Deep coal reservoir true triaxial hydraulic fracturing physical simulation method

Abstract

The invention discloses a deep coal reservoir true triaxial hydraulic fracturing physical simulation method, and relates to the technical field of coal bed gas exploitation. Comprising the following steps: S1, filling a soluble stress buffer medium, a coal matrix modified embedded steel bar adhesive and a micro-convex threaded steel pipe into a low-crack coal briquette to obtain a coal sample; s2, loading the coal sample through a true triaxial pressure chamber, and performing temperature control compensation at the same time; and S3, simulating the displacement requirement of the underground fracturing operation through the prepared fracturing fluid and the constant flow pump, determining the fracture in the coal sample through the sensor and the CT scanner, and obtaining the three-dimensional expansion path of the fracture. According to the invention, the reinforcing material is matched with the mechanical property of the coal body, the stress concentration is reduced, the breakage rate of the coal sample in the experiment is reduced, meanwhile, the micro-convex threaded steel pipe is combined with the raw material belt for sealing implantation, and the compactness of the embedded steel bar is verified through CT scanning, so that the integrity of the shaft structure and the unblocked channel of the fracturing fluid are further ensured.

Description

Technical Field

[0001] The present invention relates to the technical field of coalbed methane extraction, and particularly to a true triaxial hydraulic fracturing physical simulation method for deep coal reservoirs. Background Technique

[0002] The hydraulic fracturing technology is a key means to improve the gas extraction effect of low-permeability coal reservoirs and the development efficiency of coalbed methane. However, deep coal reservoirs (generally with a burial depth exceeding 1000 meters) usually have a complex geomechanical environment, including high stress, high temperature, strong heterogeneity, developed natural fractures, and formation fluid coupling effects. The traditional uniaxial or biaxial hydraulic fracturing simulation methods are difficult to truly reflect the fracture propagation law and reservoir stimulation effect of deep coal reservoirs under real geological conditions, resulting in the physical simulation results being difficult to well guide the on-site fracturing operation and causing great uncertainty in the on-site fracturing design.

[0003] The Chinese invention patent with the publication number CN119643304A discloses a true triaxial hydraulic fracturing fault slip casing deformation monitoring method and system. The method includes: preparing a target reservoir simulation specimen; arranging and installing a wellbore and sensors; carrying out a water pressure physical model experiment and recording the experimental data; analyzing and processing the experimental data, and quantitatively evaluating in combination with the final state of the target reservoir simulation specimen. The invention can quantitatively evaluate the shear deformation effect of fault slip on the wellbore casing under true triaxial hydraulic fracturing conditions; provides a scientific basis for the optimization and selection of fracturing design parameters, and has engineering practical significance for the optimization of the horizontal well casing damage prevention theory and on-site construction design.

[0004] However, most of the current physical simulation technologies for deep coal reservoir hydraulic fracturing adopt the conventional operation of wrapping coal samples with cement or epoxy resin. Due to the essential differences in mechanical properties between the reinforcement material and the coal body, it will not only cause serious distortion of test data, but also trigger the dual risks of seal failure and wellbore blockage during the preparation process of the borehole open hole section. At the same time, the material compatibility problem will further exacerbate the experimental difficulty under high stress conditions. When the stress exceeds 30 MPa, the coal sample crushing rate will increase significantly, and due to the coupling effect between temperature and stress, it will further increase the simulation difficulty of the true fracturing response characteristics of deep coal reservoirs. Summary of the Invention

[0005] The purpose of the present invention is to provide a true triaxial hydraulic fracturing physical simulation method for deep coal reservoirs to solve the problems proposed in the above background technique.

[0006] To achieve the above purpose, the present invention provides the following technical solution: A true triaxial hydraulic fracturing physical simulation method for deep coal reservoirs, including:

[0007] S1: Coal sample preparation and consolidation: Fill soluble stress buffer medium, coal matrix modified reinforcing glue, and micro-convex threaded steel pipes into the interior of low-fracture coal blocks to obtain coal samples;

[0008] S2: True triaxial loading and temperature control: Load the coal samples through a true triaxial pressure chamber while performing temperature control compensation;

[0009] S2.1: Specimen installation: Lay a silicone rubber gasket, a carbon fiber grid, and a polyether ether ketone plate on the surface of the cushion block in the true triaxial pressure chamber in sequence to construct a buffer layer. At the same time, insert the steel pipe in the coal sample into the central hole of the buffer layer, and fix the coal sample to the cushion block in the true triaxial pressure chamber through fixing bolts;

[0010] S2.2: Stress loading: In the true triaxial pressure chamber, perform stress loading on the coal sample in stages through a hydraulic pump and sensors;

[0011] S2.3: Temperature control: Wind a ring-shaped electric heating tube around the metal shell loaded with the coal sample, and obtain the target temperature according to the relationship formula between the vertical stress and the formation temperature. At the same time, perform temperature adjustment through the ring-shaped electric heating tube and the target temperature. The formula for calculating the target temperature is specifically:

[0012] ,

[0013] Where: is the target temperature, is the initial temperature, is the temperature-stress coupling coefficient, is the current vertical stress value;

[0014] S3: Fracturing implementation and monitoring: Simulate the displacement requirements of downhole fracturing operations through the configured fracturing fluid and constant flow pump. At the same time, determine the fractures in the coal sample through sensors, and obtain the three-dimensional propagation path of the fractures through a CT scanner.

[0015] Furthermore, obtaining the coal samples includes:

[0016] S1.1: Coal block pretreatment: Collect original coal blocks from the coal wall with a geological hammer and cut the original coal blocks to obtain cut coal blocks;

[0017] S1.2: Buffer medium filling: Drill a central hole in the cut coal block along the bedding plane direction of the cut coal block, and fill soluble stress buffer medium at the bottom of the central hole;

[0018] S1.3: Consolidation with planting bar adhesive: Add coal matrix modified planting bar adhesive into the inner part of the central hole filled with the soluble stress buffer medium, and implant a micro-convex threaded steel pipe into the inner part of the central hole to obtain an initial coal sample;

[0019] S1.4: Quality verification: Scan the initial coal sample with a CT scanner to obtain the pore area of the central hole, determine the planting bar filling density, and at the same time compare the planting bar filling density with a preset density threshold, and determine the final coal sample according to the comparison result. Specifically:

[0020] When the planting bar filling density is greater than the preset density threshold, repeat steps S1.3 and S1.4 to re-obtain the initial coal sample until the planting bar filling density is not greater than the preset density threshold. Otherwise, the initial coal sample is the final coal sample.

[0021] Furthermore, mix the raw materials NaCl, raw material SiO2 and silane coupling agent ethanol solution to obtain a soluble stress buffer medium, and at the same time fill the soluble stress buffer medium into the bottom of the central hole in stages. Specifically:

[0022] Introduce the soluble stress buffer medium into the inner part of the central hole through a powder filling funnel and a cemented carbide punch, and the compacted thickness is not less than 5 mm for the first-stage filling;

[0023] Continue to introduce the soluble stress buffer medium into the inner part of the central hole filled in the first stage through a powder filling funnel and a punch with an exhaust groove structure, and the total compacted thickness is not less than 10 mm for the second-stage filling.

[0024] Furthermore, prepare coal matrix modified planting bar adhesive from epoxy resin, curing agent and coal powder, including:

[0025] SA1.3.1: Premixing of coal powder: Stir and mix the epoxy resin and coal powder until the coal powder is evenly dispersed to obtain premixed coal powder;

[0026] SA1.3.2: Adding curing agent: Add the curing agent to the premixed coal powder and continue to stir and mix to obtain a mixed colloid;

[0027] SA1.3.3: Defoaming treatment: Subject the mixed colloid to degassing treatment with a vacuum degassing machine to obtain coal matrix modified planting bar adhesive.

[0028] Furthermore, implanting a micro-convex threaded steel pipe into the inner part of the central hole includes:

[0029] SB1.3.1: Steel pipe pretreatment: After threading the stainless steel pipe, it is ultrasonically cleaned with pure acetone and dried simultaneously to obtain the pretreated steel pipe.

[0030] SB1.3.2: Inserting the steel pipe: Wrap the end of the pretreated steel pipe with raw tape. At the same time, after injecting the coal matrix modified rebar glue into the inner part of the central hole, insert the end of the steel pipe wrapped with the raw tape into the inner part of the central hole until the top of the pretreated steel pipe is flush with the surface of the cut coal block.

[0031] SB1.3.3: Curing and maintenance: Cure and maintain the cut coal block implanted with the steel pipe to obtain the initial coal sample.

[0032] Furthermore, the stress is loaded on the coal sample in stages, including:

[0033] S2.2.1: Preloading: Connect the high-pressure hydraulic pump to the three-axis independent oil circuit. At the same time, set the high-pressure hydraulic pump to the synchronous loading mode. Synchronously increase the three-way stress to the preset initial stress, obtain the fluctuation change of the stress, and determine the completion state of the preloading stage according to the fluctuation change.

[0034] S2.2.2: Main loading: Synchronously increase the vertical stress, the maximum horizontal principal stress, and the minimum horizontal principal stress of the high-pressure hydraulic pump in equal proportion to the preset loading stress value and perform constant loading to obtain the strain value data. At the same time, determine the state of constant loading according to the convergence state of the strain value data. Specifically:

[0035] When the strain value data converges, perform gradient loading on the maximum horizontal principal stress, the minimum horizontal principal stress, and the vertical stress synchronously according to the preset loading stress value. Otherwise, continue with constant loading until the strain value data converges.

[0036] Furthermore, determining the completion state of the preloading stage according to the fluctuation change includes:

[0037] W1: Determine the preloading state: Obtain the real-time pressure value through a three-way pressure gauge, compare the real-time pressure value with the preset pressure value, and determine the preloading state according to the comparison result. Specifically:

[0038] When the real-time pressure value is the same as the preset pressure value, the preloading is completed and the initial strain value is obtained. Otherwise, continue with pressure loading until the real-time pressure value is the same as the preset pressure value.

[0039] W2: Determine the fluctuation threshold: Obtain the strain change amount threshold according to the initial strain value. Specifically:

[0040] ,

[0041] Wherein: is the strain change amount threshold value, is the initial strain value;

[0042] W3: Determine the stress fluctuation state: After the preloading is completed, perform constant loading, obtain the strain values at different times, and at the same time, according to the strain values and the strain change amount threshold value, respectively perform longitudinal strain comparison and transverse strain comparison, and determine the strain fluctuation state according to the comparison results;

[0043] W4: Abnormal handling: Perform strain drift handling and unidirectional abnormal handling according to the strain fluctuation state.

[0044] Furthermore, determining the strain fluctuation state includes:

[0045] W3.1: Longitudinal comparison: According to the initial strain value, obtain the strain change difference between the strain values at different times and the initial strain value, and compare the strain change difference with the strain change amount threshold value, and at the same time, determine the strain fluctuation state according to the comparison results, specifically:

[0046] When the strain change difference is greater than the strain change amount threshold value, the strain fluctuation state is abnormal, and the next step is executed; otherwise, the strain fluctuation state is normal;

[0047] W3.2: Transverse comparison: According to the strain values at different times, draw a three-way strain-time curve, and determine the strain fluctuation state according to the trend change of the three-way strain-time curve, specifically:

[0048] When the three curves in the three-way strain-time curve rise or fall in parallel, the strain fluctuation state is normal; otherwise, the strain fluctuation state is abnormal, and the next step is executed.

[0049] Furthermore, obtaining the three-dimensional expansion path of the crack includes:

[0050] S3.1: Fracturing fluid injection: Configure fracturing fluid through pure water, KCl solution and a drainage aid, and inject the fracturing fluid through an intelligent fracturing fluid injection unit;

[0051] S3.2: Crack monitoring: Uniformly arrange a plurality of sensors on the outer wall of the pressure chamber, and at the same time, according to the propagation speed of sound waves in the coal sample, the positions and time differences of each sensor, determine the position coordinates of the rupture point, and at the same time, obtain the CT image of the coal sample through a CT scanner, and obtain a three-dimensional model according to the CT image of the coal sample and the position coordinates of the rupture point;

[0052] The position coordinates of the rupture point are specifically as follows:

[0053] ,

[0054] Where: is the X-axis coordinate of the acoustic emission sensor, is the Y-axis coordinate of the acoustic emission sensor, is the Z-axis coordinate of the acoustic emission sensor, is the X-axis coordinate of the rupture point, is the Y-axis coordinate of the rupture point, is the Z-axis coordinate of the rupture point, is the propagation speed of sound waves in the final coal sample, is the time difference of the i-th sensor.

[0055] Furthermore, injecting the fracturing fluid through the intelligent fracturing fluid injection unit includes:

[0056] S3.1.1: Configure the fracturing fluid: Stir and mix pure water and KCl solution, add a drainage aid, and obtain the fracturing fluid through magnetic stirring;

[0057] S3.1.2: Pipeline exhaust treatment: Close the outlet ball valve of the fracturing fluid, start the plunger pump, and perform cyclic flushing at a preset flow rate. At the same time, ensure that there are no bubbles in the fracturing fluid in the pipeline through the transparent polycarbonate observation window;

[0058] S3.1.3: Flow control: Inject the fracturing fluid through a constant flow pump according to the preset flow rate value. At the same time, compare the real-time pressure in the wellbore with the preset pressure threshold, and adjust the liquid flow rate of the fracturing fluid according to the comparison result. Specifically:

[0059] When the real-time pressure in the wellbore is not greater than the preset pressure threshold, maintain the current operating state of the constant flow pump. Otherwise, stop the constant flow pump, and at the same time, adjust the liquid flow rate of the fracturing fluid according to the current liquid flow rate to obtain a new adjusted liquid flow rate. Specifically:

[0060] ,

[0061] Where: is the new adjusted flow rate, is the current initial flow rate, is the base of the natural logarithm, is the duration of pressure fluctuation, is the time constant.

[0062] Compared with the prior art, the beneficial effects of the present invention are:

[0063] First: By filling the soluble stress buffer medium and modifying the grouting adhesive for reinforcing bars with coal matrix, the mechanical properties of the reinforcement material and the coal body are matched, thus reducing stress concentration and the coal sample breakage rate in the experiment. At the same time, the micro-convex threaded steel pipe is combined with raw tape for sealed implantation, and the density of the implanted reinforcing bars is verified by CT scanning, further ensuring the integrity of the shaft structure and the smoothness of the fracturing fluid channel;

[0064] Second: The invention disperses stress through the buffer layer and dynamically adjusts the temperature of the electric heating pipe according to the vertical stress, thus realizing the coordinated control between stress and temperature, and being able to be closer to the actual geological conditions;

[0065] Third: The invention combines a multi-sensor array and a CT scanner, locates the coordinates of the fracture point through the acoustic wave transit time, and generates a three-dimensional model of the fracture, thus providing high-precision data support for the fracturing design, and automatically optimizing the injection displacement according to the real-time pressure threshold;

[0066] Fourth: The invention arranges a buffer layer between the coal sample and the cushion block in the true triaxial pressure chamber for hydraulic fracturing, without using cement or the like to pour the sample, which can not only save the time for cement pouring and curing, but also improve the efficiency of the true triaxial hydraulic fracturing physical simulation experiment. Description of the Drawings

[0067] Figure 1 It is the CT scan pore distribution diagram of the coal sample in the present invention;

[0068] Figure 2 It is the compaction characteristic distribution diagram of the coal sample in the present invention;

[0069] Figure 3 It is the triaxial stress loading and temperature response distribution diagram in the present invention;

[0070] Figure 4 It is the stress distribution nephogram of the buffer layer in the present invention;

[0071] Figure 5 It is the flow schematic diagram of the true triaxial hydraulic fracturing physical simulation method for deep coal reservoirs in the present invention. Detailed Embodiments

[0072] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the 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.

[0073] Most of the current physical simulation technologies for hydraulic fracturing of deep coal reservoirs adopt the conventional operation of wrapping coal samples with cement or epoxy resin. Due to the essential differences in mechanical properties between the reinforcement materials and the coal body, it will not only lead to serious distortion of test data, but also cause the dual risks of seal failure and wellbore blockage during the preparation of the borehole open hole section. At the same time, the material compatibility problem will further exacerbate the experimental difficulty in high-stress environments. When the stress exceeds 30 MPa, the crushing rate of coal samples will increase significantly, and due to the coupling effect between temperature and stress, it will further increase the simulation difficulty of the true fracturing response characteristics of deep coal reservoirs. The technical solution of this application prepares coal samples through soluble stress buffer media, coal matrix modified reinforcing glue, and micro-convex threaded steel pipes, and conducts staged stress loading on the coal samples in a true triaxial pressure chamber, and dynamically adjusts the temperature through annular electric heating pipes and vertical stress. At the same time, simulate downhole operations according to the configured fracturing fluid, and real-time monitor the crack propagation path through sensors and CT scanners and construct a three-dimensional model, thus solving the problems of poor material compatibility, easy breakage of coal samples under high stress, and temperature-stress coupling effect simulation in traditional simulations, and then improving the authenticity of the fracturing response of deep coal reservoirs.

[0074] Example 1

[0075] Reference Figures 1 - 5 , this embodiment provides a true triaxial hydraulic fracturing physical simulation method for deep coal reservoirs. The simulation method specifically includes the following steps:

[0076] Step S1: Coal sample preparation and consolidation. That is, fill the inside of the low-fracture coal block with soluble stress buffer media, coal matrix modified reinforcing glue, and micro-convex threaded steel pipes to obtain coal samples. Specifically as follows:

[0077] Step S1.1: Coal block pretreatment. That is, gently strike the coal wall with a geological hammer until the surface layer of the coal wall peels off and a fresh cross-section appears. At the same time, collect the original coal block from the fresh cross-section. Further, cut the collected original coal block with a bridge stone cutting machine to obtain the cut coal block.

[0078] In the process of specific implementation, a 0.5 kg geological hammer strikes the coal wall at an angle of 45° until the surface layer of the coal wall peels off and a fresh cross-section appears. At the same time, obtain 25 kg of original coal blocks from the fresh cross-section, and the size of the original coal blocks is 35 cm * 32 cm * 30 cm. Further, cut the original coal block with a bridge stone cutting machine, where the feed speed is set to 2 mm / s and the cooling water flow rate is set to 5 L / min to obtain the cut coal block, and the size of the cut coal block is 100.2 mm * 99.8 mm * 100.1 mm.

[0079] Step S1.2: Buffer medium filling. That is, drill a central hole along the bedding plane direction of the cut coal block, and fill a soluble stress buffer medium at the bottom of the drilled central hole. Specifically, place the cut coal block in a three-dimensional positioning fixture, and correct the position through a dial indicator. At the same time, start the drilling machine and drill a hole towards the center of the cut coal block. The diameter of the central hole is 10 mm and the depth is 50 mm. After drilling is completed, clean and blow it through a wire brush and a high-pressure air gun to ensure the internal cleanliness of the central hole.

[0080] During the specific implementation process, place the cut coal block with dimensions of 100.2 mm * 99.8 mm * 100.1 mm in a three-dimensional positioning fixture, and use a dial indicator with an accuracy of 0.01 mm for calibration, so that the flatness deviation of the upper surface of the cut coal block is not greater than 0.02 mm / 100 mm, and the perpendicularity between the drilling axis and the bedding plane is not greater than 0.5°. At the same time, the three-dimensional positioning fixture clamps and fixes the cut coal block with a lateral clamping force of 8 - 10 N.

[0081] Furthermore, use a Sherline 5400 type bench precision drilling machine to drill a central hole in the fixed cut coal block. Specifically, at the initial stage of drilling, the drill bit contacts the coal sample surface at a rate of 0.05 mm / s, and when the pressure gauge reaches 5 N, the drill bit starts formal drilling. During the formal drilling process, use a feed rate of 0.1 mm / s, that is, the drill bit rotates at a speed of 300 rpm for central drilling. After drilling is completed, use a wire brush with a diameter of 11.8 mm to reciprocally clean the central hole, and at the same time, use a high-pressure air gun to blow the inside of the central hole at a pressure of 0.5 MPa to remove the residues in the hole.

[0082] In this embodiment, mix the raw material NaCl and the raw material SiO2 to obtain a mixed particle filler, and at the same time, through hierarchical compaction, fill the obtained mixed particle filler into the inside of the central hole in stages.

[0083] During the specific implementation process, set the mass ratio between the raw material NaCl and the raw material SiO2 to 19:1. At the same time, dry mix the raw material NaCl and the raw material SiO2 in a mixer at a rate of 60 rpm for 15 minutes, add 3 ml of 0.5% silane coupling agent ethanol solution, and continue to mix for 10 minutes. Then, dry it at a temperature of 80 °C for 2 hours until the moisture content of the obtained mixed particle filler is not greater than 0.1%.

[0084] Furthermore, during the hierarchical filling process in this embodiment, it is filled in two stages, specifically:

[0085] During the first-stage filling, the obtained mixed granular filler is introduced into the inner part of the central hole through a powder filling funnel and a cemented carbide punch, and pre-pressing is carried out. Specifically, pre-pressing is carried out at an initial pressure of 0.5 MPa, and at the same time, stage pressurization is carried out until the pressure is increased to 2 MPa and the pressure is maintained for 60 seconds until the compaction thickness is 5 mm.

[0086] During the second-stage filling, the obtained mixed granular filler is continuously introduced into the inner part of the central hole through a powder filling funnel and a punch with an exhaust groove structure, and compaction is carried out. Specifically, repeated pressurization and depressurization are carried out at a pressure of 5 MPa until the total filling thickness is 10 mm.

[0087] Step S1.3: Consolidation of the planting bar adhesive. That is, in the central hole filled with the mixed granular filler in step S1.2, a coal matrix modified planting bar adhesive is added, and at the same time, a micro-convex threaded steel pipe is implanted into the inner part of the central hole. Specifically, after mixing epoxy resin, curing agent and pulverized coal, a coal matrix modified planting bar adhesive is obtained. At the same time, the micro-convex threaded steel pipe is implanted into the inner part of the central hole through the coal matrix modified planting bar adhesive.

[0088] In this embodiment, the epoxy resin, curing agent and pulverized coal are configured in a mass ratio of 4:1:1 to obtain a coal matrix modified planting bar adhesive, specifically as follows:

[0089] Step SA1.3.1: Premixing of pulverized coal. That is, the epoxy resin and pulverized coal are premixed by a planetary mixer until the pulverized coal is evenly dispersed. Specifically, in a constant temperature environment of 25 °C, 80 g of epoxy resin and 20 g of pulverized coal are premixed by a planetary mixer at a rotation speed of 300 rpm for 10 minutes until the pulverized coal is evenly dispersed. It should be noted that during the premixing process, the dispersion degree of the pulverized coal is measured by a rotational viscometer until its dispersion degree reaches the preset target dispersion degree. In this embodiment, until the dispersion degree of the pulverized coal reaches 850 cP, premixed pulverized coal is obtained.

[0090] Step SA1.3.2: Adding the curing agent. That is, 20 g of the curing agent is added to the premixed pulverized coal obtained in step S1.3.1, and the mixture is continuously stirred by a planetary mixer at a rotation speed of 300 rpm for 5 minutes to ensure that there are no bubbles in the material, and a mixed colloid is obtained.

[0091] Step SA1.3.3: Defoaming treatment. That is, the mixed colloid obtained in step S1.3.2 is transferred to a vacuum defoaming machine and degassed for 5 minutes to obtain a coal matrix modified planting bar adhesive.

[0092] In this embodiment, the micro-convex threaded steel pipe is implanted into the inner part of the central hole through the obtained coal matrix modified planting bar adhesive, specifically as follows:

[0093] Step SB1.3.1: Steel pipe pretreatment. That is, the stainless steel pipe is subjected to thread machining. The pitch of the external thread is set to 2 mm, the thread angle is set to 60°, and the protrusion height is set to 0.5 mm. The internal thread is set as a taper seal thread, and the taper angle is set to 5° for connecting the fracturing pipeline. Further, the steel pipe after thread machining is immersed in pure acetone for ultrasonic cleaning, and after ultrasonic cleaning at a frequency of 40 kHz for 10 minutes, it is dried in a hot air circulation oven at a temperature of 80 °C for 15 minutes.

[0094] Step SB1.3.2: Insert the steel pipe. That is, apply PTFE tape to the end of the steel pipe pretreated in Step SB1.3.1. It should be noted that during the process of wrapping the PTFE tape, 3 layers of PTFE tape are wrapped around the insertion end of the pretreated steel pipe to ensure its sealing performance.

[0095] Further, the coal matrix modified reinforcing glue obtained in Step SA1.3.3 is injected into the interior of the glue gun, and through the glue gun, the coal matrix modified reinforcing glue is injected into the interior of the central hole. At the same time, the insertion end of the pretreated steel pipe wrapped with PTFE tape is inserted into the interior of the central hole until the top of the pretreated steel pipe is flush with the surface of the coal sample.

[0096] It should be noted that when injecting the coal matrix modified reinforcing glue into the interior of the central hole, the injection amount of the coal matrix modified reinforcing glue is determined according to the volume of the central hole and the volume of the steel pipe, specifically:

[0097] ,

[0098] where: is the volume of the injected coal matrix modified reinforcing glue, is the volume of the central hole, is the volume of the steel pipe after pretreatment.

[0099] Step SB1.3.3: Curing and maintenance. That is, at a constant temperature of 25 °C, the coal sample implanted with the steel pipe is placed horizontally for at least 48 hours for curing and maintenance.

[0100] Step S1.4: Quality verification. That is, the cured and maintained coal sample is scanned by a CT scanner to obtain the pore area at the central hole, and the corresponding reinforcing filling density is determined, specifically:

[0101] ,

[0102] where: is the percentage of the pore volume in the total filling volume, is the void volume within the filling area of the coal matrix modified reinforcing glue, is the geometric volume of the filling area of the coal matrix modified rebar adhesive.

[0103] In the process of specific implementation, the cured coal sample is scanned by a CT scanner to obtain 5000 pore pixels and a voxel size of 9.8 μm. Then the void volume in the filling area of the coal matrix modified rebar adhesive is: 5000 * (9.8 * 10 -3 ) 3 = 0.47 mm 3 . Further, the geometric volume of the filling area of the coal matrix modified rebar adhesive is 1900 mm 3 , and the corresponding rebar filling density is 0.025%.

[0104] Further, the obtained rebar filling density is compared with the preset density threshold, and according to the comparison result, the final coal sample is determined. Specifically:

[0105] When the obtained rebar filling density is greater than the preset density threshold, steps S1.3 and S1.4 are repeated to re-consolidate the rebar adhesive until the obtained rebar filling density is not greater than the preset density threshold. Otherwise, the obtained coal sample is the final coal sample.

[0106] Refer to Figure 1 , Figure 1 which is the CT scan pore distribution map of the coal sample in this embodiment. It can be seen from Figure 1 that: Figure 1 The pore distribution in

[0107] is uniform and there is no obvious aggregation phenomenon. Therefore, the rebar filling density in this embodiment meets the standard, that is, the coal matrix modified rebar adhesive is tightly combined with the coal body and meets the requirements of the preset density threshold.

[0108] Step S2: True triaxial loading and temperature control. That is, through the true triaxial pressure chamber, the final coal sample obtained in step S1.4 is loaded, and temperature control compensation is carried out during the loading process. Specifically as follows:

[0109] Step S2.1: Specimen installation. That is, the final coal sample obtained in step S1.4 is set in the true triaxial pressure chamber, and a buffer layer is set between the final coal sample and the cushion block in the true triaxial pressure chamber, that is, a buffer layer with a three-layer structure of silicone rubber gasket - carbon fiber grid - polyether ether ketone plate is constructed.

[0109] In this embodiment, the surface of the cushion block of the true triaxial pressure chamber is wiped with absolute ethanol to ensure no particle residue. Meanwhile, on the surface of the wiped cushion block, a silicone rubber gasket, a carbon fiber grid, and a polyether ether ketone plate are laid in sequence. Specifically, the silicone rubber gasket is laid on the surface of the wiped cushion block, and bubbles are removed by pressing with hands. At the same time, the carbon fiber grid is placed on the upper end of the silicone rubber gasket with tweezers until it completely covers the silicone rubber gasket, and the polyether ether ketone plate is installed on the upper end of the carbon fiber grid, thereby obtaining a buffer layer.

[0110] Furthermore, the steel pipe in the final coal sample is inserted into the central hole of the buffer layer, and the final coal sample and the buffer layer are positioned and aligned by a laser locator. At the same time, the final coal sample and the cushion block of the true triaxial pressure chamber are fixed by fixing bolts.

[0111] Reference Figure 2 , Figure 2 is the distribution diagram of the compaction characteristics of the coal sample in this embodiment. It can be seen from Figure 2 that: the thickness of the buffer layer decreases with the increase of the compaction pressure, and at a pressure of 5 MPa, the compaction thickness is stable at about 0.8 mm.

[0112] Step S2.2: Stress loading. That is, in the true triaxial pressure chamber, the final coal sample is stress-loaded in stages through a hydraulic pump and sensors. Specifically as follows:

[0113] Step S2.2.1: Preloading. That is, the high-pressure hydraulic pump is connected to the triaxial independent oil circuits to ensure that each direction pressure can be independently controlled. Specifically, through manual adjustment, the piston is slowly brought into contact with the final coal sample until the reading shown on the pressure gauge is 0.1 MPa, thereby eliminating the mechanical gap between the piston and the final coal sample.

[0114] Furthermore, the high-pressure hydraulic pump is set to the synchronous loading mode, and the three-direction stresses are synchronously increased at the same rate. At the same time, the readings shown on the three-direction pressure gauges are monitored in real time. When the three-direction stresses all reach the preset initial stress, that is, when the three-direction stresses all reach 1 MPa, it is maintained for 20 minutes, and the fluctuating changes of the stresses are determined according to the magnitudes of the obtained three-direction stresses. And when the stress fluctuation reaches a stable state, the preloading stage is completed.

[0115] Step S2.2.2: Main loading. That is, the vertical stress, the maximum horizontal principal stress, and the minimum horizontal principal stress of the high-pressure hydraulic pump are synchronously increased in equal proportion to the preset loading stress value. At the same time, based on the preset loading stress value as the base stress value, the three-direction stress synchronous loading of the maximum horizontal principal stress, the minimum horizontal principal stress, and the vertical stress is carried out.

[0116] Specifically, the high-pressure hydraulic pump is set to the single-axis priority mode, and the vertical stress is loaded at a set rate, with a 10-second pause for every 1 MPa of loading. Meanwhile, an acoustic emission sensor is used to monitor the micro-cracking signals of the final coal sample. After the stress is loaded to 10 MPa, a constant load is applied for 5 minutes, the strain value during the constant loading is obtained, and it is determined whether the obtained strain value data converges. Specifically, when the strain value data converges, a gradient loading of the maximum horizontal principal stress, the minimum horizontal principal stress, and the vertical stress is synchronously carried out based on the 10 MPa as the base stress value. Conversely, when the strain value data does not converge, the constant loading continues until the strain value data converges.

[0117] Step S2.3: Temperature control. That is, according to the stress value of the vertical stress, the built-in electric heating tube is started for temperature adjustment, and the temperature fluctuation is maintained by a PID controller. Specifically, the annular electric heating tube is wound around the metal shell loaded with the final coal sample. Meanwhile, according to the relationship formula between the vertical stress and the formation temperature, the target temperature is obtained, and temperature adjustment is carried out through the annular electric heating tube and the target temperature.

[0118] In this embodiment, the formula for obtaining the target temperature is specifically:

[0119] ,

[0120] Where: is the target temperature, is the initial temperature, is the temperature-stress coupling coefficient, is the current vertical stress value.

[0121] Refer to Figure 3 , Figure 3 is the distribution diagram of triaxial stress loading and temperature response in this embodiment. It can be seen from Figure 3 that: the three-way stresses increase synchronously, and the vertical stress is dominant. Therefore, the true triaxial loading and temperature control process in this embodiment conforms to the stress characteristics of deep coal reservoirs. At the same time, the horizontal stress ratio is fixed, and the temperature increases linearly with the minimum horizontal stress. Thus, the temperature control of the annular electric heating tube can effectively achieve dynamic compensation. Further, under the high stress of 30 MPa, the temperature fluctuation range is consistent with the actual downhole working conditions, and there is no sudden change in the stress-temperature curve. Therefore, the buffer layer in this embodiment can effectively disperse stress concentration.

[0122] Refer to Figure 4 , Figure 4 is the stress distribution nephogram of the buffer layer in this embodiment. It can be seen from Figure 4It can be known that the minimum value of the central stress concentration coefficient is 0.5, and the maximum value at the edge is 1.2. Therefore, the stress gradually decreases from the center to the edge, and the gradient change is gentle. That is to say, the setting of the three-layer buffer layer can effectively disperse the stress.

[0123] Step S3: Fracturing implementation and monitoring. That is, by using pure water, KCl solution, and a flowback aid, a fracturing fluid is configured, and through the fracturing fluid and a constant flow pump, the displacement requirements of downhole fracturing operations are simulated. At the same time, through sensors installed on the outer wall of the pressure chamber, fractures in the coal sample are determined, and the three-dimensional expansion path of the fractures is captured in real time by a CT scanner. Specifically as follows:

[0124] Step S3.1: Injection of fracturing fluid. That is, by using pure water, KCl solution, and a flowback aid, a fracturing fluid is configured, and at the same time, the fracturing fluid is injected through an intelligent fracturing fluid injection unit. Specifically as follows:

[0125] Step S3.1.1: Configuration of fracturing fluid. That is, the mass ratio between pure water and KCl solution is set at 1:0.2%, and the volume ratio between pure water and the flowback aid is set at 1:0.1%. At the same time, pure water, KCl solution, and the flowback aid are mixed and stirred to obtain the fracturing fluid.

[0126] During the specific implementation process, while stirring, 36 g of KCl solution is added to 18 L of pure water, and then 18 ml of the flowback aid is slowly added, and magnetic stirring is carried out at a speed of 300 rpm for 30 minutes.

[0127] Step S3.1.2: Pipeline exhaust treatment. That is, the outlet ball valve of the fracturing fluid is closed, the plunger pump is started, and circulating flushing is carried out at a preset flow rate. At the same time, through a transparent polycarbonate observation window, it is determined whether there are any bubbles remaining in the fracturing fluid in the pipeline.

[0128] Furthermore, after closing the circulation pump and allowing it to stand, the exhaust valve is opened. After determining that no gas is ejected, the fracturing fluid is extracted from the sampling port using a medical syringe. Specifically, when there are no bubbles in the extracted fracturing fluid, the pipeline exhaust is completed. Otherwise, corresponding treatments are carried out according to the abnormal conditions. Specifically:

[0129] When there are continuously large bubbles and the pump head seal ring is worn, the new seal ring is soaked in hydraulic oil for 30 minutes and then the pump head seal assembly is replaced.

[0130] When there are continuously large bubbles and there is gas accumulation at the high point of the pipeline, the high-pressure exhaust mode is started, and the outlet valve is intermittently switched.

[0131] When air leakage is detected by negative pressure, the ferrule of each quick-connect joint is replaced, and at the same time, the observation window seal ring is smeared with silicone grease and then tightened again.

[0132] Step S3.1.3: Flow control. That is, according to the preset flow value, the fracturing fluid is injected through a constant flow pump to simulate the displacement requirement of downhole fracturing operations. Meanwhile, the liquid flow rate of the fracturing fluid is obtained through a flow meter, and the real-time pressure in the wellbore is obtained through a pressure sensor.

[0133] Furthermore, the real-time pressure in the wellbore is compared with the preset pressure threshold, and the liquid flow rate of the fracturing fluid is adjusted according to the comparison result. Specifically:

[0134] When the real-time pressure in the wellbore is not greater than the preset pressure threshold, the current operating state of the constant flow pump is maintained. Otherwise, the constant flow pump is directly stopped, and at the same time, according to the currently obtained liquid flow rate, the liquid flow rate of the fracturing fluid is adjusted to obtain a new adjusted liquid flow rate. Specifically:

[0135] ,

[0136] Where: is the new adjusted flow rate, is the current initial flow rate, is the base of the natural logarithm, is the duration of pressure fluctuation, is the time constant.

[0137] Step S3.2: Fracture monitoring. That is, a plurality of sensors are evenly arranged on the outer wall of the pressure chamber. Meanwhile, according to the propagation speed of sound waves in the final coal sample and the positions and time differences of each sensor, the position coordinates of the fracture point are determined. Specifically:

[0138] ,

[0139] Where: is the X-axis coordinate of the acoustic emission sensor, is the Y-axis coordinate of the acoustic emission sensor, is the Z-axis coordinate of the acoustic emission sensor, is the X-axis coordinate of the fracture point, is the Y-axis coordinate of the fracture point, is the Z-axis coordinate of the fracture point, is the propagation speed of sound waves in the final coal sample, is the time difference of the i-th sensor.

[0140] Furthermore, according to the cumulative amount of the injected fracturing fluid, the coal sample is scanned by a CT scanner to obtain the corresponding CT image. Meanwhile, according to the obtained CT image, a corresponding three-dimensional model is generated.

[0141] Embodiment 2

[0142] This embodiment provides a physical simulation method for true triaxial hydraulic fracturing of deep coal reservoirs. The specific implementation method is the same as that of Embodiment 1, except that in step S2.2.1, according to the stable state of stress fluctuation, the completion state of the preloading stage is determined. The following is an example of the present invention in combination with the specific implementation manner of this embodiment.

[0143] In this embodiment, according to the stable state of stress fluctuation, the completion state of the preloading stage is determined as follows:

[0144] Step W1: Determine the preloading state. That is, through the three-way pressure gauge, obtain the real-time pressure value, compare the obtained real-time pressure value with the preset pressure value, and determine the preloading state according to the comparison result. Specifically:

[0145] When the real-time pressure value is the same as the preset pressure value, that is, when the display readings of the three-way pressure gauge are all 1 MPa, the preloading is completed. At the same time, when the preloading is completed, record the corresponding initial strain value. Otherwise, continue to apply pressure until the obtained real-time pressure value is the same as the preset pressure value.

[0146] Step W2: Determine the fluctuation threshold. That is, according to the initial strain value obtained in step W1, obtain the corresponding strain change amount threshold. Specifically:

[0147] ,

[0148] Where: is the strain change amount threshold, is the initial strain value.

[0149] In the process of specific implementation, when the initial strain value is 502 με, the corresponding strain change amount threshold is 0.025 με.

[0150] Step W3: Determine the stress fluctuation state. That is, after the preloading is completed, obtain the strain values at different times at intervals. At the same time, according to the strain values at different times and the strain change amount threshold in step W2, conduct longitudinal strain comparison and transverse strain comparison respectively. And determine the strain fluctuation state according to the comparison result. Specifically as follows:

[0151] Step W3.1: Longitudinal comparison. That is, according to the three-way initial strain values obtained in step W1, obtain the strain change difference between the strain values at different times and the initial strain value. At the same time, compare the obtained strain change difference with the strain change amount threshold in step W2, and determine whether there is a difference in the strain fluctuation state according to the comparison result. Specifically:

[0152] When the obtained strain change difference is greater than the strain change amount threshold, the strain fluctuation state is abnormal, and step W4 is executed. When the obtained strain change difference is not greater than the strain change amount threshold, the strain fluctuation state is normal.

[0153] Step W3.2: Lateral comparison. That is, according to the strain values obtained at different times, a three-way strain-time curve is plotted, and based on the trend change of the three-way strain-time curve, it is determined whether the strain fluctuation state is abnormal. Specifically:

[0154] When the three curves in the three-way strain-time curve rise or fall in parallel, the strain fluctuation state is normal. Otherwise, the strain fluctuation state is abnormal, and step W4 is executed.

[0155] Step W4: Abnormal handling. That is, according to the abnormal strain fluctuation state determined in step W3, corresponding abnormal handling is carried out. Specifically as follows:

[0156] Step W4.1: Strain drift handling. That is, when the obtained strain change difference is greater than the strain change amount threshold, the high-pressure hydraulic pump system is inspected to determine whether the displayed reading of the pressure gauge is consistent with the preset initial stress, and whether there is oil stain at the inspection joint between the high-pressure hydraulic pump and the three-axis independent oil circuit. Specifically, when the displayed reading is inconsistent with the preset initial stress, the pressure is continuously applied to make the displayed reading of the pressure gauge consistent with the preset initial stress. When there is oil stain at the inspection joint between the high-pressure hydraulic pump and the three-axis independent oil circuit, the existing oil stain is removed.

[0157] Furthermore, strain gauge verification is carried out to measure the resistance value of the strain gauge with a multimeter, and at the same time, based on the comparison between the measured resistance value of the strain gauge and the normal resistance value range of the strain gauge, the state of the strain gauge is determined and processed. Specifically:

[0158] When the resistance value of the strain gauge is not within the normal resistance value range of the strain gauge, the state of the strain gauge is debonding, that is, the old glue layer is removed, the surface is cleaned with acetone, and at the same time, the strain gauge is re-pasted. Otherwise, the state of the strain gauge is not debonding, that is, the stable time of preloading is extended.

[0159] Step W4.2: Unidirectional abnormal handling. That is, when the three curves in the three-way strain-time curve do not rise or fall in parallel, the fine-tuning knob of the oil pressure valve for the minimum horizontal principal stress is manually adjusted to return the strain value to the normal range. At the same time, a dial indicator is installed, and the probe of the dial indicator is brought into contact with the surface of the minimum horizontal principal stress of the final coal sample, and the corresponding displacement change amount is obtained. Furthermore, based on the obtained displacement change amount, the corresponding theoretical strain value is determined. Specifically:

[0160] ,

[0161] Wherein: is the theoretical strain value, is the deformation amount in the length direction of the final coal sample obtained by the dial gauge, is the original gauge length of the final coal sample.

[0162] Furthermore, compare the actually obtained strain value with the theoretical strain value to obtain the strain difference, and compare the obtained strain difference with the preset difference threshold. At the same time, based on the comparison result, determine the data abnormal state of the strain value. Specifically:

[0163] When the obtained strain difference is greater than the preset difference threshold, the strain value corresponding to the strain difference is abnormal. That is, detect the oil pressure valve and the sensor. When the oil pressure valve fails, directly replace the oil pressure valve. When the sensor fails, enable the standby strain channel. Otherwise, the strain value corresponding to the strain difference is normal.

[0164] Although the embodiments of the present invention have been shown and described, those of ordinary skill in the art can understand that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principle and spirit of the present invention. The scope of the present invention is defined by the appended embodiments and their equivalents.

Claims

1. A physical simulation method for true triaxial hydraulic fracturing of deep coal reservoirs, characterized in that, It includes: S1: Coal sample preparation and consolidation: Fill soluble stress buffer medium, coal matrix modified reinforcing glue and micro-convex threaded steel pipes into the interior of low-fracture coal blocks to obtain coal samples; S2: True triaxial loading and temperature control: Load the coal samples through a true triaxial pressure chamber while performing temperature control compensation; S2.1: Specimen installation: Lay a silicone rubber gasket, a carbon fiber grid and a polyetheretherketone plate on the surface of the cushion block in the true triaxial pressure chamber in sequence to construct a buffer layer. At the same time, insert the steel pipe in the coal sample into the central hole of the buffer layer, and fix the coal sample with the cushion block in the true triaxial pressure chamber through fixing bolts; S2.2: Stress loading: In the true triaxial pressure chamber, stress load the coal samples in stages through a hydraulic pump and sensors; S2.3: Temperature control: Wind a ring-shaped electric heating pipe around the metal shell loaded with the coal sample, and obtain the target temperature according to the relationship formula between the vertical stress and the formation temperature. At the same time, perform temperature regulation through the ring-shaped electric heating pipe and the target temperature. The formula for obtaining the target temperature is specifically: ， Wherein: is the target temperature, is the initial temperature, is the temperature-stress coupling coefficient, is the current vertical stress value; S3: Fracturing implementation and monitoring: Simulate the displacement requirements of downhole fracturing operations through the configured fracturing fluid and a constant flow pump. At the same time, determine the fractures in the coal samples through sensors, and obtain the three-dimensional expansion path of the fractures through a CT scanner.

2. The physical simulation method of true triaxial hydraulic fracturing for deep coal reservoirs according to claim 1, characterized in that The obtained coal samples include: S1.1: Coal block pretreatment: Collect original coal blocks from the coal wall with a geological hammer and cut the original coal blocks to obtain cut coal blocks; S1.2: Buffer medium filling: Drill a central hole in the cut coal block along the bedding plane direction of the cut coal block, and fill soluble stress buffer medium at the bottom of the central hole; S1.3: Reinforcing glue consolidation: Add coal matrix modified reinforcing glue into the interior of the central hole filled with the soluble stress buffer medium, and implant a micro-convex threaded steel pipe into the interior of the central hole to obtain an initial coal sample; S1.4: Quality verification: Scan the initial coal sample with a CT scanner to obtain the pore area of the central hole, determine the filling density of the reinforcement, and compare the filling density of the reinforcement with a preset density threshold. According to the comparison result, determine the final coal sample, specifically: When the filling density of the reinforcement is greater than the preset density threshold, repeat steps S1.3 and S1.4 to re-obtain the initial coal sample until the filling density of the reinforcement is not greater than the preset density threshold. Otherwise, the initial coal sample is the final coal sample.

3. A true triaxial hydraulic fracturing physical simulation method for deep coal reservoirs according to claim 2, characterized in that Mix raw material NaCl, raw material SiO2 and a silane coupling agent ethanol solution to obtain a soluble stress buffer medium, and fill the soluble stress buffer medium into the bottom of the central hole in stages, specifically: Introduce the soluble stress buffer medium into the interior of the central hole through a powder filling funnel and a cemented carbide punch, and the thickness after compaction is not less than 5 mm for the first-stage filling; Through the powder filling funnel and the punch with a vent groove structure, the soluble stress buffer medium is continuously introduced into the inner part of the central hole after the first-stage filling, and the total thickness after compaction is not less than 10 mm, and the second-stage filling is carried out.

4. A true triaxial hydraulic fracturing physical simulation method for deep coal reservoirs according to claim 2, characterized in that Prepare the coal matrix modified rebar glue through epoxy resin, curing agent and pulverized coal, including: SA1.3.1: Premixing of pulverized coal: Stir and mix the epoxy resin and pulverized coal until the pulverized coal is evenly dispersed to obtain premixed pulverized coal. SA1.3.2: Adding curing agent: Add the curing agent to the premixed pulverized coal and continue to stir and mix to obtain a mixed colloid. SA1.3.3: Defoaming treatment: Subject the mixed colloid to degassing treatment by a vacuum degassing machine to obtain the coal matrix modified rebar glue.

5. A true triaxial hydraulic fracturing physical simulation method for deep coal reservoirs according to claim 2, characterized in that Insert a micro-convex threaded steel pipe into the inner part of the central hole, including: SB1.3.1: Pretreatment of the steel pipe: After threading the stainless steel pipe, perform ultrasonic cleaning with pure acetone and simultaneously perform drying treatment to obtain the pretreated steel pipe. SB1.3.2: Inserting the steel pipe: Wrap the end of the pretreated steel pipe with raw tape. At the same time, after injecting the coal matrix modified rebar glue into the inner part of the central hole, insert the end of the steel pipe wrapped with the raw tape into the inner part of the central hole until the top of the pretreated steel pipe is flush with the surface of the cut coal block. SB1.3.3: Curing and maintenance: Curing and maintaining the cut coal block implanted with the steel pipe to obtain the initial coal sample.

6. The physical simulation method of true triaxial hydraulic fracturing for deep coal reservoirs according to claim 1, characterized in that Apply stress loading to the coal sample in stages, including: S2.2.1: Preloading: Connect the high-pressure hydraulic pump to the three-axis independent oil circuit, and set the high-pressure hydraulic pump to the synchronous loading mode. Synchronously increase the three-way stress to the preset initial stress, obtain the fluctuation change of the stress, and determine the completion state of the preloading stage according to the fluctuation change. S2.2.2: Main loading: Increase the vertical stress, maximum horizontal principal stress and minimum horizontal principal stress of the high-pressure hydraulic pump proportionally and synchronously to the preset loading stress value, and perform constant loading to obtain the strain value data. At the same time, determine the state of constant loading according to the convergence state of the strain value data. Specifically: When the strain value data converges, then perform gradient loading of the maximum horizontal principal stress, minimum horizontal principal stress and vertical stress according to the preset loading stress value. Otherwise, continue to perform constant loading until the strain value data converges.

7. A true triaxial hydraulic fracturing physical simulation method for deep coal reservoirs according to claim 6, characterized in that Determine the completion state of the preloading stage according to the fluctuation change, including: W1: Determine the preloading state: Obtain the real-time pressure value through a three-way pressure gauge, compare the real-time pressure value with the preset pressure value, and determine the preloading state according to the comparison result. Specifically: When the real-time pressure value is the same as the preset pressure value, the preloading is completed, and the initial strain value is obtained. Otherwise, continue to perform pressure loading until the real-time pressure value is the same as the preset pressure value. W2: Determine the fluctuation threshold: Obtain the strain change amount threshold according to the initial strain value. Specifically: ， Wherein: is the strain change amount threshold value, is the initial strain value; W3: Determine the stress fluctuation state: After the preloading is completed, a constant load is applied, and the strain values at different times are obtained. At the same time, according to the strain values and the strain change amount threshold, longitudinal strain comparison and transverse strain comparison are respectively carried out, and according to the comparison results, the strain fluctuation state is determined; W4: Abnormality handling: According to the strain fluctuation state, strain drift handling and unidirectional abnormality handling are carried out.

8. The physical simulation method for true triaxial hydraulic fracturing of deep coal reservoirs according to claim 7, wherein The determined strain fluctuation state includes: W3.1: Longitudinal comparison: According to the initial strain value, obtain the strain change difference between the strain values at different times and the initial strain value, and compare the strain change difference with the strain change amount threshold. At the same time, according to the comparison results, determine the strain fluctuation state, specifically: When the strain change difference is greater than the strain change amount threshold, the strain fluctuation state is abnormal, and the next step is executed; otherwise, the strain fluctuation state is normal; W3.2: Transverse comparison: According to the strain values at different times, draw a three-way strain-time curve, and determine the strain fluctuation state according to the trend change of the three-way strain-time curve, specifically: When the three curves in the three-way strain-time curve rise or fall in parallel, the strain fluctuation state is normal; otherwise, the strain fluctuation state is abnormal, and the next step is executed.

9. The physical simulation method for true triaxial hydraulic fracturing of deep coal reservoirs according to claim 1, characterized in that Obtaining the three-dimensional expansion path of the crack includes: S3.1: Fracturing fluid injection: Configure the fracturing fluid with pure water, KCl solution and a drainage aid, and inject the fracturing fluid through the intelligent fracturing fluid injection unit; S3.2: Crack monitoring: Uniformly arrange a plurality of sensors on the outer wall of the pressure chamber. At the same time, according to the propagation speed of sound waves in the coal sample, the positions of each sensor and the time difference, determine the position coordinates of the rupture point. At the same time, obtain the CT image of the coal sample through a CT scanner, and obtain a three-dimensional model according to the CT image of the coal sample and the position coordinates of the rupture point; The position coordinates of the rupture point are specifically: ， Wherein: is the X-axis coordinate of the acoustic emission sensor, is the Y-axis coordinate of the acoustic emission sensor, is the Z-axis coordinate of the acoustic emission sensor, is the X-axis coordinate of the rupture point, is the Y-axis coordinate of the rupture point, is the Z-axis coordinate of the rupture point, is the propagation speed of the sound wave in the final coal sample, is the time difference of the i-th sensor.

10. A true triaxial hydraulic fracturing physical simulation method for deep coal reservoirs according to claim 9, characterized in that, Injecting the fracturing fluid through the intelligent fracturing fluid injection unit includes: S3.1.1: Configure the fracturing fluid: Stir and mix pure water and KCl solution, add a drainage aid, and obtain the fracturing fluid after magnetic stirring; S3.1.2: Pipeline exhaust treatment: Close the outlet ball valve of the fracturing fluid, start the plunger pump, and perform circulating flushing at a preset flow rate. At the same time, through the transparent polycarbonate observation window, ensure that there are no bubbles in the fracturing fluid in the pipeline; S3.1.3: Flow control: According to the preset flow value, inject the fracturing fluid through a constant flow pump. At the same time, compare the real-time pressure in the wellbore with the preset pressure threshold, and adjust the liquid flow rate of the fracturing fluid according to the comparison results, specifically: When the real-time pressure in the wellbore is not greater than the preset pressure threshold, maintain the current operating state of the constant flow pump; otherwise, stop the constant flow pump, and at the same time adjust the liquid flow rate of the fracturing fluid according to the current liquid flow rate to obtain a new adjusted liquid flow rate, specifically: ， Wherein: is the new adjusted flow rate, is the current initial flow rate, is the base of the natural logarithm, is the pressure fluctuation duration, is the time constant.

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