Simulation experimental device and method for weak surface sliding friction characteristics in shale gas development

Abstract

The invention relates to the technical field of shale gas geology and development, and in particular to a weak surface sliding friction characteristic simulation experimental device and method in shale gas development. A positioning sleeve is arranged in a heating chamber; two test pieces are slidably assembled inside the positioning sleeve, and two pore fluid channels extending at different angles are drilled on the inclined surface of the test piece to simulate the shale fracture weak surface; a liquid supply pump supplies liquid to the gap of the two test pieces through a liquid supply hole to simulate the pore pressure; a pressure piece extends from one end of the positioning sleeve and then contacts the corresponding test piece to simulate the ground stress; pressure is applied to the test piece, and the two test pieces slide through the inclined surface, and the sliding friction coefficient change and the critical sliding water pressure are obtained through the shear displacement and shear rate change of the test piece, and the action process of the shale weak surface sliding friction is restored in a real-life simulation manner, so that the shale weak surface sliding friction model under different in-situ temperatures and stress states, different occurrence shale weak surfaces, different fracturing fluids and pore pressures can be studied.

Description

Technical Field

[0001] The invention belongs to the technical field of shale gas geology and development, and in particular relates to a weak surface sliding friction characteristic simulation experimental device and method in shale gas development. Background Art

[0002] In the development of shale gas, there is a problem that the sliding of shale weak surface causes casing deformation, which seriously affects the fracturing effect. In view of the ultra-low permeability and low porosity characteristics of shale reservoirs, horizontal wells and volume fracturing construction methods are required during development. With the large-scale volume fracturing construction of various shale oil and gas fields, if the reservoir area is in a high stress difference environment, the geological movement is frequent, and the bedding, lithology interface, natural fractures and faults are developed. During the hydraulic fracturing process of shale gas horizontal wells, the fracturing fluid pumped into the weak surface of shale causes the reservoir to produce an uneven shear stress field, the net pressure in the fracture increases, the weak surface is activated, and the casing deformation caused by the sliding of the weak surface occurs frequently, resulting in the frequent phenomenon that the bridge plug cannot be lowered to the designated position. The problem of formation shear slip in the volume fracturing construction of shale reservoirs has seriously affected the on-site construction and production, and has become a bottleneck restricting the development of shale oil and gas resources.

[0003] The frequent occurrence of the phenomenon that the bridge plug cannot be lowered due to casing deformation in shale gas horizontal wells is largely due to the fact that during the hydraulic fracturing process, the fracturing fluid pumped into the weak surface of the shale produces an uneven shear stress field in the reservoir, the net pressure in the fracture increases, the weak surface is activated, and the weak surface slip is triggered. Since deep shales are mostly in a high stress difference environment, the ambient temperature is high, the occurrence of weak surface cracks is complex, and different fracturing engineering factors have a great influence, which increases the difficulty of studying the sliding friction characteristics of shale weak surfaces and proposing effective prevention and control measures.

[0004] Therefore, it is necessary to provide an improved technical solution to address the above-mentioned deficiencies in the prior art. Summary of the invention

[0005] The purpose of the present invention is to overcome the deficiencies in the above-mentioned prior art. The present invention provides a device and method for simulating the weak surface sliding friction characteristics in shale gas development.

[0006] In order to achieve the above object, the present invention provides the following technical solutions:

[0007] A simulation experimental device for weak surface sliding friction characteristics in shale gas development, comprising:

[0008] A heating furnace, wherein the heating furnace has a heating cavity extending through the heating furnace in its axial direction, and a positioning sleeve is arranged in the heating cavity;

[0009] Test pieces, two of the test pieces are slidably assembled inside the positioning sleeve, and the opposing inclined surfaces are used as sliding surfaces, and two pore fluid channels extending at different angles are drilled on the inclined surfaces of the test pieces to simulate the weak surface of shale fractures;

[0010] A liquid supply pump, wherein the test piece with holes on the inclined surface is provided with a liquid supply hole extending along the axis thereof, the liquid supply hole is communicated with the pore fluid channel, and the liquid supply pump supplies liquid to the gap between the two test pieces through the liquid supply hole to simulate the pore pressure;

[0011] A pressure piece, which extends from one end of the positioning sleeve and contacts the corresponding test piece to simulate ground stress;

[0012] In response to the pressure applied by the pressure member to the test piece, the two test pieces slide through the inclined surface, and the change of the sliding friction coefficient and the critical sliding water pressure are obtained through the shear displacement and shear rate change of the test piece.

[0013] Preferably, the positioning sleeve is made of plastic material, and boron nitride powder is provided between the inner wall of the heating chamber and the positioning sleeve.

[0014] Preferably, the positioning sleeve is a copper tube, and the shear stress of the specimen is corrected by calculating and subtracting the shear strength of the positioning sleeve.

[0015] Preferably, a pressurized container is provided outside the heating furnace, the pressurized container has a loading and unloading port facing the heating furnace, and the pressurized container is correspondingly connected to an air pump.

[0016] Preferably, two ends of the positioning sleeve are respectively provided with a tungsten carbide cylinder, a corundum cylinder and a plunger corresponding to the test piece, so that the force distribution on the end surface of the test piece is uniform;

[0017] The plunger corresponding to the test piece located at the top is slidably assembled on the inner wall of the access opening through a sealing ring;

[0018] The pressure member applies pressure to the plunger from top to bottom, and a liquid supply hole corresponding to the test piece is provided in the middle of the upper plunger.

[0019] The plunger is a metal part, and its outer wall is provided with a sealing ring corresponding to the positioning sleeve;

[0020] A polytetrafluoroethylene gasket is provided between the plunger corresponding to the test piece located below and the bottom surface of the pressurized container.

[0021] The pore fluid channel on each inclined surface of the specimen includes a first pore fluid channel and a second pore fluid channel, the first pore fluid channel extends along the axial direction of the specimen, and the second pore fluid channel extends obliquely and intersects with an end of the first pore fluid channel away from the inclined surface, so that the pore pressure of the sliding surface is evenly distributed;

[0022] When the inclined surface has an inclination angle of 30°, the first pore fluid channel is eccentrically distributed with respect to the specimen, and the second pore fluid channel is vertically distributed with respect to the inclined surface;

[0023] When the inclined surface has an inclination angle of 60°, the first pore fluid channel is concentrically distributed with the test piece, and the second pore fluid channel is perpendicularly distributed with the inclined surface.

[0024] A method for simulating the sliding friction characteristics of weak surfaces in shale gas development is provided, wherein the method is carried out by using any of the above-mentioned experimental devices, and the method is characterized in that the method comprises the following steps:

[0025] Step S1, selecting equipment according to the test purpose;

[0026] Step S2, collecting shale oriented samples as test pieces, and making pore fluid channels and fluid supply holes on the inclined surface of the test pieces;

[0027] Step S3, designing a simulation experiment scheme to simulate the weak plane sliding friction in shale gas development, and setting the experimental parameters including temperature, stress, weak plane inclination, fracturing fluid type, and pore pressure;

[0028] Step S4, applying pressure to the test pieces so that the two test pieces slide through the inclined surface, and obtaining the change of the sliding friction coefficient and the critical sliding water pressure through the shear displacement and shear rate change of the test pieces;

[0029] Step S5, repeating steps S3-S4, establishing a shale weak surface sliding friction model under different data parameters according to the experimental results, and preliminarily establishing the nonlinear correlation between temperature, stress, weak surface inclination, fracturing fluid type, pore pressure and shale weak surface friction strength.

[0030] In step S3, an acidic solution is prepared as a fracturing fluid to immerse the test piece, so as to analyze the effect of hydration on the friction and sliding characteristics of shale, and reveal the law of the friction coefficient increasing with the shear displacement and the shear rate change in two types of media: neutral deionized water and acidic slick water.

[0031] The shear stress of the specimen is corrected by calculating and subtracting the shear strength of the positioning sleeve.

[0032] Beneficial effects: By restoring the action process of shale weak surface sliding friction in a real-life simulation, we can study the shale weak surface sliding friction model under the in-situ temperature and stress state of reservoirs at different depths, shale weak surfaces of different occurrences, different fracturing fluids and pore pressures, and preliminarily establish the nonlinear correlation between temperature, stress, weak surface inclination, fracturing fluid, pore pressure and shale weak surface friction strength. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The drawings constituting a part of the present application are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. Among them:

[0034] Figure 1 A simplified structural diagram of an experimental device in a specific embodiment provided by the present invention;

[0035] Figure 2 This is a schematic structural diagram of a specific embodiment provided by the present invention in which the inclined surface is 30 degrees;

[0036] Figure 3 A schematic diagram of the structure of a specific embodiment of the present invention in which the inclined surface is 60 degrees.

[0037] In the figure: 1. Heating furnace; 2. Boron nitride powder; 3. Porous fluid channel; 4. Positioning sleeve; 5. Plunger; 6. Liquid supply hole; 7. Tungsten carbide cylinder; 8. Test piece; 9. Corundum column; 10. Polytetrafluoroethylene gasket; 11. Sealing ring. DETAILED DESCRIPTION

[0038] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention belong to the scope of protection of the present invention.

[0039] In the description of the present invention, the terms "longitudinal", "lateral", "upper", "lower", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings. They are only for the convenience of describing the present invention and do not require that the present invention must be constructed and operated in a specific orientation. Therefore, they should not be understood as limitations on the present invention. The terms "connected" and "connected" used in the present invention should be understood in a broad sense. For example, they can be fixed connections or detachable connections; they can be directly connected or indirectly connected through intermediate components. For ordinary technicians in this field, the specific meanings of the above terms can be understood according to specific circumstances.

[0040] The present invention will be described in detail below with reference to the accompanying drawings and in combination with embodiments. It should be noted that the embodiments and features in the embodiments of the present invention can be combined with each other without conflict.

[0041] In the prior art, different experimental schemes and test devices are used to study earthquake fault nucleation, mine pressure control and other aspects. The existing technical problems are as follows:

[0042] (1) The effects of the original reservoir production state, reservoir temperature, stress environment, porosity, pressure, fracturing fluid type, etc. on the friction strength are not considered, which lacks specificity and universality.

[0043] (2) Failure to physically simulate the formation and evolution of deep shale weak planes in the reservoir;

[0044] (3) There is no comparative study on the sliding friction characteristics of shale weak surfaces under different geological and engineering factors.

[0045] In view of the above technical defects, the present invention provides a simulation experimental device for weak surface sliding friction characteristics in shale gas development.

[0046] like Figure 1-3 As shown, it includes a heating furnace 1, a test piece 8, a liquid supply pump and a pressurizing member. The heating furnace 1 is a cylindrical structure, and its side wall is provided with a spirally distributed heating wire, so as to heat the test piece 8, so as to simulate the in-situ temperature of reservoirs at different depths. The heating furnace 1 has a heating chamber that runs through its axial direction, and the heating chamber is columnar. A positioning sleeve 4 is provided in the heating chamber, and the positioning sleeve 4 is used to position the two test pieces 8 and maintain the relative positions of the two test pieces 8 in the initial state. The two test pieces 8 are slidably assembled inside the positioning sleeve 4. The two test pieces 8 use the opposing inclined surfaces as sliding surfaces. The inclination angle of the inclined surface is set according to the actual experimental purpose, so as to position the test pieces. The specimen 8 is processed, and the specimen 8 is directionally collected from the shale core drilled in the deep formation of the shale gas block. The abutting surfaces of the two specimens 8 are located in the middle of the positioning sleeve 4, so that they can slide along the inclined surface when mutual extrusion occurs, thereby simulating the sliding effect of the weak surface in shale gas development. In order to analyze the influence of static geological factors and dynamic engineering factors on the friction sliding of shale weak surfaces, shale cores with different reservoir temperatures and stress states are selected to make artificial fracture core specimens 8 with different inclination angles; in order to make the pore pressure on the sliding surface uniformly distributed, the inclined surface of the prepared specimen 8 is perforated, and two pore fluid channels 3 extending at different angles are drilled to simulate the weak surface of shale fractures.

[0047] The test piece 8 with holes on the inclined surface is provided with a liquid supply hole 6 extending along its axis, the lower end of the liquid supply hole 6 extends to the top of the pore fluid channel 3 and is interconnected with the pore fluid channel 3, the liquid supply hole 6 is connected to the liquid supply pump through a liquid supply pipe, and a pressure gauge is provided on the liquid supply pipe to monitor the interstitial pressure, and the liquid supply pump supplies liquid to the gap between the two test pieces 8 through the liquid supply hole 6 to simulate the pore pressure; the pressurizing member extends from one end of the positioning sleeve 4 and then contacts the corresponding test piece 8, and simulates the ground stress by applying axial pressure, so as to adjust the data such as temperature, stress state, weak surface inclination, pore pressure, etc., so as to design a corresponding experimental plan, and study the sliding friction characteristics of shale weak surface and its influencing factors from the perspective of forward analysis.

[0048] During the specific experiment, the pressure piece is used to apply pressure to the test piece 8. Under the action of the inclined surface, the two test pieces 8 overcome the friction force, so that the test piece 8 slides through the inclined surface to simulate the real shale weak surface sliding. A stress gauge is set on the pressure piece, and the movement state of the pressure piece is monitored, for example, by an industrial camera or a displacement sensor, so as to obtain the shear displacement and shear rate changes of the two test pieces 8. Then, the pressure parameters of the pressure piece and the shear displacement and shear rate changes can be used to obtain the changes in the sliding friction coefficient and the critical sliding water pressure of the shale sliding surface under different set parameters.

[0049] In an optional embodiment, the positioning sleeve 4 is made of a plastic metal material, so that during the test, the specimen 8 can generate radial shear through the sliding of the inclined surface, so that the movement of the specimen 8 can be displayed through the plastic deformation of the positioning sleeve 4. Boron nitride powder 2 is provided between the inner wall of the heating chamber and the positioning sleeve 4. The boron nitride powder 2 is similar to graphite in nature and is known as "white graphite". It has good electrical insulation, thermal conductivity and chemical corrosion resistance, and can conduct heat inward to the specimen 8 without affecting the radial displacement of the specimen 8, thereby simulating the real formation temperature.

[0050] In this embodiment, the positioning sleeve 4 is a copper tube. The copper tube has a low cost and has good plasticity, ductility and thermal conductivity. In order to ensure the accuracy of the test results, the test data is corrected for shear stress by calculation. Specifically, in the actual experimental process, when friction occurs between the inclined surfaces of the two test pieces 8, the copper tube wrapped around the test piece 8 undergoes shear deformation, which will generate resistance to the test piece 8 during the deformation process. Therefore, it is necessary to deduct the resistance generated by the deformation of the copper tube. After subtracting the shear strength of the copper tube from the shear strength, the shear strength of the friction sliding sample is obtained.

[0051] Calculate the shear stress (τ 1st ) is as follows:

[0052]

[0053] Among them, the axial pressure value (σ1) of the pressure piece on the specimen 8, the confining pressure value (σ3), and θ are the angles between the axial direction and the weak surface of the crack.

[0054] The experiment shows that the shear strength of the copper tube is approximately linear within the experimental shear distance, and the average slope of the shear stress axial displacement curve after the onset of shear deformation is (6.1 ± 0.49) MPa / mm.

[0055] Therefore, by subtracting the shear strength from the shear strength of the copper tube under the corresponding sliding displacement (ΔL), the actual shear strength of the shale fracture weak surface sample (τ) can be obtained, that is:

[0056] τ=τ 1st—6.1*ΔL

[0057] In an optional embodiment, a pressurized container is provided outside the heating furnace 1. The pressurized container may be a metal can having a take-in / take-out port facing the heating furnace 1. The take-in / take-out port has a cylindrical smooth inner wall extending axially along the heating furnace 1. A pressurized pipe corresponding to an air pump is provided on the side wall of the pressurized container. A pressure gauge is provided between the air pump and the pressurized pipe, thereby meeting the needs of simulating actual formation confining pressure.

[0058] Furthermore, the two ends of the positioning sleeve 4 are respectively provided with a tungsten carbide cylinder 7, a corundum column 9 and a plunger 5 corresponding to the test piece 8. The tungsten carbide cylinder 7, the corundum column 9 and the plunger 5 are distributed in sequence along the end away from the sliding surface and are slidably assembled in the positioning sleeve 4. The tungsten carbide cylinder 7 has heat conduction capability and high strength, so that the end face of the test piece 8 can be heated. The diameter of the tungsten carbide cylinder 7 is adapted to the diameter of the test piece 8, so that the stress transmitted from the corundum column 9 to the test piece 8 is evenly distributed.

[0059] In order to maintain the seal, the plunger 5 corresponding to the test piece 8 located above is slidably assembled on the inner wall of the access port through a sealing ring 11. The upper end of the plunger 5 is provided with an enlarged end corresponding to the access port, and the enlarged end is adapted to the aperture of the access port. Its smaller end extends into the interior of the positioning sleeve 4 and is slidably assembled in the positioning sleeve 4. In order to meet the gap pressure simulation, the pressure member applies pressure to the plunger 5 from top to bottom. The plunger 5 is a steel column. An industrial camera corresponding to the pressure member is provided outside the pressurized container, so that the longitudinal displacement of the test piece 8 is fed back through the running trajectory of the pressure member. The pressure member can be a hydraulic cylinder or a triaxial experimental system. The plunger 5, the corundum column 9, and the tungsten carbide cylinder 7 corresponding to the upper test piece 8 are all provided with a liquid supply hole 6 corresponding to the test piece 8 in the middle to achieve the purpose of liquid supply. In this embodiment, the contact surface between the hydraulic column of the pressure member and the plunger 5 is provided with a groove corresponding to the liquid supply pipe. The liquid supply hole 6 is also used as a channel for measuring the sample temperature with a thermocouple during the experiment, and is used according to the experimental setting conditions.

[0060] In an optional embodiment, the plunger 5 is a metal part, and a sealing ring 11 corresponding to the positioning sleeve 4 is provided on its outer wall, thereby performing a sliding seal with the positioning sleeve 4 to ensure that the internal liquid does not leak outward and maintain the gap pressure. A polytetrafluoroethylene gasket 10 is provided between the plunger 5 corresponding to the test piece 8 located below and the bottom surface of the pressurized container, and a lubricant is applied to the bottom end of the plunger 5, and then the polytetrafluoroethylene gasket 10 is affixed to reduce the friction of the horizontal sliding of the plunger 5.

[0061] In an optional embodiment, the pore fluid channel 3 on the inclined surface of each specimen 8 includes a first pore fluid channel and a second pore fluid channel, the first pore fluid channel extends along the axial direction of the specimen 8, and the second pore fluid channel extends obliquely and intersects with an end of the first pore fluid channel away from the inclined surface, so that the pore pressure on the sliding surface is evenly distributed.

[0062] When the inclined surface has an inclination angle of 30°, the first pore fluid channel is eccentrically distributed with respect to the test piece 8, and the second pore fluid channel is vertically distributed with respect to the inclined surface;

[0063] When the inclined surface has an inclination angle of 60°, the first pore fluid channel is concentrically distributed with the test piece 8, and the second pore fluid channel is perpendicularly distributed with the inclined surface.

[0064] In an optional embodiment, for the above experimental device, the present invention also provides a simulation experimental method for weak surface sliding friction characteristics in shale gas development. The experiment is carried out through any of the above-mentioned experimental devices, including the following steps: Step S1, equipment is selected according to the purpose of the experiment to ensure that all indicators of the equipment can meet the requirements of the simulation experiment. The coupled three-axis sliding friction physical simulation experiment can be used as the experimental equipment.

[0065] Step S2, collecting directional shale samples as test pieces 8, the test pieces 8 are obtained by directional collection of shale cores drilled in deep formations of shale gas blocks, and porous fluid channels 3 and liquid supply holes 6 are made on the inclined surface of the test piece 8. The inclination angle of the inclined surface is made and selected according to actual conditions.

[0066] Step S3, designing a simulation experiment scheme to simulate the weak plane sliding friction in shale gas development, wherein the simulation experiment scheme needs to set experimental parameters including temperature, stress, weak plane inclination, fracturing fluid type, and pore pressure.

[0067] Step S4, applying pressure to the test piece 8, monitoring the movement trajectory and applied stress of the test piece during the pressurization process, and causing the two test pieces 8 to slide through the inclined surface as the test piece is continuously pressurized. The change in sliding friction coefficient and critical sliding water pressure are obtained based on the detected shear displacement and shear rate change of the test piece 8. The experimental structure corrects the shear stress of the test piece 8 by calculating and subtracting the shear strength of the positioning sleeve 4.

[0068] Step S5, repeat steps S3-S4, and establish a shale weak surface sliding friction model under different data parameters according to the experimental results. The experimental results are processed and correlated using computer technology (multiple regression analysis, fuzzy cluster analysis, hierarchical analysis and other mathematical geological methods) to preliminarily establish the nonlinear correlation between temperature, stress, weak surface inclination, fracturing fluid type, pore pressure and shale weak surface friction strength. Based on the influence of different reservoir geological factors (in-situ temperature, stress state, weak surface occurrence) and engineering factors (fracturing fluid type, fracturing pressure) on the shale weak surface sliding friction strength, comprehensive geological-engineering-drainage and production prevention and control measures for alleviating shale weak surface sliding are proposed.

[0069] In an optional embodiment, in step S3, an acidic solution is prepared as a fracturing fluid to immerse the test piece 8, so as to analyze the influence of hydration on the friction and sliding characteristics of shale, and reveal the law of the friction coefficient increasing with the shear displacement and the shear rate change in two types of media, neutral deionized water and acidic slick water. In the simulated experiment, an acidic liquid is selected to immerse the parallel samples of the weak surface of the fracture. The laboratory configures an acidic solution with a pH value of about 3, and the immersion time is 24 hours, so that the pores inside the sample are fully saturated with the acidic solution.

[0070] In another optional embodiment, a friction sliding experiment is carried out on the deep layers of the Longmaxi Formation-Wufeng Formation in the Luzhou block in southern Sichuan. A high-pressure triaxial experimental system is used for the friction sliding experiment. Its designed maximum temperature is 700°C, the control accuracy is ±1°C, the confining pressure is designed to be 500MPa, the control accuracy is ±0.5MPa, the maximum pore pressure is 200Mpa, and other indicators can meet the requirements of the simulation experiment.

[0071] According to the experimental purpose, specimen 8 is a deep shale core of the Longmaxi Formation-Wufeng Formation in the Luzhou block in southern Sichuan. By drilling sampling, a stone column with a diameter of 20 mm and a length of 40 mm was drilled as specimen 8. Specimen 8 was polished on both sides and then cut into prefabricated fracture weak surfaces with different angles to the horizontal plane according to the design. After drilling holes on the inclined surface of specimen 8, it was immersed in an acidic solution so that the pores inside specimen 8 were fully saturated with the acidic solution.

[0072] According to the actual geological environment of the reservoir, engineering conditions, and the principle and purpose of the simulation experiment, the experimental parameters such as constants and variables in each group of experiments are set. According to the data, the core samples of the experimental objects are stored at a depth range of 3500-4500 meters, the corresponding formation temperature range is about 120-150℃, the overlying formation pressure is about 90-120Mpa, the confining pressure range is about 90-110MPa, and the hydraulic fracturing pressure of shale gas wells is about 100Mpa.

[0073] The variable parameters in the experimental parameters include reservoir temperature, stress state, weak surface inclination, fracturing fluid type, pore pressure, etc. Three types of experimental research contents are designed:

[0074] a Analysis of friction characteristics of shales with different occurrences (setting weak surface high angle and low angle series inclination) at different temperatures (designed according to different reservoir temperatures) and different stress states (according to actual reservoir conditions): the law of friction coefficient (friction strength) changing with increasing shear displacement and shear rate.

[0075] b The friction characteristics of single-occurrence shale under different pore fluid media (design parameter ratio of solution supplied by the fluid supply pump) and pore pressure conditions are analyzed to reveal the law of friction coefficient increasing with shear displacement and shear rate change in two types of media: neutral deionized water and acidic slick water.

[0076] c Analysis of friction characteristics of shales of different occurrences under different temperatures and pore pressures. According to the actual set temperature, stress state and pore pressure of the reservoir in the study area, the critical sliding water pressure under the set conditions is obtained.

[0077] According to the above three types of experimental results, the effects of temperature, stress, weak surface inclination, fracturing fluid and pore pressure on the friction strength of shale weak surface during the simulation experiment are analyzed, and the law of the change of friction (friction strength) with shear displacement and shear rate is quantitatively analyzed to obtain the critical sliding water pressure under the set conditions. In this way, the variation characteristics of friction strength, shear displacement and shear rate under different conditions and the critical sliding water pressure under the set conditions are obtained, and then, for the shale gas fracturing production practice, prevention and control measures to reduce the friction and sliding of shale weak surfaces can be proposed.

[0078] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention are within the scope of protection of the pending claims of the present invention.

Claims

1. A simulation experimental device for weak surface sliding friction characteristics in shale gas development, characterized in that: include: A heating furnace, wherein the heating furnace has a heating cavity extending through the heating furnace in its axial direction, and a positioning sleeve is arranged in the heating cavity; Test pieces, two of the test pieces are slidably assembled inside the positioning sleeve, and the opposing inclined surfaces are used as sliding surfaces, and two pore fluid channels extending at different angles are drilled on the inclined surfaces of the test pieces to simulate the weak surface of shale fractures; A liquid supply pump, wherein the test piece with holes on the inclined surface is provided with a liquid supply hole extending along the axis thereof, the liquid supply hole is communicated with the pore fluid channel, and the liquid supply pump supplies liquid to the gap between the two test pieces through the liquid supply hole to simulate the pore pressure; A pressure piece, which extends from one end of the positioning sleeve and contacts the corresponding test piece to simulate ground stress; In response to the pressure member applying pressure to the test piece, the two test pieces slide through the inclined surface, and the change of the sliding friction coefficient and the critical sliding water pressure are obtained through the shear displacement and shear rate change of the test piece; The pore fluid channel on each inclined surface of the specimen includes a first pore fluid channel and a second pore fluid channel, the first pore fluid channel extends along the axial direction of the specimen, and the second pore fluid channel extends obliquely and intersects with an end of the first pore fluid channel away from the inclined surface, so that the pore pressure of the sliding surface is evenly distributed; When the inclined surface has an inclination angle of 30°, the first pore fluid channel is eccentrically distributed with respect to the specimen, and the second pore fluid channel is vertically distributed with respect to the inclined surface; When the inclined surface has an inclination angle of 60°, the first pore fluid channel is concentrically distributed with the test piece, and the second pore fluid channel is perpendicularly distributed with the inclined surface.

2. The weak surface sliding friction characteristics simulation experimental device for shale gas development according to claim 1 is characterized in that: The positioning sleeve is made of plastic material, and boron nitride powder is arranged between the inner wall of the heating cavity and the positioning sleeve.

3. The weak surface sliding friction characteristics simulation experimental device for shale gas development according to claim 2 is characterized in that: The positioning sleeve is a copper tube, and the shear stress of the test piece is corrected by calculating and subtracting the shear strength of the positioning sleeve.

4. The weak surface sliding friction characteristics simulation experimental device for shale gas development according to claim 1 is characterized in that: A pressurized container is arranged outside the heating furnace, the pressurized container has a taking-in and putting-out port facing the heating furnace, and the pressurized container is correspondingly connected to an air pump.

5. The weak surface sliding friction characteristics simulation experimental device for shale gas development according to claim 4 is characterized in that: The two ends of the positioning sleeve are respectively provided with a tungsten carbide cylinder, a corundum cylinder and a plunger corresponding to the test piece, so that the force distribution on the end surface of the test piece is uniform; The plunger corresponding to the test piece located at the top is slidably assembled on the inner wall of the access opening through a sealing ring; The pressure member applies pressure to the plunger from top to bottom, and a liquid supply hole corresponding to the test piece is provided in the middle of the upper plunger.

6. The weak surface sliding friction characteristics simulation experimental device for shale gas development according to claim 5 is characterized in that: The plunger is a metal part, and its outer wall is provided with a sealing ring corresponding to the positioning sleeve; A polytetrafluoroethylene gasket is provided between the plunger corresponding to the test piece located below and the bottom surface of the pressurized container.

7. A method for simulating the sliding friction characteristics of weak surfaces in shale gas development, wherein the experiment is carried out by using the experimental device described in any one of claims 1 to 6, characterized in that: The following steps are involved: Step S1, selecting equipment according to the test purpose; Step S2, collecting shale oriented samples as test pieces, and making pore fluid channels and fluid supply holes on the inclined surface of the test pieces; Step S3, designing a simulation experiment scheme to simulate the weak plane sliding friction in shale gas development, and setting the experimental parameters including temperature, stress, weak plane inclination, fracturing fluid type, and pore pressure; Step S4, applying pressure to the test pieces so that the two test pieces slide through the inclined surface, and obtaining the change of the sliding friction coefficient and the critical sliding water pressure through the shear displacement and shear rate change of the test pieces; Step S5, repeating steps S3-S4, establishing a shale weak surface sliding friction model under different data parameters according to the experimental results, and preliminarily establishing the nonlinear correlation between temperature, stress, weak surface inclination, fracturing fluid type, pore pressure and shale weak surface friction strength.

8. The method for simulating the weak surface sliding friction characteristics in shale gas development according to claim 7, characterized in that: In step S3, an acidic solution is prepared as a fracturing fluid to immerse the test piece, so as to analyze the effect of hydration on the friction and sliding characteristics of shale, and reveal the law of the friction coefficient increasing with the shear displacement and the shear rate change in two types of media: neutral deionized water and acidic slick water.

9. The method for simulating the weak surface sliding friction characteristics in shale gas development according to claim 7, characterized in that: The shear stress of the specimen is corrected by calculating and subtracting the shear strength of the positioning sleeve.

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