Device and method for simulating crack morphology caused by hot dry rock fracturing

Abstract

The present invention provides a device and method for simulating the crack morphology formed by hot dry rock fracturing, relates to the field of rock mass mechanics experimental technology, and provides a new simulation device and method for simulating the crack network morphology caused by expansion of hot dry rock under the thermal-mechanical coupling. The device includes a frame, a core limiting structure, and a heating device, wherein the core limiting structure is arranged on the frame, and a receiving cavity for placing the core is formed in the core limiting structure, and the core limiting structure is used to simulate the surrounding ground stress on the core when the core in the receiving cavity expands and cracks; the heating device is connected to the core limiting structure and the heating device is used to heat the core in the receiving cavity to simulate the geothermal temperature. The present invention is used to simulate the crack network morphology caused by expansion of hot dry rock under the thermal-mechanical coupling.

Description

Technical Field

[0001] The present invention relates to the field of rock mechanics experimental technology, and in particular to a device and method for simulating the formation of crack morphology by fracturing of hot dry rock, and specifically to a device and method for simulating the formation of crack network morphology by fracturing of hot dry rock due to expansion under the action of thermal-mechanical coupling. Background Art

[0002] Hot dry rock (HDR) is a new type of high-efficiency, low-carbon, clean energy with the characteristics of large reserves, wide distribution, and stable energy supply. The first task in the development process is to form an artificial heat reservoir in the rock reservoir thousands of meters underground through hydraulic fracturing technology, and then inject cold fluid into the formation, and circulate it between the injection well, artificial heat reservoir and production well, and exchange heat with the hot dry rock reservoir, so as to extract the heat energy in the hot dry rock for ground heating, power generation, etc.

[0003] Enhanced geothermal system (EGS) is an effective method to develop hot dry rock resources. Since the 1970s, European and American countries have successively carried out practical projects for the development of hot dry rocks. The results show that the temperature of hot dry rock reservoirs shows various changes with the increase of formation depth. At the same time, the artificial fractures produced by fracturing have directionality and uncertainty, which makes it impossible to produce satisfactory hydraulic conductivity between injection wells and production wells. The geological conditions of hot dry rock reservoirs also cause the development of fractures to vary. Among them, brittleness is a necessary factor affecting the formation of fracture networks after reservoir fracturing. In addition, the bedding structure and tensile strength of hot dry rocks are also two key factors affecting the formation of fracture networks. Therefore, the key technology in the development of hot dry rock thermal energy is to form a complex artificial fracture network system that is connected in the underground high-temperature reservoir.

[0004] Traditional hydraulic fracturing technology originated from the development of oil and gas fields, and mainly considers the influence of fluid-solid coupling on crack expansion. In the process of hydraulic fracturing of hot dry rocks, the influence of deep geothermal high temperature must also be fully considered. Therefore, the hot dry rock fracturing process is mainly thermal fluid-solid coupling. During the hydraulic fracturing of hot dry rocks, the effective stress is directly obtained through the water injection pressure. When the water injection pressure continues to increase in the wellbore and exceeds the ground stress and the tensile strength of the rock mass, the hot dry rock cracks to produce a fracture network structure. When simulating this process, the sealing of the hot dry rock wellbore is very high, mainly to prevent the water injection pressure from being too high to damage the wellbore but fail to achieve the fracturing effect. The expansion fracturing method can not only meet the water injection pressure effect during hydraulic fracturing, but also save the process of sealing the wellbore to prevent pressure relief. Moreover, this method is stable and convenient and can achieve the same effect. Therefore, a new simulation device and method are needed to simulate the expansion-induced fracture network morphology under the thermal-mechanical coupling of hot dry rocks. Summary of the invention

[0005] The purpose of the present invention is to provide a device and method for simulating the crack morphology formed by hot dry rock fracturing, so as to provide effective technical support for the efficient development of hot dry rock thermal energy. The preferred technical solutions among the many technical solutions provided by the present invention can produce many technical effects as described below.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] The present invention provides a device for simulating the crack morphology formed by the fracturing of hot dry rocks, comprising a frame, a core limiting structure and a heating device, wherein the core limiting structure is arranged on the frame, and a receiving cavity for placing the core is formed in the core limiting structure, and the core limiting structure is used to simulate the surrounding ground stress to which the core is subjected when the core in the receiving cavity expands and fractures; the heating device is connected to the core limiting structure and is used to heat the core in the receiving cavity to simulate the geothermal temperature.

[0008] Preferably, the core limiting structure includes a thick-walled tube, which is placed on the frame and the core can be placed in the thick-walled tube. The thick-walled tube is a split structure and the core crack morphology can be easily observed when the thick-walled tube is disassembled; the device also includes a filling material, which is used to fill the gap between the thick-walled tube and the core so as to simulate the effect of circumferential geostress on the core during expansion and fracturing.

[0009] Preferably, the thick-walled tube comprises two semicircular thick-walled tubes, each of which has connecting parts at both ends in the height direction, the connecting parts extending in a direction away from the center position of the thick-walled tube, and the corresponding connecting parts of the two semicircular thick-walled tubes are detachably connected through a connecting piece; a top opening hole is provided on the connecting part of one of the two semicircular thick-walled tubes, and the top opening hole is used for disassembly of the two semicircular thick-walled tubes; the filling material is red copper skin and / or thin iron sheet.

[0010] Preferably, the core limiting structure includes an upper pad and a lower pad, the lower pad is arranged on the frame and the core can be supported on the lower pad, the upper pad is suspended on the frame and the height of the upper pad is adjustable, the upper pad is used to contact the upper end surface of the core placed in the thick-walled tube, when the upper pad is adjusted to contact the upper end surface of the core in the thick-walled tube, the upper pad and the lower pad are used to simulate the vertical ground stress on the core during the expansion and fracturing process.

[0011] Preferably, the upper pad is connected to a screw rod, the screw rod passes through a fixing nut on the frame body, and the screw rod is threadedly connected to the fixing nut, so that the height of the upper pad can be changed when the screw rod is rotated; a limiting groove is provided on the frame body, the lower pad is located in the limiting groove, the lower pad is inserted into the thick-walled tube, and the sum of the core height and the thickness of the upper pad and the lower pad is not greater than the height of the thick-walled tube; the diameter of the lower pad is consistent with the diameter of the upper pad.

[0012] Preferably, the frame includes an upper plate, a lower plate and a fixing rod, the upper plate is supported above the lower plate by the fixing rod, a limiting groove is provided on the lower plate, the thick-walled tube is placed on the lower plate, the upper plate is located above the thick-walled tube, and the screw of the device passes through the upper plate.

[0013] Preferably, the heating device includes a heating sleeve and a temperature controller, the heating sleeve is connected to the temperature controller, the heating sleeve is wrapped around the outside of the thick-walled tube of the core limiting structure to heat the core in the thick-walled tube; the temperature controller is connected to two temperature sensors, the two temperature sensors are arranged between the core and the inner wall of the thick-walled tube, and the two temperature sensors are distributed opposite to each other in the diameter direction of the thick-walled tube; through the control of the temperature controller, different rates of temperature rise can be achieved and a heating method with multiple alternating temperature rise rates can be achieved.

[0014] Preferably, the device also includes an acoustic emission device, which includes an acoustic emission controller and an acoustic emission probe, the acoustic emission controller is connected to the acoustic emission probe, and the acoustic emission probe is distributed on the outer wall of the thick-walled tube of the core limiting structure; probe mounting holes are provided on the outer wall of the thick-walled tube, and the acoustic emission probe is located in the corresponding probe mounting holes; there are multiple acoustic emission probes, and the acoustic emission probes are distributed along the height direction of the thick-walled tube, and the acoustic emission probes at the same height are evenly distributed along the circumferential direction of the thick-walled tube.

[0015] A method for expanding and fracturing hot dry rock using the device for simulating the crack morphology formed by fracturing hot dry rock is characterized by comprising the following steps: placing a drilled core in the core limiting structure, starting a heating device to heat the core to simulate the core environment temperature; after heating to a predetermined temperature value, pouring an expansion agent slurry into the drilled hole of the core, and using the core limiting structure to simulate the effect of surrounding ground stress on the core during expansion and fracturing.

[0016] Preferably, the method specifically includes the following contents:

[0017] Step S1, drilling a bare core, drilling a hole in the middle of the core, measuring the core diameter with a measuring tool, determining the amount of filling material to be wrapped around the outside of the core according to the difference between the measured minimum core diameter and the inner diameter of the thick-walled tube, wrapping the filling material around the core and tightening it with a rubber band;

[0018] Step S2, placing the lower pad on the lower plate limit groove, placing the rock core on the lower pad, and wrapping the two semicircular thick-walled tubes around the rock core and connecting them;

[0019] Step S3, installing an acoustic emission device, setting the acoustic emission probe on the outer wall of the thick-walled tube; installing a heating sleeve, wrapping the heating sleeve around the outer side of the thick-walled tube, and connecting a temperature sensor;

[0020] Step S4, connecting the power supply, starting the temperature controller, setting the corresponding heating mode and heating time, performing the preset temperature mode treatment on the core, and starting the acoustic emission controller;

[0021] Step S5, injecting a slurry expansion agent into the core borehole through an iron pipe, wherein the proportion of the expansion agent is as follows: selecting a HSCA-1 type high-efficiency expansion agent, mixing the expansion agent with water at a mass ratio of 4:1, and then fully stirring to form a mixed slurry expansion agent;

[0022] Step S6, twisting the screw rod to drive the upper pad to move downward, so that the upper pad contacts the upper end surface of the core, and applies axial stress to the core;

[0023] Step S7: After the core is fractured, it is cooled to room temperature, the heating jacket is opened, the acoustic emission probe is unloaded, the screw is turned to lift the upper pad, the two semicircular thick-walled cylinders are opened, and the broken core is taken out;

[0024] Step S8, after taking out the broken core, the internal crack extension morphology of the broken core can be obtained, and the cracks on the surface of the core can be depicted with a colored oil pen (which can be a red oil pen); the fracture network topology structure is reconstructed in three dimensions by CT scanning to characterize the fracture morphology, and can also be characterized by volume density and fractal dimension.

[0025] The present invention provides a device for simulating the crack morphology formed by fracturing of hot dry rocks. The device simulates the surrounding ground stress on the rock core when the rock core in the accommodating cavity expands and cracks through a core limiting structure, and simulates the geothermal temperature through an external heating device, so as to simulate the morphology of the crack network caused by the expansion of the hot dry rock under the thermal-mechanical coupling effect, and provide effective technical support for the efficient development of hot dry rock thermal energy.

[0026] The preferred technical solution of the present invention can at least produce the following technical effects:

[0027] The thick-walled tube is divided into two halves. Compared with the common cylindrical thick-walled tube, it is easy to operate and can be reused. After the core is fractured, not only can the fracture morphology at both ends of the core be observed, but also the thick-walled tube can be disassembled to obtain the overall fracture crack morphology of the core.

[0028] The upper and lower pads can provide upper and lower clamping forces, which can not only simulate the vertical ground stress on the rock sample, but also prevent the fragments from the upper and lower surfaces from splashing during the core fracturing process.

[0029] The external heating device heats the core. Through the digital temperature controller, different heating modes, different heating rates and core fracturing conditions under multi-cycle heating modes can be achieved, and different thermal-mechanical coupling modes can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0031] Figure 1 is a schematic structural diagram of a device for simulating the formation of crack morphology by fracturing of hot dry rock provided by an embodiment of the present invention;

[0032] Figure 2 It is a structural schematic diagram of a frame and a thick-walled tube provided in an embodiment of the present invention;

[0033] Figure 3 It is a schematic structural diagram of a thick-walled tube provided by an embodiment of the present invention;

[0034] Figure 4 is a schematic top view of a device for simulating the formation of crack morphology by fracturing of hot dry rock provided by an embodiment of the present invention;

[0035] Figure 5 yes Figure 4 AA cross-sectional view of ;

[0036] Figure 6 is a schematic diagram of the distribution of acoustic emission probes provided in an embodiment of the present invention;

[0037] Figure 7 It is a simplified flow chart provided by an embodiment of the present invention.

[0038] In the figure, 1-frame; 11-upper plate; 12-lower plate; 13-fixing rod; 14-fixing nut; 2-temperature controller; 3-thick-walled cylinder; 31-semicircular thick-walled cylinder; 32-connecting part; 4-upper pad; 5-lower pad; 6-screw; 7-acoustic emission device; 71-acoustic emission controller; 72-acoustic emission probe. DETAILED DESCRIPTION

[0039] To make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be described in detail below. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other implementation methods obtained by ordinary technicians in this field without creative work belong to the scope of protection of the present invention.

[0040] See also Figure 1-Figure 7 The present invention provides a device for simulating the crack morphology caused by hot dry rock fracturing, comprising a frame 1, a core limiting structure and a heating device, wherein the core limiting structure is arranged on the frame 1, and a receiving cavity for placing the core is formed in the core limiting structure, and the core limiting structure is used to simulate the surrounding geostress to which the core is subjected when the core in the receiving cavity expands and fractures; the heating device is connected to the core limiting structure and the heating device is used to heat the core in the receiving cavity to simulate the geothermal temperature. The present invention adopts an expansion fracturing method, that is, drilling a hole on the core, injecting a high-efficiency expansion agent into the borehole, simulating the surrounding geostress to which the core is subjected when it expands and fractures through the core limiting structure, and simulating the geothermal temperature of the core through an external heating device, so as to simulate the morphology of the expansion-induced crack network under the thermal-mechanical coupling of hot dry rock, and provide effective technical support for the efficient development of hot dry rock thermal energy.

[0041] As an optional implementation method of an embodiment of the present invention, the core limiting structure includes a thick-walled tube 3, which is placed on the frame 1 and the core can be placed in the thick-walled tube 3. The thick-walled tube 3 is a split structure and the crack morphology of the core can be easily observed when the thick-walled tube 3 is disassembled; the thick-walled tube 3 provided by the present invention is a split structure, which is easy to operate and can be reused compared with an ordinary cylindrical thick-walled tube; after the core is fractured, not only can the fracture morphology at both ends of the core be observed, but the thick-walled tube can also be disassembled to obtain the overall fracture crack morphology of the core.

[0042] The device also includes a filling material, which is used to fill the gap between the thick-walled tube 3 and the core to simulate the circumferential stress on the core during expansion and fracturing (equivalent to giving the core a circumferential confining pressure). The gap between the core and the thick-walled tube 3 is filled with hard material, and the amount of filling material can be freely changed according to the size of the gap between the core and the thick-walled tube to ensure that the core is in full contact with the thick-walled tube 3 during the fracturing process to prevent the rock sample (core) from being fractured due to local circumferential force; the filling material is generally made of copper skin and thin iron sheet, and the thickness of the filling material ranges from 1 mm to 2 mm. Before the core is fracturing, the difference between the core diameter and the inner diameter of the thick-walled tube 3 must be actually measured, and several layers of filling material are wrapped on the surface of the core according to the measured difference, and the material thickness is not less than the measured difference.

[0043] As an optional implementation method of the present invention, see Figure 2 and Figure 3 The specific structure of the thick-walled tube 3 can be as follows: the thick-walled tube 3 includes two semicircular thick-walled tubes 31, each of which has a connecting portion 32 at both ends along the height direction. The connecting portion 32 can be an extended iron plate and welded on the semicircular thick-walled tube 31. The connecting portion 32 extends in a direction away from the center position of the thick-walled tube 3. The corresponding connecting portions 32 of the two semicircular thick-walled tubes 31 are detachably connected through a connecting piece. A rubber gasket with a thickness of 5 mm is inserted between the two butted connecting portions 32. The connecting portion 32 is provided with There are tapping holes, and screws are screwed into the tapping holes to achieve the connection of the two semicircular thick-walled tubes 31 (7 to 8 holes are drilled on each connecting part 32, and the holes are screwed with hexagon socket screws or ordinary screws); one of the two semicircular thick-walled tubes 31 is provided with top opening holes on the two connecting parts 32 (one top opening hole can be provided on both connecting parts 32, and the top opening hole is a tapping hole), and the top opening hole is used for the disassembly of the two semicircular thick-walled tubes 31, that is, it is helpful to facilitate the disassembly of the thick-walled tube 3 after the core fracturing is completed. In addition, the screwing depth of the screws on the connecting part 32 can also make the thick-walled tube 3 apply annular confining pressure to the core.

[0044] The dimensions of the thick-walled tube 3 are as follows: the height of the thick-walled tube 3 can be 200 mm, the inner diameter can be 110 mm, and the wall thickness can be 10 mm. The thick-walled tube 3 mainly bears the stress in the radial direction after the core is fractured.

[0045] As an optional implementation method of an embodiment of the present invention, the core limiting structure includes an upper pad 4 and a lower pad 5, the lower pad 5 is arranged on the frame 1 and the core can be supported on the lower pad 5, the upper pad 4 is suspended on the frame 1 and the height of the upper pad 4 is adjustable, the upper pad 4 is used to contact the upper end surface of the core placed in the thick-walled tube 3, when the upper pad 4 is adjusted to contact the upper end surface of the core in the thick-walled tube 3 (applying axial stress to the core), the upper pad 4 and the lower pad 5 are used to simulate the vertical ground stress applied to the core during the expansion and fracturing process.

[0046] Regarding the height adjustment structure of the upper pad 4, it can be specifically as follows: the upper pad 4 is connected to the screw 6, the screw 6 passes through the fixing nut 14 on the frame 1 (the fixing nut 14 is embedded after the center hole of the upper plate 11 of the frame 1 is drilled, and the screw passes vertically through the upper plate 11 through the fixing nut 14), the screw 6 is threadedly connected with the fixing nut 14, and the height of the upper pad 4 can be changed when the screw 6 is rotated; a limiting groove is provided on the frame 1 (a limiting groove is provided on the lower plate 12 of the frame 1), the lower pad 5 is located in the limiting groove, the lower pad 5 is inserted into the thick-walled tube 3, and the sum of the core height and the thickness of the upper pad 4 and the lower pad 5 is not greater than the height of the thick-walled tube 3. The dimensions of the upper pad 4 and the lower pad 5 can be as follows: the diameters of the upper pad 4 and the lower pad 5 can be 106 mm and the thicknesses can be 6 mm; the diameters of the upper pad 4 and the lower pad 5 are the same as the diameter of the core, the core height is 170 to 180 mm, a hole with a diameter of 10 mm is drilled in the middle of the core, but do not drill through it, the drilling depth can be 160 mm, and the sum of the core height and the thickness of the upper pad 4 and the lower pad 5 is not greater than the height of the thick-walled tube 3.

[0047] As an optional implementation of the embodiment of the present invention, the frame 1 includes an upper plate 11, a lower plate 12 and a fixing rod 13. The upper plate 11 is supported on the upper part of the lower plate 12 by the fixing rod 13. A limiting groove is provided on the lower plate 12. The thick-walled tube 3 is placed on the lower plate 12. The upper plate 11 is located above the thick-walled tube 3. The screw 6 of the device passes through the upper plate 11. The upper plate 11 and the lower plate 12 are connected by the fixing rod 13. The entire frame 1 provides an action force and a reaction force for the screw 6 to be screwed into the clamping core. The dimensions of the frame 1 can be as follows: the length of the lower plate 12 is 160 mm, the width is 120 mm, and the thickness is 10 mm. There is a limiting groove with a diameter of 106 mm in the middle of the lower plate 12, and the extension of the limiting groove is 5 mm. The fixing rod is 260 mm high and has a diameter of 10 mm.

[0048] As an optional implementation method of the present invention, the heating device includes a heating sleeve and a temperature controller 2, the heating sleeve is connected to the temperature controller 2, the heating sleeve is wrapped on the outside of the thick-walled tube 3 of the core limiting structure to heat the core in the thick-walled tube 3, the heating sleeve is wrapped on the outside of the thick-walled tube to form a cylindrical closed space, mainly for heating the internal core; the temperature controller 2 is connected to two temperature sensors, the two temperature sensors are arranged between the core and the inner wall of the thick-walled tube 3, and the two temperature sensors are distributed opposite to each other in the diameter direction of the thick-walled tube 3 (the two temperature sensors are symmetrically distributed); the temperature controller 2 can be controlled to achieve different rates of temperature rise and multiple heating methods with alternating temperature rise rates. The digital display temperature controller is internally programmed with programs related to the temperature control mode, and multiple temperature rise modes of cyclic heating can be realized. For example, the temperature rise rate can be set to 1 to 10 degrees Celsius per minute, and alternating temperature control (such as sawtooth temperature control) can also be performed to control the temperature rise and fall of the hot dry rock core for multiple cycles, so that different thermal load modes can be achieved during the simulation process; at the same time, with the fracturing effect of high-efficiency expansion agents, different thermal-mechanical coupling modes can be achieved. The hot dry rock after different heat treatments can also be cold-treated with low-temperature liquid nitrogen to form a cold-heat shock cycle thermal-mechanical coupling effect, simulating the morphology of the crack network caused by hot dry rocks after various temperature load treatments.

[0049] Regarding high-efficiency slurry expansion agent: HSCA-Ⅰ type high-efficiency expansion agent is selected. The expansion agent is made of quicklime calcined at high temperature (above 1400 degrees Celsius) in a rotary kiln and a small amount of additives. Before use, the expansion agent and water are mixed in a mass ratio of 4:1, and then fully stirred to form a mixed slurry. The high-efficiency expansion agent should be added after the core is heated to the preset temperature.

[0050] As an optional implementation method of an embodiment of the present invention, the device also includes an acoustic emission device 7, which includes an acoustic emission controller 71 and an acoustic emission probe 72. The acoustic emission controller 71 is connected to the acoustic emission probe 72, and the acoustic emission probe 72 is distributed on the outer wall of the thick-walled tube 3 of the core limiting structure; a probe mounting hole is provided on the outer wall of the thick-walled tube 3 (a hole with a depth of 5 mm and a diameter of 8 mm can be drilled on the outer wall surface of the thick-walled tube 3 to form a probe mounting hole), and the acoustic emission probe 72 is located in the corresponding probe mounting hole; there are multiple acoustic emission probes 72, and the acoustic emission probes 72 are distributed along the height direction of the thick-walled tube 3, and the acoustic emission probes 72 at the same height are evenly distributed along the circumferential direction of the thick-walled tube 3. The 12 acoustic emission probes 72 can be divided into three groups and inserted into the thick-walled tube 3. Different groups are distributed at different heights, and the four probes in the same group are evenly distributed around the thick-walled tube 3 in a 360° circumferential direction to ensure uniform acoustic emission monitoring. The probes are connected to the acoustic emission controller 71, the acoustic emission device 7 is turned on, and the cracking of the core is monitored using the acoustic emission device 7.

[0051] The present invention provides a device for simulating the crack morphology formed by hot dry rock fracturing under the action of thermal coupling, which can be used to simulate the morphology of artificial fracture network formed by hot dry rock fracturing. The rock core drilled on site is placed in a thick-walled tube 3, and the thick-walled tube 3 is used to apply circumferential stress to the rock core, and a certain vertical stress is applied to the upper and lower bottom surfaces of the rock core. Under different heating modes and heating temperatures, the hot dry rock is expanded and fractured by a high-efficiency expansion agent, and the fracturing process is monitored by an acoustic emission device 7. After fracturing, the two halves of the thick-walled tube are disassembled to obtain the fractured rock core fragments, and the crack morphology and internal fracture structure of the rock core can be clearly observed. The topological structure of the artificial fracture network is reconstructed and imaged in three dimensions by a CT scanning method to characterize the crack morphology, and the crack body density can also be used to characterize the crack morphology.

[0052] A method for expanding and fracturing hot dry rock using a device that simulates the crack morphology formed by fracturing hot dry rock, comprising the following steps: placing a drilled rock core in a rock core limiting structure, starting a heating device to heat the rock core to simulate the rock core ambient temperature; after heating to a predetermined temperature value, pouring an expansion agent slurry into the drilled hole of the rock core, and using the rock core limiting structure to simulate the effect of surrounding ground stress on the rock core during expansion and fracturing. The above method specifically includes the following contents:

[0053] Step S1, drill an exposed core (the core size is 106 mm in diameter and 170 mm in height), drill a hole in the middle of the core (drilling a hole with a diameter of 10 mm, the drilling depth can be 160 mm), measure the core diameter with a measuring tool (which can be a vernier caliper), determine the amount of filling material that needs to be wrapped around the outside of the core according to the difference between the measured minimum core diameter and the inner diameter of the thick-walled tube 3 (the filling material can be a thin iron sheet with a thickness ranging from 1 mm to 2 mm), wrap the filling material around the core and tighten it with a rubber band;

[0054] Step S2, place the lower pad 5 on the limiting groove of the lower plate 12, place the rock core on the lower pad 5 and align them, screw the screw 6 to drive the upper pad 4 to rise to the top, wrap the two semicircular thick-walled tubes 31 around the rock core and connect them with screws to clamp the rock core;

[0055] Step S3, installing the acoustic emission device 7, setting the acoustic emission probe 72 on the outer wall of the thick-walled tube 3 (12 acoustic emission probes 72 are divided into three groups and inserted into the thick-walled tube 3, different groups are distributed at different heights, and the 4 probes of the same group are evenly distributed around the thick-walled tube 3 in a 360° circumferential direction to ensure uniform acoustic emission monitoring); installing a heating sleeve, wrapping the heating sleeve around the outside of the thick-walled tube 3, and connecting a temperature sensor, two temperature sensors are arranged between the core and the inner wall of the thick-walled tube 3, and the two temperature sensors are distributed opposite to each other in the diameter direction of the thick-walled tube 3;

[0056] Step S4, connect the power supply, start the temperature controller 2, set the corresponding heating mode and heating time (the heating mode can be set to single heating, the heating rate is set to 5 degrees Celsius / minute, that is, increase by 5 degrees Celsius per minute, increase the temperature to 200 degrees Celsius, and keep warm for 12 hours), perform the preset temperature mode processing on the core, and start the acoustic emission controller 71;

[0057] Step S5, after keeping warm for 12 hours (the temperature reaches 200 degrees Celsius), inject the slurry expansion agent into the core borehole through the iron pipe, wherein the proportion of the expansion agent is as follows: select HSCA-1 type high-efficiency expansion agent, mix the expansion agent and water in a mass ratio of 4:1, and then stir well to form a mixed slurry expansion agent;

[0058] Step S6, screwing the screw 6 to drive the upper pad 4 to move downward, so that the upper pad 4 contacts the upper end surface of the core, applying axial stress to the core;

[0059] Step S7: After the core is fractured, it is cooled to room temperature, the heating jacket is opened, the acoustic emission probe is unloaded, the screw 6 is turned to lift the upper pad 4, the two semicircular thick-walled tubes 31 are opened, and the broken core is taken out;

[0060] Step S8, after taking out the broken core, the internal crack extension morphology of the broken core can be obtained, and the cracks on the surface of the core can be depicted with a colored oil pen (for example, a red oil pen); the fracture network topology structure is reconstructed in three dimensions by CT scanning to characterize the fracture morphology, and can also be characterized by volume density and fractal dimension.

[0061] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art can easily think of changes or substitutions within the technical scope disclosed by the present invention, which should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A device for simulating the crack morphology caused by hot dry rock, characterized in that: It comprises a frame (1), a core limiting structure and a heating device, wherein: The core limiting structure is arranged on the frame (1), and a receiving cavity for placing the core is formed in the core limiting structure, and the core limiting structure is used to simulate the surrounding ground stress effect on the core when the core in the receiving cavity expands and cracks; the heating device is connected to the core limiting structure and is used to heat the core in the receiving cavity to simulate the geothermal temperature; The core limiting structure comprises a thick-walled tube (3), the thick-walled tube (3) is placed on the frame (1) and the core can be placed in the thick-walled tube (3), the thick-walled tube (3) is a split structure and the core crack morphology can be easily observed when the thick-walled tube (3) is disassembled; The device further comprises a filling material, wherein the filling material is used to fill the gap between the thick-walled tube (3) and the rock core so as to simulate the effect of circumferential ground stress on the rock core during expansion and fracturing; The thick-walled tube (3) comprises two semicircular thick-walled tubes (31), each of the semicircular thick-walled tubes (31) having connecting parts (32) at both ends in the height direction, the connecting parts (32) extending in a direction away from the center position of the thick-walled tube (3), and the corresponding connecting parts (32) of the two semicircular thick-walled tubes (31) are detachably connected via a connecting piece; a top opening hole is provided on the connecting part (32) of one of the two semicircular thick-walled tubes (31), and the top opening hole is used for disassembling the two semicircular thick-walled tubes (31); The filling material is copper sheet and / or thin iron sheet; The core limiting structure comprises an upper pad (4) and a lower pad (5), wherein the lower pad (5) is arranged on the frame (1) and the core can be supported on the lower pad (5), the upper pad (4) is suspended on the frame (1) and the height of the upper pad (4) is adjustable, and the upper pad (4) is used to contact the upper end surface of the core placed in the thick-walled tube (3). When the upper pad (4) is adjusted to contact the upper end surface of the core in the thick-walled tube (3), the upper pad (4) and the lower pad (5) are used to simulate the vertical ground stress applied to the core during expansion and fracturing; The frame (1) comprises an upper plate (11), a lower plate (12) and a fixing rod (13); the upper plate (11) is supported above the lower plate (12) by the fixing rod (13); a limiting groove is provided on the lower plate (12); the thick-walled tube (3) is placed on the lower plate (12); the upper plate (11) is located above the thick-walled tube (3); and the screw rod (6) of the device passes through the upper plate (11).

2. The device for simulating crack morphology caused by hot dry rock fracturing according to claim 1, characterized in that: The upper pad (4) is connected to a screw rod (6), the screw rod (6) passes through a fixing nut (14) on the frame (1), the screw rod (6) is threadedly connected to the fixing nut (14), and the height of the upper pad (4) can be changed when the screw rod (6) is rotated; The frame (1) is provided with a limiting groove, the lower pad (5) is located in the limiting groove, the lower pad (5) is inserted into the thick-walled tube (3), and the sum of the height of the core and the thickness of the upper pad (4) and the lower pad (5) is not greater than the height of the thick-walled tube (3); The diameter of the lower pad (5) is consistent with the diameter of the upper pad (4).

3. The device for simulating crack morphology caused by hot dry rock fracturing according to any one of claims 1 to 2, characterized in that: The heating device comprises a heating sleeve and a temperature controller (2), the heating sleeve being connected to the temperature controller (2), and the heating sleeve being wrapped around the outside of the core limiting structure thick-walled tube (3) to heat the core in the thick-walled tube (3); The temperature controller (2) is connected to two temperature sensors, the two temperature sensors are arranged between the rock core and the inner wall of the thick-walled tube (3), and the two temperature sensors are distributed opposite to each other in the diameter direction of the thick-walled tube (3); Through the control of the temperature controller (2), temperature rises at different rates can be achieved, and a heating method with multiple alternating temperature rise rates can be achieved.

4. The device for simulating crack morphology caused by hot dry rock fracturing according to any one of claims 1 to 2, characterized in that: The device further comprises an acoustic emission device (7), the acoustic emission device (7) comprising an acoustic emission controller (71) and an acoustic emission probe (72), the acoustic emission controller (71) being connected to the acoustic emission probe (72), and the acoustic emission probe (72) being distributed on the outer wall of the thick-walled tube (3) of the core limiting structure; A probe mounting hole is provided on the outer wall of the thick-walled cylinder (3), and the acoustic emission probe (72) is located in the corresponding probe mounting hole; There are a plurality of acoustic emission probes (72), the acoustic emission probes (72) are distributed along the height direction of the thick-walled tube (3), and the acoustic emission probes (72) at the same height are evenly distributed along the circumferential direction of the thick-walled tube (3).

5. A method for performing expansion fracturing on hot dry rock using the device for simulating the formation of crack morphology by fracturing of hot dry rock according to any one of claims 1 to 4, characterized in that: The following steps are involved: Placing the drilled core in the core limiting structure, and starting the heating device to heat the core to simulate the core environment temperature; After being heated to a predetermined temperature, the expansion agent slurry is poured into the drill hole of the core, and the core limiting structure is used to simulate the effect of the surrounding ground stress on the core when it expands and fractures.

6. The method according to claim 5, characterized in that Specifically include the following: Step S1, drilling a bare core, drilling a hole in the middle of the core, measuring the diameter of the core with a measuring tool, determining the amount of filling material to be wrapped around the outside of the core according to the difference between the measured minimum core diameter and the inner diameter of the thick-walled tube (3), wrapping the filling material around the core and tightening it with a rubber band; Step S2, placing the lower pad (5) on the limiting groove of the lower plate (12), placing the rock core on the lower pad (5), and wrapping the two semicircular thick-walled cylinders (31) around the rock core and connecting them; Step S3, installing the acoustic emission device (7), setting the acoustic emission probe (72) on the outer wall of the thick-walled tube (3); installing the heating sleeve, wrapping the heating sleeve around the outer side of the thick-walled tube (3), and connecting the temperature sensor; Step S4, connecting the power supply, starting the temperature controller (2), setting the corresponding heating mode and heating time, subjecting the core to the preset temperature mode, and starting the acoustic emission controller (71); Step S5, injecting a slurry expansion agent into the core borehole through an iron pipe, wherein the proportion of the expansion agent is as follows: selecting a HSCA-1 type high-efficiency expansion agent, mixing the expansion agent with water at a mass ratio of 4:1, and then fully stirring to form a mixed slurry expansion agent; Step S6, turning the screw rod (6) to drive the upper pad (4) to move downward, so that the upper pad (4) contacts the upper end surface of the rock core, and applies axial stress to the rock core; Step S7: After the core is fractured, it is cooled to room temperature, the heating jacket is opened, the acoustic emission probe (72) is unloaded, the screw (6) is turned to lift the upper pad (4), the two semicircular thick-walled cylinders (31) are opened, and the broken core is taken out; Step S8, after taking out the broken core, obtain the internal crack extension morphology of the broken core, and use a colored oil pen to depict the cracks on the surface of the core; use a CT scanning method to perform three-dimensional reconstruction imaging of the fracture network topology to characterize the fracture morphology, or characterize it through volume density and fractal dimension.

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