Tight rock fracturing device and fracturing method based on CO2 phase change

Abstract

The present invention discloses a dense rock fracturing device and a fracturing method based on CO2 phase change. The dense rock fracturing device includes a dense rock sample with a central through hole, and also includes an injection module, which is configured to inject fluid into the dense rock sample; an upper sealing assembly and a lower sealing pipe, both of which are arranged in the central through hole, and the upper sealing assembly and the lower sealing pipe are arranged relative to each other, and the upper sealing assembly, the lower sealing pipe and the central through hole are enclosed to form an open hole section, and the fluid is injected into the open hole section; a temperature control module, including a cold control unit and a heat control unit, and the cold control unit and the heat control unit are connected to the open hole section; a fluid monitoring module, which is used to monitor the pressure and temperature of the fluid in the open hole section. The alternating effect of fluid temperature and pressure can degrade the mechanical properties of the dense rock sample; the phase change of the fluid induces the generation of dominant fractures, and the two work together to achieve the purpose of strengthening the transformation of the dense rock sample.

Description

Technical Field

[0001] The present invention relates to the technical field of dense rock material fracturing, and in particular to a dense rock fracturing device and a fracturing method based on CO2 phase change. Background Art

[0002] In the field of efficient unconventional oil and gas extraction, hydraulic fracturing technology is one of the commonly used methods for increasing the permeability of tight reservoirs. This technology uses a ground pump truck group to inject a mixture of water, proppants, chemical reagents, etc. into the deep formation at high pressure to fracture the reservoir, providing a large range of seepage channels for oil and gas extraction. CO2, as a water-free fracturing fluid, provides a green and efficient method for the development of oil and gas reservoirs. It should be noted that when the ground pump truck group injects liquid CO2 directly into the deep reservoir, its temperature and pressure will change simultaneously, during which there will be phase changes in the wellbore, and CO2 exists in a multi-phase form. In addition, once the reservoir rock at a specified depth cracks, the volume of the fracture changes after the transformation, and the temperature and pressure of the injected CO2 also change accordingly, which will also cause the CO2 to undergo a phase change.

[0003] According to the CO2 phase diagram, when the temperature exceeds 31.1°C and the pressure exceeds 7.38 MPa, CO2 enters a supercritical state, which is common in reservoirs deeper than 800 meters. Therefore, based on the advantages of supercritical CO2 fracturing and the physical property of volume expansion induced by phase changes (i.e., the transformation from liquid or supercritical state to gas), it can be found that in order to increase reservoir productivity, reduce surface pump group pressure, and respond to the concept of green ecology, the use of CO2 phase changes can be used to efficiently fracture tight reservoir rocks.

[0004] At present, there are two main forms of using CO2 phase change for rock breaking: one is CO2 directional fracturing device, and the other is "cold shock-endothermic heating". The first working mode is to fill the liquid storage tube with liquid CO2 under normal temperature and high pressure conditions, which can obtain a physical explosion effect, but the directional fracturing device contains multiple components, and some components cannot be reused; the second working mode is to inject low-temperature liquid CO2 into the rock, then empty the CO2, and then heat the frozen rock to achieve the fracturing effect through this cycle. By analyzing the working principles and usage systems of the above two forms, it can be found that: (1) The directional fracturing device is an assembled composite structure, which is difficult to adapt to the open hole section packaging in small physical simulation tests and the real-time control of the temperature parameters in the hole; (2) For the "cold shock-endothermic heating" split cycle form, although this form can obtain a certain fracturing effect, it is still different from the continuous fluid injection form used in actual engineering applications; (3) The process of cold injection of liquid CO2 into the actual formation corresponds to the wellbore-rock heat exchange of the indoor test system. This process is a real-time change rather than an artificial split. Therefore, both methods are unable to achieve sealing within the open hole section and meet the requirements of multi-mode precise control of fluid phase changes within the open hole section. Summary of the Invention

[0005] The present invention aims to solve at least one of the technical problems existing in the prior art or related art.

[0006] In view of this, the present invention provides a dense rock fracturing device and fracturing method based on CO2 phase change, wherein the dense rock fracturing device can degrade the mechanical properties of dense rock samples through the cooperation of a temperature control module and an injection module; the fluid phase change induces the generation of dominant fractures, thereby achieving the purpose of strengthening the fracturing transformation of dense rock samples.

[0007] Specifically, the following technical solutions are included:

[0008] An embodiment of a first aspect of the present invention provides a dense rock fracturing device based on CO2 phase change, comprising a dense rock sample having a central through hole, the dense rock pressure device comprising:

[0009] an injection module configured to inject fluid into the dense rock sample;

[0010] An upper sealing assembly and a lower sealing pipe are both disposed in the central through hole. The upper sealing assembly and the lower sealing pipe are disposed opposite to each other. The upper sealing assembly, the lower sealing pipe and the central through hole together form an open hole section, and the fluid is injected into the open hole section.

[0011] a temperature control module, comprising a cold control unit and a heat control unit, wherein the cold control unit and the heat control unit are connected to the open hole section;

[0012] The fluid monitoring module is used to monitor the pressure and temperature of the fluid in the open hole section.

[0013] Optionally, the dense rock pressure device further comprises:

[0014] A height-fixing component, wherein the first end of the height-fixing component is detachably connected to the upper sealing component, the second end of the height-fixing component is detachably connected to the lower sealing pipe, and an output hole is provided on the height-fixing component, and the output hole is connected to the upper sealing component and the injection module.

[0015] Optionally, the height setting component includes: a base, a blind hole is provided in the base, the first end of the base and the second end of the base are provided with external threads, an output hole is provided on the base, the output hole is connected to the blind hole, and the blind hole is connected to the upper sealing assembly and the injection module.

[0016] Optionally, a first fixing nut is provided on the first end of the base, and a second fixing nut is provided on the second end of the base. The first fixing nut is used to limit the connection length between the first end of the base and the upper sealing assembly, and the second fixing nut is used to limit the connection length between the second end of the base and the lower sealing pipe.

[0017] Optionally, the upper sealing assembly includes:

[0018] An outer tube is disposed in the central through hole, an end of the outer tube close to the lower sealing tube is provided with an internal thread, and the outer tube is connected to the first end of the height-fixing assembly through the internal thread;

[0019] an inner tube, the inner tube being disposed within the outer tube, the inner tube being in communication with the output hole and the injection module, the inner tube forming a fluid injection channel, the inner tube being shorter than the outer tube, the inner tube and the outer tube being aligned at ends away from the lower sealing tube, and a temperature measuring element being disposed within the inner tube;

[0020] Wherein, an upper pressure head is provided at one end of the upper sealing component away from the lower sealing pipe.

[0021] Optionally, a first groove and a second groove are provided on a side of the outer tube facing away from the inner tube, and the upper sealing assembly further comprises:

[0022] a first sealing member, disposed in the first groove;

[0023] a second sealing member disposed in the second groove, with a preset distance between the second sealing member and the first sealing member;

[0024] Wherein, the first sealing member and the second sealing member are located at one end of the outer tube close to the lower sealing tube member.

[0025] Optionally, the lower sealing pipe comprises:

[0026] A sealing body, wherein the sealing body is provided with four through holes;

[0027] a heat medium U-shaped tube, disposed in two of the through holes, the open end of the heat medium U-shaped tube being connected to the temperature control module, and the closed end of the heat medium U-shaped tube being located in the open hole section;

[0028] a refrigerant U-shaped tube disposed in the other two through holes, the open end of the refrigerant U-shaped tube being connected to the temperature control module, and the closed end of the refrigerant U-shaped tube being located in the open hole section;

[0029] Wherein, a lower pressure head is provided between the lower sealing pipe and the temperature control module.

[0030] Optionally, a third groove and a fourth groove are provided on the side wall of the sealing body facing the central through hole, and the lower sealing pipe further comprises:

[0031] a third sealing member, disposed in the third groove;

[0032] a fourth sealing member disposed in the fourth groove, with a preset distance between the fourth sealing member and the third sealing member;

[0033] Wherein, the third sealing member and the fourth sealing member are located at one end of the lower sealing tube member close to the upper sealing assembly.

[0034] Optionally, the temperature control module includes: a cold control unit and a heat control unit, the cold control unit is connected to the refrigerant U-shaped tube, and the heat control unit is connected to the heat medium U-shaped tube.

[0035] Optionally, the cold control unit includes: a refrigerant storage tank, a refrigerant medium, a cold circulation pump, a cold exchanger and a cold control system. The cold circulation pump inputs the refrigerant medium in the refrigerant storage tank into the refrigerant U-shaped tube. After passing through the refrigerant U-shaped tube, the refrigerant passes through the cold exchanger and returns to the refrigerant storage tank. The circulation of the refrigerant is controlled by a first reversing valve.

[0036] Optionally, the thermal control unit includes: a heat medium storage tank, a heat medium, a heat circulation pump, a heat exchanger and a thermal control system. The heat circulation pump inputs the heat medium in the heat medium storage tank into the heat medium U-shaped tube. After passing through the heat medium U-shaped tube, the heat medium passes through the heat exchanger and returns to the heat medium storage tank. The circulation of the heat medium is controlled by a second reversing valve.

[0037] Optionally, the injection module includes: a gas cylinder and a fluid injection pump, the gas cylinder is connected to the fluid injection pump through a pipeline, the fluid injection pump is connected to the fluid injection channel through a pipeline, and is communicated with the height-fixing component.

[0038] Optionally, the fluid monitoring module includes:

[0039] a temperature monitoring unit connected to the temperature measuring element, wherein the temperature monitoring unit is configured to obtain the temperature of the fluid in the open hole section;

[0040] A pressure monitoring unit is provided at one end of the fluid injection channel close to the upper pressure head, and is configured to obtain the pressure of the fluid in the open hole section.

[0041] An embodiment of the second aspect of the present invention provides a method for fracturing dense rock based on CO2 phase change, using the above-mentioned dense rock fracturing device, the method comprising the following steps:

[0042] Preparation of dense rock specimens with central through-holes;

[0043] Adjust the height of the open hole section as needed, and form a sealed space in the open hole section through the upper sealing component and the lower sealing component;

[0044] Test the sealing of the open hole section. If the sealing is effective, proceed to the next step. If the sealing is not effective, return to the previous step.

[0045] According to the rock fracturing test requirements, applying a preset axial pressure, confining pressure and setting an ambient temperature to the dense rock sample;

[0046] Turning on the temperature control module to adjust the temperature change in the open hole section through the temperature control module, and at the same time turning on the injection module to inject fluid into the open hole section for intermittent pressurization; or

[0047] Only the injection module is opened, and the fluid volume is changed to obtain different fluid pressure combinations in a manner of dynamically distributing the flow rate, and the fluid is injected into the open hole section for pressurization;

[0048] During the test, the fluid pressure curve and temperature were monitored synchronously, and the fracture network morphology was finally obtained.

[0049] The embodiments of the present invention provide a dense rock fracturing device and a fracturing method based on CO2 phase change, wherein the dense rock fracturing device includes an upper sealing assembly and a lower sealing pipe, which enclose an open hole section with a central through hole of a dense rock sample. Fluid is injected into the open hole section through an injection module, the temperature change in the open hole section is regulated by a temperature control module, and the pressure in the open hole section is regulated by dynamically allocating the injection flow rate. Phase change dominated by pressure is achieved by fluid volume change, and phase change dominated by temperature is achieved by heat transfer between different media in the open hole section. Under the premise of efficient sealing of the open hole section, the rock pore wall is subjected to the alternating action of fluid temperature and pressure. On the one hand, the mechanical properties of the dense rock sample are degraded, thereby promoting the preferential generation of multiple initial induced cracks inside the dense rock sample and reducing the pumping pressure for the initiation of cracking of the dense rock sample. On the other hand, the phase change effect can induce the generation of more dominant fractures, thereby forming a more complex fracture network, thereby achieving the purpose of enhancing the transformation effect of the dense rock sample. The sealing of the open hole section within the central through-hole of a dense rock specimen in this application differs from conventional glue injection. This assembly structure simplifies the sealing operation of the dense rock specimen and achieves reusability in an integrated form. Furthermore, the method of controlling the phase change of CO2 within the open hole section in this application overcomes the limitations of conventional directional fracturing devices in dense rock specimens containing central through-holes and provides a new injection method for fracturing dense rock specimens. This method has important reference value for indoor simulation experiments to study the phase change of CO2 coupled with cyclic injection and improve the effectiveness of reservoir fracturing.

[0050] The above description is only an overview of the technical solution of the present application. In order to more clearly understand the technical means of the present application, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. 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 creative work.

[0052] Figure 1 is a schematic diagram of a dense rock fracturing device according to one embodiment of the present invention;

[0053] Figure 2 is a schematic diagram of a central through hole according to one embodiment of the present invention;

[0054] Figure 3 is a schematic diagram of a temperature control module according to an embodiment of the present invention;

[0055] Figure 4 is a schematic diagram of a dense rock sample having a central through hole according to one embodiment of the present invention;

[0056] Figure 5a is an exemplary cross-sectional view of a rock fracturing effect near an open hole section according to an embodiment of the present invention;

[0057] Figure 5b is an exemplary cross-sectional view of a rock fracturing effect near an open hole section according to an embodiment of the present invention;

[0058] Figure 6 Schematic diagram of a fluid pressure curve for fracturing a dense rock sample according to one embodiment of the present invention;

[0059] Figure 7 Schematic diagram of a fluid temperature curve for fracturing a dense rock sample according to one embodiment of the present invention;

[0060] Figure 8 The figure is a flowchart of the steps of a dense rock fracturing method according to one embodiment of the present invention.

[0061] in, Figures 1 to 5b The corresponding relationship between the reference numerals and component names is as follows:

[0062] 1 dense rock sample, 101 central through hole, 102 initial induced fracture, 103 dominant pressure fracture, 2 gas cylinder, 3 fluid injection pump, 4 upper sealing assembly, 401 outer tube, 402 inner tube, 403 temperature measuring component, 404 first sealing component, 405 second sealing component, 5 lower sealing pipe, 501 sealing body, 502 third sealing component, 503 fourth sealing component, 6 joint assembly, 7 fluid injection channel, 8 temperature control module, 801 refrigerant medium, 802 cold circulation pump, 803 cooler, 8 04 Cooling control system, 805 Heat medium, 806 Impurity filter, 807 Overflow valve, 808 Heat exchanger, 809 Thermal control system, 810 First reversing valve, 811 Heat circulation pump, 812 Second reversing valve, 9 Upper pressure head, 10 Lower pressure head, 11 Temperature monitoring unit, 12 Pressure monitoring unit, 14 Height setting component, 1401 Base, 1402 Blind hole, 1403 Output hole, 15 Heat medium U-tube, 16 Refrigerant U-tube, 17 Naked eye section, 18 First fixing nut, 19 Second fixing nut. DETAILED DESCRIPTION

[0063] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0064] Before further describing the embodiments of the present invention in detail, the directional terms involved in the embodiments of the present invention, such as "upper part", "lower part" and "side part", do not have the meaning of limiting the scope of protection of the present invention.

[0065] To make the technical solutions and advantages of the present invention more clear, the embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0066] Figure 1 is a schematic diagram of a dense rock fracturing device according to one embodiment of the present invention; Figure 4 Schematic diagram of a dense rock sample containing a central through hole according to one embodiment of the present invention.

[0067] like Figure 1 and Figure 4 As shown, one embodiment of the present invention provides a dense rock fracturing device based on CO2 phase change, including a dense rock sample 1 having a central through hole 101. The dense rock pressure device includes:

[0068] an injection module configured to inject fluid into the dense rock sample 1;

[0069] The upper sealing assembly 4 and the lower sealing pipe 5 are both disposed in the central through hole 101. The upper sealing assembly 4 and the lower sealing pipe 5 are disposed opposite to each other. The upper sealing assembly 4, the lower sealing pipe 5 and the central through hole 101 enclose an open hole section 17. The fluid is injected into the open hole section 17.

[0070] The temperature control module 8 includes a cold control unit and a heat control unit, and the cold control unit and the heat control unit are connected to the open hole section 17;

[0071] The fluid monitoring module is used to monitor the pressure and temperature of the fluid in the open hole section 17.

[0072] Among them, the dense rock fracturing device includes an upper sealing component 4 and a lower sealing pipe 5, which together with the central through hole 101 of the dense rock sample 1 form an open hole section 17. Fluid is injected into the open hole section 17 through the injection module, and the temperature change in the open hole section 17 is adjusted by the temperature control module 8. The phase change of temperature control is realized by heat transfer of different media in the open hole section 17. The pressure in the open hole section 17 is adjusted by dynamically distributing the injection flow rate, and the phase change of pressure control is realized by the change of fluid volume. Under the premise of efficient sealing, the open hole section 17 is subjected to the alternating temperature-pressure effect of the fluid on the rock pore wall. On the one hand, the mechanical properties of the dense rock sample 1 are degraded, and multiple initial induced cracks 102 are preferentially generated inside the dense rock sample 1, thereby reducing the pumping pressure for the initiation of cracking of the dense rock sample 1. On the other hand, the phase change effect induces the generation of more dominant fractures 103, thereby forming a more complex fracture network, thereby achieving the purpose of strengthening the transformation effect of the dense rock sample 1. The sealing of the open hole section 17 within the central through hole 101 of the dense rock sample 1 in this application differs from conventional glue injection. This assembly structure simplifies the sealing operation of the dense rock sample 1 and achieves reusability in an integrated form. Furthermore, the method of controlling the phase change of CO2 within the open hole section 17 in this application overcomes the limitations of conventional directional fracturing devices in dense rock samples 1 containing central through hole 101 and provides a new injection method for fracturing dense rock samples 1. This has important reference value for indoor simulation experiments to study the phase change of CO2 coupled with cyclic injection and to improve the effectiveness of reservoir fracturing.

[0073] It should be noted that the dense rock fracturing device based on CO2 phase change of this embodiment can not only solve the adaptation problem of the size of the open hole section 17 in the indoor small-scale physical simulation test and make the open hole section 17 completely sealed, but also actively induce the CO2 phase change in the open hole section 17 while ensuring the sealing of the dense rock sample 1, and finally achieve the goal of strengthening the fracturing transformation of the dense rock sample 1.

[0074] Figure 2 Schematic diagram of the center through hole according to one embodiment of the present invention.

[0075] In one possible implementation, Figure 2 As shown, the dense rock pressure device also includes:

[0076] The height-fixing component 14, the first end of the height-fixing component 14 is detachably connected to the upper sealing component 4, the second end of the height-fixing component 14 is detachably connected to the lower sealing pipe 5, and an output hole 1403 is provided on the height-fixing component 14, and the output hole 1403 is connected to the upper sealing component 4 and the injection module.

[0077] It should be noted that the upper sealing component 4 and the lower sealing pipe fitting 5 are respectively connected at both ends of the height determining component 14. Usually, the first end of the height determining component 14 is detachably connected to the upper sealing component 4, and the second end of the height determining component 14 is detachably connected to the lower sealing pipe fitting 5. In this embodiment, the detachable connection is a threaded connection. By screwing in the length of the threaded section, the distance between the upper sealing component 4 and the lower sealing pipe fitting 5 is controlled, that is, the height of the open hole section 17 is controlled. Therefore, through the setting of the height determining component 14, open hole sections 17 of different heights can be set according to test requirements, thereby realizing the adjustability of the height of the open hole section 17 and improving the applicability of the dense rock fracturing device.

[0078] Among them, since the height-fixing component 14 is used to support the upper sealing component 4 and the lower sealing pipe 5, the middle section of the height-fixing component 14 is located in the open hole section 17, and an output hole 1403 needs to be set on the height-fixing component 14. At the same time, the fluid of the injection module enters the height-fixing component 14 after passing through the upper sealing component 4, and is output from the output hole 1403 of the height-fixing component 14, thereby achieving the purpose of the input module injecting fluid into the open hole section 17.

[0079] In a feasible embodiment, the height fixing component 14 includes: a base 1401, a blind hole 1402 is provided in the base 1401, the first end of the base 1401 and the second end of the base 1401 are provided with external threads, an output hole 1403 is provided on the base 1401, the output hole 1403 is connected to the blind hole 1402, and the blind hole 1402 is connected to the upper sealing component 4 and the injection module.

[0080] Among them, the height determination component 14 includes a base 1401, and a blind hole 1402 and an output hole 1403 are provided in the base 1401. The output hole 1403 is used to connect the blind hole 1402 and the open hole section 17. The blind hole 1402 is connected to the upper sealing component 4. The injection module injects fluid into the upper sealing component 4. The fluid then enters the blind hole 1402 and enters the open hole section 17 through the output hole 1403 connected to the blind hole 1402, which is convenient for subsequent fracturing tests.

[0081] It should be noted that the first and second ends of the base 1401 are both provided with external threads, and the ends at which the upper sealing component 4 and the lower sealing pipe 5 are connected to the base 1401 are provided with internal threads. Through the cooperation of the internal and external threads, on the one hand, the connection between the height fixing component 14 and the upper sealing component 4 and the lower sealing pipe 5 is realized, and on the other hand, the height of the open hole section 17 can be adjusted by adjusting the screw-in length of the external thread, so that the height of the open hole section 17 meets different test requirements.

[0082] In a feasible embodiment, a first fixing nut 18 is provided on the first end of the base 1401, and a second fixing nut 19 is provided on the second end of the base 1401. The first fixing nut 18 is used to limit the connection length between the first end of the base 1401 and the upper sealing assembly 4, and the second fixing nut 19 is used to limit the connection length between the second end of the base 1401 and the lower sealing pipe 5.

[0083] It should be noted that after the first end of the base 1401 is connected to the upper sealing assembly 4, the length of the base 1401 screwed into the upper sealing assembly 4 can be locked by the first fixing nut 18, so that even when the fluid pressure increases, the base 1401 will not be deflected or moved to a certain extent; similarly, after the second end of the base 1401 is connected to the lower sealing tube 5, the length of the base 1401 screwed into the lower sealing tube 5 can be locked by the second fixing nut 19, so that even when the fluid pressure increases, the base 1401 will not be deflected or moved to a certain extent.

[0084] In one possible implementation, Figure 2 As shown, the upper sealing assembly 4 includes:

[0085] The outer tube 401 is disposed in the central through hole 101. An inner thread is provided on one end of the outer tube 401 close to the lower sealing tube 5. The outer tube 401 is connected to the first end of the height-fixing assembly 14 via the inner thread.

[0086] Inner tube 402, inner tube 402 is disposed within outer tube 401, and is in communication with output port 1403 and the injection module. Inner tube 402 forms fluid injection channel 7. The length of inner tube 402 is less than that of outer tube 401. The ends of inner tube 402 and outer tube 401 away from lower sealing tube 5 are aligned. A temperature measuring element 403 is disposed within inner tube 402;

[0087] An upper pressure head 9 is provided at one end of the upper sealing component 4 away from the lower sealing pipe 5 .

[0088] It should be noted that the upper sealing assembly 4 includes an outer tube 401 adapted to the central through hole 101 and an inner tube 402 serving as a fluid injection channel 7. The length of the inner tube 402 is smaller than the length of the outer tube 401, and the inner tube 402 and the outer tube 401 are aligned at one end away from the lower sealing pipe 5. The outer tube 401 and the inner tube 402 are fixed by welding the aligned ends. After the upper sealing assembly 4 enters the central through hole 101, the aligned end is pressed by the upper pressure head 9 to achieve the fixation and sealing of the upper sealing assembly 4. A joint assembly 6 is provided on the side of the upper pressure head 9 facing away from the upper sealing assembly 4. The pipeline of the injection module passes through the joint assembly 6 and the upper pressure head 9 in turn, and enters the inner tube 402 serving as the fluid injection channel 7 to achieve fluid injection.

[0089] In a feasible embodiment, a first groove and a second groove are provided on a side of the outer tube 401 facing away from the inner tube 402, and the upper sealing assembly 4 further includes:

[0090] A first sealing member 404 is disposed in the first groove;

[0091] The second sealing member 405 is disposed in the second groove, with a preset distance between the second sealing member 405 and the first sealing member 402;

[0092] The first sealing member 404 and the second sealing member 405 are located at one end of the outer tube 401 close to the lower sealing tube 5 .

[0093] It should be noted that a first groove and a second groove are provided on the side of the outer tube 401 facing away from the inner tube 402. A first seal 404 is provided in the first groove, and a second seal 405 is provided in the second groove. The first seal 404 is an O-ring, and the second seal 405 is a Y-ring. The O-ring can play a role in straightening the upper seal assembly 4 during the installation process of the upper seal assembly 4, ensuring that the structure of the upper seal assembly 4 floats in the center position of the central through hole 101; the Y-ring is used to adapt to rock hole walls with different processing roughness. The arrangement of the two rings can ensure that the fluid in the open hole section 17 cannot leak from the inner wall of the dense rock sample 1 during the injection process. In this embodiment, the O-ring is arranged above the Y-ring, and a distance of 1mm to 2mm is left between the two, that is, the preset distance is 1mm to 2mm. Among them, since the cross-section of the Y-ring is Y-shaped, it is a typical lip-shaped sealing ring. It relies on its open lip to stick to the coupling surface of the sealing pair, and the action of the lip effectively applies fluid pressure to the seal. This design makes the lip of the Y-ring subjected to circumferential compression when sealing, and the contact with the sealing surface becomes wider, and the contact stress increases, thereby providing better sealing performance. Therefore, the step shape is also suitable for the lip-shaped installation method; the O-ring cross-section is circular, and its sealing performance mainly depends on the preload force of the O-ring in the groove and the interference fit between the groove and the O-ring. For rocks with different processing roughness, this may affect the interference fit between the O-ring and the groove, and thus affect its sealing performance. Therefore, in this embodiment, the O-ring and the Y-ring are used together to effectively improve the sealing performance of the upper sealing assembly 4.

[0094] It is understood that the second groove forms a stepped shape with the bottom end of the outer tube 401, which facilitates the installation and removal of the Y-shaped sealing ring. Furthermore, an internally threaded hole is provided within the outer tube 401, which is used to connect to the first end of the height control assembly 14. The internally threaded hole communicates with the inner tube 402, facilitating the fluid in the inner tube 402 to enter the blind hole 1402 of the height control assembly 14 and flow to the open-hole section 17. A stepped shape can be provided at the end of the internally threaded hole away from the inner tube 402, recessed into the lower end of the outer tube 401, to accommodate the first fixing nut 18.

[0095] In a feasible embodiment, the lower sealing pipe 5 includes:

[0096] The sealing body 501 is provided with four through holes;

[0097] A heat medium U-shaped tube 15 is disposed in two of the through holes. The open end of the heat medium U-shaped tube 15 is connected to the temperature control module 8, and the closed end of the heat medium U-shaped tube 15 is located in the open hole section 17;

[0098] The refrigerant U-shaped tube 16 is disposed in the other two through holes, the open end of the refrigerant U-shaped tube 16 is connected to the temperature control module 8, and the closed end of the refrigerant U-shaped tube 16 is located in the open hole section 17;

[0099] A lower pressure head 10 is provided between the lower sealing pipe 5 and the temperature control module 8 .

[0100] It should be noted that the four through holes provided on the sealing body 501 are used to insert the heat medium U-shaped tube 15 and the refrigerant U-shaped tube 16. The four through holes are evenly distributed on the sealing body 501, and the distance between the through holes and the outer wall of the sealing body 501 is selected according to actual needs and is not restricted.

[0101] It is understood that the lower sealing tube 5 is pressed into the central through hole 101 by the lower pressure head 10 at the end away from the upper sealing assembly 4. In other words, the arrangement of the upper pressure head 9 and the lower pressure head 10 ensures the sealing of the central through hole 101. The lower pressure head 10 is provided with four through holes, which are arranged opposite to the four through holes of the sealing body 501, facilitating the connection of the refrigerant U-shaped tube 16 and the heating medium U-shaped tube 15 to the temperature control module 8.

[0102] In a feasible embodiment, a third groove and a fourth groove are provided on the side wall of the sealing body 501 facing the central through hole 101, and the lower sealing pipe 5 further includes:

[0103] A third sealing member 502 is disposed in the third groove;

[0104] The fourth sealing member 503 is disposed in the fourth groove, with a preset distance between the fourth sealing member 503 and the third sealing member 502;

[0105] The third sealing member 502 and the fourth sealing member 503 are located at one end of the lower sealing tube 5 close to the upper sealing assembly 4 .

[0106] It should be noted that a third groove and a fourth groove are provided on the side wall of the sealing body 501 facing the central through hole 101. A third sealing member 502 is provided in the third groove, and a fourth sealing member 503 is provided in the fourth groove. The third sealing member 502 and the fourth sealing member 503 are located at the end of the lower sealing tube 5 close to the upper sealing assembly 4. The third sealing member 502 is an O-ring, and the fourth sealing member 503 is a Y-ring. The O-ring can straighten the lower sealing tube 5 during installation, ensuring that the structure of the lower sealing tube 5 floats in the center of the central through hole 101. The Y-ring is used to adapt to rock hole walls with different processing roughness. The arrangement of the two sealing rings can ensure that the fluid in the open hole section 17 cannot leak from the inner wall of the dense rock sample 1 during the injection process. In this embodiment, the O-ring is provided below the Y-ring, and a distance of 1 mm to 2 mm is left between the two, that is, the preset distance is 1 mm to 2 mm. The principle is the same as that of the first seal 404 and the second seal 405. The cooperation of the third seal 502 and the fourth seal 503, that is, the O-ring and the Y-ring, can improve the sealing effect of the lower sealing pipe 5, so that the open hole section 17 meets the sealing requirements.

[0107] It is understood that the fourth groove forms a step with the top of the sealing body 501, which facilitates the installation and removal of the Y-shaped sealing ring. Furthermore, an internally threaded hole is provided at the center of the end of the sealing body 501 facing the upper sealing assembly 4. This internally threaded hole is used to connect to the second end of the height control assembly 14. A step can be provided at the end of the internally threaded hole facing away from the lower pressure head 10, recessed into the top of the sealing body 501, to accommodate the second fixing nut 19.

[0108] The upper sealing assembly 4 and the lower sealing pipe 5 are tightened by the first fixing nut 18 and the second fixing nut 19 to ensure that the length of the open hole section 17 remains unchanged.

[0109] Figure 3 FIG. 1 is a schematic diagram of a temperature control module according to an embodiment of the present invention.

[0110] In one possible implementation, Figure 3 As shown, the temperature control module 8 includes: a cold control unit and a heat control unit. The cold control unit is connected to the refrigerant U-shaped tube 16 , and the heat control unit is connected to the heat medium U-shaped tube 15 .

[0111] The temperature control module 8 is used to adjust the temperature of the fluid in the open hole section 17 , ensuring that heat transfer in the open hole section 17 achieves temperature-controlled fluid phase changes.

[0112] It is understandable that the fluid in the present application refers to CO2, which is a common gas with a variety of different phases. At normal temperature and pressure, CO2 is mainly in a gaseous state. When the pressure and temperature change, CO2 may be converted into a liquid, solid, and supercritical state. Among them, when the temperature exceeds 31.1°C and the pressure exceeds 7.38MPa, supercritical CO2 has both the density of a liquid and the fluidity of a gas. The high solubility, low viscosity, and high diffusivity of supercritical CO2 are suitable for fracturing dense rocks. Through the adjustment of the injection module and the adjustment of the temperature control module 8 in the present application, the phase of the fluid in the open hole section 17 is changed, which is convenient for enhancing the effect of fracturing the dense rock sample 1.

[0113] It should be noted that the upper sealing component 4, the height fixing component 14 and the lower sealing pipe 5 form a sealing module, which is placed in the central through hole 101 of the dense rock sample 1 to carry out tests on the dense rock sample 1. The detachable form allows the sealing module to be used for multiple dense rock samples 1 and can be reused, thereby reducing the test cost.

[0114] In a feasible embodiment, the cold control unit includes: a refrigerant storage tank, a refrigerant 801, a cold circulation pump 802, a cooler 803 and a cold control system 804. The cold circulation pump 802 inputs the refrigerant 801 in the refrigerant storage tank into the refrigerant U-shaped tube 16. After passing through the refrigerant U-shaped tube 16, the refrigerant 801 passes through the cooler 803 and returns to the refrigerant storage tank. The circulation of the refrigerant 801 is controlled by the first reversing valve 810.

[0115] Among them, the cooling control unit provides a medium for cooling and heat exchange for the fluid in the open hole section 17. The preset temperature and circulation mode are set by the cooling control system 804, and the refrigerant 801 is input to the cold circulation pump 802. It enters the open hole section 17 through the refrigerant U-shaped pipe 16. After passing through the refrigerant U-shaped pipe 16, the refrigerant 801 is output to the cold exchanger 803 for cooling, and then returned to the refrigerant storage tank for the next recycling of the refrigerant 801. It should be noted that the refrigerant 801 will heat up after passing through the refrigerant U-shaped pipe 16, so it needs to be cooled by the cold exchanger 803 and then returned to the refrigerant storage tank to ensure the repeated use of the refrigerant 801.

[0116] In a feasible embodiment, the thermal control unit includes: a heat medium storage tank, a heat medium 805, a heat circulation pump 811, a heat exchanger 808 and a thermal control system 809. The heat circulation pump 811 inputs the heat medium 805 in the heat medium storage tank into the heat medium U-shaped tube 15. After passing through the heat medium U-shaped tube 15, the heat medium 805 passes through the heat exchanger 808 and returns to the heat medium storage tank. The circulation of the heat medium 805 is controlled by the second reversing valve 812.

[0117] The thermal control unit provides a medium for heating and heat exchange for the fluid in the open hole section 17. The thermal control system 809 sets a preset temperature and circulation mode, inputs heat medium 805 into the heat circulation pump 811, and enters the open hole section 17 through the heat medium U-shaped pipe 15. After passing through the heat medium U-shaped pipe 15, the heat medium 805 is output to the heat exchanger 808 for heating, and then returns to the heat medium storage tank for the next recycling of the heat medium 805. It should be noted that after passing through the heat medium U-shaped pipe 15, the heat medium 805 will cool down, so it needs to be heated by the heat exchanger 808 before returning to the heat medium storage tank to ensure the repeated use of the heat medium 805.

[0118] It should be noted that an impurity filter 506 is provided on the heat medium 805. The heat medium 805 is preferably thermal oil, and the cold medium 801 is preferably Freon. When the heat transfer oil comes into contact with air or oxidizing substances under high temperature conditions (above 60°C), an oxidation reaction will occur. That is to say, during use, the heat transfer oil may produce some non-metallic impurities, such as colloids and sediments, due to high-temperature oxidation. These impurities affect the heat transfer effect and the safety of the dense rock fracturing device. Therefore, an impurity filter 506 is required to prevent impurities from entering the dense rock fracturing device and participating in heat exchange. In contrast, Freon has a high chemical stability and is not easy to react chemically with other substances. Therefore, it is not easy to produce impurities generated by chemical reactions, nor will it produce soluble impurities. Therefore, no impurity filter is required in the cold medium 801.

[0119] The relief valve 807 is added to the thermal oil circulation to control the maximum pressure within the system, preventing system damage or accidents caused by excessive pressure. The Freon refrigeration cycle, on the other hand, controls the internal pressure of the system through regulation of the compressor and condenser, eliminating the need for a separate relief valve 807. Specifically, the following operating principles are explained: 1) In the heat medium 805 circulation, the relief valve 807 primarily controls the maximum pressure within the temperature control module 8, preventing damage or accidents caused by excessive pressure. When the pressure within the temperature control module 8 exceeds the set value, the relief valve 807 automatically opens, discharging excess heat medium 805 (thermal oil) to maintain stable operation of the temperature control module 8. 2) In the refrigerant 801 refrigeration cycle, the refrigerant 801 absorbs heat and vaporizes in the evaporator. It is then compressed into a high-temperature, high-pressure gas in the compressor, where it releases heat and condenses into a liquid through the condenser. During the entire process, the pressure inside the temperature control module 8 is controlled by adjusting the compressor and the condenser, and no additional overflow valve 807 is required to control the system pressure.

[0120] It can be understood that the heat medium 805, heated to a preset temperature, passes through the heat medium U-shaped tube 15, transfers heat with the fluid in the open hole section 17, and is then circulated by the second reversing valve 812 to allow the heat medium 805 to flow in and out. Similarly, the refrigerant 801, cooled to a preset temperature, passes through the refrigerant U-shaped tube 16, transfers heat with the fluid in the open hole section 17, and is then circulated by the first reversing valve 810 to allow the refrigerant 801 to flow in and out.

[0121] It should be noted that the injection module regulates the pressure of the injected fluid, achieving a pressure-dominated phase change through fluid volume changes, and the temperature control module 8 regulates the temperature change within the openhole section 17 to achieve a temperature-dominated phase change. This allows the openhole section 17 to be effectively sealed, and the rock pore wall to be subjected to the alternating temperature-pressure effects of the fluid. On the one hand, the mechanical properties of the dense rock sample 1 are degraded, prompting the preferential generation of multiple initial induced fractures 102 within the dense rock sample 1, thereby reducing the pumping pressure required for the initiation of fractures in the dense rock sample 1. On the other hand, the CO2 phase change effect induces the generation of more dominant fractures 103, thereby achieving the purpose of enhancing the transformation effect of the dense rock sample 1. The method of controlling the phase change of CO2 within the openhole section 17 of the present application not only overcomes the limitations of conventional directional fracturing devices in dense rock samples 1 containing central through-holes 101, but also provides a new injection method for fracturing dense rock samples 1. This method has important reference value for indoor simulation experiments to study the phase change of CO2 coupled with cyclic injection and improve reservoir transformation effects.

[0122] In a feasible embodiment, the injection module includes: a gas cylinder 2 and a fluid injection pump 3, the gas cylinder 2 is connected to the fluid injection pump 3 through a pipeline, the fluid injection pump 3 is connected to the fluid injection channel 7 through a pipeline, and is connected to the height setting component 14.

[0123] The injection module consists of a gas cylinder 2 and a fluid injection pump 3, connected in sequence. The fluid injection pump 3 operates in constant flow and constant displacement volume modes and can be controlled manually or automatically. The fluid injection pump 3 is used to continuously or cyclically inject CO2 gas into the openhole section 17 of the dense rock sample 1. The injection flow rate is designed to dynamically adjust the fluid pressure in the openhole section 17 to below the CO2 critical point, achieving phase changes in the CO2 through pressure control.

[0124] In one feasible embodiment, the fluid monitoring module includes:

[0125] The temperature monitoring unit 11 is connected to the temperature measuring element 403 and is configured to obtain the temperature of the fluid in the open hole section 17;

[0126] The pressure monitoring unit 12 is disposed at one end of the fluid injection channel 7 close to the upper pressure head 9 . The pressure monitoring unit 12 is configured to obtain the pressure of the fluid in the open hole section 17 .

[0127] The fluid monitoring module is used to monitor temperature and pressure during fluid injection. The temperature measuring element 403 is located within the inner tube 402, i.e., within the fluid injection channel 7. The pressure monitoring unit 12 (pressure sensor) is located at the fluid injection port, i.e., on the connector assembly 6. The temperature measuring element 403 is connected to the temperature monitoring unit 11 through the upper pressure head 9 and the connector assembly 6, enabling external reading of the fluid temperature. The pressure monitoring unit 12 monitors the fluid pressure within the open hole section 17.

[0128] It should be noted that pressure detection unit 12 is typically a pressure sensor connected to fluid injection channel 7 (at the upper end of fluid injection channel 7) to measure injection pressure. Specifically, as fluid passes through the injection pipeline and into pressure detection unit 12, pressure detection unit 12 displays a specific pressure value. Because pressure detection unit 12 is located very close to open hole section 17, the value reported by pressure detection unit 12 is generally considered to be the fluid pressure within open hole section 17.

[0129] It can be understood that the changes in the pressure and temperature of the injected fluid cause the phase of the fluid CO2 to change, eventually causing cracks to form in the dense rock sample 1 near the open hole section 17, as shown in FIG. Figure 5a and Figure 5b As shown, first generate Figure 5a The initial crack 102 is induced, and the pressure and / or temperature are continuously increased to produce the following Figure 5b As shown in the preferred fractures 103, under ideal conditions, assuming that the rock wall within the open hole section 17 is free of initial defects, the alternating temperature and pressure will degrade the mechanical properties of the surrounding rock, causing initial induced fractures 102 to form around the hole wall that are equivalent to natural initial defects within the rock. Preferred fractures 103 are fractures that preferentially expand based on the initial induced fractures 102. As long as the initial induced fractures 102 are generated in the first step, the second step will expand from these initial induced fractures 102. There is a sequence between the initial induced fractures 102 and the preferred fractures 103. According to stress concentration theory, crack expansion preferentially begins from the initial induced fractures 102, followed by the formation of the preferred fractures 103. The method of controlling the phase change of CO2 in the open hole section 17 of the present application not only overcomes the limitations of the use of traditional directional fracturing devices in the dense rock sample 1 containing the central through hole 101, but also provides a new injection method for fracturing the dense rock sample 1. It has important reference value for indoor simulation experiments to study the phase change of CO2 coupled with cyclic injection and improve the reservoir transformation effect.

[0130] It should be noted that the location of fracture initiation is not necessarily the initial defect within the openhole section 17. However, initial defects (such as cracks, pores, etc.) may become the preferred path for fracturing because these areas often experience stress concentration, making the rock more susceptible to fracture. In the absence of obvious initial defects, fractures may also form in areas of stress concentration (such as the point of maximum circumferential stress at the wellbore wall). In other words, in this application, the pressurization process before rock fracturing involves phase changes coupled with temperature and pressure alternation. This complex synergistic effect can degrade the mechanical properties of the rock in the early stages, forming some initial induced fractures 102 that resemble the initial defects. However, because the failure conditions are not reached, the rock will not be fractured. If multiple small defects exist within the openhole section 17, they may affect the fracturing process. The specific impact depends on the distribution, size, shape of these small defects, and their relationship to the formation stress field. In some cases, multiple small defects may interconnect to form a major fracture network (dominant fractures 103). This usually occurs when the small defects are close to each other and the formation stress field is conducive to their interconnection, which directly forms dominant fractures 103. However, if the small defects are far apart, or the formation stress field is not conducive to their interconnection, they may develop independently, forming multiple independent fractures, which are referred to as dominant fractures 103 in this application. The "small defects" here can be considered as some small initial induced fractures 102 caused by the deterioration of rock properties during the pressurization process.

[0131] Figure 8 The figure is a flowchart of the steps of a dense rock fracturing method according to one embodiment of the present invention.

[0132] like Figure 8As shown, another embodiment of the present invention provides a dense rock fracturing method based on CO2 phase change, using the above-mentioned dense rock fracturing device, the dense rock fracturing method includes the following steps:

[0133] Step 1, preparing a dense rock sample with a central through hole;

[0134] Step 2: Adjust the height of the open hole section as needed, and form a sealed space in the open hole section through the upper sealing component and the lower sealing component;

[0135] Step 3: Test the sealing of the open hole section. If the sealing is effective, proceed to the next step. If the sealing is not effective, return to the previous step.

[0136] Step 4: Apply preset axial pressure, confining pressure and set ambient temperature to the dense rock sample according to the rock fracturing test requirements;

[0137] Step 5: Turn on the temperature control module to adjust the temperature change in the open hole section through the temperature control module, and at the same time turn on the injection module to inject fluid into the open hole section for intermittent pressurization; or

[0138] Step 6: Open only the injection module, change the fluid volume to obtain different fluid pressure combinations in a dynamic flow distribution manner, and inject fluid into the open hole section for pressurization;

[0139] Step 7: During the test, the fluid pressure curve and temperature are monitored synchronously to finally obtain the fracture network morphology. Specifically, in step 1, the dense rock sample 1 is usually prepared into a cylindrical shape. The regular shape is more convenient for placement and installation. Then, a hole is drilled in the center of the dense rock sample 1 to form a central through hole 101. In step 2, before the central through hole 101 is installed, the upper sealing component 4, the height-fixing component 4 and the lower sealing pipe 5 are installed, and the height of the height-fixing component 14 located between the upper sealing component 4 and the lower sealing pipe 5 is made to meet the height of the open hole section 17 required by the experiment. At the same time, the first sealing member 404 and the second sealing member 40 are installed on the upper sealing component 4. 5. Install the third seal 502 and the fourth seal 503 on the lower sealing pipe 5 to form a sealing module, and install the sealing module into the central through hole 101 to form a sealed space in the open hole section 17; at the same time, install the upper pressure head 9 and the lower pressure head 10 at both ends of the central through hole 101, and install the joint assembly 6 on the upper pressure head 9, and install the injection module, temperature control module 8 and fluid monitoring module together; Step 3. Test the sealing performance of the open hole section 17. Pre-introduce 0.5MPa of carbon dioxide into the open hole section 17 of the dense rock sample 1 to test the sealing performance of the open hole section 17. Check the sealing effectiveness of the open hole section 17 in the central through hole 101 of the dense rock sample 1. If the injection pressure is stable and does not drop (observed by the pressure monitoring unit 12), it means that the sealing is effective. If the sealing is effective, go to step 4. If the sealing is not effective, repeat step 2 to recheck the installation of the sealing module; Step 4, according to the requirements of the rock fracturing test, apply the preset axial pressure, confining pressure and set the ambient temperature to the dense rock sample 1; Step 5, turn on the temperature control module, adjust the temperature change in the open hole section through the temperature control module, and at the same time turn on the injection module to inject fluid into the open hole section for intermittent pressurization. The two modules (injection module and temperature control module) cooperate with each other to achieve the purpose of temperature-dominated phase change to fracture the dense rock sample; Alternatively, in step 6, only turn on the injection module, change the fluid volume according to the dynamic flow distribution method to obtain different fluid pressure combinations, and inject fluid into the open hole section for pressurization to achieve pressure-dominated phase change to fracture the dense rock sample 1; Step 7, synchronously monitor the fluid pressure curve and temperature during the test, and finally obtain a complex fracture network morphology, such as Figure 5a and Figure 5b shown.

[0140] It should be noted that the axial pressure and confining pressure are applied separately through an external servo system. The axial pressure is applied first, then the confining pressure, and the two are loaded alternately until the preset value of the experimental plan is reached. Note: There are many ways to apply, but this method and steps are routine operations, so they are not drawn in the figure. The ambient temperature is that the entire specimen is surrounded by an oil bath medium (or other medium), and the medium temperature is changed by external heating or cooling equipment. Then, through heat transfer, the dense rock specimen reaches the same temperature as the medium surrounding it. Note: This part is also a routine operation, so it is not drawn in the figure. The axial pressure, confining pressure and ambient temperature are equivalent to the initial setting values ​​of the experimental parameters.

[0141] It is understandable that Figure 6 The CO2 pressure change curve is actively controlled by dynamically allocating flow. Figure 7 The CO2 temperature change curve actively controlled by the temperature control module is shown for easier understanding.

[0142] Example 1

[0143] The pressure-controlled phase change mode: The fluid injection pump 3 has a pump cavity solvent V0. The injection flow rate Q is input manually or automatically. The flow rate Q has positive and negative values. A positive value indicates an upward state, which means the fluid volume decreases. A negative value indicates a downward state, which means the fluid volume increases. Specifically, the fluid pressurization method in the fracturing test is changed by dynamically allocating the flow rate Q. Q1, -Q2, Q3, -Q4, ..., Q n , -Q n+1 The flow rate combination represented by the subscripts 1, ..., n is an arbitrary value, and the actual value depends on the working efficiency and range of the fluid injection pump 3. The sum of the pump chamber volume, the pipeline volume, and the volume inside the open hole section 17 cavity is V. Since the sum of the pipeline volume and the volume inside the open hole section 17 cavity is a constant value, and the permeability of the dense rock sample 1 is poor, the change in the total volume of the injected fluid can be approximated as the change in the pump chamber volume V0 of the fluid injection pump 3. Corresponding to the flow rate combination Q1, -Q2, Q3, -Q4, ..., Q n , -Q n+1 The pump chamber volume combination is V1, V2, V3, V4, ..., V n , V n+1 ,V0≥(V1,V2,V3,V4,...,V n , V n+1 ). Through negative flow distribution and time control, a variety of CO2 injection pressure combinations can be obtained. The fluid pressure diagram obtained by monitoring is as follows: Figure 6As shown, it is only necessary to ensure that the pressure drops below the critical pressure of CO2 (7.38MPa) each time, and 7.0MPa can be selected. It should be noted that the prerequisite for ensuring that the pressure cycle drops to 7.0MPa is that the initial pressure value of the pump chamber is less than 7.0MPa, thereby achieving the phase change of CO2 in the open hole section 17. Considering the fluid compressibility and the working efficiency of the fluid injection pump 3, the preferred initial pressure is 6.5MPa to 7.0MPa.

[0144] It should be noted that the pressure can be changed by setting different flow rates. Figure 6 It is a schematic diagram of monitoring pressure under multiple pressure combinations. The pressure can be changed by setting different flow rates. This is the working principle of the injection module. The principle is similar to that of a piston or plunger injection pump. The piston (plunger) pushes to reduce the volume, and the fluid is compressed, then the pressure will increase. If the piston (plunger) moves backward to increase the volume, the pressure will decrease. At the same time, due to the extremely low permeability of the dense rock sample 1, the fluid injected in a short time will not directly seep from the open hole section 17 to the side of the dense rock sample 1, so the open hole section 17 can achieve pressure holding. Until the stress generated by the fluid pressure (usually the injection pressure) in the open hole section 17 at a certain point in the dense rock sample 1 exceeds the tensile strength of the dense rock sample 1, the dense rock sample 1 will break, that is, cracks (initial induced cracks 102 and dominant pressure cracks 103) are formed.

[0145] Example 2

[0146] The temperature-controlled phase change mode: The injection flow rate Q set by the fluid injection pump 3 is a constant value, ensuring that the CO2 is pressurized in the form of intermittent injection, that is, the injection is stopped after the specified pressure is injected. The method of inducing the phase change of CO2 in the open hole section 17 is mainly to adjust the temperature control module 8 so that the fluid temperature in the open hole section 17 changes. The curve diagram obtained by monitoring is shown in FIG. Figure 7 As shown. It should be noted that, during the actual experimental operation, the premise of applying the temperature control module 8 is that the dense rock sample 1 is preset in a stable ambient temperature, which can be 40°C. Correspondingly, the temperature of the thermal control unit can be set to 80°C and the temperature of the cold control unit can be set to 0°C, so as to quickly change the CO2 temperature in the open hole section 17 near the critical point and realize the phase change of CO2. The cold medium 801 is preferably Freon, and the hot medium 805 is preferably heat-conducting oil with good thermal conductivity. Taking into account the heat transfer characteristics between different media, as well as the working efficiency of the heat exchanger 808 and the cooler 803, the injection time of the fluid pressure change needs to match the temperature change rate.

[0147] It should be noted that constant-flow injection is intended to increase pressure, but if coupled with temperature increases and decreases, the process becomes significantly more complicated. However, because the temperature and pressure within the openhole section 17 can be continuously monitored in real time, the phase of the CO2 within the openhole section 17 can be inferred. Therefore, the CO2 preferably transitions from a supercritical state to a gaseous state. Considering only the fluid within the openhole section 17, assuming that the fluid at a 40°C ambient temperature is pressurized to 10 MPa and injection is stopped, then the temperature of the CO2 within the openhole section 17 is reduced to 25°C solely by the temperature control module 8. According to the principle of fluid PVT, during this process, the small volume of the openhole section 17 will experience simultaneous temperature and pressure changes, causing the CO2 phase to change. Depending on the changes between different phases, the resulting effects may include expansion effects, thermal effects, and other factors, which in turn may have certain impacts on the dense rock sample 1 within the openhole section 17.

[0148] It's understandable that "openhole section 17" is a term used in physical simulation tests in the oil industry's fracturing chambers. The upper sealing assembly 4 and the lower sealing pipe 5 together form the openhole section 17. The fluid injection port is located in this openhole area. This means that after fluid injection, it is sealed within the openhole section 17, where the fluid is pressurized.

[0149] In the present invention, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance. The term "plurality" refers to two or more than two, unless otherwise clearly defined.

[0150] Other embodiments of the present invention will readily occur to those skilled in the art upon consideration of the specification and practice of the invention disclosed herein. The present invention is intended to cover any variations, uses, or adaptations of the present invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein. The description and examples are to be considered as illustrative only.

[0151] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A dense rock fracturing device based on CO2 phase change, comprising a dense rock sample having a central through hole, characterized in that: The dense rock fracturing device comprises: an injection module configured to inject fluid into the dense rock sample; An upper sealing assembly and a lower sealing pipe are both disposed in the central through hole. The upper sealing assembly and the lower sealing pipe are disposed opposite to each other. The upper sealing assembly, the lower sealing pipe and the central through hole together form an open hole section, and the fluid is injected into the open hole section. a temperature control module, comprising a cold control unit and a heat control unit, wherein the cold control unit and the heat control unit are connected to the open hole section; a fluid monitoring module, configured to monitor the pressure and temperature of the fluid in the open hole section; The dense rock fracturing device further comprises: a height-fixing assembly, wherein a first end of the height-fixing assembly is detachably connected to the upper sealing assembly, a second end of the height-fixing assembly is detachably connected to the lower sealing pipe, and an output hole is provided on the height-fixing assembly, the output hole being in communication with the upper sealing assembly and the injection module; The height setting component includes: a base, a blind hole is provided in the base, the first end of the base and the second end of the base are provided with external threads, an output hole is provided on the base, the output hole is connected to the blind hole, and the blind hole is connected to the upper sealing component and the injection module The upper sealing assembly comprises: An outer tube is disposed in the central through hole, an end of the outer tube close to the lower sealing tube is provided with an internal thread, and the outer tube is connected to the first end of the height-fixing assembly through the internal thread; an inner tube, the inner tube being disposed within the outer tube, the inner tube being in communication with the output hole and the injection module, the inner tube forming a fluid injection channel, the inner tube being shorter than the outer tube, the inner tube and the outer tube being aligned at ends away from the lower sealing tube, and a temperature measuring element being disposed within the inner tube; Wherein, an upper pressure head is provided at one end of the upper sealing component away from the lower sealing pipe; The lower sealing pipe comprises: A sealing body, wherein the sealing body is provided with four through holes; a heat medium U-shaped tube, disposed in two of the through holes, the open end of the heat medium U-shaped tube being connected to the temperature control module, and the closed end of the heat medium U-shaped tube being located in the open hole section; a refrigerant U-shaped tube disposed in the other two through holes, the open end of the refrigerant U-shaped tube being connected to the temperature control module, and the closed end of the refrigerant U-shaped tube being located in the open hole section; Wherein, a lower pressure head is provided between the lower sealing pipe and the temperature control module.

2. The dense rock fracturing device based on CO2 phase change according to claim 1 is characterized in that: A first fixing nut is provided on the first end of the base, and a second fixing nut is provided on the second end of the base. The first fixing nut is used to limit the connection length between the first end of the base and the upper sealing assembly, and the second fixing nut is used to limit the connection length between the second end of the base and the lower sealing pipe.

3. The dense rock fracturing device based on CO2 phase change according to claim 1, characterized in that: The outer tube is provided with a first groove and a second groove on a side facing away from the inner tube, and the upper sealing assembly further comprises: a first sealing member, disposed in the first groove; a second sealing member disposed in the second groove, with a preset distance between the second sealing member and the first sealing member; Wherein, the first sealing member and the second sealing member are located at one end of the outer tube close to the lower sealing tube member.

4. The dense rock fracturing device based on CO2 phase change according to claim 1, characterized in that: The side wall of the sealing body facing the central through hole is provided with a third groove and a fourth groove, and the lower sealing pipe further comprises: a third sealing member, disposed in the third groove; a fourth sealing member disposed in the fourth groove, with a preset distance between the fourth sealing member and the third sealing member; Wherein, the third sealing member and the fourth sealing member are located at one end of the lower sealing tube member close to the upper sealing assembly.

5. The dense rock fracturing device based on CO2 phase change according to claim 1 is characterized in that: The temperature control module includes: a cold control unit and a heat control unit, the cold control unit is connected to the refrigerant U-shaped tube, and the heat control unit is connected to the heat medium U-shaped tube.

6. The dense rock fracturing device based on CO2 phase change according to claim 5, characterized in that: The cold control unit includes: a refrigerant storage tank, a refrigerant medium, a cold circulation pump, a cold exchanger and a cold control system. The cold circulation pump inputs the refrigerant medium in the refrigerant storage tank into the refrigerant U-shaped tube. After passing through the refrigerant U-shaped tube, the refrigerant passes through the cold exchanger and returns to the refrigerant storage tank. The circulation of the refrigerant is controlled by a first reversing valve.

7. The dense rock fracturing device based on CO2 phase change according to claim 5, characterized in that: The thermal control unit includes: a heat medium storage tank, a heat medium, a heat circulation pump, a heat exchanger and a thermal control system. The heat circulation pump inputs the heat medium in the heat medium storage tank into the heat medium U-shaped pipe. After passing through the heat medium U-shaped pipe, the heat medium passes through the heat exchanger and returns to the heat medium storage tank. The circulation of the heat medium is controlled by a second reversing valve.

8. The dense rock fracturing device based on CO2 phase change according to claim 1, characterized in that: The injection module includes: a gas cylinder and a fluid injection pump, the gas cylinder is connected to the fluid injection pump through a pipeline, the fluid injection pump is connected to the fluid injection channel through a pipeline, and is communicated with the height-fixing component.

9. The dense rock fracturing device based on CO2 phase change according to claim 1, characterized in that: The fluid monitoring module includes: a temperature monitoring unit connected to the temperature measuring element, wherein the temperature monitoring unit is configured to obtain the temperature of the fluid in the open hole section; A pressure monitoring unit is provided at one end of the fluid injection channel close to the upper pressure head, and is configured to obtain the pressure of the fluid in the open hole section.

10. A method for fracturing dense rock based on CO2 phase change, using the dense rock fracturing device according to any one of claims 1 to 9, characterized in that: The dense rock fracturing method comprises the following steps: Preparation of dense rock specimens with central through-holes; Adjust the height of the open hole section as needed, and form a sealed space in the open hole section through the upper sealing component and the lower sealing component; Test the sealing of the open hole section. If the sealing is effective, proceed to the next step. If the sealing is not effective, return to the previous step. According to the rock fracturing test requirements, applying a preset axial pressure, confining pressure and setting an ambient temperature to the dense rock sample; Turning on the temperature control module to adjust the temperature change in the open hole section through the temperature control module, and at the same time turning on the injection module to inject fluid into the open hole section for intermittent pressurization; or Only the injection module is opened, and the fluid volume is changed to obtain different fluid pressure combinations in a manner of dynamically distributing the flow rate, and the fluid is injected into the open hole section for pressurization; During the test, the fluid pressure curve and temperature were monitored synchronously, and the fracture network morphology was finally obtained.

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