A true triaxial hydraulic fracturing seepage fluid collection and boundary regulation device and analysis method

Abstract

The application discloses a true triaxial hydraulic fracturing seepage liquid collecting and boundary regulating device and an analysis method, which comprises six partitioned flow guide loading plates, six sets of boundary pressure regulating units, a high-precision metering device and a data acquisition system. The inner side of each loading plate is provided with a flow guide groove network and a flexible rubber ring, which are used for independently guiding the seepage liquid on the six surfaces of the sample to a liquid collecting cavity. The collecting pipeline is provided with a three-way joint, which is connected with the liquid outlet hole, the metering balance and the regulating unit respectively, so that the real-time metering of the seepage liquid and the regulating of the boundary pressure are synchronously performed; the regulating unit adjusts the internal pressure of the pipeline, and six directions can be independently set to construct a homogeneous or inhomogeneous boundary field. The data acquisition system records the injection parameters, the boundary pressure and the seepage liquid parameters, calculates the fracturing efficiency according to the flow conservation, and determines the preferential expansion direction of the crack in combination with the seepage response characteristics of each direction. The application realizes the independent metering of the seepage liquid on the surface and the flexible regulating of the boundary, avoids the pollution of the equipment, and improves the evaluation precision of the fracturing efficiency.

Description

Technical Field

[0001] This invention relates to the field of indoor hydraulic fracturing experimental technology, specifically to optimizing hydraulic fracturing experiments. More specifically, this invention relates to an experimental system capable of independently controlling the seepage capacity of six boundary surfaces of a rock sample under true triaxial loading conditions, and simultaneously realizing fracturing fluid seepage collection, flow rate measurement, fracturing efficiency evaluation, and fracture propagation behavior analysis. Background Technology

[0002] Unconventional oil and gas reservoirs can be improved through special methods to maximize the exploitation of oil and gas resources that cannot be directly obtained using traditional extraction techniques. Hydraulic fracturing is a key measure for enhancing production in unconventional tight oil and gas reservoirs. During hydraulic fracturing, the injected high-pressure fluid drives the continuous expansion of hydraulic fractures, connecting anisotropic fracture groups in the reservoir and ultimately forming a complex network of crisscrossing fractures. Subsequently, under the influence of pressure differential, free and adsorbed oil and gas within the reservoir rock are released and flowed back with the fracturing fluid, achieving the oil and gas extraction process. The morphology of hydraulic fracture expansion has a significant impact on the formation of the complex fracture network and the final extraction efficiency of oil and gas.

[0003] Indoor true triaxial hydraulic fracturing simulation is the main method for studying the initiation and propagation of hydraulic fractures in reservoirs. During the test, after high-pressure fracturing fluid is injected into the rock sample, part of it is used for fracture propagation and fracture volume formation, while the other part seeps out from the sample surface through the rock matrix or after the fracture is connected.

[0004] Current true triaxial hydraulic fracturing experiments typically focus on parameters such as injection pressure, injection flow rate, and acoustic emission, while neglecting the collection and quantitative analysis of exudate from the sample surface. Exudate generated on the sample surface often remains directly inside the pressure chamber or spreads to the loading device, displacement sensors, and sealing components, not only affecting the experimental environment but also potentially causing equipment corrosion, sensor failure, and subsequent experimental errors.

[0005] At the same time, existing technologies generally have these shortcomings:

[0006] (1) It is impossible to accurately measure the volume of fluid seeping out of the sample in all directions, making it difficult to establish the conservation relationship of fracturing fluid flow rate;

[0007] (2) The effective fracturing fluid volume used for fracture propagation and the leakage fluid volume cannot be distinguished, resulting in a large error in the evaluation of fracturing efficiency;

[0008] (3) There is a lack of monitoring methods for the distribution characteristics of exudate in different directions, making it difficult to analyze the direction of crack propagation;

[0009] (4) Existing experimental setups typically employ fixed boundary flow conditions, which cannot simulate the effects of differences in permeability in different directions in actual reservoirs;

[0010] (5) If the exudate remains in the pressure chamber for a long time, it will easily cause pollution and corrosion to the loading mechanism, sealing components and measuring instruments.

[0011] Therefore, it is necessary to develop a true triaxial hydraulic fracturing test system that can control the six-sided boundary conditions of the sample and collect and measure the exudate in real time, so as to provide a reliable experimental platform for fracturing efficiency evaluation, crack propagation law research and boundary seepage effect analysis. Summary of the Invention

[0012] In view of the above-mentioned problems in the existing technology, the technical problem to be solved by the present invention is that in the existing true triaxial hydraulic fracturing test, it is difficult to effectively collect the exudate on the sample surface, it is impossible to measure it in real time, it is difficult to control the boundary seepage conditions, and it is difficult to accurately evaluate the fracturing efficiency.

[0013] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: a true triaxial hydraulic fracturing exudate collection and boundary control device, comprising:

[0014] Six zoned flow-guiding loading plates are respectively positioned on the outer sides of the six stress surfaces of the rock sample. Each zoned flow-guiding loading plate has multiple flow-guiding grooves on the side closest to the rock sample. These grooves are interconnected to form a flow-guiding groove network. A flexible rubber ring is provided around the periphery of the flow-guiding groove network. The flexible rubber ring and the flow-guiding groove network together constitute a flow-guiding constraint structure. The flow-guiding groove network connects to a pre-set liquid collection chamber inside each zoned flow-guiding loading plate. The liquid collection chamber is connected to the outside through a liquid outlet. The flow-guiding groove network is used to guide and collect the fracturing fluid seeping from the sample surface into the liquid collection chamber. Each flow-guiding loading plate serves as both a loading component and a flow-guiding and collecting component for the seeping fluid. A flexible square rubber ring is installed inside the flow-guiding groove. The flow-guiding groove forms a liquid confluence channel for guiding the fracturing fluid seeping from the sample surface into the liquid collection chamber. The flexible flow-guiding constraint structure uses a flexible rubber ring made of fluororubber. The flexible rubber ring and the flow guide groove together form a flow guiding and constraint structure, which is used to suppress the lateral diffusion of liquid, guide the exudate to flow preferentially along the preset path, and improve the liquid collection efficiency.

[0015] Six sets of boundary pressure control units, each of which is connected to the liquid outlet of a corresponding partitioned flow guide loading plate through a tee joint on an independent collection pipeline, are used to independently adjust the internal pressure of each flow guide network and independent collection pipeline to form a flow-promoting boundary, a natural flow boundary, or a flow-impeding boundary at the corresponding stress surface.

[0016] The high-precision metering device includes six high-precision metering balances. The independent collection pipeline is equipped with a three-way connector. The first port of the three-way connector is connected to the liquid outlet, the second port is connected to the high-precision metering balance, and the third port is connected to the boundary pressure control unit. The high-precision metering balance is used to measure the flow rate and cumulative leakage of the exudate in six directions in real time.

[0017] The data acquisition and processing system is connected to the fracturing fluid injection pump, the boundary pressure control unit, and the high-precision metering balance, respectively, to synchronously record the injection pressure, injection flow rate, boundary pressure, and parameters of the exudate in each direction, and to calculate the fracturing efficiency based on the flow conservation relationship.

[0018] Furthermore, the flexible rubber ring is made of fluororubber, with a height of 2.5 mm and a width of 2.0 mm; the guide groove has a groove height of 2.0 mm and a groove width of 3.0 mm.

[0019] Furthermore, the edge of the partitioned flow guiding loading plate is provided with a 45° structural extension, and the 45° structural extensions of adjacent partitioned flow guiding loading plates fit together during assembly to prevent exudate from leaking from the joint of the loading plate.

[0020] The edge of the partitioned flow guide loading plate has a 45° extension structure, which makes the loading plates fit more tightly together.

[0021] Furthermore, the boundary pressure control unit adjusts the pressure in the following way:

[0022] When the internal pressure of the flow channel network and the independent collection pipeline is adjusted to be lower than the ambient pressure, a boundary that promotes seepage is formed on the corresponding stress surface.

[0023] When the internal pressure of the flow channel network and the independent collection pipeline is adjusted to equal the ambient pressure, a natural seepage boundary is formed on the corresponding stress surface.

[0024] When the pressure inside the flow channel network and the independent collection pipeline is higher than the ambient pressure, a boundary that hinders seepage is formed on the corresponding stress surface.

[0025] The boundary conditions of the six stress surfaces can be independently set to the same or different pressure states to construct homogeneous or heterogeneous boundary pressure fields. The boundary pressure control unit is used to adjust the internal pressure of the guide channel network and collection pipeline, thereby constructing different boundary conditions. When the internal pressure is lower than the ambient pressure, a seepage-promoting boundary is formed; when the internal pressure is equal to the ambient pressure, a natural seepage boundary is formed; and when the internal pressure is higher than the ambient pressure, a seepage-impeding boundary is formed. The six boundary surfaces can be independently set to different pressure states to construct heterogeneous boundary pressure conditions.

[0026] A method for surface-level collection, measurement, and fracture propagation analysis of permeate in hydraulic fracturing physical simulation tests using the aforementioned true triaxial hydraulic fracturing permeate collection and boundary control device includes the following steps:

[0027] S1: Place the rock sample in the true triaxial fracturing chamber of the true triaxial hydraulic fracturing physical simulation test seepage surface collection and measurement and fracture propagation analysis device, so that the six stress surfaces of the rock sample are in contact with the corresponding zone flow guiding loading plates, apply the preset triaxial stress and keep it stable.

[0028] S2: Start the fracturing fluid injection pump to inject fracturing fluid into the rock sample.

[0029] S3: The leachate is collected by the flow channel network, and after being restricted from lateral diffusion by the flow constraint structure, it is transported to the corresponding high-precision metering balance through an independent collection pipeline to record the flow rate and cumulative leachate in each direction in real time.

[0030] S4: The data acquisition and processing system synchronously acquires injection parameters, boundary pressure parameters, and exudate parameters in all directions, and calculates fracturing efficiency based on flow conservation.

[0031] S5: Compare the flow response characteristics of the exudate in each direction, determine the direction in which the flow rate first increases and the growth rate is the preferred direction for fracture propagation, and analyze the fracture propagation behavior in combination with the trend of fracturing efficiency changes.

[0032] Furthermore, the calculation model for the fracturing efficiency is as follows:

[0033]

[0034] in: For fracturing efficiency; For the cumulative injection volume; Let be the cumulative seepage volume at the i-th boundary surface; The pre-calibrated system compression compensation volume.

[0035] Furthermore, before step S1, a boundary condition setting step is included: the boundary pressure values ​​of the six stress surfaces are set by six sets of boundary pressure control units, so that each stress surface is independently in a state of promoting seepage, natural seepage, or hindering seepage, so as to simulate the heterogeneous seepage environment of the reservoir.

[0036] Furthermore, step S6 is included: performing correlation analysis between the fracturing efficiency calculation results and the boundary pressure conditions in each direction to evaluate the influence of different boundary seepage conditions on the fracture initiation pressure, propagation rate and final fracture penetration mode.

[0037] Furthermore, in step S3, the flexible rubber ring is compressed and tightened under axial stress, which enhances the sealing effect between the guide channel network and the surface of the rock sample, and prevents the exudate from flowing along the contact surface between the loading plate and the sample.

[0038] Compared with the prior art, the present invention has at least the following advantages:

[0039] 1. This invention forms a flow channel network by creating multiple interconnected flow channels on the inner surface of six partitioned flow-guiding loading plates, and surrounding the network with flexible rubber rings. Together, they constitute a flow-guiding constraint structure. This flow channel network independently guides the fracturing fluid seeping from each of the six surfaces of the sample to a pre-set collection chamber inside the loading plate, and then outputs it through an independent collection pipeline via an outlet hole. This design physically isolates and independently guides the seepage fluid in six directions, fundamentally avoiding the mixing and cross-flow of seepage fluid from different directions. At the same time, the flexible rubber rings are compressed and tightened under axial stress, forming an effective seal against the sample surface. This not only inhibits the lateral diffusion of the liquid but also prevents the seepage fluid from leaking into the pressure chamber along the joints or contact surfaces of the loading plate. Thus, it achieves fully enclosed, directional, and pollution-free real-time collection of the seepage fluid from the six boundary surfaces of the sample.

[0040] 2. This invention constructs a "collection-metering-control" fluid loop by setting a three-way connector on an independent collection pipeline, simultaneously connecting the outlet to a high-precision metering balance and a boundary pressure control unit. The high-precision metering balance independently weighs the exudate in each direction, outputting the flow rate and cumulative exudate volume in real time, enabling precise control of the leakage distribution of fracturing fluid in each direction of the sample. Meanwhile, the boundary pressure control unit actively adjusts the pressure inside the guide channel network and collection pipeline through the same pipeline, keeping it below, equal to, or above the ambient pressure, thereby forming boundaries that promote seepage, allow natural seepage, or impede seepage at the corresponding stress surfaces. This parallel structural design allows for simultaneous and non-interfering real-time weighing and metering of the exudate and active intervention in boundary pressure, solving the problem of equipment contamination and corrosion caused by long-term retention of exudate inside the pressure chamber in traditional devices, and providing a feasible technical means to independently set different boundary seepage conditions for each direction.

[0041] 3. This invention also synchronously records injection pressure, injection flow rate, boundary pressure in each direction, and exudate flow rate and cumulative exudate volume in each direction through a data acquisition and processing system, and constructs a real-time calculation model for fracturing efficiency based on the principle of flow conservation. Since the exudate volume in each of the six directions is independently and accurately measured, the system can establish a dynamic balance relationship between the fracturing fluid injection volume and the leakage volume in each direction in real time, thereby achieving a quantitative distinction between "effective fluid volume for fracture propagation" and "ineffective fluid volume lost through the matrix or fractures".

[0042] 4. Because independent collection and real-time metering of permeate from six directions are achieved, the evaluation of fracturing efficiency no longer relies on rough estimates or empirical assumptions about leakage, but is based on precise mass conservation calculations, significantly improving the objectivity and accuracy of fracturing efficiency evaluation. At the experimental functional level, the seepage capacity of the six boundary surfaces can be independently set to promote, natural, or hindered states, enabling flexible construction of homogeneous or heterogeneous boundary pressure fields. This realistically simulates the anisotropic seepage environment of the reservoir, making the conditions of indoor physical simulation tests closer to actual underground working conditions. This allows for a systematic study of the influence of boundary seepage conditions on fracture initiation pressure, differences in propagation rates in each direction, and the final fracture penetration behavior. Attached Figure Description

[0043] Figure 1 This is a top view of the overall structure of the system in this patent.

[0044] Figure 2 This is a cross-sectional view of the overall structure of the system in this patent.

[0045] Figure 3 This is a side view of the overall structure of the system in this patent.

[0046] Figure 4 A schematic diagram of the flow guiding loading plate in the X and Y directions.

[0047] Figure 5 A schematic diagram of a Z+ direction partitioned flow guide loading plate.

[0048] Figure 6 A schematic diagram of a Z-direction partitioned flow guide loading plate. Detailed Implementation

[0049] The present invention will now be described in further detail.

[0050] See Figure 1-6 The apparatus and testing method of the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments, but the embodiments of the present invention are not limited thereto. Various substitutions and modifications made based on ordinary technical knowledge and conventional methods in the art without departing from the above-described technical concept of the present invention should be included within the protection scope of the present invention.

[0051] Example 1: Test for collecting exudate and determining crack propagation direction under standard homogeneous boundary conditions

[0052] This embodiment aims to illustrate the specific operation process of the device of the present invention in independently collecting and measuring the exudate from six surfaces of a rock sample in real time under homogeneous boundary conditions, and determining the preferred direction of crack propagation based on the exudate response characteristics.

[0053] Before the experiment, a shale outcrop or artificial rock core was selected as rock sample 1 and processed into a standard cubic specimen with a side length of 300mm × 300mm × 300mm. The parallelism and perpendicularity of the end faces met the requirements of the true triaxial test. Rock sample 1 was connected to the fracturing fluid injection pump through a high-pressure pipeline, and an injection pipe and sealing assembly were installed in the injection hole at the center of rock sample 1. Then, rock sample 1 was placed in the true triaxial fracturing chamber of the true triaxial hydraulic fracturing physical simulation test seepage surface collection and metering and fracture propagation analysis device, ensuring that the six stress surfaces of rock sample 1 were in full contact with the inner surfaces of the corresponding six partitioned flow guiding loading plates 3, and ensuring that there were no visible gaps between each stress surface and the corresponding loading plate. Finally, the top cover of the true triaxial fracturing chamber was closed and locked tightly.

[0054] The electro-hydraulic servo control system is activated, and preset axial pressure, confining pressure, and pore pressure are applied to the three principal stress directions of rock sample 1 according to the preset triaxial stress loading path until the triaxial stress reaches the target value and remains stable. During this process, the flexible rubber ring 5, located on the outer periphery of the inner surface of the partitioned flow guiding loading plate 3, undergoes elastic compression deformation under the squeezing action of axial stress, tightly adhering to the surface of rock sample 1 to form an initial sealing boundary. This self-tightening action ensures that after subsequent fracturing fluid seepage, the liquid will not laterally flow or leak along the contact surface between the partitioned flow guiding loading plate 3 and rock sample 1 to the outside of the partitioned flow guiding loading plate 3.

[0055] Meanwhile, the six sets of boundary pressure control units 8 control the internal pressure of the flow channel network 4 and the independent collection pipeline 7 to be equal to the ambient atmospheric pressure, without applying additional suction or back pressure to the seepage process of each stress surface of the sample, thereby forming a homogeneous boundary pressure field with consistent boundary conditions in six directions in this test example.

[0056] After the triaxial stress stabilizes, the fracturing fluid injection pump is started, and fracturing fluid is continuously pumped into the rock sample 1 at a preset constant injection flow rate. The fracturing fluid enters the central hole of the rock sample 1 through the injection pipe, where it is pressurized at the bottom of the hole. When the pressure inside the hole reaches the rock fracturing pressure, hydraulic fractures are generated inside the rock sample 1. As the fractures further expand, some of the fracturing fluid seeps along the fracture wall or micro-fractures in the rock matrix to the surface of the rock sample 1, eventually reaching the six stress surfaces of the sample and forming exudate. After the fracturing fluid seeps out from the surface of the rock sample 1, the exudate from each stress surface first enters the flow channel network 4 on the inner surface of the corresponding zoned flow guiding loading plate 3. During this process, the flexible rubber ring 5 surrounding the flow channel network 4 acts as a physical barrier, effectively inhibiting the lateral diffusion of the exudate to the outside of the zoned flow guiding loading plate 3 or adjacent areas, and forcing the liquid to flow towards the collection chamber along the preset path of the flow channel network 4. After the exudate enters the pre-set collection chamber inside the loading plate, it is discharged from the zoned flow guiding loading plate 3 through the outlet hole 6.

[0057] The exudate discharged from the outlet 6 flows along the independent collection pipe 7. A three-way connector is installed on the independent collection pipe 7. The first port of the three-way connector connects to the outlet 6, the second port connects to the inlet of the high-precision measuring balance 9, and the third port connects to the boundary pressure control unit 8. Since the boundary pressure control unit 8 does not actively apply additional positive or negative pressure, but only keeps the pipe open to the atmosphere, the exudate, driven by gravity and the residual pressure inside the sample, flows through the second port into the collection container on the high-precision measuring balance 9. Six high-precision measuring balances 9, corresponding to six directions respectively, continuously weigh the mass increment of the liquid in the collection container and convert it into volumetric flow rate and cumulative exudate. This data is transmitted in real time to the data acquisition and processing system 10 in the form of electrical signals.

[0058] The data acquisition and processing system 10 synchronously acquires and records the injection pressure and flow rate of the injection pump, the set pressure values ​​of the six boundary pressure control units 8, and the real-time flow rate and cumulative volume of the six high-precision metering balances 9, using a unified time reference. When a sudden increase in flow rate with the largest growth rate occurs first in a certain direction, it can be determined that the crack preferentially extends in that direction.

[0059] After the test, the fracturing fluid injection pump was stopped first. After the injection pressure dropped to a safe range, the triaxial stress was released in sequence, the fracturing chamber cover was opened, and rock sample 1 was taken out. At the same time, the boundary pressure control unit 8 was switched to the venting mode to drain all the residual liquid in the pipeline and the guide channel into the collection container to avoid the liquid from remaining in the loading plate or pressure chamber for a long time.

[0060] Example 2: Correlation Analysis Experiment of Crack Propagation Behavior and Boundary Seepage Conditions under Heterogeneous Boundary Conditions

[0061] This embodiment aims to illustrate the specific implementation of how the device of the present invention analyzes the influence of boundary seepage conditions on crack initiation, propagation and penetration behavior by using the exudate response characteristics and fracturing efficiency calculation results when different boundary pressure conditions are set independently in six directions.

[0062] Before the experiment began, boundary conditions were first set. Six boundary pressure control units 8 were used to set different boundary pressure states on the six stress surfaces of the rock sample 1: the X-direction stress surface was set as a seepage-promoting boundary, meaning the internal pressure of the guide channel network 4 and independent collection pipe 7 was adjusted to be lower than the ambient pressure, for example, a relative vacuum of -20 kPa to -50 kPa. Active suction reduced the seepage back pressure on the sample surface in this direction, accelerating liquid discharge in that direction. The X-direction stress surface was set as a seepage-impeding boundary, meaning the internal pressure of the guide channel network 4 and independent collection pipe 7 was adjusted to be higher than the ambient pressure, for example, a gauge pressure of +100 kPa to +200 kPa. A reverse pressure barrier was formed on the sample surface in this direction, inhibiting liquid seepage. The Y-direction and Z-direction stress surfaces were both set as seepage-promoting boundaries, with different negative pressure values ​​set to simulate different degrees of promoting effect. The Y-direction and Z-direction stress surfaces were both set as natural seepage boundaries, meaning the internal pressure was equal to the ambient atmospheric pressure. Therefore, a heterogeneous boundary pressure field with different pressure states in six directions is constructed to simulate the differences in permeability of actual reservoirs in different directions.

[0063] After setting the boundary conditions, following the same operating steps as in Example 1, the rock sample 1 was placed in the true triaxial fracturing chamber, ensuring that the six stress surfaces were in contact with the corresponding zoned flow-guiding loading plates 3, and a preset triaxial stress was applied and maintained. During the stress application process, the flexible rubber rings 5 ​​at the six stress surfaces were all compressed and tightened, independently ensuring the sealing effect between the corresponding loading plate and the sample surface. After the stress stabilized, the fracturing fluid injection pump was started, and fracturing fluid was injected into the rock sample 1 at a constant injection flow rate.

[0064] After the fracturing fluid is injected, the liquid seeping from the surface of the rock sample 1 is collected by the flow channel network 4 inside the flow-guiding loading plates 3 of each zone. Constrained by their respective flexible rubber rings 5, it avoids lateral flow and flows into the independent collection pipeline 7 through the outlet holes 6. The seepage fluid from each direction is split at the tee joint; one path flows into the high-precision weighing balance 9 for real-time weighing, while the other path is regulated by the pressure of the corresponding boundary pressure control unit 8. In this embodiment, due to the different set values ​​of the boundary pressure control units 8 in each direction, the pressure states inside the flow channel network 4 and the independent collection pipeline 7 are different in each direction, resulting in differences in the seepage back pressure on the sample surface in each direction. In the direction promoting seepage, the negative pressure suction accelerates the process of the seepage fluid desorbing from the sample surface and flowing into the flow channel network 4; in the direction hindering seepage, the positive back pressure inhibits the discharge of the seepage fluid and may even force some fracturing fluid to seep out in other low-resistance directions; in the direction of natural seepage, the seepage process is not subject to additional intervention, reflecting the natural seepage characteristics of the sample in that direction. A high-precision weighing balance 9 continuously weighs in all directions and transmits the data to a data acquisition and processing system 10 in real time.

[0065] The data acquisition and processing system 10 simultaneously acquires injection parameters, boundary pressure setpoints in six directions, and the flow rate and cumulative volume of exudate in each direction. On one hand, the system calculates fracturing efficiency in real time, which is calculated as the cumulative injected volume minus the cumulative exudate volume in the six directions, minus the system compression compensation volume, and then divided by the cumulative injected volume. On the other hand, the system correlates the flow rate response curves of exudate in each direction with the corresponding boundary pressure conditions. Specifically, the system compares the time sequence of the first appearance of exudate flow rate, the order of peak flow rates, and the final proportion of cumulative exudate volume in each direction. When the earliest appearance of exudate, the fastest flow rate increase, and the largest proportion of cumulative exudate volume are observed in the direction promoting seepage, it indicates that the fracture experiences the least seepage resistance in that direction, and the fracture preferentially extends towards this low back pressure direction. When the appearance of exudate is significantly delayed and the peak flow rate is significantly lower than in other directions in the direction hindering seepage, it indicates that the high back pressure boundary in that direction effectively inhibits fracture extension, forcing the fracture to extend in other directions. When one direction is a promoting boundary and the other is a hindering boundary, if a large amount of seepage is observed in the promoting boundary direction while almost no seepage is observed in the hindering boundary direction, it is determined that the crack has undergone obvious asymmetric expansion and the crack has not penetrated the hindering boundary direction.

[0066] The data acquisition and processing system 10 further correlates the fracturing efficiency calculation results with the boundary pressure conditions in each direction to quantitatively evaluate the crack propagation behavior under different boundary seepage conditions.

[0067] Specifically, firstly, the crack initiation pressure under each test condition was determined based on the injection pressure-time curve, and the variation law of the initiation pressure under different boundary pressure combinations was compared to evaluate the influence of promoting seepage boundary, natural seepage boundary, and hindering seepage boundary on the ease of crack initiation. Subsequently, combined with the exudate flow response curves in each direction, the time of first appearance of exudate, flow rate increase rate, time to peak flow, and cumulative exudate growth rate in each direction were statistically analyzed and used as characterizing parameters of crack propagation rate. The propagation speed and propagation advantage of cracks in each direction under different boundary conditions were compared and analyzed.

[0068] After the test, based on the cumulative exudate distribution in six directions, the duration of exudation, and whether a continuous and stable seepage channel was formed in each direction, combined with the CT scan results after sample fracturing, sample dissection observation results, or acoustic emission localization results, a comprehensive judgment was made as to whether the crack penetrated the corresponding boundary surface. When a stable seepage continuously occurs at a certain boundary surface, the cumulative exudate significantly increases, and the corresponding crack is connected to the boundary surface in the CT image or sample cross-section, it is determined that the crack has penetrated the boundary surface; conversely, when the exudate response in that direction is weak or there is no continuous seepage, and the crack does not extend to the corresponding boundary surface, it is determined that the crack has penetrated the boundary surface.

[0069] Based on the above analysis, the correspondence between boundary seepage conditions and crack initiation pressure, crack propagation rate and crack penetration mode is established, providing experimental basis for evaluating the influence of different boundary seepage conditions on crack propagation behavior.

[0070] After the test, stop the injection pump, relieve the triaxial stress, and remove rock sample 1. Switch each boundary pressure control unit 8 to the venting mode to drain the residual liquid in the pipeline and guide channel, clean all components, and complete the test.

[0071] Example 3: Pre-calibration method for system compression compensation volume

[0072] This embodiment aims to illustrate the specific implementation of the pre-calibration of the system compression compensation volume Vcomp(t) in this invention. The calibration result is an important basic data for the accurate calculation of fracturing efficiency.

[0073] Before the formal test begins, the compression volume of the entire injection pipeline system, the guide channel network 4, the liquid collection chamber, and the independent collection pipeline 7 needs to be calibrated. The calibration steps are as follows: First, without placing the rock sample 1, install the six zoned guide loading plates 3 in the true triaxial fracturing chamber according to the same assembly method as in the actual test, and completely seal all the liquid outlets 6. Then, start the fracturing fluid injection pump and inject fracturing fluid into the closed pipeline system at the same injection rate as in the actual test. At the same time, the pressure response curve and cumulative injection volume of the injection port are continuously recorded by the data acquisition and processing system 10. Since the system is in a completely closed state and there is no compressible rock sample, all the liquid pumped by the injection pump is used to compress the residual air in the pipeline, cause micro-elastic deformation of the pipeline wall, and fill the volume of the guide channel network 4 and the liquid collection chamber. This part of the volume is the system compression compensation volume Vcomp(t). The calibration process was repeated under different injection pressure gradients, and a calibration curve showing the change in system compression compensation volume with injection pressure was plotted. The relationship between Vcomp and injection pressure was fitted using a second-order polynomial or piecewise linear function, and the fitted formula was stored in the data acquisition and processing system 10. In subsequent actual tests, the data acquisition and processing system 10 automatically retrieved the corresponding Vcomp(t) value based on the real-time monitored injection pressure value and substituted it into the fracturing efficiency calculation formula. This deducted the contribution of the system's own compression deformation to the injection volume, ensuring that the injection volume used in the fracturing efficiency calculation truly reflects the amount of liquid entering the rock sample 1, thus improving the accuracy of fracturing efficiency evaluation.

[0074] Example 4: Description of the assembly and sealing structure of the partitioned flow guiding loading plate

[0075] This embodiment aims to explain in detail the sealing fit relationship of the 45° structural extension 2 set on the edge of the partitioned flow guide loading plate 3 during the assembly process.

[0076] As shown in the attached diagram, the six partitioned flow-guiding loading plates 3 correspond to the six stress surfaces of the rock sample 1, and each partitioned flow-guiding loading plate 3 has a 45° structural extension 2 along its edge. When the six loading plates are assembled around the rock sample 1 in the true triaxial fracturing chamber, the 45° structural extensions 2 of adjacent partitioned flow-guiding loading plates 3 fit together during assembly. That is, the 45° bevel of one loading plate forms a surface contact fit with the 45° bevel of the adjacent loading plate, forming a labyrinthine sealing path. Compared with right-angle butt joints, the 45° bevel fit generates a component force along the bevel direction when the loading plate is subjected to axial clamping force, making the fit between two adjacent partitioned flow-guiding loading plates 3 tighter and further reducing the gap between the contact surfaces. At the same time, this bevel fit structure effectively extends the path length of the exudate leaking outward from the sample surface along the joint of the partitioned flow-guiding loading plate 3, increasing the leakage resistance. Even in the edge area of ​​the loading plate without the flexible rubber ring 5, it can effectively prevent the exudate from leaking into the pressure chamber along the joint between the loading plates. The 45° extension 2 and the flexible rubber ring 5 on the inner surface together form a double sealing barrier. The former prevents leakage at the joint between the loading plates, while the latter inhibits lateral flow on the sample surface. The two work together to ensure that the exudate in the six directions is completely discharged through the guide channel network 4 and the independent collection pipeline 7 without leakage loss, thereby ensuring the integrity and accuracy of the exudate measurement in the six directions.

[0077] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A true triaxial hydraulic fracturing exudate collection and boundary control device, characterized in that, include: Six partitioned flow-guiding loading plates (3) are respectively set on the outer side of the six stress surfaces of the rock sample (1). Each partitioned flow-guiding loading plate (3) has multiple flow-guiding grooves on the side close to the rock sample (1). The multiple flow-guiding grooves are interconnected to form a flow-guiding groove network (4). A flexible rubber ring (5) is provided around the periphery of the flow-guiding groove network (4). The flexible rubber ring (5) and the flow-guiding groove network (4) together constitute a flow-guiding constraint structure. The flow-guiding groove network (4) is connected to a pre-set liquid collection cavity inside the partitioned flow-guiding loading plate (3). The liquid collection cavity is connected to the outside through a liquid outlet hole (6). The flow-guiding groove network (4) is used to guide and collect the fracturing fluid seeping from the sample surface into the liquid collection cavity. Six sets of boundary pressure control units (8), each of the boundary pressure control units (8) is connected to the liquid outlet (6) of a corresponding partitioned flow guide loading plate (3) through a three-way connector provided on the independent collection pipeline (7) connected to it, and is used to independently adjust the internal pressure of each flow guide network (4) and independent collection pipeline (7) to form a flow-promoting boundary, a natural flow boundary or a flow-impeding boundary at the corresponding force surface; The high-precision metering device includes six high-precision metering balances (9). The independent collection pipeline (7) is equipped with a three-way connector. The first port of the three-way connector is connected to the liquid outlet (6), the second port is connected to the high-precision metering balance (9), and the third port is connected to the boundary pressure control unit (8). The high-precision metering balance (9) is used to measure the flow rate and cumulative leakage of the exudate in six directions in real time. The data acquisition and processing system (10) is connected to the fracturing fluid injection pump, the boundary pressure control unit (8) and the high-precision metering balance (9) respectively, and is used to synchronously record the injection pressure, injection flow rate, boundary pressure and exudate parameters in each direction, and calculate the fracturing efficiency according to the flow conservation relationship.

2. The true triaxial hydraulic fracturing exudate collection and boundary control device according to claim 1, characterized in that, The flexible rubber ring (5) is made of fluororubber and has a height of 2.5 mm and a width of 2.0 mm; the guide groove has a groove height of 2.0 mm and a groove width of 3.0 mm.

3. The true triaxial hydraulic fracturing exudate collection and boundary control device according to claim 1, characterized in that, The edge of the partitioned flow guiding loading plate (3) is provided with a 45° structural extension (2), and the 45° structural extension (2) of adjacent partitioned flow guiding loading plates (3) fit together during assembly to prevent exudate from leaking from the joint of the loading plate.

4. The true triaxial hydraulic fracturing exudate collection and boundary control device according to claim 1, characterized in that, The boundary pressure control unit (8) adjusts the pressure in the following way: When the internal pressure of the flow channel network (4) and the independent collection pipeline (7) is lower than the ambient pressure, a seepage-promoting boundary is formed on the corresponding force-bearing surface; When the internal pressure of the flow channel network (4) and the independent collection pipeline (7) is equal to the environmental pressure, a natural seepage boundary is formed on the corresponding stress surface; When the internal pressure of the flow channel network (4) and the independent collection pipeline (7) is higher than the ambient pressure, a boundary that hinders seepage is formed on the corresponding force-bearing surface.

5. A method for surface-level collection, metering, and fracture propagation analysis of permeate in hydraulic fracturing physical simulation tests using the true triaxial hydraulic fracturing permeate collection and boundary control device described in any one of claims 1 to 4, characterized in that, Includes the following steps: S1: Place the rock sample (1) in the true triaxial fracturing chamber of the true triaxial hydraulic fracturing physical simulation test seepage surface collection and measurement and fracture propagation analysis device, so that the six stress surfaces of the rock sample (1) are in contact with the corresponding partition flow guiding loading plate (3), apply the preset triaxial stress and keep it stable; S2: Start the fracturing fluid injection pump and inject fracturing fluid into the rock sample (1); S3: The leachate is collected by the flow channel network (4), and after being restricted by the flow constraint structure to diffuse laterally, it is transported to the corresponding high-precision metering balance (9) through the independent collection pipeline (7) to record the flow rate and cumulative leachate in each direction in real time. S4: Data acquisition and processing system (10) synchronously acquires injection parameters, boundary pressure parameters and permeate parameters in each direction, and calculates fracturing efficiency based on flow conservation. S5: Compare the flow response characteristics of the exudate in each direction, determine the direction in which the flow rate first increases and the growth rate is the preferred direction for fracture propagation, and analyze the fracture propagation behavior in combination with the trend of fracturing efficiency changes.

6. The method for collecting and measuring seepage from different surfaces and analyzing fracture propagation in a hydraulic fracturing physical simulation test according to claim 5, characterized in that, The calculation model for fracturing efficiency is as follows: in: For fracturing efficiency; For the cumulative injection volume; Let be the cumulative seepage volume at the i-th boundary surface; The pre-calibrated system compression compensation volume.

7. The method for collecting and measuring seepage from different surfaces and analyzing fracture propagation in a hydraulic fracturing physical simulation test according to claim 6, characterized in that, Before step S1, a boundary condition setting step is also included: the boundary pressure values ​​of the six stress surfaces are set by six sets of boundary pressure control units (8) respectively, so that each stress surface is independently in the state of promoting seepage, natural seepage or hindering seepage, so as to simulate the heterogeneous seepage environment of the reservoir.

8. The method for collecting and measuring seepage from different surfaces and analyzing fracture propagation in a hydraulic fracturing physical simulation test according to claim 7, characterized in that, It also includes step S6: performing correlation analysis between the fracturing efficiency calculation results and the boundary pressure conditions in each direction, and evaluating the influence of different boundary seepage conditions on the fracture initiation pressure, propagation rate and final fracture penetration mode.

9. The method for collecting and measuring seepage from different surfaces and analyzing fracture propagation in a hydraulic fracturing physical simulation test according to claim 5, characterized in that, In step S3, the flexible rubber ring (5) is compressed and tightened under axial stress, which enhances the sealing effect between the guide groove network (4) and the surface of the rock sample (1).

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