage pipe (5) at the same time, spray out through the fracturing hole seepage pipe (4) of the upper pressure head (1a) under the constraint of the filter ring (2), and inject into the interior of the prefabricated specimen (8), thereby generating fracturing cracks and forming a seepage channel; the fracturing fluid flows through the prefabricated specimen (8) and reaches the top of the lower pressure head (1b), the fluid in the fracturing hole seepage area (10) flows out from the fracturing hole seepage pipe (4), and the fluid in the fracture seepage area (11) flows out from the fracture seepage pipe (5) of the lower pressure head (1b) through the convergence of the groove (3); after fracturing, inject proppant to fill; Step 6: Determine the diameter of the filter ring (2) of the lower pressure head (1b) by using an isotope determination method, and install the filter ring (2) of the selected diameter onto the front end surface of the lower pressure head (1b), including the following steps: Step (1): After fracturing the prefabricated test piece (8), different isotopes of the same fluid are respectively injected into the fracturing hole seepage pipe (4) and the fracture seepage pipe (5) of the upper pressure head (1a) to perform a seepage test; Step (ii): detecting the content of the non-corresponding isotopic fluid in the fracturing hole seepage pipe (4) or the fracture seepage pipe (5) of the lower pressure head (1b), thereby determining the seepage effect of the fracturing hole seepage zone (10) on the fracture seepage zone (11); Step (iii): when the non-corresponding isotope content of the fracture seepage pipe (5) is large, radial cracks around the fracturing hole exist in the fracture seepage zone (11), thereby increasing the radius of the filter ring (2), that is, reducing the range of the fracture seepage zone; the expansion of the radius of the filter ring (2) is determined according to the non-corresponding isotope content of the fracturing hole seepage pipe (4); when the fracturing hole seepage pipe (4) does not contain the non-corresponding isotope content, the influence of the fracturing hole on the fracture seepage zone is eliminated, and at the same time, the diameter of the filter ring (2) is determined, and the filter ring (2) of the selected diameter is installed; Step 7: Without unloading the prefabricated specimen (8) during the whole process, the permeability of the prefabricated specimen (8) after fracturing is measured by the steady-state method. The fracture seepage pipes (5) of the pressure heads (1) on both sides are opened, and the fracture hole seepage pipes (4) are closed. The fluid without proppant enters from the fracture seepage pipe (5) of the upper pressure head (1a) and is evenly distributed on the surface of the specimen through the groove (3). It flows from the fracture seepage channel of the fracture seepage zone (11), passes through the entire prefabricated specimen (8), reaches the top surface of the lower pressure head (1b), and then flows out of the fracture seepage pipe (5) of the lower pressure head (1b) through the convergence of the groove. After the seepage flow rate stabilizes, the seepage data is collected.

2. The method for using the filter ring partition double seepage test pressure head assembly under true triaxial stress according to claim 1, characterized in that: The hole (9) has a depth of 55 mm.

3. The method for using the filter ring partition double seepage test pressure head assembly under true triaxial stress according to claim 1, characterized in that: In step six, the isotope determination method uses C12 and C14.

4. The method for using the filter ring partition double seepage test pressure head assembly under true triaxial stress according to claim 1 is characterized in that: A sealing gasket is provided in the filter ring (2) provided on the upper pressure head (1a) to isolate the fluid from flowing outwards, so that the fluid is accurately injected into the hole (9) during the fracturing process.

5. The method for using the filter ring partition double seepage test pressure head assembly under true triaxial stress according to claim 1, characterized in that: The pressure head (1) has a narrow front end and a wide rear end, and the top portion matches the cross-sectional dimensions of the prefabricated test piece (8) and is a square.

6. The method for using the filter ring partition double seepage test pressure head assembly under true triaxial stress according to claim 5 is characterized by: The middle and rear part of the pressure head (1) is arranged as a cylinder, and four cut surfaces are arranged according to the rectangular edge of the top of the pressure head (1) as transition surfaces with the middle and rear cylinder, so that the front end surface of the pressure head (1) is narrow and the rear end surface is wide.

7. The method for using the filter ring partition double seepage test pressure head assembly under true triaxial stress according to claim 1 is characterized in that: The groove (3) is formed by a plurality of annular grooves coaxial with the fracturing hole seepage pipe (4), and a plurality of linear grooves uniformly distributed in an annular direction with the fracturing hole seepage pipe (4) as the center.

8. The method for using the filter ring partition double seepage test pressure head assembly under true triaxial stress as described in claim 1 is characterized by: The fracturing hole seepage pipe (4) and the crack seepage pipe (5) are connected to the permeability testing system via a first bolt (6) and a second bolt (7), respectively.

9. The method for using the filter ring partition double seepage test pressure head assembly under true triaxial stress as described in claim 1 is characterized by: The rear end surface of the pressure head (1) is marked with a crosshair for positioning the pressure head (1) when it is installed in a permeability testing system.

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