Compact rock fracturing device and method based on CO2 phase state change

Abstract

The invention discloses a compact rock fracturing device and method based on CO2 phase state change. The compact rock fracturing device comprises a compact rock sample with a central through hole, and further comprises an injection module configured to inject fluid into the compact rock sample; the upper sealing assembly and the lower sealing pipe fitting are both arranged in the center through hole, the upper sealing assembly and the lower sealing pipe fitting are oppositely arranged, an open hole section is defined by the upper sealing assembly, the lower sealing pipe fitting and the center through hole, and fluid is injected into the open hole section; the temperature control module comprises a cold control unit and a thermal control unit, and the cold control unit and the thermal control unit are communicated into the open hole section; and the fluid monitoring module is used for monitoring the pressure and temperature of the fluid in the naked eye section. The fluid temperature and pressure alternating effect can degrade the mechanical property of the compact rock sample; the fluid phase state change induces the generation of dominant fracturing fractures, and the two synergistically act to achieve the purpose of strengthening the reconstruction of the compact 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 exploitation of unconventional oil and gas, 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 through a high-pressure pump to fracture the reservoir, providing a large range of seepage channels for oil and gas production. 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 at the same time, 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 begins to crack, 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 change in phase.

[0003] According to the CO2 phase diagram, when the temperature exceeds 31.1°C and the pressure exceeds 7.38MPa, CO2 is in a supercritical state, which is common in reservoirs over 800m deep. Therefore, based on the advantages of supercritical CO2 fracturing and the physical properties of volume expansion induced by phase changes (i.e., liquid or supercritical state becomes gaseous), it can be found that in order to increase reservoir production capacity, reduce surface pump group pressure, and respond to the concept of green ecology, the tight reservoir rock can be efficiently fractured by utilizing the CO2 phase change.

[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 a 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 packaging of the open hole section in small physical simulation tests and the real-time regulation 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 of actual engineering applications; (3) The process of cold injection of liquid CO2 to 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 precisely controlling the phase changes of the fluid within the open hole section in multiple ways. 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 a 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 the 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 component and a lower sealing pipe are both arranged in the central through hole. The upper sealing component and the lower sealing pipe are arranged opposite to each other. The upper sealing component, the lower sealing pipe and the central through hole enclose a naked hole section. The fluid is injected into the naked 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 fitting, 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, the base is provided with an output hole, 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 comprises:

[0018] An outer tube is arranged 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 is arranged inside the outer tube, the inner tube is connected with the output hole and the injection module, the inner tube forms a fluid injection channel, the length of the inner tube is less than the length of the outer tube, the inner tube and the outer tube are aligned at one end away from the lower sealing pipe member, and a temperature measuring member is arranged inside 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 is disposed in the first groove;

[0023] A second sealing member is disposed in the second groove, and a preset distance is left 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 is disposed in two of the through holes, the open end of the heat medium U-shaped tube is connected to the temperature control module, and the closed end of the heat medium U-shaped tube is located in the open hole section;

[0028] A refrigerant U-shaped tube is disposed in the other two through holes, the open end of the refrigerant U-shaped tube is connected to the temperature control module, and the closed end of the refrigerant U-shaped tube is located in the naked eye 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 is 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, a cold circulation pump, a cold exchanger and a cold control system. The cold circulation pump inputs the refrigerant 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, wherein the heat circulation pump inputs the heat medium in the heat medium storage tank into the heat medium U-shaped tube, and the heat medium passes through the heat medium U-shaped tube and then passes through the heat exchanger and returns to the heat medium storage tank, and 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 gas injection pump through a pipeline, the gas 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 comprises:

[0039] a temperature monitoring unit connected to the temperature measuring component, 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 disposed at one end of the fluid injection channel close to the upper pressure head, and the pressure monitoring unit is configured to obtain the pressure of the fluid in the open hole section.

[0041] The embodiment of the second aspect 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 comprises the following steps:

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

[0043] Adjust the height of the open hole section according to the demand, 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 ineffective, 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 by 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 embodiment of the present invention provides a dense rock fracturing device and a fracturing method based on CO2 phase change, wherein the dense rock fracturing device includes an upper sealing component and a lower sealing pipe, which are combined with the central through hole of the dense rock sample to form an open hole section, and fluid is injected into the open hole section through an injection module, and the temperature change in the open hole section is adjusted by a temperature control module. The pressure in the open hole section is adjusted by dynamically allocating the injection flow rate, and the phase change of pressure-dominated control is achieved by fluid volume change, and the phase change of temperature-dominated control is achieved by heat transfer of different media in the open hole section. Under the premise of efficient sealing of the open hole section, and the rock pore wall under the action of fluid temperature-pressure alternation, on the one hand, the mechanical properties of the dense rock sample are degraded, and multiple initial induced cracks are preferentially generated inside the dense rock sample, thereby 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 strengthening the transformation effect of the dense rock sample. The sealing of the open hole section in the central through hole of the dense rock sample in the present application is different from the conventional glue injection treatment. The assembly structure simplifies the sealing operation of the dense rock sample and realizes reusability in an integrated form. At the same time, the method of controlling the phase change of CO2 in the open hole section of the present application not only overcomes the limitations of the use of traditional directional fracturing devices in dense rock samples with central through holes, but also provides a new injection method for the fracturing of dense rock samples. It has important reference value for indoor simulation experiments to study the phase change of CO2 coupled with cyclic injection and improve the effect of reservoir fracturing transformation.

[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 an embodiment of the present invention;

[0053] Figure 2 is a schematic diagram of a central through hole according to an 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 A schematic diagram of a dense rock sample having a central through hole according to an embodiment of the present invention;

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

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

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

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

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

[0062] 1 dense rock sample, 101 central through hole, 102 initial induced fracture, 103 dominant 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 fixed height 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 be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0064] Before further describing the embodiments of the present invention in detail, the directional nouns 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 protection scope 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 an embodiment of the present invention; Figure 4 Schematic diagram of a dense rock sample containing a central through hole according to an 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, and 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 arranged in the central through hole 101. The upper sealing assembly 4 and the lower sealing pipe 5 are arranged 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] The dense rock fracturing device comprises 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 an injection module, and the temperature change in the open hole section 17 is adjusted through a 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 has the fluid temperature-pressure alternatingly acting 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 fracturing of the dense rock sample 1. On the other hand, the phase change effect induces more dominant fracturing cracks 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 in the central through hole 101 of the dense rock sample 1 in the present application is different from the conventional glue injection process. The assembly structure simplifies the sealing operation of the dense rock sample 1 and realizes reusability in an integrated form. At the same time, 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 the fracturing of the dense rock sample 1, which has important reference value for indoor simulation experiments to study the phase change of CO2 coupled with cyclic injection and improve the effect of reservoir fracturing transformation.

[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 a possible implementation, Figure 2 As shown, the dense rock pressure device also includes:

[0076] The height determining component 14, the first end of the height determining component 14 is detachably connected to the upper sealing component 4, the second end of the height determining component 14 is detachably connected to the lower sealing pipe 5, and an output hole 1403 is provided on the height determining 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 achieving 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 determining component 14 is used to support the upper sealing component 4 and the lower sealing pipe 5, the middle section of the height determining component 14 is located in the open hole section 17, and an output hole 1403 is required to be set on the height determining component 14. At the same time, the fluid of the injection module enters the height determining component 14 after passing through the upper sealing component 4, and is output from the output hole 1403 of the height determining 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, and 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, so as to facilitate subsequent fracturing tests.

[0081] It should be noted that the first end and the second end of the base 1401 are both provided with external threads, and the end at which the upper sealing component 4 and the lower sealing pipe fitting 5 are connected to the base 1401 is 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 fitting 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 component 4, the length of the base 1401 screwed into the upper sealing component 4 can be locked by the first fixing nut 18, so that even when the fluid pressure increases, the base 1401 will not deflect or move to a certain extent; similarly, after the second end of the base 1401 is connected to the lower sealing pipe fitting 5, the length of the base 1401 screwed into the lower sealing pipe fitting 5 can be locked by the second fixing nut 19, so that even when the fluid pressure increases, the base 1401 will not deflect or move to a certain extent.

[0084] In a 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 component 14 through the inner thread.

[0086] Inner tube 402, inner tube 402 is arranged in outer tube 401, inner tube 402 is connected with output hole 1403 and injection module, inner tube 402 forms fluid injection channel 7, length of inner tube 402 is less than length of outer tube 401, inner tube 402 and outer tube 401 are aligned at one end away from lower sealing pipe 5, and temperature measuring element 403 is arranged in 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 component 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 less 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 fitting 5. The outer tube 401 and the inner tube 402 are fixed by welding the aligned ends. After the upper sealing component 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 component 4. A joint assembly 6 is provided on the side of the upper pressure head 9 facing away from the upper sealing component 4. After the pipeline of the injection module passes through the joint assembly 6 and the upper pressure head 9 in turn, it enters the inner tube 402 serving as the fluid injection channel 7 to achieve fluid injection.

[0089] In a feasible implementation manner, a first groove and a second groove are provided on a side of the outer tube 401 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, and a preset distance is left 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 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. Among them, 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 at the center position of the central through hole 101; the Y-ring is used to adapt to the rock hole wall with different processing roughness. Through the setting of the two sealing rings, it can be ensured 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-type sealing ring is Y-shaped, it is a typical lip-shaped sealing ring. It relies on its open lip edge to stick to the sealing pair coupling surface, and the fluid pressure is effectively applied to the seal through the action of the lip edge. This design makes the lip of the Y-type sealing ring subject 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 adapted to 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 then 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 component 4.

[0094] It is understandable that the second groove and the bottom end of the outer tube 401 form a step shape, and such a setting can facilitate the installation and removal of the Y-shaped sealing ring. At the same time, an internal threaded hole is provided in the outer tube 401, and the internal threaded hole is used to connect the first end of the height setting component 14. The internal threaded hole is connected to the inner tube 402, so that the fluid in the inner tube 402 can enter the blind hole 1402 of the height setting component 14 and flow to the open hole section 17. A step shape can be set at the end of the internal threaded hole away from the inner tube 402, and the step shape is recessed at the lower end of the outer tube 401, and the step shape is used to accommodate the first fixing nut 18.

[0095] In a feasible implementation manner, the lower sealing pipe member 5 comprises:

[0096] A sealing body 501, wherein 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] A 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] Wherein, 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 without limitation.

[0101] It is understandable that the lower sealing pipe 5 is pressed into the central through hole 101 by the lower pressure head 10 at one end away from the upper sealing assembly 4, that is, the sealing of the central through hole 101 can be ensured by the arrangement of the upper pressure head 9 and the lower pressure head 10. Four through holes are arranged on the lower pressure head 10, which are arranged opposite to the four through holes of the sealing body 501, so as to facilitate the connection between the refrigerant U-shaped tube 16 and the heat medium U-shaped tube 15 and the temperature control module 8.

[0102] In a feasible implementation manner, 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, and a preset distance is left 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 the sealing body 501 is provided with a third groove and a fourth groove on the side wall facing the central through hole 101, the third groove is provided with a third seal 502, the fourth groove is provided with a fourth seal 503, and the third seal 502 and the fourth seal 503 are located at one end of the lower sealing pipe 5 close to the upper sealing assembly 4, the third seal 502 is an O-ring, and the fourth seal 503 is a Y-ring. Among them, the O-ring can play a role in straightening the lower sealing pipe 5 during the installation of the lower sealing pipe 5, ensuring that the structure of the lower sealing pipe 5 floats at the center of the central through hole 101; the Y-ring is used to adapt to the rock hole wall with different processing roughness. Through the setting of the two seals, it can be ensured 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 below the Y-ring, and a distance of 1mm to 2mm is left between the two, that is, the preset distance is 1mm to 2mm. The principle is the same as that of the first seal 404 and the second seal 405 . The third seal 502 and the fourth seal 503 , that is, the cooperation of 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 understandable that the fourth groove and the top of the sealing body 501 form a step shape, and such a setting can facilitate the installation and removal of the Y-shaped sealing ring. At the same time, an internal threaded hole is provided at the center of one end of the sealing body 501 facing the upper sealing component 4, and the internal threaded hole is used to connect the second end of the height setting component 14. A step shape can be provided at the end of the internal threaded hole away from the lower pressure head 10, and the step shape is recessed at the top of the sealing body 501, and the step shape is used 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. 4 is a schematic diagram of a temperature control module according to an embodiment of the present invention.

[0110] In a 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 of the fluid in the open hole section 17 is adjusted by the temperature control module 8 to ensure that the heat transfer in the open hole section 17 realizes the temperature-controlled fluid phase change.

[0112] It is understandable that the fluid of 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 temperature control module 8 of the present application, the phase of the fluid in the open hole section 17 changes, 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, and the test of the dense rock sample 1 can be carried out. 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 cold control unit provides a medium for cooling and heat exchange for the fluid in the open hole section 17, and the preset temperature and circulation mode are set by the cold control system 804, and the refrigerant 801 is input to the cold circulation pump 802, and enters the open hole section 17 through the refrigerant U-shaped tube 16. After passing through the refrigerant U-shaped tube 16, the refrigerant 801 output is cooled by the cold exchanger 803, and then returned to the refrigerant storage tank to prepare for the next circulation of the refrigerant 801. It should be noted that the refrigerant 801 will heat up after passing through the refrigerant U-shaped tube 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 implementation, 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] Among them, the heat control unit provides a medium for heating and heat exchange for the fluid in the open hole section 17, and the preset temperature and circulation mode are set by the thermal control system 809, and the heat medium 805 is input to the heat circulation pump 811, and enters the open hole section 17 through the heat medium U-shaped pipe 15. The heat medium 805 output after passing through the heat medium U-shaped pipe 15 is heated by the heat exchanger 808 and then returned to the heat medium storage tank to prepare for the next recycling of the heat medium 805. It should be noted that the heat medium 805 will cool down after passing through the heat medium U-shaped pipe 15, so it needs to be heated by the heat exchanger 808 and then returned 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 heat transfer 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 (above 60°C), an oxidation reaction will occur. In other words, 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 to participate in heat exchange. In contrast, Freon has a high chemical stability and is not easy to react chemically with other substances, so 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] Among them, the overflow valve 807 is added to the heat transfer oil circulation to control the maximum pressure in the system to prevent system damage or accidents caused by excessive pressure; while the Freon refrigeration cycle controls the pressure inside the system through the adjustment of the compressor and the condenser, and does not require an additional overflow valve 807 to control the system pressure. Specifically from the working principle: 1) In the circulation of the hot medium 805, the main function of the overflow valve 807 is to control the maximum pressure in the temperature control module 8 to prevent the temperature control module 8 from being damaged or accidents caused by excessive pressure. When the pressure in the temperature control module 8 exceeds the set value, the overflow valve 807 will automatically open to discharge the excess hot medium 805 (heat transfer oil) to maintain the stable operation of the temperature control module 8. 2) In the refrigeration cycle of the cold medium 801, the cold medium 801 absorbs heat and vaporizes in the evaporator, and then is compressed into a high-temperature and high-pressure gas in the compressor, and then releases heat and condenses into a liquid through the condenser. During the whole 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 hot medium 805 heated to the preset temperature passes through the hot medium U-shaped tube 15, and heat is transferred with the fluid in the open hole section 17, and the inflow and outflow of the hot medium 805 are realized through the second reversing valve 812, so that the hot medium 805 forms a cycle. Similarly, the cold medium 801 cooled to the preset temperature passes through the cold medium U-shaped tube 16, and heat is transferred with the fluid in the open hole section 17, and the inflow and outflow of the cold medium 801 are realized through the first reversing valve 810, so that the cold medium 801 forms a cycle.

[0121] It should be noted that the pressure of the injected fluid is adjusted by the injection module to achieve the phase change of pressure control by the change of fluid volume, and the temperature change in the open hole section 17 is adjusted by the temperature control module 8 to achieve the phase change of temperature control, so that the open hole section 17 is under the premise of efficient sealing, and the rock pore wall is under the action of fluid temperature-pressure alternation, 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, and the pumping pressure for the initiation of fracturing of the dense rock sample 1 is reduced, and on the other hand, the CO2 phase change effect induces more dominant fractures 103, so as to achieve the purpose of strengthening the transformation effect of the dense rock sample 1. 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 dense rock samples 1 containing central through holes 101, but also provides a new injection method for fracturing dense rock samples 1, which 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.

[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 gas injection pump 3 through a pipeline, the gas 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 is formed by connecting a gas cylinder 2 and an injection fluid pump 3 in sequence. The fluid injection pump 3 is configured with a constant flow rate and a constant displacement volume working mode, which can be manually controlled or automatically controlled. The fluid injection pump 3 is used to continuously or cyclically inject gas CO2 into the open hole section 17 of the dense rock sample 1. The fluid pressure value in the open hole section 17 is dynamically adjusted to below the critical point of CO2 by designing the injection flow rate, and the phase change of CO2 is realized by the form of pressure master control.

[0124] In a feasible implementation, the fluid monitoring module includes:

[0125] The temperature monitoring unit 11 is connected to the temperature measuring component 403, and the temperature monitoring unit 11 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 the temperature and pressure during the fluid injection process. The temperature measuring element 403 is arranged in the inner tube 402, that is, in the fluid injection channel 7, and the pressure monitoring unit 12 (pressure sensor) is arranged at the fluid injection port, that is, on the joint assembly 6. The temperature measuring element 403 is led out from the upper pressure head 9 and the joint assembly 6 and connected to the temperature monitoring unit 11 to realize the external reading of the fluid temperature, and the pressure monitoring unit 12 realizes the monitoring of the fluid pressure in the open hole section 17.

[0128] It should be noted that the pressure detection unit 12 is usually a pressure sensor, which is connected to the fluid injection channel 7 (at the upper end of the fluid injection channel 7) to obtain the injection pressure. Specifically, the fluid passes through the pressure detection unit 12 via the injection pipeline, and the pressure detection unit 12 will display a specific pressure value. Since the arrangement position of the pressure detection unit 12 is very close to the open hole section 17, it is approximately considered that the value fed back by the pressure detection unit 12 is the fluid pressure value in the 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, and eventually cracks are generated in the dense rock sample 1 near the open hole section 17, such as Figure 5a and Figure 5b As shown, first generate Figure 5a The initial induced crack 102 shown in FIG. 1 is then continued with pressurization and / or temperature adjustment to produce the following Figure 5b The dominant pressure fractures 103 shown, under ideal conditions, assuming that there are no initial defects in the rock wall in the open hole section 17, will degrade the mechanical properties of the surrounding rocks under the action of alternating temperature and pressure, causing some initial induced cracks 102 equivalent to the natural initial defects inside the rock to be generated around the hole wall. The dominant pressure fractures 103 refer to cracks that preferentially expand based on the initial induced cracks 102. As long as the initial induced cracks 102 are generated in the first step, the second step will first expand from these initial induced cracks 102. There is a sequence of initial induced cracks 102 and dominant pressure fractures 103. According to the stress concentration theory, it is known that the crack expansion starts from the initial induced cracks 102 first, and then the dominant pressure fractures 103 are formed. 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 the fracturing of 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 the fracture is not necessarily the initial defect in the open hole section 17. However, initial defects (such as cracks, pores, etc.) may become the preferred path of fracturing, because these areas usually have stress concentration, making the rock more likely to fracture in these places. In the absence of obvious initial defects, cracks may also form in areas of stress concentration (such as the maximum circumferential stress point at the well wall). That is, in this application, the pressurization process before rock fracturing involves temperature and pressure alternating coupled phase changes. This synergistic complex effect can degrade the mechanical properties of the rock in the initial stage, that is, some initial induced cracks 102 similar to initial defects are formed, but because the destruction conditions are not reached, the rock will not be fractured. If there are multiple small defects in the open hole section 17, they may have an impact on the fracturing process. The specific impact depends on the distribution, size, shape of these small defects and their relationship with the formation stress field. In some cases, multiple small defects may be connected to each other to form a main fracture network (dominant fracture 103). This usually occurs when the distance between the small defects is close and the formation stress field is conducive to their connection with each other, and then the dominant fracture 103 will be directly formed. However, if the small defects are far apart, or the formation stress field is not conducive to their interconnection, they may develop separately to form multiple independent fractures, which are referred to as the dominant fractures 103 in this application. The "small defects" here can be considered as some small initial induced fractures 102 generated by the deterioration of rock properties during the pressurization process.

[0131] Figure 8 The present invention is a flowchart of the steps of a dense rock fracturing method according to an 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, adjusting the height of the open hole section according to the demand, and forming 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 ineffective, return to the previous step.

[0136] Step 4, according to the rock fracturing test requirements, applying a preset axial pressure, confining pressure and setting an ambient temperature to the dense rock sample;

[0137] Step 5, turning on the temperature control module, adjusting the temperature change in the open hole section through the temperature control module, and at the same time turning on the injection module, injecting fluid into the open hole section for intermittent pressurization; or

[0138] Step 6, only opening the injection module, changing the fluid volume to obtain different fluid pressure combinations in a manner of dynamically allocating flow, and injecting fluid into the open hole section for pressurization;

[0139] Step 7, during the test, the fluid pressure curve and temperature are monitored synchronously, and the fracture network morphology is finally obtained. Specifically, step 1, usually the dense rock sample 1 is first prepared into a cylindrical shape, and the regular shape is more convenient for placement and installation, and then a hole is drilled at the center of the dense rock sample 1 to form a central through hole 101; 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, and at the same time, the first sealing component 404 and the second sealing component 40 are installed on the upper sealing component 4. 5. Install the third sealing member 502 and the fourth sealing member 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 for 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, the temperature control module 8 and the 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, and check 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 a 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 turn on the injection module at the same time, inject fluid into the open hole section for intermittent pressurization, and the two modules (injection module and temperature control module) cooperate with each other to adjust to achieve the purpose of temperature-controlled phase change to fracture the dense rock sample; or, Step 6, only turn on the injection module, change the fluid volume in a dynamic flow distribution manner to obtain different fluid pressure combinations, and inject fluid into the open hole section for pressurization to achieve the purpose of pressure-controlled 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 by an external servo system. The axial pressure is applied first, and then the confining pressure. 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 step are routine operations, so they are not drawn in the figure. The ambient temperature is that the entire sample is surrounded by an oil bath medium (or other medium), and the medium temperature is changed by external heating or cooling equipment, and then the dense rock sample reaches the same temperature as the medium surrounding it through heat transfer. Note: This part is also a routine operation, so it is not drawn in the figure. Axial pressure, confining pressure and ambient temperature are equivalent to the initial setting values ​​of the experimental parameters.

[0141] Understandably, Figure 6 The CO2 pressure 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 mode of phase change controlled by pressure: the solvent in the pump cavity of the fluid injection pump 3 is V0, and the injection flow rate Q is input through manual control or automatic control. The flow rate Q has positive and negative values. The positive value is the upward state, which means that the fluid volume decreases, and the negative value is the downward state, which means that the fluid volume increases. Specifically, the fluid pressurization method in the fracturing test is changed by dynamically allocating the flow rate Q, and 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 of the open hole section 17 cavity is V. Since the sum of the pipeline volume and the volume of the open hole section 17 cavity is a constant, and the permeability of the dense rock sample 1 is poor, the change in the total volume of the injected fluid can be approximated by the change in the pump chamber volume V0 of the fluid injection pump 3. Corresponding to the flow combinations 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 regulation, a variety of CO2 injection pressure combinations can be obtained. The schematic diagram of the fluid pressure 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 premise 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, so as to achieve the phase change of CO2 in the open hole section 17. Considering the fluid compression properties 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 change can be achieved by setting different flow rates. Figure 6 It is a schematic diagram of monitoring pressure under various 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 of 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 mode of temperature-controlled phase change: the injection flow rate Q set by the fluid injection pump 3 is a constant value, ensuring that CO2 is pressurized in the form of intermittent injection, that is, injection stops 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 the figure below: Figure 7 As shown. It should be noted that, in the actual experimental operation process, 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 cold exchanger 803, the injection time of the fluid pressure change needs to match the temperature change rate.

[0147] It should be noted that the constant flow injection is for pressurization, but if the operation of heating and cooling is coupled, the process will become very complicated. However, because the temperature and pressure in the open hole section 17 can be continuously monitored in real time, the phase state of CO2 in the open hole section 17 can be inferred. Therefore, CO2 preferably changes from supercritical state to gas state. Under the premise of considering only the fluid in the open hole section 17, assuming that the fluid at an ambient temperature of 40°C is pressurized to 10MPa and then injection is stopped, at this time, only the temperature control module 8 is relied on to reduce the temperature of CO2 in the open hole section 17 to 25°C. According to the fluid PVT principle, in this process, the experimental phenomenon of simultaneous changes in temperature and pressure will occur in the open hole section 17 with a very small volume, causing the phase state of CO2 to change. According to the changes between different phases, the impacts may include expansion effect, thermal effect, etc., which will have a certain impact on the dense rock sample 1 of the open hole section 17.

[0148] It is understandable that the open hole section 17 is a term used in the physical simulation test in the fracturing chamber of the petroleum industry. The upper sealing assembly 4 and the lower sealing pipe 5 enclose the open hole section 17. The fluid injection hole is opened in the open hole section area, which means that after the fluid is injected, the fluid is sealed in the open hole section 17, and the fluid will be pressurized here.

[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] Those skilled in the art will readily appreciate other embodiments of the present invention after considering the specification and practicing 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 present invention and include common knowledge or customary techniques in the art that are not disclosed by the present invention. The description and examples are to be considered exemplary only.

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

Claims

1. A dense rock fracturing device based on CO2 phase change, comprising a dense rock sample with a central through hole, characterized in that: The dense rock pressure device comprises: an injection module configured to inject fluid into the dense rock sample; An upper sealing component and a lower sealing pipe are both arranged in the central through hole. The upper sealing component and the lower sealing pipe are arranged opposite to each other. The upper sealing component, the lower sealing pipe and the central through hole enclose a naked hole section. The fluid is injected into the naked 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; The fluid monitoring module is used to monitor the pressure and temperature of the fluid in the open hole section.

2. The dense rock fracturing device based on CO2 phase change according to claim 1 is characterized in that: The dense rock pressure device also includes: 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 fitting, 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.

3. The dense rock fracturing device based on CO2 phase change according to claim 2 is characterized in that: 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.

4. The dense rock fracturing device based on CO2 phase change according to claim 3 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.

5. The dense rock fracturing device based on CO2 phase change according to claim 2 is characterized in that: The upper sealing assembly comprises: An outer tube is arranged 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 is arranged inside the outer tube, the inner tube is connected with the output hole and the injection module, the inner tube forms a fluid injection channel, the length of the inner tube is less than the length of the outer tube, the inner tube and the outer tube are aligned at one end away from the lower sealing pipe member, and a temperature measuring member is arranged inside the inner tube; Wherein, an upper pressure head is provided at one end of the upper sealing component away from the lower sealing pipe.

6. The dense rock fracturing device based on CO2 phase change according to claim 2, characterized in that: The lower sealing pipe comprises: A sealing body, wherein the sealing body is provided with four through holes; A heat medium U-shaped tube is disposed in two of the through holes, the open end of the heat medium U-shaped tube is connected to the temperature control module, and the closed end of the heat medium U-shaped tube is located in the open hole section; A refrigerant U-shaped tube is disposed in the other two through holes, the open end of the refrigerant U-shaped tube is connected to the temperature control module, and the closed end of the refrigerant U-shaped tube is located in the naked eye section; Wherein, a lower pressure head is provided between the lower sealing pipe and the temperature control module.

7. The dense rock fracturing device based on CO2 phase change according to claim 6 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.

8. The dense rock fracturing device based on CO2 phase change according to claim 5, characterized in that: The injection module includes: a gas cylinder and a fluid injection pump, the gas cylinder is connected to the gas injection pump through a pipeline, the gas 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 5, characterized in that: The fluid monitoring module comprises: a temperature monitoring unit connected to the temperature measuring component, wherein the temperature monitoring unit is configured to obtain the temperature of the fluid in the open hole section; A pressure monitoring unit is disposed at one end of the fluid injection channel close to the upper pressure head, and the pressure monitoring unit 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 samples with central through-holes; Adjust the height of the open hole section according to the demand, 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 ineffective, 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 by 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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