Physical simulation experiment method for hydraulic fracturing crack propagation under excavated roadway condition

Abstract

The invention discloses a physical simulation experiment method for hydraulic fracturing crack propagation under a dug roadway condition. The method comprises the following steps: determining relative positions, thicknesses and size parameters of an overlying strata, a horizontal well, a coal seam and a roadway under actual fracturing working conditions and geological conditions; the obtained parameters are equivalently proportioned into the test piece model; determining the ratio of each material in the test piece model; manufacturing a physical model similar test piece; calculating and determining the stress required to be loaded in the triaxial direction in the true triaxial hydraulic fracturing physical model experiment process; placing the manufactured and checked similar test piece into true triaxial high-temperature high-stress hydraulic fracturing simulation equipment; fracturing the test piece, and monitoring the crack initiation and expansion rule of a fracturing crack in real time by using acoustic emission; and sectioning the fracturing test piece to observe the flow track of the fracturing fluid and the crack propagation condition. According to the method, whether the hydraulic fracturing can press through the roadway under each given construction parameter in the hydraulic fracturing period under the excavated roadway condition can be determined, and the hydraulic fracturing crack extension rule can be determined, so that the construction parameters in the fracturing process can be optimized in real time.

Description

Technical Field

[0001] The invention belongs to the field of disaster management in coal mining areas and relates to a physical simulation experimental method for hydraulic fracturing crack expansion under excavated tunnel conditions. Background Art

[0002] Hydraulic fracturing technology was initially used to transform oil and gas reservoirs to increase oil and gas production. As staged hydraulic fracturing technology continues to mature and develop, it has gradually been introduced to the management of disasters such as gas and rock burst in coal mining areas. Based on the actual working conditions of hydraulic fracturing technology in coal mining areas, hydraulic fracturing technology is divided into three technical application modes: ground horizontal well hydraulic fracturing technology, underground directional long-hole hydraulic fracturing technology in coal mines, and well-ground combined hydraulic fracturing technology. Among them, the two modes of underground directional long-hole hydraulic fracturing technology in coal mines and the well-ground combined hydraulic fracturing technology must be implemented only when the underground tunnels in coal mines have been excavated when applying them to the management of disasters such as gas and rock burst. They are mainly used in the preparation stage of the entire life cycle of coal mining. For mines with relatively tight mining continuity, in the early stage of using ground horizontal well hydraulic fracturing technology to control gas and rock burst in the coal seam recovery working face, it is possible that all or part of the tunnels around the coal seam recovery working face have been mined. It can mainly be applied to the planning and preparation stages of the entire life cycle of coal mining.

[0003] In summary, the three modes of hydraulic fracturing technology in coal mining areas to control gas, rock burst and other disasters will be applied to the conditions where the tunnels around the coal seam mining face in the coal mine have been excavated. The so-called tunnel excavation conditions refer to the state where the coal seam working face to be treated is adjacent to the main tunnel that has been excavated, and one, two or three tunnels of the return air tunnel, machine tunnel or cut-eye of the coal seam working face to be treated have been excavated. Under the condition that the underground tunnels in the coal mine have been excavated, the process of using segmented hydraulic fracturing technology to control gas and rock burst disasters may cause the hydraulic fracturing cracks to transition and extend through the tunnels, and even cause the fracturing fluid to flood the tunnels. Therefore, in order to effectively prevent the transitional expansion of the fracturing cracks during hydraulic fracturing and press through the tunnels, it is necessary to clarify the expansion law of hydraulic fracturing cracks under the conditions of the excavated tunnels. Among them, similar physical simulation experiments are used as a direct and intuitive way to study the expansion law of cracks in the hydraulic fracturing process. So far, hydraulic fracturing physical simulation experiments have mainly focused on the non-excavation state where the original stress field of the formation is in equilibrium. However, there have never been any hydraulic fracturing physical simulation experiments targeting the non-equilibrium ground stress field under excavation tunnel conditions.

[0004] Therefore, in order to fill the gap in the physical simulation experiment of hydraulic fracturing under the conditions of an excavated roadway, clarify the law of fracture propagation during hydraulic fracturing under the conditions of an excavated roadway, optimize and give appropriate construction parameters during fracturing, and prevent the roadway from being broken through, the present invention proposes a physical simulation experiment method for fracture propagation during hydraulic fracturing under the conditions of an excavated roadway, so as to clarify whether the roadway can be broken through during hydraulic fracturing under each given construction parameter during hydraulic fracturing under the conditions of an excavated roadway, and then optimize the construction parameters during the fracturing process in real time. Summary of the Invention

[0005] Aiming at the deficiencies in the prior art, the purpose of the present invention is to provide a physical simulation experiment method for fracture propagation during hydraulic fracturing under the conditions of an excavated roadway, so as to clarify whether the roadway can be broken through during hydraulic fracturing under each given construction parameter during hydraulic fracturing under the conditions of an excavated roadway, and then optimize the construction parameters during the fracturing process in real time; thereby filling the gap in the physical simulation experiment of hydraulic fracturing under the conditions of an excavated roadway, clarifying the law of fracture propagation during hydraulic fracturing under the conditions of an excavated roadway, optimizing and giving appropriate construction parameters during fracturing, and preventing the roadway from being broken through.

[0006] In order to solve the above technical problems, the present invention is implemented by adopting the following technical solutions:

[0007] A physical simulation experiment method for fracture propagation during hydraulic fracturing under the conditions of an excavated roadway includes the following steps:

[0008] Step 1: Determine the relative position relationship parameters between the overlying strata, horizontal well, coal seam, and roadway, the thickness parameters of the coal seam and overlying strata, and the roadway size parameters under the actual fracturing working conditions and geological conditions;

[0009] Step 2: Scale the parameters obtained in Step 1 to the specimen model in equivalent proportion, and calculate and obtain the relative position relationship parameters between the overlying strata, horizontal well, coal seam, and roadway, the thickness parameters of the coal seam and overlying strata, and the roadway size parameters in the specimen model;

[0010] Step 3: Determine the materials and their ratios used for the coal seam similar specimens and overlying strata similar specimens in the specimen model;

[0011] Step 4: Fabricate a physical simulation similar specimen based on the parameters in Step 2 and the material ratios determined in Step 3;

[0012] Step 5: Check whether the production of the coal seam specimen is successful, check whether the prefabricated roadway is complete through CT scanning, and at the same time perform CT scanning on other overlying strata specimens to record the original fracture development in the specimen before fracturing for comparing the fracture propagation after fracturing;

[0013] Step 6: Calculate and determine the stresses required to be loaded in the three-axis directions during the true triaxial hydraulic fracturing physical model experiment;

[0014] Step 7: Install acoustic emission probes on the test piece to monitor in real time the initiation and propagation of cracks in the rock sample during fracturing;

[0015] Step 8: Place the fabricated and inspected similar test pieces into the true triaxial high-temperature and high-stress hydraulic fracturing simulation equipment;

[0016] Step 9: Put a fluorescent green tracer into the fracturing fluid so that when the similar test piece model is cut open after fracturing to observe the propagation trajectory of the fracturing fluid and the law of crack propagation, the propagation law of the fluorescent green tracer in the sandstone can directly observe the propagation law of the fracturing fluid therein;

[0017] Step 10: Implement the fracturing of the test piece, and conduct experiments on the initiation and propagation laws of fractures during fracturing under different fracturing displacement and volume parameters by using the single control variable method; at the same time, use acoustic emission to monitor in real time the initiation and propagation laws of fracturing cracks during fracturing;

[0018] Step 11: Cut open the test piece after fracturing to observe the flow trajectory of the fracturing fluid and the crack propagation situation: Cut along the roadway trend to observe the crack propagation situation around the roadway rib; cut along the roadway axis to observe the crack propagation situation along the roadway axis.

[0019] The present invention further includes the following technical features:

[0020] Specifically, in the above Step 1, under the actual fracturing working conditions and geological conditions, the overlying strata of the coal seam are overlying stratum I and overlying stratum II in sequence; the horizontal well is arranged in overlying stratum II, and the roadways in the coal seam are roadway I and roadway II; the respective parameters are: the vertical distance H from the horizontal well arranged in overlying stratum II to the projection of the top of the roadway in the coal seam, the vertical distance Q from the horizontal well to the projection of the coal seam roof, and the distances L1 and L2 from the horizontal projection of the horizontal well to the roadway on the plane where the roadway height is located, the distances M1, M2, N1, N2, X1, X2, Y1, Y2 from roadway I and roadway II to the coal seam floor, left and right, front and back boundaries, and the thicknesses C, F1, F2 of the coal seam, overlying stratum I and overlying stratum II.

[0021] Specifically, step 2 includes: Based on the relative position relationships of the horizontal well, roadway I, roadway II, coal seam, overlying stratum I, and overlying stratum II under the actual working conditions and geological conditions obtained in step 1, as well as the thickness parameters of the coal seam, overlying stratum I, and overlying stratum II, and the sizes of roadway I and roadway II; then design the same equivalent similarity ratio to scale the horizontal well, roadway I, roadway II, coal seam, overlying stratum I, and overlying stratum II under the actual working conditions and geological conditions to a 300*300*300 mm specimen size model; calculate and obtain various parameters including: the vertical distance h from the horizontal well arranged in overlying stratum II to the projection of the top of the roadway in the coal seam, the vertical distance q from the horizontal well to the projection of the coal seam roof, and the distances l1 and l2 from the horizontal projection of the horizontal well to the roadway on the plane where the roadway height is located, the distances m1, m2, n1, n2, x1, x2, y1, y2 from roadway I and roadway II to the coal seam floor and the left and right boundaries of the model, and the thicknesses c, f1, f2 of the coal seam, overlying stratum I, and overlying stratum II.

[0022] Specifically, in step 3, select pulverized coal, cement, mudstone powder, gypsum, and quartz sand materials to make artificial coal seams and their overlying rock similar specimens for physical simulation of fracturing; step 3 includes: According to the test results of the mechanical parameters of the simulated specimens under different material ratios, determine the material ratio method, and at the same time, based on this material ratio method, the mechanical parameters of the fabricated simulated specimens are the same as or close to the actual geological parameters, and then prepare the specimens for physical simulation of fracturing according to this material ratio method; design that the mudstone adopts a similar ratio of mudstone powder, cement, and gypsum, the sandstone adopts a similar ratio of cement and sand, and the coal adopts a similar ratio of pulverized coal, cement, and gypsum.

[0023] Specifically, in step 4, use a 300*300*300 mm mold to make it. The mold is composed of five square steel plates of 300*300 mm combined by external clamps. Small cylindrical protrusions are reserved on the mold used to make the rock sample similar specimens. The small cylindrical protrusions will leave small round holes on the fabricated specimens for installing acoustic emission probes. Square small protrusions are arranged in parallel on the mold at the position of the small cylindrical protrusions for prefabricating the wire grooves on the similar specimens.

[0024] Specifically, step 4 includes:

[0025] Step 4.1: Precast a coal seam specimen with an excavated roadway: Add the prepared coal seam material into a 300*300*300 mold. When the flat height of the material in the mold reaches m1, according to the positional distance relationship between roadway I and the boundaries n1, x1, y1 of the model and the coal seam floor boundary m1, place a plastic model into the mold as roadway I; continue to add material and level it. When the flat height of the material in the mold reaches m2, according to the positional distance relationship between roadway II and the boundaries n2, x2, y2 of the model and the coal seam floor boundary m2, place a plastic model into the mold as roadway II; continue to add material and level it. After the material height reaches c, place a pressing plate and apply a pressure of 0.5 Mpa on the pressing plate. After the material is coagulated for 14 days, the precast coal seam specimen with an excavated roadway is completed;

[0026] Step 4.2: Precast the overlying rock stratum I specimen above the coal seam: Add the proportioned overlying rock stratum material into the mold and level it. When the added material height reaches f1, place a pressing plate and apply a pressure of 0.5 Mpa on the pressing plate. After the material is coagulated for 14 days, the similar specimen of the overlying rock stratum I above the coal seam is completed;

[0027] Step 4.3: Precast the fractured overlying rock stratum II specimen above the coal seam: Add the proportioned overlying rock stratum material into the mold and level it. When the added material height reaches q - f1, according to the positional distance relationship between the horizontal well and the boundaries l1 + n1, q - f1, l2 + n2 of the model, place the horizontal well into the mold. Continue to add material and level it. After the material height reaches f2, place a pressing plate and apply a pressure of 0.5 Mpa on the pressing plate. After the material is coagulated for 14 days, the overlying rock stratum II specimen above the coal seam is completed; The horizontal well is made of steel pipe with a diameter of 8 - 10 mm, the perforation density is 9 holes / 1 cm, and the perforation holes are spirally distributed on the horizontal well.

[0028] Specifically, in step 5, if the precast roadway is incomplete, step 4 needs to be continued until the precast specific roadway is complete as checked by CT scanning.

[0029] Specifically, in step 6, the vertical stress is calculated according to the burial depth from the ground to the overlying rock stratum to be fractured. The maximum horizontal principal stress and the minimum horizontal principal stress are obtained from the rock stratum in-situ stress profile acquired by acoustic cross-dipole logging. Select the respective lithological in-situ stress values of the section from the coal seam to the overlying rock stratum to be fractured, and calculate the average value of the maximum horizontal principal stress and the minimum horizontal principal stress. Then, use the obtained average value as the loaded maximum horizontal principal stress and minimum principal stress.

[0030] Specifically, in step 7: The acoustic emission probes are designed to be installed on three surfaces of the specimen model. Select one surface from each of the opposite surfaces among the six surfaces, and install six on each plane. The acoustic emission probes and the monitoring lines are fixed with foaming glue.

[0031] Specifically, in step 11, after the designed volume of fracturing fluid is injected into the specimen and the injection is completed, the equipment pump for injecting the fracturing fluid is closed, and the acoustic emission real-time monitoring continues to observe the crack propagation situation. After no crack propagation events are detected by the acoustic emission monitoring, the acoustic emission equipment monitoring is stopped. When the pressure on the fracturing simulation equipment drops to zero, the fracturing experimental simulation equipment is opened and the pre-fractured rock sample is removed; the fractured specimen is scanned by CT to obtain the crack propagation morphology of the fractured specimen and compare it with the crack distribution of the similar specimen before fracturing; the densely populated area of hydraulic fracturing crack propagation in the similar specimen is determined, and the crack propagation morphology is observed by cutting open the crack-dense area of the specimen.

[0032] Compared with the prior art, the present invention has the following technical effects:

[0033] The present invention can clarify each currently given construction parameter during hydraulic fracturing under the conditions of the existing roadway, give a reasonable judgment on whether the fracturing cracks can penetrate the roadway during hydraulic fracturing, and then optimize the safety construction parameters of the fracturing process in real time to ensure the safety of the existing roadway. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is the front view of the relative position relationship among the coal seam, overlying rock, roadway, and horizontal well under actual fracturing conditions and geological conditions.

[0035] Figure 2 It is the side view of the relative position relationship among the coal seam, overlying rock, roadway, and horizontal well under actual fracturing conditions and geological conditions.

[0036] Figure 3 It is the top view projection diagram of the relative position relationship among the coal seam, roadway, and horizontal well under actual fracturing conditions and geological conditions located on the coal seam working face.

[0037] Figure 4 It is the front view of the relative position relationship among the coal seam, overlying rock, roadway, and horizontal well under the conditions of the fracturing physical model similar specimen.

[0038] Figure 5 It is the side view of the relative position relationship among the coal seam, overlying rock, roadway, and horizontal well under the conditions of the fracturing physical model similar specimen.

[0039] Figure 6 It is the top view projection diagram of the relative position relationship among the coal seam, roadway, and horizontal well under the conditions of the fracturing physical model similar specimen located on the coal seam working face.

[0040] Figure 7 It is the structure diagram of the 300*300*300mm mold.

[0041] Figure 8 It is the special mold for the roadway.

[0042] Figure 9 It is the coal seam specimen with an existing roadway.

[0043] Figure 10 It is a structural diagram of the pressing plate.

[0044] Figure 11 It is a schematic diagram of a prefabricated horizontal well.

[0045] Figure 12 It is a layout diagram of the installation points of acoustic emission probes.

[0046] Figure 13 It is a schematic diagram of the superposition sequence of specimens of similar coal seams, overlying strata I and overlying strata II. Specific implementation manners

[0047] The following are specific embodiments of the present invention. It should be noted that the present invention is not limited to the following specific embodiments, and all equivalent transformations made on the basis of the technical solution of this application fall within the protection scope of the present invention.

[0048] Embodiment 1:

[0049] As Figures 1 to 13 shown, this embodiment provides a physical simulation experiment method for the crack propagation of hydraulic fracturing under the conditions of an excavated roadway, which is characterized by including the following steps:

[0050] Step 1: Determine the relative position relationship parameters between the overlying strata to be fractured, the horizontal well, the coal seam, and the roadway under the actual fracturing working conditions and geological conditions, the thickness parameters of the coal seam and the overlying strata, and the size parameters of the roadway; specifically, under the actual fracturing working conditions and geological conditions, the overlying strata on the coal seam are overlying strata I and overlying strata II in sequence; the horizontal well is arranged in overlying strata II, and the roadways in the coal seam are roadway I and roadway II; Step 1 specifically includes: determining, under the actual fracturing working conditions and geological conditions, the vertical distance H from the horizontal well arranged in the overlying strata II to be fractured to the projection of the top of the roadway in the coal seam, the vertical distance Q from the horizontal well to the projection of the coal seam roof, and the distances L1 and L2 from the horizontal projection of the horizontal well to the distance from the roadway on the plane where the height of the roadway is located, the distances M1, M2, N1, N2, X1, X2, Y1, Y2 from roadway I and roadway II to the coal seam floor, left and right, front and back boundaries, and the thicknesses C, F1, F2 of the coal seam, overlying strata I and overlying strata II, as Figures 1 - 3 shown.

[0051] Step 2: Equivalently scale the parameters obtained in Step 1 to the specimen model, and calculate and obtain the relative position relationship parameters between the overlying strata, horizontal well, coal seam, and roadway in the specimen model, the thickness parameters of the coal seam and overlying strata, and the roadway size parameters. Step 2 specifically includes: Based on the relative position relationship between the horizontal well, Roadway I, Roadway II, coal seam, Overlying Stratum I, and Overlying Stratum II under the actual working conditions and geological conditions obtained in Step 1, as well as the thickness parameters of the coal seam, Overlying Stratum I, and Overlying Stratum II, and the sizes of Roadway I and Roadway II; then design the same equivalent similarity ratio to equivalently scale the horizontal well, Roadway I, Roadway II, coal seam, Overlying Stratum I, and Overlying Stratum II under the actual working conditions and geological conditions to a specimen size model of 300*300*300 mm; Based on the 300*300*300 mm specimen size model, calculate and obtain the relative position relationship and size after each equivalent similarity ratio. The parameters include: the vertical distance h from the horizontal well arranged in the Overlying Stratum II for fracturing to the projection of the top of the roadway in the coal seam, the vertical distance q from the horizontal well to the projection of the coal seam roof, and the distances l1 and l2 from the horizontal projection of the horizontal well to the roadway on the plane where the roadway height is located, the distances m1, m2, n1, n2, x1, x2, y1, y2 from Roadway I and Roadway II to the coal seam floor and the left and right boundaries of the model, and the thicknesses c, f1, f2 of the coal seam, Overlying Stratum I, and Overlying Stratum II, as Figures 4 - 6 shown.

[0052] Step 3: Determine the material mixing ratios of each material in the specimen model; specifically, based on the thickness parameters of the coal seam, Overlying Stratum I, and Overlying Stratum II in the 300*300*300 mm specimen size model determined in Step 2, the sizes and position relationships of Roadway I and Roadway II, make similar rock specimen. For the specimens used in the physical simulation of fracturing similarity tests, it is difficult to obtain complete specimens from natural coal seams and their overlying strata. Therefore, artificial coal seams and their overlying strata similar specimens made of materials such as coal powder, cement, mudstone powder, gypsum, and quartz sand are selected for physical simulation of fracturing. In order to determine that the specimens made can better match the mechanical properties of the coal seam and overlying strata, different material mixing ratios are used to make simulated specimens. After the simulated specimens are cured for 14 days, cylindrical cores of 25*50 mm are taken from the simulated specimens for uniaxial compression tests to measure mechanical parameters such as elastic modulus, Poisson's ratio, and uniaxial compressive strength. According to the test results of the mechanical parameters of the simulated specimens under different material mixing ratios, determine and optimize a specific material mixing ratio. At the same time, based on this material mixing ratio, the mechanical parameters of the simulated specimens made are the same as or close to the actual geological parameters, and then prepare the specimens for physical simulation of fracturing according to this specific material mixing ratio; Since the overlying strata of the coal seam are mainly mudstone and sandstone, it is designed that the mudstone uses a similar mixing ratio of mudstone powder, cement, and gypsum, the sandstone uses a similar mixing ratio of cement and sand, and the coal uses a similar mixing ratio of coal powder, cement, and gypsum.

[0053] Step 4: Based on the parameters in Step 2 and the material mix ratio of the specimen model determined in Step 3, fabricate physical simulation similar specimens; specifically including: Based on the specific material mix ratio method of the similar mode specimens, fabricate physical model similar specimens. The physical model similar specimens are fabricated using a 300*300*300mm mold, which is composed of five square steel plates of 300*300mm combined by external clamps. Small cylindrical protrusions are reserved on the mold used for fabricating rock sample similar specimens, and the small cylindrical protrusions will leave small round holes on the fabricated specimens for installing acoustic emission probes. Square small protrusions are arranged in parallel on the mold at the position of the small cylindrical protrusions, as Figure 7 shown, for prefabricating the wire grooves on the similar specimens.

[0054] Specifically, Step 4 includes:

[0055] Step 4.1: Prefabricate a coal seam specimen with an excavated roadway. The special roadway mold is as Figure 8 shown. Fabricate a specific plastic model with certain plasticity according to the geometric shape and size of the roadway section. The geometric shape of the roadway section can be circular, rectangular, right-angled semi-circular arch, etc. The prefabricated roadway is located inside the coal seam. The fabricated similar coal seam specimen with the prefabricated excavated roadway cannot see the internal roadway from the outside, and the designated excavated roadway is in a sealed state, as Figure 9 shown. The process of prefabricating a coal seam specimen with an excavated roadway is as follows: Obtain the material for fabricating the coal seam specimen using a specific mix ratio method. Add the prepared material into the 300*300*300 mold. Each time the added material is spread flat in the mold. When the flat height of the material in the mold reaches m1, according to the position distance relationship between Roadway I and the boundaries n1, x1, y1 of the model and the coal seam floor boundary m1, place the specific plastic model into the 300*300*300 mold for fabricating Roadway I. Continue to add and spread the material. When the flat height of the material in the mold reaches m2, according to the position distance relationship between Roadway II and the boundaries n2, x2, y2 of the model and the coal seam floor boundary m2, place the specific plastic model into the 300*300*300 mold for fabricating Roadway II. Continue to add and spread the material. After adding and spreading the material until the material height reaches c, stop adding the material. Then place a pressing plate with an engineering size fit into the 300*300*300 mold, as Figure 10 shown, which can be lowered into the 300*300*300 mold. Apply a pressure of 0.5 Mpa on the pressing plate. After waiting for the material to coagulate for 14 days, the prefabrication of the coal seam specimen with an excavated roadway is successful. Two iron cylinders are welded on the pressing plate to facilitate lowering the pressing plate into the 300*300*300 mold.

[0056] Step 4.2: Precast the similar specimen of the non-fractured overlying stratum I above the coal seam. Based on the material mixing ratio determined in Step 3, use a 300*300*300 mold to make the similar specimen of the non-fractured overlying stratum I above the coal seam. Pour the mixed materials into the 300*300*300 mold and level them. When the height of the leveled materials reaches f1, stop adding materials. Then, place a pressing plate with an engineering size fit into the 300*300*300 mold and apply a pressure of 0.5 Mpa on the pressing plate. After waiting for the materials to set for 14 days, the precast of the similar specimen of the non-fractured overlying stratum I above the coal seam is successful.

[0057] Step 4.3: Precast the similar specimen of the fractured overlying stratum II above the coal seam. The fractured overlying stratum II is the stratum at the top of the similar model. During the process of making the similar specimen of the fractured overlying stratum II, a horizontal well needs to be precast in it in advance. The horizontal well is as Figure 11 shown. The horizontal well is made of steel pipe. Based on the overall similar model specimen with a specification of 300*300*300 mm, the diameter of the horizontal well is designed to be 8 - 10 mm, the perforation density is designed to be 9 holes / cm, and the perforation holes are spirally distributed on the horizontal well. The number of perforation clusters is determined according to the specific actual research situation. Based on the material mixing ratio of the fractured overlying stratum II determined in Step 3, use a 300*300*300 mold to make the similar specimen of the fractured overlying stratum II. Pour the mixed materials into the 300*300*300 mold and level them. When the height of the leveled materials reaches q - f1, according to the position distance relationship of the horizontal well from the boundaries l1 + n1, q - f1, l2 + n2 of the model, place the horizontal well into the 300*300*300 mold, continue to add materials and level them. After the materials are added and leveled until the material height reaches f2, stop adding materials. Then, place a pressing plate with an engineering size fit into the 300*300*300 mold and apply a pressure of 0.5 Mpa on the pressing plate. After waiting for the materials to set for 14 days, the precast of the similar specimen of the fractured overlying stratum above the coal seam is successful.

[0058] Step 5: Check whether the precast coal seam specimen with an excavated roadway is successfully made. Mainly check whether the precast roadway with specific geometric dimensions is complete through CT scanning technology. At the same time, other overlying stratum specimens are also scanned by CT to record the original fracture development in the specimen before fracturing, which is used to analyze the crack propagation after fracturing. Among them, if the precast roadway is incomplete, continue Step 4 until the precast specific roadway is checked to be complete by CT scanning.

[0059] Step 6: Calculate and determine the three-direction stresses required to be loaded in the three axes during the true triaxial hydraulic fracturing physical simulation experiment. Specifically, the vertical stress is calculated according to the burial depth from the ground to the overlying strata to be fractured. The maximum horizontal principal stress and the minimum horizontal principal stress are obtained from the formation stress profile acquired by acoustic cross-dipole logging. The lithology formation stress values of the respective sections from the coal seam to the overlying strata to be fractured are selected, and the average value of the maximum horizontal principal stress and the minimum horizontal principal stress is calculated. Then, the obtained average value is used as the maximum horizontal principal stress and the minimum principal stress loaded in the three-direction stresses.

[0060] Step 7: Install acoustic emission probes on the prefabricated similar specimens to monitor the occurrence of crack initiation and propagation events on the rock samples in real time during the fracturing process. Among them, the acoustic emission probes are designed to be installed on only 3 faces, that is, one face is selected from each of the opposite faces among the six faces of the 300*300*300 model, and six are installed on each plane. As Figure 12 shown, the acoustic emission probes and the monitoring lines are fixed on the similar specimens using foaming glue.

[0061] Step 8: Use the true triaxial fracturing physical simulation test equipment, namely the high-temperature and high-stress hydraulic fracturing simulation equipment, to conduct an indoor similar physical simulation experiment on the specimens. Place the similar specimens made in accordance with Steps 4 and 5 into the true triaxial high-temperature and high-stress hydraulic fracturing simulation equipment in the order from bottom to top. The stacking order of the specimens is as Figure 13 shown. The made rock samples should be handled gently during transportation and placement, and any knocking or bumping is strictly prohibited to prevent new cracks from occurring in the rock samples.

[0062] Step 9: Optimization of the fracturing fluid for the hydraulic fracturing physical simulation experiment. A tracer with fluorescent green is put into the fracturing fluid. When observing the fracturing fluid expansion trajectory and the crack propagation law after fracturing by cutting open the similar specimen model, the expansion law of the fluorescent green tracer in the sandstone can directly observe the expansion law of the fracturing fluid in it. For other colors of tracers used in the fracturing fluid in the sandstone, it can also be seen, but for other colors of tracers used in the mudstone and coal seam, the fracturing fluid expansion trajectory and the expansion law cannot be clearly observed. Only by using a tracer with fluorescent green can the flow trajectory of the fracturing fluid and the crack propagation law be observed.

[0063] Step 10: Implement the fracturing of the similar specimens. Use the single control variable method to conduct experiments on the crack initiation and propagation laws during the fracturing process under different fracturing displacement, volume and other parameters. If there is fracturing seepage around the fracturing equipment during the fracturing experiment, there is no need to stop the fracturing experiment until all the designed fracturing fluid volume is pumped in, or until the fracturing fluid jets out under high pressure during the fracturing, then stop the fracturing experiment. At the same time, use acoustic emission to monitor the crack initiation and propagation laws of the fracturing cracks in real time during the fracturing.

[0064] Step 11: Cut open the fractured specimen to observe the flow trajectory of the fracturing fluid and the crack propagation. After fracturing the coal seam specimen prefabricated with an excavated roadway, it is cut open into two cases: cut along the roadway strike to observe the crack propagation around the roadway rib; cut along the roadway axis to observe the crack propagation along the roadway axis. Specifically, after the designed volume of fracturing fluid is injected into the specimen and the injection is completed, turn off the equipment pump for injecting the fracturing fluid, and continue to observe the crack propagation through real-time acoustic emission monitoring. When no crack propagation events are detected by the acoustic emission monitoring, stop the acoustic emission equipment monitoring. After the pressure on the fracturing simulation equipment drops to zero, open the fracturing experiment simulation equipment and remove the pre-fractured rock sample; then perform a CT scan on the fractured specimen to obtain the crack propagation morphology of the fractured specimen and compare it with the crack distribution of the similar specimen before fracturing; based on the CT scan, determine the densely populated area of hydraulic fracturing crack propagation in the similar specimen, and then cut open the specimen in the crack-dense area to observe the crack propagation morphology.

[0065] The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0066] In addition, it should be noted that, in the above specific embodiments, the various specific technical features described can be combined in any suitable manner without conflict. To avoid unnecessary repetition, the present invention does not separately describe various possible combination methods.

[0067] Furthermore, any combination can be made between different embodiments of the present invention as long as it does not violate the idea of the present invention, and it should also be regarded as the content disclosed by the present invention.

Claims

1. A physical simulation experimental method for hydraulic fracturing crack expansion under excavated tunnel conditions, characterized in that: The following steps are involved: Step 1: Determine the relative position relationship parameters between the overburden layer, horizontal well, coal seam and tunnel, the thickness parameters of the coal seam and overburden layer, and the size parameters of the tunnel under the actual fracturing conditions and geological conditions; Step 2: Equivalently proportion the parameters obtained in step 1 to the specimen model, and calculate the relative position relationship parameters between the overburden layer, horizontal well, coal seam, and roadway in the specimen model, the thickness parameters of the coal seam and overburden layer, and the size parameters of the roadway; Step 3: Determine the materials and proportions used for the coal seam similar specimens and overburden strata similar specimens in the specimen model; Step 4: Based on the parameters in step 2 and the material ratio determined in step 3, a physical simulation similar test piece is manufactured; Step 5: Check whether the coal seam specimen is successfully made, and check whether the prefabricated tunnel is complete through CT scanning. At the same time, other overburden specimens are also CT scanned to record the original fracture development in the specimen before fracturing, which is used to compare the fracture expansion after fracturing; Step 6: Calculate and determine the stress required to be loaded in the three-axis directions during the true triaxial hydraulic fracturing model test; Step 7: Install an acoustic emission probe on the specimen to monitor the initiation and expansion of rock cracks in real time during the fracturing process; Step 8: Place the manufactured and inspected similar specimens into a true triaxial high temperature and high stress hydraulic fracturing simulation device; Step 9: Put a fluorescent green tracer into the fracturing fluid, and when a similar specimen model is cut open after fracturing to observe the expansion trajectory of the fracturing fluid and the law of crack expansion, the expansion law of the fluorescent green tracer in the sandstone can directly observe the expansion law of the fracturing fluid therein; Step 10: Implementing fracturing of the specimen, using a single control variable method to test the crack initiation and expansion laws of the fracturing process under different fracturing displacements and volume parameters; at the same time, using acoustic emission to monitor the crack initiation and expansion laws of the fracturing cracks in real time during the fracturing process; Step 11: Cut open the specimen after fracturing to observe the flow trajectory of the fracturing fluid and the expansion of the cracks: cut along the direction of the tunnel to observe the expansion of the fracturing cracks around the tunnel wall; cut along the axial direction of the tunnel to observe the expansion of the axial fracturing cracks in the tunnel.

2. The physical simulation experimental method for hydraulic fracturing crack expansion under excavated tunnel conditions according to claim 1, characterized in that: In the step 1, under actual fracturing conditions and geological conditions, the overburden layers on the coal seam are overburden layer I and overburden layer II, respectively; the horizontal well is arranged in overburden layer II, and the tunnels in the coal seam are tunnel I and tunnel II; the parameters are: the vertical distance H between the horizontal well arranged in the overburden layer II and the projection of the top of the tunnel in the coal seam, the vertical distance Q between the horizontal well and the projection of the coal seam roof, and the distance L1 and L2 from the horizontal projection of the horizontal well to the plane where the tunnel height is located, the distance M1, M2, N1, N2, X1, X2, Y1, Y2 between tunnel I and tunnel II and the bottom plate, left and right and front and rear boundaries, and the thickness C, F1, F2 of the coal seam, overburden layer I and overburden layer II.

3. The physical simulation experimental method for hydraulic fracturing crack expansion under excavated tunnel conditions as claimed in claim 2, characterized in that: The step 2 comprises: based on the relative position relationship of the horizontal well, tunnel I, tunnel II, coal seam, overburden layer I and overburden layer II under the actual working conditions and geological conditions obtained in step 1, as well as the thickness parameters of the coal seam, overburden layer I and overburden layer II, and the size of the tunnel I and tunnel II; then designing the same equivalent similarity ratio to convert the horizontal well, tunnel I, tunnel II, coal seam, overburden layer I and overburden layer II under the actual working conditions and geological conditions to an equivalent proportion of 300*300*300mm specimen size; In the 2-inch model, the parameters calculated include: the vertical distance h between the horizontal well arranged in the fracturing overburden layer II and the projection of the top of the roadway in the coal seam, the vertical distance q between the horizontal well and the projection of the coal seam roof, and the distance l1 and l2 from the horizontal projection of the horizontal well to the plane where the roadway height is located, the distances m1, m2, n1, n2, x1, x2, y1, y2 between the roadway I and roadway II and the bottom of the coal seam and the left and right boundaries of the model, and the thicknesses c, f1, f2 of the coal seam, overburden layer I and overburden layer II.

4. The physical simulation experimental method for hydraulic fracturing crack expansion under excavated tunnel conditions as claimed in claim 3, characterized in that: In the step 3, coal powder, cement, mudstone powder, gypsum, and quartz sand are selected as materials to make artificial coal seams and similar specimens of their overlying rocks for physical simulation of fracturing; step 3 includes: determining the material ratio method according to the mechanical parameter test results of the simulated specimens under different material ratios, and at the same time, based on the material ratio method, the mechanical parameters of the simulated specimens made are the same or close to the actual geological parameters, and then preparing the specimens for physical simulation of fracturing according to this material ratio method; designing mudstone to adopt a similar ratio of mudstone powder, cement, and gypsum, sandstone to adopt a similar ratio of cement and sand, and coal to adopt a similar ratio of coal powder, cement, and gypsum.

5. The physical simulation experimental method for hydraulic fracturing crack expansion under excavated tunnel conditions as claimed in claim 4, characterized in that: In step 4, a 300*300*300mm mold is used. The mold is composed of five 300*300mm square steel plates combined by external clips. A small cylindrical protrusion is reserved on the mold used in making similar test pieces of rock samples. The small cylindrical protrusion will leave a small round hole on the manufactured test piece for installing the acoustic emission probe. A parallel square small protrusion is set on the mold at the position of the small cylindrical protrusion for prefabricating the wire groove on the similar test piece.

6. The physical simulation experimental method for hydraulic fracturing crack expansion under excavated tunnel conditions according to claim 5, characterized in that: The step 4 comprises: Step 4.1: Prefabricate the coal seam specimen with excavated tunnels: add the prepared coal seam material into a 300*300*300 mold. When the material paving height in the mold reaches m1, according to the position distance relationship between tunnel I and each boundary n1, x1, y1 of the model and the boundary m1 of the coal seam floor, put the plastic model into the mold as tunnel I; continue to add materials and spread them. When the material paving height in the mold reaches m2, according to the position distance relationship between tunnel II and each boundary n2, x2, y2 of the model and the boundary m2 of the coal seam floor, put the plastic model into the mold as tunnel II; continue to add materials and spread them until the material height is c, put in a pressing plate, apply a pressure of 0.5Mpa on the pressing plate, and wait for the material to solidify for 14 days to complete the prefabrication of the coal seam specimen with excavated tunnels; Step 4.2: Prefabricate the test piece of the overburden layer I on the upper part of the coal seam: add the well-proportioned overburden layer material into the mold and flatten it. When the height of the added material reaches f1, put it into the pressing plate, apply a pressure of 0.5Mpa on the pressing plate, and wait for the material to solidify for 14 days to prefabricate a similar test piece of the overburden layer I on the upper part of the coal seam; Step 4.3: Prefabricate the overburden II specimen for fracturing the upper part of the coal seam: add the well-proportioned overburden material into the mold and spread it flat. When the height of the added material reaches q-f1, place the horizontal well into the mold according to the position distance relationship between the horizontal well and each boundary of the model l1+n1, q-f1, l2+n2, continue to add material and spread it flat. When the material height reaches f2, put it into the pressing plate, apply a pressure of 0.5Mpa on the pressing plate, and wait for the material to solidify for 14 days to prefabricate the overburden II specimen of the upper part of the coal seam; the horizontal well is made of steel pipe with a diameter of 8-10mm and a perforation density of 9 holes / 1cm, and the perforations are spirally distributed on the horizontal well.

7. The physical simulation experimental method for hydraulic fracturing crack expansion under excavated tunnel conditions according to claim 1, characterized in that: In step 5, if the prefabricated tunnel is incomplete, step 4 needs to be continued until the prefabricated specific tunnel is checked to be complete by CT scanning.

8. The physical simulation experimental method for hydraulic fracturing crack expansion under excavated tunnel conditions according to claim 1, characterized in that: In step 6, the vertical stress is calculated according to the burial depth from the ground to the overburden to be fractured, and the maximum horizontal principal stress and the minimum horizontal principal stress are obtained by the rock formation in-situ stress profile obtained by the acoustic cross-dual-level logging. The respective lithological in-situ stress values ​​from the coal seam to the overburden to be fractured are selected, and the average value of the maximum horizontal principal stress and the minimum horizontal principal stress is obtained, and then the obtained average value is used as the loaded horizontal maximum principal stress and the minimum principal stress.

9. The physical simulation experimental method for hydraulic fracturing crack expansion under excavated tunnel conditions according to claim 1, characterized in that: In step 7: the acoustic emission probes are designed and installed on three surfaces of the specimen model, one surface is selected from each of the six opposite surfaces, and six are installed on each plane. The acoustic emission probes and the monitoring lines are fixed with foam glue.

10. The physical simulation experimental method for hydraulic fracturing crack expansion under excavated tunnel conditions according to claim 1, characterized in that: In the step 11, after the designed amount of fracturing fluid is injected into the test piece, the pump of the equipment for injecting the fracturing fluid is turned off, and the real-time acoustic emission monitoring continues to observe the crack expansion. When the acoustic emission monitors no crack expansion event, the acoustic emission equipment monitoring is stopped. When the pressure on the fracturing simulation device drops to zero, the fracturing experiment simulation device is opened to remove the prefabricated rock sample for fracturing; the test piece after fracturing is scanned by CT to obtain the crack expansion morphology of the test piece after fracturing, and Compare the distribution of cracks in similar specimens without hydraulic fracturing; determine the areas with dense hydraulic fracturing crack expansion in similar specimens, The crack-dense area of ​​the specimen was cut open to observe the crack propagation morphology.

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