Dense-cut hydraulic fracturing simulation system and method based on true triaxial experiment

Abstract

The present invention relates to a close-cut hydraulic fracturing and fracture-forming simulation system and method based on a true triaxial experiment. The close-cut hydraulic fracturing and fracture-forming simulation system includes a true triaxial stress loading subsystem, an artificial core, and an artificial wellbore. The artificial wellbore is cast in the artificial core, and a fracturing fluid containing at least two temporary plugging agents with different particle sizes and densities is injected into the artificial wellbore. The artificial wellbore is provided with symmetrically arranged slits, and copper sheets are inserted into the two symmetrical slits and connected together to form a circular structure for simulating perforation on the artificial wellbore. The true triaxial stress loading subsystem includes a stress loading hydraulic source and stress loading plates distributed in the three principal stress directions of the artificial core. The method includes preparing an artificial wellbore, an artificial core, and loading the artificial core with triaxial stress and conducting a close-cut fracturing experiment. The present invention simulates perforation by arranging copper sheets on the artificial wellbore, and in combination with a temporary plugging agent, can effectively achieve a multi-cluster high-efficiency fracture fracturing effect in the laboratory.

Description

Technical Field

[0001] The present invention relates to the technical field of hydraulic fracturing for unconventional tight oil and gas production, and in particular to a close-cutting hydraulic fracturing fracture simulation system and method based on true triaxial experiments. Background Art

[0002] my country is rich in unconventional tight oil and gas resources, including shale gas, shale oil, tight sandstone gas, and coalbed methane. Unconventional oil and gas are a key alternative for increasing reserves and production in my country. Tight oil and gas reservoirs typically exhibit low porosity, low permeability, and poor reservoir connectivity. Consequently, conventional drilling and completion methods have low natural productivity, making hydraulic fracturing a crucial tool for increasing the productivity of these resources. Hydraulic fracturing involves creating a flow channel through a surface fracturing vehicle, surface pipelines, and a wellbore. The surface fracturing vehicle pumps fracturing fluid into the wellbore through the pipeline. When the fluid pressure in the wellbore exceeds the fracture pressure of the formation rock, fractures form in the formation. Fracturing fluid is then injected into the formation to propel the fractures. Subsequently, proppant of a specific amount and particle size is pumped into the existing fractures along with the fracturing fluid, creating proppant fractures that maintain a certain level of conductivity after the fracturing operation. The more fractures created by fracturing, the more complex the fracture morphology, and the larger the effective swept volume of the fracturing treatment, the higher the post-fracturing production capacity. Therefore, with technological advancements, the distances between segments and clusters in fracturing wells have become increasingly smaller, moving from single-stage, single-cluster to multi-stage, multi-cluster approaches. This is currently known as close-cut fracturing. This technology aims to achieve more hydraulic fractures by reducing the distance between fracturing stages while increasing the number of perforation clusters, thereby creating a complex post-fracturing fracture network and ultimately increasing production.

[0003] While reducing the interval between stages and increasing the number of perforation clusters can, to a certain extent, promote the formation of more fractures and create a more complex fracture network around the wellbore, due to stress interference between hydraulic fractures, fractures may deflect, leading to the aggregation or arrest of multiple fractures. Therefore, smaller intervals between fracture stages are not necessarily better, nor are more clusters necessarily better. Currently, close-cut hydraulic fracturing is still in the engineering exploration stage. The mechanism of this process, the characteristics of multiple fracture initiation during fracturing construction, the propagation patterns of fractures after fracturing, and the evaluation of post-fracturing fracture effects are all unclear. Therefore, conducting indoor experiments with close-cut hydraulic fracturing is of great significance for understanding the effectiveness and regularity of this process. Summary of the Invention

[0004] The purpose of the present invention is to provide a close-cut hydraulic fracturing and fracture-forming simulation system and method based on true triaxial experiments, so as to solve the problem in the prior art that the close-cut hydraulic fracturing technology cannot effectively achieve the fracturing effect of multi-cluster efficient fracturing due to the lack of indoor experimental simulation analysis.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] The present invention provides a close-cut hydraulic fracturing and fracture-making simulation system based on a true triaxial experiment, comprising a true triaxial stress loading subsystem, the close-cut hydraulic fracturing and fracture-making simulation system also comprising an artificial core and an artificial wellbore;

[0007] One end of the artificial wellbore is an open injection end, and the other end of the artificial wellbore is a closed end. A fracturing fluid containing at least two temporary plugging agents with different particle sizes and densities is injected into the artificial wellbore. A symmetrically arranged slit is opened on the cylinder body of the artificial wellbore near the closed end. A semicircular copper sheet is inserted into the slit. The copper sheets at two symmetrical slits are spliced ​​and connected to form a circular structure for simulating perforations on the artificial wellbore. A gap is formed between the copper sheet and the slit for the temporary plugging agent to pass through.

[0008] The true triaxial stress loading subsystem includes a stress loading hydraulic source and stress loading plates distributed in the three principal stress directions of the artificial core. The three stress loading plates are respectively connected to the stress loading hydraulic source through hydraulic high-pressure pipelines, and the artificial wellbore is cast in the artificial core.

[0009] Furthermore, the copper sheet has a groove that is engaged with the slit, and the groove is a "C"-shaped structure with arc edges on both sides and a straight edge in the middle. The copper sheets at the two symmetrical slits are spliced ​​together to form a circular structure with an irregular hole in the middle. The vertical distance between the notch of the groove and the straight edge in the middle is equal to the depth of the slit, and the inner wall of the arc edge of the groove forms a contact fit with the outer wall of the artificial wellbore.

[0010] Furthermore, the thickness of the copper sheet is smaller than the width of the slit, and the width of the gap between the copper sheet and the slit is greater than 3 times the particle size of the maximum particle size temporary plugging agent, and there are several symmetrically arranged two slits distributed along the length direction of the cylinder of the artificial wellbore, and an internal thread is provided on the inner wall of the artificial wellbore from the injection end along the length direction of the cylinder, and a spiral anti-slip external thread is provided on the outer wall of the cylinder of the artificial wellbore between the internal thread section and the closed end, and the closed end of the artificial wellbore is a sealing plate structure welded at the port of the artificial wellbore.

[0011] Furthermore, the dense cutting hydraulic fracturing simulation system also includes a fracturing control subsystem, which includes a fracturing control terminal, a controller, a high-pressure servo injection pump and an intermediate container. The fracturing control terminal, the controller and the high-pressure servo injection pump are connected in sequence through signal lines. The intermediate container is injected with fracturing fluid containing at least two temporary plugging agents with different particle sizes and densities. The high-pressure servo injection pump and the intermediate container are connected through a first fracturing fluid delivery pipeline, the intermediate container and the injection end of the artificial wellbore are connected through a liquid injection high-pressure pipeline, the high-pressure servo injection pump and the liquid injection high-pressure pipeline are connected through a second fracturing fluid delivery pipeline, and the first fracturing fluid delivery pipeline and the second fracturing fluid delivery pipeline are respectively installed with high-pressure valves, wherein the diameter of the liquid injection high-pressure pipeline is greater than 3 times the particle size of the maximum particle size temporary plugging agent.

[0012] Furthermore, the dense-cutting hydraulic fracturing simulation system also includes an acoustic emission monitoring subsystem, which includes an acoustic emission monitoring terminal and an acoustic emission probe. The artificial core has a hexahedral structure, and at least two acoustic emission probes are arranged on the six faces of the artificial core. The acoustic emission monitoring terminal is connected to each of the acoustic emission probes through an acoustic emission cable, and a pre-differential amplifier is installed on the acoustic emission cable.

[0013] Based on the above-mentioned close-cut hydraulic fracturing and fracture-making simulation system based on true triaxial experiment, the present invention also provides a close-cut hydraulic fracturing and fracture-making simulation method, comprising:

[0014] preparing an artificial wellbore and an artificial core, and providing a fracturing fluid containing at least two temporary plugging agents of different particle sizes and densities;

[0015] The prepared artificial core is placed in the stress loading area of ​​the true triaxial stress loading subsystem, the sealing of the artificial wellbore is checked, and the input parameters of the acoustic emission monitoring subsystem are calibrated. The artificial core is then subjected to triaxial stress loading by the true triaxial stress loading subsystem.

[0016] After stress loading is completed, a fracturing fluid containing at least two temporary plugging agents with different particle sizes and densities is injected into the artificial wellbore through the fracturing control subsystem. After the fracturing experiment is completed, the initiation and expansion of the artificial core at the wellbore position of the artificial wellbore are observed and studied by core sectioning.

[0017] Furthermore, the invention also includes a method for preparing an artificial core:

[0018] Identify the main mineral composition and proportion of the target rock, and select the corresponding mineral rock powder based on the clear mineral composition and proportion;

[0019] Quartz sand, Portland cement, selected mineral rock powders and aqueous solution are uniformly mixed in a preset ratio to prepare concrete, and the prepared concrete is poured into a mold for making artificial cores and stirred thoroughly;

[0020] Lubricating oil is evenly applied to both sides of the copper sheet on the artificial well shaft to form an oil film layer, and the artificial well shaft is poured into the concrete along the center position of the mold during the concrete pouring process;

[0021] After the concrete is poured, the mold is removed at a predetermined time, and the demoulded artificial core is placed under preset temperature conditions for water curing until the artificial core reaches the required strength.

[0022] Furthermore, it also includes a method for detecting the sealing of the artificial wellbore: the artificial wellbore is connected to the fracturing control subsystem, and the high-pressure servo injection pump is controlled to inject a predetermined amount of fracturing fluid into the artificial wellbore through the second fracturing fluid delivery pipeline. According to the pressure value fed back on the fracturing control terminal, if the pressure in the artificial wellbore reaches the preset pressure and remains unchanged for a certain period of time, it is judged that the sealing of the artificial wellbore meets the standards.

[0023] Furthermore, it also includes a calibration method for the input parameters of the acoustic emission monitoring subsystem: the artificial core is connected to the acoustic emission monitoring subsystem, and based on the lead breaking experiment of the artificial core, the acoustic emission parameters input on the acoustic emission monitoring terminal are calibrated according to the acoustic emission event positioning point monitored on the artificial core, until the acoustic emission event positioning point monitored after the artificial core breaks the lead is close to the actual break position.

[0024] Furthermore, it also includes a step of loading the artificial core triaxial stress;

[0025] S1. The true triaxial stress loading subsystem is used to simultaneously load the stress in three directions of the artificial core to the preset minimum principal stress value;

[0026] S2. Simultaneously loading the stresses in the intermediate principal stress direction and the maximum principal stress direction to a preset intermediate principal stress value;

[0027] S3, increasing the stress in the maximum principal stress direction to a preset maximum principal stress value;

[0028] S4. After the stress loading is completed, the pressure is stabilized for a certain period of time. If the stress in the three directions decreases, the stress value in the decreasing stress direction is added until the preset stress value is reached.

[0029] The present invention adopts the above technical solution, which has the following beneficial effects:

[0030] 1. The present invention combines the structure of copper sheet simulated perforation and symmetrical slits at the perforation positions of the artificial wellbore, uses two semicircular copper sheets to be clamped at two symmetrical slit positions of the artificial wellbore, and the two copper sheets are spliced ​​and connected to form a circular structure and used to simulate the perforation on the artificial wellbore. The prefabricated cracks have regular and flat shapes and good cracking effect, which fully plays the role of prefabricated cracks simulating perforation, and can accurately determine the slits and cracking initiation positions, so that the cluster spacing and slit depth are adjustable and controllable, and the cracking initiation position is reliable.

[0031] 2. The present invention sets temporary plugging agents with variable particle size and density to ensure the injection volume and temporary plugging performance of the fracturing fluid with temporary plugging agents. Based on the temporary plugging principle, dense cutting experiments are carried out by prefabricating cracks, which is close to the on-site fracturing process, and realizes the study of the influence of dense cutting technology on crack initiation pressure and initiation degree under indoor conditions. The process does not require the use of mechanical separation tools other than temporary plugging agents. The multi-cluster initiation effect can be achieved by injecting the fracturing fluid with temporary plugging agents once. Compared with the traditional multi-cluster fracturing physical simulation method, which has insufficient initiation and difficulty in simulating the simultaneous initiation and expansion of multiple clusters, the present invention can fully simulate the initiation and expansion of multiple cracks under different cluster spacing and different temporary plugging agent conditions, and has a good effect in simulating the extension of multiple cracks. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Various other advantages and benefits will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiment below. The accompanying drawings are for illustration purposes only and are not to be considered as limiting the present invention. Throughout the drawings, the same reference numerals are used to denote the same components. In the drawings:

[0033] Figure 1 This is a schematic diagram of the overall structure of a close-cut hydraulic fracturing simulation system based on a true triaxial test provided by an embodiment of the present invention;

[0034] Figure 2 Schematic diagram of an artificial wellbore structure of a close-cut hydraulic fracturing and fracture-making simulation system based on a true triaxial test provided by an embodiment of the present invention;

[0035] Figure 3 Figure (a) is a side view of an artificial wellbore and artificial core assembly structure provided by an embodiment of the present invention, and Figure (b) is a front view of an artificial wellbore and artificial core assembly structure provided by an embodiment of the present invention;

[0036] Figure 4 Figure (a) is a schematic diagram of the copper sheet structure provided by an embodiment of the present invention, and Figure (b) is a schematic diagram of the bonding structure of two symmetrical copper sheets provided by an embodiment of the present invention.

[0037] The symbols in the accompanying drawings represent the following:

[0038] 1. True triaxial stress loading subsystem; 11. Stress loading hydraulic source; 12. Stress loading plate; 2. Artificial core; 3. Artificial wellbore; 31. Sealing plate; 32. Slit; 33. Copper sheet; 331. Groove; 34. Internal thread; 35. Anti-slip external thread; 4. Fracturing control subsystem; 41. Fracturing control terminal; 42. Controller; 43. High-pressure servo injection pump; 44. Intermediate container; 45. First fracturing fluid delivery pipeline; 46. Injection high-pressure pipeline; 47. Second fracturing fluid delivery pipeline; 48. High-pressure valve; 5. Acoustic emission monitoring subsystem; 51. Acoustic emission monitoring terminal; 52. Acoustic emission probe; 53. Acoustic emission cable; 54. Pre-differential amplifier. DETAILED DESCRIPTION

[0039] Exemplary embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. Although exemplary embodiments of the present invention are shown in the accompanying drawings, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments described herein. Rather, these embodiments are provided to enable a more thorough understanding of the present invention and to fully convey the scope of the present invention to those skilled in the art.

[0040] Due to the lack of indoor experimental simulation analysis in the existing close-cut hydraulic fracturing technology, the technology cannot effectively guide the on-site implementation or optimization of close-cut fracturing with multi-cluster high-efficiency fracturing. The present invention provides a close-cut hydraulic fracturing fracture simulation system and method based on true triaxial experiments. The close-cut hydraulic fracturing fracture simulation system includes a true triaxial stress loading subsystem, an artificial core, and an artificial wellbore. The artificial wellbore is cast in the artificial core, and a fracturing fluid containing at least two temporary plugging agents with different particle sizes and densities is injected into the artificial wellbore. The artificial wellbore is provided with symmetrically arranged slits, and copper sheets are inserted into the two symmetrical slits and spliced ​​together to form a circular structure for simulating perforation on the artificial wellbore. The true triaxial stress loading subsystem includes a stress loading hydraulic source and stress loading plates distributed in the three principal stress directions of the artificial core. The method includes preparing an artificial wellbore and an artificial core, loading the artificial core with triaxial stress, and conducting a close-cut fracturing experiment. The present invention simulates perforation by arranging copper sheets on an artificial wellbore and uses a temporary plugging agent to effectively achieve a multi-cluster high-efficiency fracturing effect in the laboratory.

[0041] The scheme of the present invention is described in detail below through examples.

[0042] Example

[0043] like Figure 1 As shown, the present invention provides a close-cut hydraulic fracturing simulation system based on true triaxial experiment, including a true triaxial stress loading subsystem 1, an artificial core 2 and an artificial wellbore 3, and the specific structure is as follows:

[0044] Combine Figure 2 and Figure 3 As shown, one end of the artificial wellbore 3 is an open injection end, and the other end of the artificial wellbore 3 is a closed end. Preferably, the closed end of the artificial wellbore 3 is a closed structure formed by welding a sealing plate 31 with the same inner diameter as the artificial wellbore 3 at the end, wherein the sealing plate 31 is a circular plate structure. Fracturing fluid containing at least two temporary plugging agents with different particle sizes and densities is injected into the artificial wellbore 3, and symmetrically arranged slits 32 are opened on the cylinder of the artificial wellbore 3 near the closed end, and semicircular copper sheets 33 are stuck in the slits 32, and the copper sheets 33 at the two symmetrical slits 32 are spliced ​​and connected to form a circular structure and used to simulate the perforation on the artificial wellbore 3. A gap is formed between the copper sheet 33 and the slit 32 for the temporary plugging agent to pass through. Among them, Figure 3 Figure (a) is a side view of the artificial core 2 with the artificial wellbore 3 cast. Figure 3 Figure (b) is a front view of the artificial core 2 with the artificial wellbore 3 cast.

[0045] The true triaxial stress loading subsystem 1 includes a stress loading hydraulic source 11 and stress loading plates 12 distributed in the three principal stress directions of the artificial core 2. The three stress loading plates 12 are respectively connected to the stress loading hydraulic source 11 through hydraulic high-pressure pipelines 13. The artificial wellbore 3 is pre-cast in the artificial core 2. Among them, there are several symmetrically arranged two-slits 32 distributed along the length direction of the cylinder of the artificial wellbore 3. The slits 32 are symmetrically cut on the cylinder of the artificial wellbore 3 using a wire cutting method, and the depth of the slits 32 is less than the outer radius of the artificial wellbore 3, and multiple symmetrical two-slits 32 are cut uniformly or non-uniformly along the length direction of the artificial wellbore 3. Among them, the artificial wellbore 3 is preferably a seamless steel pipe with a certain wall thickness, and the length and specific diameter of the artificial wellbore 3 can be designed according to the size of the specific experimental sample.

[0046] The above-described structure, by arranging copper sheets 33 on the artificial wellbore 3 to simulate perforation, fully achieves the effect of prefabricated fractures to simulate perforation, and can accurately determine the location of the slits 32 and the crack initiation position, making the cluster spacing and the depth of the slits 32 adjustable and controllable. The cluster spacing is the distance between two adjacent slits 32 along the length of the artificial wellbore 3.

[0047] Further, combined with Figure 4As shown, the copper sheet 33 has a groove 331 that engages with the slit 32. The groove 331 is a "C"-shaped structure with arc edges on both sides and a straight edge in the middle. The copper sheets 33 at the two symmetrical slits 32 are pieced together to form a circular structure with an irregular hole in the middle. The vertical distance between the notch of the groove 331 and the straight edge in the middle is equal to the depth of the slit 32, and the inner wall of the arc edge of the groove 331 forms a contact fit with the outer wall of the artificial wellbore 3. Among them, multiple copper sheets 33 are used to simulate the perforation cluster around the artificial wellbore 3 at the initial moment. The material is preferably hard copper, and since the cracks are generally very small in width, the thickness of the copper sheet 33 should be relatively thin, and the preferred thickness range of the copper sheet 33 is 0.2 to 0.3 mm.

[0048] like Figure 4 As shown, when processing the copper sheet 33, the outer radius R1 of the copper sheet 33 should be determined according to the specific design, and the inner radius R2 of the arc edge of the groove 331 on the copper sheet 33 should be determined according to the outer radius of the artificial well 3, so that R2 and the outer radius of the artificial well 3 can match each other. When the two semicircular copper sheets 33 are put together, they can just be stuck at the position of the two symmetrical slits 32 on the artificial well 3, and when the two copper sheets 33 of the same size are put together, they form a complete circular structure with an irregular hole in the middle. It is preferred to use strong glue to glue the seams of the two copper sheets 33 together to prevent the copper sheet 33 from shifting or falling during the preparation of the artificial core 2. Among them, Figure 4 Figure (a) is a schematic diagram of the structure of the copper sheet 33. Figure 4 Figure (b) is a schematic structural diagram of two symmetrical copper sheets 33 after being joined.

[0049] Furthermore, the thickness of the copper sheet 33 is less than the width of the slit 32, and the width of the gap between the copper sheet 33 and the slit 32 is greater than three times the particle size of the temporary plugging agent with the largest particle size. When the copper sheet 33 is inserted into the slit 32 on the artificial wellbore 3, the gap remaining between the copper sheet 33 and the slit 32 is sufficient to allow the temporary plugging agent with the largest particle size to pass smoothly. An internal thread 34 is provided on the inner wall of the artificial wellbore 3 along its length from the injection end, and a spiral anti-slip external thread 35 is provided on the outer wall of the artificial wellbore 3 between the internal thread 34 section and the closed end. The internal thread 34 section is used to connect to the fracturing fluid injection pipeline and act as a seal, while the anti-slip external thread 35 acts as a seal during the process of pouring the artificial wellbore 3 into the artificial core 2, and prevents the artificial wellbore 3 from rotating due to excessive force during the connection process of the fracturing fluid injection pipeline, thereby acting as an anti-slip agent.

[0050] Further, look back Figure 1The dense-cutting hydraulic fracturing simulation system also includes a fracturing control subsystem 4. The fracturing control subsystem 4 comprises a fracturing control terminal 41, a controller 42, a high-pressure servo injection pump 43, and an intermediate container 44. The fracturing control terminal 41, the controller 42, and the high-pressure servo injection pump 43 are sequentially connected via signal lines. The intermediate container 44 is injected with fracturing fluid containing at least two temporary plugging agents of different particle sizes and densities. A first fracturing fluid delivery pipeline 45 connects the high-pressure servo injection pump 43 and the intermediate container 44. The intermediate container 44 is connected to the injection end of the artificial wellbore 3 via a high-pressure injection pipeline 46. A second fracturing fluid delivery pipeline 47 connects the high-pressure servo injection pump 43 and the high-pressure injection pipeline 46. High-pressure valves 48 are installed on each of the first and second fracturing fluid delivery pipelines 45 and 47. The diameter of the high-pressure injection pipeline 46 is greater than three times the maximum particle size of the temporary plugging agent to prevent blockage during injection.

[0051] Through the above-described configuration, the fracturing control terminal 41 and controller 42 control the high-pressure servo injection pump 43 to inject the fracturing fluid from the intermediate container 44 into the artificial wellbore 3 for a close-cut fracturing experiment. Combined with the copper sheet 33 simulating perforations and the symmetrical slotting of the perforation positions in the artificial wellbore 3, close-cut fracturing construction simulation experiments were conducted at different cluster spacings, initial fracture cutting depths, injection rates, and fracturing fluid viscosities. Temporary plugging agents of varying particle size and density were used to ensure the injection volume and temporary plugging performance of the fracturing fluid containing the temporary plugging agent. Based on the principle of temporary plugging, close-cut experiments were conducted by prefabricating fractures, similar to field fracturing processes. This enabled the study of the impact of the close-cut process on fracture initiation pressure and degree under indoor conditions. This process did not require mechanical separation tools other than the temporary plugging agent; multiple clusters could be initiated through a single injection of the fracturing fluid containing the temporary plugging agent. Compared to traditional multi-cluster fracturing physical simulations, this improved the effect of single-cluster fracturing with multiple clusters to complete fracturing with perforated clusters.

[0052] Furthermore, by directly connecting the high-pressure servo injection pump 43 and the high-pressure injection pipeline 46 via a second fracturing fluid delivery pipeline 47, the system's sealing performance can be checked before the experiment. For example, the fracturing control terminal 41 and controller 42 can be used to control the high-pressure servo injection pump 43 to directly pump a small amount of fracturing fluid without a temporary plugging agent into the artificial wellbore 3, raising the pressure within the artificial wellbore 3 to 0.5 MPa. If the pressure remains essentially unchanged for approximately 30 seconds, the system's sealing performance is satisfactory. The pressure within the artificial wellbore 3 can be read on the fracturing control terminal 41.

[0053] Further, look back Figure 1The dense cutting hydraulic fracturing simulation system also includes an acoustic emission monitoring subsystem 5. The acoustic emission monitoring subsystem 5 includes an acoustic emission monitoring terminal 51 and an acoustic emission probe 52. Based on the true triaxial experiment, the artificial core 2 is selected as a hexahedral structure. At least two acoustic emission probes 52 are arranged on the six faces of the artificial core 2, and the acoustic emission monitoring terminal 51 is connected to each acoustic emission probe 52 through an acoustic emission cable 53, and a pre-differential amplifier 54 is installed on the acoustic emission cable 53. Through the setting of this structure, the lead breaking experiment of the artificial core 2 can be carried out, the acoustic emission parameters can be adjusted, and the acoustic emission monitoring subsystem 5 can be calibrated so that the acoustic emission event positioning point monitored after the lead breaking of the artificial core 2 is basically consistent with the actual lead breaking position.

[0054] Based on the above-mentioned close-cut hydraulic fracturing and fracture-making simulation system based on true triaxial experiment, the present invention also provides a close-cut hydraulic fracturing and fracture-making simulation method, comprising:

[0055] An artificial wellbore 3 and an artificial core 2 are prepared, and a fracturing fluid containing at least two temporary plugging agents with different particle sizes and densities is provided;

[0056] The prepared artificial core 2 is placed in the stress loading area of ​​the true triaxial stress loading subsystem 1, the sealing of the artificial wellbore 3 is checked and the input parameters of the acoustic emission monitoring subsystem 5 are calibrated, and the artificial core 2 is loaded with triaxial stress through the true triaxial stress loading subsystem 1;

[0057] After stress loading is completed, a fracturing fluid containing at least two temporary plugging agents with different particle sizes and densities is injected into the artificial wellbore 3 through the fracturing control subsystem 4. After the fracturing experiment is completed, the initiation and expansion of the artificial core 2 at the wellbore position of the artificial wellbore 3 are observed and studied by core sectioning.

[0058] As mentioned above, temporary plugging agents contain at least two or more different particle sizes and densities, such as 40 / 70 mesh and 100 / 200 mesh. Temporary plugging agents with larger particle sizes are used to seal cracks with larger widths, while temporary plugging agents with smaller particle sizes are used to seal deeper cracks with smaller widths. Temporary plugging agents also need to have two densities: one with a specific gravity slightly higher than the fracturing fluid, and another with a specific gravity slightly lower. Adding the temporary plugging agent to the fracturing fluid requires stirring at a speed of 8,000 to 10,000 rpm for two minutes in a high-speed blender. The purpose of having two specific gravities is to ensure that the temporary plugging agent remains suspended as much as possible during the fracturing process without settling, and to prevent excessive upward or downward expansion of the cracks. Stirring ensures that the temporary plugging agent is fully suspended in the fracturing fluid. The dosage of the temporary plugging agent should be determined based on the specific configuration of the fracturing fluid to form a good suspension. It is best to use only a small amount of temporary plugging agent or no settling.

[0059] Furthermore, the invention also includes a method for preparing the artificial core 2:

[0060] Identify the main mineral composition and proportion of the target rock, and select the corresponding mineral rock powder based on the clear mineral composition and proportion;

[0061] Quartz sand, Portland cement, selected mineral rock powders and aqueous solution are uniformly mixed in a preset ratio to prepare concrete, and the prepared concrete is poured into a mold for making the artificial core 2 and stirred thoroughly;

[0062] Lubricating oil is evenly applied to both sides of the copper sheet 33 on the artificial well shaft 3 to form an oil film layer, and the artificial well shaft 3 is poured into the concrete along the center position of the mold during the concrete pouring process;

[0063] After the concrete is poured, the mold is removed at a predetermined time, and the demoulded artificial core 2 is placed under a preset temperature condition for water curing until the artificial core 2 reaches the required strength.

[0064] As described above, the artificial core 2 is prepared as closely as possible to the mineral composition and proportions of actual rock. Specifically, rock powders of various minerals selected based on their defined mineral composition and proportions are added to Portland cement, followed by a specific ratio of 40 / 70 mesh quartz sand. A small sample measuring 100 mm x 100 mm x 100 mm is first prepared. From this sample, a cylindrical sample with a diameter of 50 mm and a height of 100 mm is drilled to measure the mechanical parameters of the small sample, such as compressive strength, tensile strength, shear strength, elastic modulus, and Poisson's ratio. A ratio that matches the mineral composition and proportions of actual rock, while also approximating the strength of the actual core, is selected to produce the core for hydraulic fracturing experiments. Furthermore, the entire artificial wellbore 3 is positioned using specialized tools to ensure that it remains centered within the artificial core 2 throughout the entire process. The copper sheet 33 is also positioned as close to the center of the artificial core 2 as possible. Lubricating oil is evenly applied to both surfaces of the copper sheet 33 to form an oil film. This prevents the concrete from adhering to the copper sheet after solidification, thereby affecting the effectiveness of the prefabricated cracks.

[0065] Furthermore, it also includes a method for detecting the sealing of the artificial wellbore 3: the artificial wellbore 3 is connected to the fracturing control subsystem 4, and the high-pressure servo injection pump 43 is controlled to inject a predetermined amount of fracturing fluid into the artificial wellbore 3 through the second fracturing fluid delivery pipeline 47. According to the pressure value fed back on the fracturing control terminal 41, if the pressure in the artificial wellbore 3 reaches the preset pressure and remains unchanged for a certain period of time, it is judged that the sealing of the artificial wellbore 3 meets the standards.

[0066] Furthermore, it also includes a calibration method for the input parameters of the acoustic emission monitoring subsystem 5: the artificial core 2 is connected to the acoustic emission monitoring subsystem 5, and based on the lead breaking experiment of the artificial core 2, the acoustic emission parameters input on the acoustic emission monitoring terminal 51 are calibrated according to the acoustic emission event positioning point monitored on the artificial core 2, until the acoustic emission event positioning point monitored after the artificial core 2 breaks the lead is close to the actual break position.

[0067] Furthermore, the method further includes a step of loading the artificial core 2 with three-dimensional stress;

[0068] S1, using the true triaxial stress loading subsystem 1 to simultaneously load the stresses in three directions of the artificial core 2 to a preset minimum principal stress value;

[0069] S2. Simultaneously loading the stresses in the intermediate principal stress direction and the maximum principal stress direction to a preset intermediate principal stress value;

[0070] S3, increasing the stress in the maximum principal stress direction to a preset maximum principal stress value;

[0071] S4. After the stress loading is completed, the pressure is stabilized for a certain period of time. If the stress in the three directions decreases, the stress value in the decreasing stress direction is added until the preset stress value is reached.

[0072] The present invention discloses a close-cut hydraulic fracturing and fracture-forming simulation system and method based on a true triaxial experiment. After the experiment, the core is removed and the artificial core 2 is cut to observe the initiation and expansion of the cracks in the artificial core 2 at the wellbore position of the artificial wellbore 3. During the experiment, variables such as prefabricated fracture parameters, fracturing fluid properties, fracturing operation parameters, temporary plugging agent properties, perforation parameters, rock mechanical properties, and stress conditions can be changed to study the hydraulic fracturing initiation pressure, degree of initiation, crack expansion law, and interference mechanism under close-cut conditions. Close-cut fracturing construction simulations can be achieved under different cluster spacings, initial crack cutting depths, different injection rates, and different fracturing fluid viscosities, and a fracturing effect with efficient multi-cluster initiation can be achieved.

[0073] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A close-cut hydraulic fracturing simulation system based on true triaxial experiment, including a true triaxial stress loading subsystem, characterized by: The dense cutting hydraulic fracturing simulation system also includes an artificial core and an artificial wellbore; One end of the artificial wellbore is an open injection end, and the other end of the artificial wellbore is a closed end. A fracturing fluid containing at least two temporary plugging agents with different particle sizes and densities is injected into the artificial wellbore. A symmetrically arranged slit is opened on the cylinder body of the artificial wellbore near the closed end. A semicircular copper sheet is inserted into the slit. The copper sheets at two symmetrical slits are spliced ​​and connected to form a circular structure for simulating perforations on the artificial wellbore. A gap is formed between the copper sheet and the slit for the temporary plugging agent to pass through. The true triaxial stress loading subsystem includes a stress loading hydraulic source and stress loading plates distributed in the three principal stress directions of the artificial core. The three stress loading plates are respectively connected to the stress loading hydraulic source via hydraulic high-pressure pipelines. The artificial wellbore is pre-cast in the artificial core. The copper sheet has a groove that is engaged with the slit, and the groove is a "C"-shaped structure with arc edges on both sides and a straight edge in the middle. The copper sheets at the two symmetrical slits are pieced together to form a circular structure with an irregular hole in the middle. The vertical distance between the notch of the groove and the straight edge in the middle is equal to the depth of the slit, and the inner wall of the arc edge of the groove forms a contact fit with the outer wall of the artificial wellbore; The thickness of the copper sheet is less than the width of the slit, and the width of the gap between the copper sheet and the slit is greater than 3 times the particle size of the temporary plugging agent with the largest particle size, and a plurality of symmetrically arranged two slits are distributed on the cylinder body of the artificial wellbore along its length direction, and an internal thread is provided on the inner wall of the artificial wellbore from the injection end along its length direction, and a spiral non-slip external thread is provided on the outer wall of the cylinder of the artificial wellbore between the internal thread section and the closed end, and the closed end of the artificial wellbore is a closed structure formed by welding a blocking plate with an inner diameter equal to that of the artificial wellbore at the end; The dense cutting hydraulic fracturing simulation system also includes a fracturing control subsystem, which includes a fracturing control terminal, a controller, a high-pressure servo injection pump and an intermediate container. The fracturing control terminal, the controller and the high-pressure servo injection pump are connected in sequence through signal lines. The intermediate container is injected with fracturing fluid containing at least two temporary plugging agents with different particle sizes and densities. The high-pressure servo injection pump and the intermediate container are connected through a first fracturing fluid delivery pipeline, the intermediate container and the injection end of the artificial wellbore are connected through a liquid injection high-pressure pipeline, the high-pressure servo injection pump and the liquid injection high-pressure pipeline are connected through a second fracturing fluid delivery pipeline, and high-pressure valves are installed on the first fracturing fluid delivery pipeline and the second fracturing fluid delivery pipeline, wherein the diameter of the liquid injection high-pressure pipeline is greater than 3 times the particle size of the temporary plugging agent with the maximum particle size.

2. The close-cutting hydraulic fracturing simulation system based on true triaxial testing according to claim 1 is characterized by: The dense-cutting hydraulic fracturing simulation system also includes an acoustic emission monitoring subsystem, which includes an acoustic emission monitoring terminal and an acoustic emission probe. The artificial core has a hexahedral structure, and at least two acoustic emission probes are arranged on each of the six faces of the artificial core. The acoustic emission monitoring terminal is connected to each of the acoustic emission probes via an acoustic emission cable, and a pre-differential amplifier is installed on the acoustic emission cable.

3. A method for simulating close-cut hydraulic fracturing fracture creation, using a close-cut hydraulic fracturing fracture creation simulation system based on true triaxial testing according to claim 1 or 2, characterized in that: The method comprises: preparing an artificial wellbore and an artificial core, and providing a fracturing fluid containing at least two temporary plugging agents of different particle sizes and densities; The prepared artificial core is placed in the stress loading area of ​​the true triaxial stress loading subsystem, the sealing of the artificial wellbore is checked, and the input parameters of the acoustic emission monitoring subsystem are calibrated. The artificial core is then subjected to triaxial stress loading by the true triaxial stress loading subsystem. After stress loading is completed, a fracturing fluid containing at least two temporary plugging agents with different particle sizes and densities is injected into the artificial wellbore through the fracturing control subsystem. After the fracturing experiment is completed, the initiation and expansion of the artificial core at the wellbore position of the artificial wellbore are observed and studied by core sectioning.

4. A method for simulating close-cut hydraulic fracturing according to claim 3, characterized in that: Also included is a method for preparing an artificial core: Identify the main mineral composition and proportion of the target rock, and select the corresponding mineral rock powder based on the clear mineral composition and proportion; Quartz sand, Portland cement, selected mineral rock powders and aqueous solution are uniformly mixed in a preset ratio to prepare concrete, and the prepared concrete is poured into a mold for making artificial cores and stirred thoroughly; Lubricating oil is evenly applied to both sides of the copper sheet on the artificial well shaft to form an oil film layer, and the artificial well shaft is poured into the concrete along the center position of the mold during the concrete pouring process; After the concrete is poured, the mold is removed at a predetermined time, and the demoulded artificial core is placed under preset temperature conditions for water curing until the artificial core reaches the required strength.

5. The method for simulating close-cut hydraulic fracturing according to claim 3, characterized in that: It also includes a method for detecting the sealing of an artificial wellbore: the artificial wellbore is connected to the fracturing control subsystem, and the high-pressure servo injection pump is controlled to inject a predetermined amount of fracturing fluid into the artificial wellbore through the second fracturing fluid delivery pipeline. According to the pressure value fed back on the fracturing control terminal, if the pressure in the artificial wellbore reaches the preset pressure and remains unchanged for a certain period of time, it is judged that the sealing of the artificial wellbore meets the standards.

6. The method for simulating close-cut hydraulic fracturing according to claim 3, characterized in that: It also includes a calibration method for the input parameters of the acoustic emission monitoring subsystem: the artificial core is connected to the acoustic emission monitoring subsystem, and based on the lead breaking experiment of the artificial core, the acoustic emission parameters input on the acoustic emission monitoring terminal are calibrated according to the acoustic emission event positioning point monitored on the artificial core, until the acoustic emission event positioning point monitored after the artificial core breaks the lead is close to the actual break position.

7. The method for simulating close-cut hydraulic fracturing according to claim 3, characterized in that: It also includes the loading steps of the artificial core triaxial stress; S1. The true triaxial stress loading subsystem is used to simultaneously load the stress in three directions of the artificial core to the preset minimum principal stress value; S2. Simultaneously loading the stresses in the intermediate principal stress direction and the maximum principal stress direction to a preset intermediate principal stress value; S3, increasing the stress in the maximum principal stress direction to a preset maximum principal stress value; S4. After the stress loading is completed, the pressure is stabilized for a certain period of time. If the stress in the three directions decreases, the stress value in the decreasing stress direction is added until the preset stress value is reached.

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