An experimental testing method for multi-source monitoring of crack propagation and stress transmission in multi-medium rock mass

Abstract

This invention provides a multi-source monitoring and testing method for fracture propagation and stress transmission in multi-medium rock masses, relating to the field of experimental testing technology for oil and gas reservoir rock mechanics. The method includes the following steps: S1, preparing standard samples and constructing a multi-discontinuous media structure; S2, constructing a four-dimensional integrated monitoring system; S3, performing uniaxial compression fracturing experiments and hydraulic fracturing experiments respectively; S4, performing multi-source data fusion of the monitoring data under a unified time axis and spatial coordinate system; S5, performing analysis based on the multi-source data fusion. This invention solves the problems of existing technologies, such as the inability to simultaneously monitor multiple media environments, limited monitoring methods, and insufficient guidance of experimental results.

Description

Technical Field

[0001] This invention relates to the field of experimental testing technology for rock mechanics in oil and gas reservoirs, and in particular to an experimental testing method for multi-source monitoring of fracture propagation and stress transmission in multi-medium rock masses. Background Technology

[0002] Traditional rock mass fracturing experiments and monitoring methods often use homogeneous rocks or simple samples with only a single fracture, which cannot simultaneously simulate the multi-medium environment where natural fractures, faults, and multiple lithological interfaces coexist, resulting in significant deviations from actual reservoir geological conditions. Furthermore, traditional monitoring methods are mostly singular, often employing only one method from acoustic emission, digital image correlation, strain gauges, or distributed optical fibers, failing to achieve multi-scale, comprehensive characterization of the entire process from fracture initiation, propagation, and final penetration. Moreover, compression experiments and hydraulic fracturing experiments are usually conducted independently, lacking a coordinated loading and linkage analysis mechanism, thus failing to accurately reproduce the complex stress state of the underground reservoir under the combined effects of far-field stress and internal fluid pressure. Additionally, existing technologies cannot perform unified temporal alignment and spatial correlation analysis of local stress response, global deformation evolution, internal micro-damage evolution, and distributed strain distribution, resulting in an inability to fully reveal the coupling mechanism between fracture propagation and stress transmission. Summary of the Invention

[0003] The purpose of this invention is to provide an experimental testing method for multi-source monitoring of crack propagation and stress transmission in multi-medium rock masses. This method can integrate sample preparation, synergistic loading, multi-source monitoring, and data fusion to accurately characterize crack propagation patterns and stress transmission mechanisms.

[0004] To address the aforementioned technical problems, this invention provides a multi-source monitoring and testing method for crack propagation and stress transmission in multi-medium rock masses, comprising the following steps:

[0005] S1. Based on the mechanical parameters of unconventional reservoir rocks, standard samples were prepared using cement-based similar materials, and a multi-discontinuous medium structure was constructed.

[0006] S2. Construct a four-dimensional integrated monitoring system that integrates a digital image correlation full-field deformation monitoring subsystem, a strain gauge local stress monitoring subsystem, an acoustic emission internal micro-damage monitoring subsystem, and a distributed optical fiber full-domain strain monitoring subsystem.

[0007] S3. The method of first performing mechanical compression test and then performing fluid fracturing test is adopted to perform uniaxial compression fracture test and hydraulic fracturing fracturing test respectively;

[0008] S4. Perform multi-source data fusion on the monitoring data of digital image-related full-field deformation monitoring, strain gauge local stress monitoring, acoustic emission internal micro-damage monitoring, and distributed optical fiber full-domain strain monitoring under a unified time axis and spatial coordinate system.

[0009] S5. Perform analysis based on multi-source data fusion.

[0010] Furthermore, the cement-based similar material specifically includes: composite silicate cement, quartz sand, water, and water-reducing agent, with the ratio of cement:quartz sand:water = 2:1:0.7, and 0.5% water-reducing agent added.

[0011] Further, in step S1, the multiple discontinuous medium structure includes: a matrix sample, a precast crack sample, a fault sample, and a multi-lithological layered sample; the matrix sample is uniformly cast as a whole and serves as a reference control sample; the precast crack sample is processed with a through-type straight crack at the geometric center of the sample using water jet cutting technology; the fault sample is made by cutting a standard cylindrical sample into two segments at a set inclination angle, and the cut surfaces are bonded and repositioned with high-strength epoxy resin adhesive; the multi-lithological layered sample is formed by layering and casting with cement of different strength grades to form a layered structure.

[0012] Furthermore, in step S2, the digital image correlation full-field deformation monitoring subsystem adopts a dual high-speed camera orthogonally arranged, a matte speckle sticker is pasted on the sample observation surface, and the sampling frequency is set to 20 frames per second.

[0013] Furthermore, in step S2, the camera performs intrinsic and extrinsic parameter calibration, with a calibration error not exceeding 0.01 pixels.

[0014] Furthermore, the acoustic emission internal micro-damage monitoring subsystem in step S2 includes: a plurality of resonant acoustic emission sensors uniformly spaced circumferentially on the sample, wherein the contact surface between the resonant acoustic emission sensors and the sample is coated with a coupling agent.

[0015] Furthermore, the acoustic emission internal micro-damage monitoring subsystem in step S2 adopts a multi-channel acoustic emission acquisition instrument, and is set with a preamplifier gain of 40dB, a threshold value of 40dB, and a sampling rate of 10MHz.

[0016] Furthermore, in step S2, the distributed optical fiber global strain monitoring subsystem processes a spiral groove on the sample surface, embeds a single-mode optical fiber into the groove, and fixes it with quick-drying epoxy resin. The optical fiber is connected to the distributed optical fiber shape monitoring and analysis instrument and operates in the optical frequency domain reflection mode.

[0017] Furthermore, the uniaxial compression fracture loading test in step S3 adopts a displacement control mode, with the loading rate set to 0.05 mm / min. Throughout the loading process, axial load, axial displacement, strain gauge timing signals, acoustic emission parameters, and full-field image sequences are simultaneously acquired.

[0018] Furthermore, in the hydraulic fracturing loading test in step S3, an injection hole is drilled in the central axis of the sample, a stainless steel injection tube is inserted, a constant axial pressure is applied to the sample, and clean water is pumped in at a constant speed using a horizontal flow pump. The pump pressure-time curve is collected in real time, and pump pressure, acoustic emission signals and fiber optic distributed strain signals are collected simultaneously.

[0019] Based on the above technical features, the beneficial effects of the present invention are as follows:

[0020] (1) The present invention provides a multi-source monitoring and testing method for fracture propagation and stress transmission in multi-medium rock mass, which can simultaneously simulate the multi-medium structure of matrix, natural fracture, fault, and multi-lithological interface, and is highly matched with the real geological conditions of unconventional reservoirs such as shale and tight sandstone.

[0021] (2) The present invention provides a multi-source monitoring experimental test method for crack propagation and stress transmission in multi-medium rock mass. It constructs a four-dimensional integrated monitoring system that integrates a digital image correlation full-field deformation monitoring subsystem, a strain gauge local stress monitoring subsystem, an acoustic emission internal micro-damage monitoring subsystem, and a distributed optical fiber full-domain strain monitoring subsystem. It can realize multi-scale monitoring of crack propagation from micro-damage to macro-fracture and from single-point stress to full-field deformation. Attached Figure Description

[0022] Figure 1 A schematic diagram of the method flow is provided for this invention;

[0023] Figure 2 This is a schematic diagram illustrating the types of multiple discontinuous media structures in this invention;

[0024] Figure 3 This is a schematic diagram of the overall layout of the four-dimensional integrated monitoring system in this invention;

[0025] Figure 4 This is a schematic diagram of the setup of the uniaxial compression fracture test and monitoring system in this invention;

[0026] Figure 5 This is a schematic diagram of the hydraulic fracturing-induced fracture experiment and monitoring system setup in this invention;

[0027] Figure 6 The full-field principal strain contour plot of the 45° pre-cracked specimen in Experiment Example 1 is shown in the DIC diagram.

[0028] Figure 7 This is a schematic diagram of the crack in the interlayer sample of Experiment Example 2;

[0029] Figure 8 This is a distribution diagram of the high strain zone in the optical fiber of the multi-lithological sample in Experiment Example 3. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0031] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0032] To better understand this invention, it should be noted that unconventional oil and gas reservoirs generally develop multiple discontinuous media, such as natural fractures, faults, bedding, and lithological interfaces, resulting in a complex structure where continuous media (matrix) and discontinuous media coexist. These discontinuous surfaces significantly alter fracture propagation paths and stress transmission patterns, easily leading to engineering problems such as unbalanced fracture propagation, uncontrolled fracture network complexity, and increased inter-well interference, directly impacting reservoir volumetric fracturing effectiveness and ultimate recovery rate.

[0033] Example

[0034] Please refer to the above as well. Figures 1-8 The present invention provides an experimental testing method for multi-source monitoring of crack propagation and stress transmission in multi-medium rock masses, the method comprising the following steps:

[0035] S1. Based on the mechanical parameters of unconventional reservoir rocks, standard samples were prepared using cement-based similar materials, and a multi-discontinuous medium structure was constructed.

[0036] S2. Construct a four-dimensional integrated monitoring system that integrates a digital image correlation full-field deformation monitoring subsystem, a strain gauge local stress monitoring subsystem, an acoustic emission internal micro-damage monitoring subsystem, and a distributed optical fiber full-domain strain monitoring subsystem.

[0037] S3. The method of first performing mechanical compression test and then performing fluid fracturing test is adopted to perform uniaxial compression fracture test and hydraulic fracturing fracturing test respectively;

[0038] S4. Perform multi-source data fusion on the monitoring data of digital image-related full-field deformation monitoring, strain gauge local stress monitoring, acoustic emission internal micro-damage monitoring, and distributed optical fiber full-domain strain monitoring under a unified time axis and spatial coordinate system.

[0039] S5. Perform analysis based on multi-source data fusion.

[0040] It is worth noting that the multi-source monitoring experimental method for fracture propagation and stress transmission in multi-medium rock masses provided by this invention uses the geological characteristics of deep unconventional oil and gas reservoirs as a prototype to prepare similar samples of four types of multi-mediums: matrix, pre-fabricated fractures, faults, and multi-lithological stratification. A four-dimensional synchronous monitoring system of digital image correlation, strain gauges, acoustic emission, and distributed optical fiber is constructed to achieve uniaxial compression and hydraulic fracturing synergistic loading tests. The multi-source monitoring data is fused and analyzed to quantitatively characterize the entire process of fracture initiation, propagation, and penetration, as well as the stress transmission law. This allows for a more realistic simulation of the coexistence environment of discontinuous media in reservoirs, enabling multi-scale full-process monitoring of fracture propagation from microscopic damage to macroscopic rupture. The experimental conditions are highly consistent with the underground stress state, providing reliable experimental basis and theoretical support for unconventional reservoir fracturing design, fracture control, and prevention of inter-well interference.

[0041] The following will, with reference to the figures and taking a specific project as an example, explain in detail the specific implementation of the present invention.

[0042] S1. Using unconventional reservoir rock mechanical parameters as a benchmark, standard samples were prepared using cement-based similar materials, and a multi-discontinuous medium structure was constructed. The preparation of multi-medium rock mass similar material samples was carried out. Using the unconventional reservoir rock mechanical parameters of a certain Xi Formation in a basin of Sichuan as a benchmark, standard samples were prepared using cement-based similar materials. Through precise processing, a multi-discontinuous medium structure was constructed to achieve indoor equivalent simulation of reservoir geological characteristics.

[0043] In some embodiments, PC42.5R composite silicate cement is preferably used as the matrix material, standard quartz sand as the aggregate, and purified tap water as the mixing water. 0.5% by mass of high-efficiency water-reducing agent is added to improve the density and uniformity of the sample. Preferably, the mix ratio is cement:quartz sand:water = 2:1:0.7. Under this mix ratio, the uniaxial compressive strength, elastic modulus, and Poisson's ratio of the sample are highly similar to those of real shale, which can ensure the representativeness and reliability of the experimental results.

[0044] In some embodiments, the standard cylindrical specimen casting and curing process is as follows: First, the weighed cement, sand, and water-reducing agent are dry-mixed for 3 minutes, and water is added and the mixture is stirred for another 5 minutes until the slurry is uniform and free of lumps; then, the slurry is slowly poured into a rigid cylindrical mold with a diameter of 50 mm and a height of 100 mm, and placed on a vibrating table and vibrated at a frequency of 50 Hz for 2 minutes to fully remove internal air bubbles; then, after standing at room temperature for 24 hours, the specimen is demolded and placed in a standard curing room with a temperature of 20 ± 2 ℃ and a relative humidity of ≥ 95% for continuous curing for 28 days to ensure stable mechanical properties and a dispersion of less than 5%.

[0045] Furthermore, the process involves the refined construction and fabrication of a multi-medium structure. Specifically, this multi-discontinuous media structure includes: a matrix sample, a pre-cast crack sample, a fault sample, and a multi-lithological stratified sample. The matrix sample is a uniformly cast specimen without any prefabrication defects, interfaces, or cracks, serving as a benchmark control sample. The pre-cast crack sample is fabricated using waterjet cutting technology to create a continuous, straight crack at the geometric center of the sample. The crack length is controlled to be 20 mm, the crack width to be 2 mm, and the crack center to coincide with the sample center. The pre-cast crack inclination angles are set to 0°, 30°, 45°, 60°, and 90°, covering common natural crack occurrences. Depending on experimental requirements, gypsum, pure cement grout, or epoxy resin can be used for filling to simulate low- and medium-strength cracks. The study explored three fracture filling states: high-intensity, low-intensity, and high-strength. The fault specimens were prepared by cutting a standard cylindrical specimen into two sections at a set angle (0°, 15°, 30°, or 45°). The cut surfaces were polished with a diamond abrasive to a flatness of ≤0.02mm, then bonded with high-strength epoxy resin and cured at room temperature for 24 hours to simulate different angle-locked fault structures. The multi-lithological layered specimens were constructed by layering different strength grades of cement, such as PC32.5R, PC42.5R, and PC52.5R, controlling the initial setting time between upper and lower layers to 0.5 hours. This resulted in three layered structures: upper strong and lower weak, upper and lower equal strong, and upper weak and lower strong, simulating the obstruction and deflection of stress transmission by interlayered lithology and mechanical interfaces within the reservoir.

[0046] Furthermore, the end face of the specimen is subjected to precision treatment. Specifically, both ends of the specimen are finely ground with a grinding stone to ensure that the unevenness of the end face is ≤ ±0.05mm and the deviation of the angle between the end face and the specimen axis is ≤ ±0.25°, which meets the experimental standards recommended by the International Society for Rock Mechanics (ISRM).

[0047] S2. Construct a four-dimensional integrated monitoring system that integrates a digital image-related full-field deformation monitoring subsystem, a strain gauge local stress monitoring subsystem, an acoustic emission internal micro-damage monitoring subsystem, and a distributed optical fiber full-domain strain monitoring subsystem, thereby achieving full-coverage monitoring from macroscopic fracture to microscopic damage, from single-point stress to full-field deformation, and from temporal characteristics to spatial distribution.

[0048] In some embodiments, the deployment and calibration of the digital image-related full-field deformation monitoring subsystem are as follows: matte speckle stickers are uniformly pasted on the sample observation surface, with speckle diameter of 0.4–0.5 mm and speckle coverage controlled at 50%–60% to ensure relevant recognition accuracy; two high-speed cameras are orthogonally arranged, with a lens focal length of 25 mm, an aperture of f / 8, and a shooting distance of 50 cm; the internal and external parameters of the cameras are calibrated, with a calibration error ≤ 0.01 pixels; the sampling frequency is set to 20 frames / s, which can completely capture the evolution of the full-field displacement and strain fields from crack initiation to crack penetration.

[0049] In some embodiments, the layout of the acoustic emission internal micro-damage monitoring subsystem is as follows: four resonant acoustic emission sensors are evenly arranged in the circumferential direction of the sample, with the center of the sensor 25 mm away from the upper and lower end faces of the sample; a coupling agent is applied to the contact surface between the sensor and the sample, preferably medical petroleum jelly, and the sensor is secured with an elastic rubber band to ensure stable coupling; the acoustic emission system uses a DS5 multi-channel acoustic emission acquisition instrument, with a preamplifier gain of 40 dB, a threshold value of 40 dB, a sampling rate of 10 MHz, and a bandwidth of 1 kHz-3 MHz, thereby enabling real-time capture of microcrack initiation, propagation, friction slip, and through-crack signals.

[0050] In some embodiments, the deployment of the distributed optical fiber global strain monitoring subsystem is as follows: a spiral groove is machined on the sample surface using a precision grooving machine. The groove depth is 0.2 mm, the pitch is 10 mm, and the groove width is consistent with the optical fiber diameter, thus ensuring that the overall stress state of the sample is not damaged; a G.652 single-mode optical fiber is embedded in the groove and fixed with quick-drying epoxy resin to ensure that the optical fiber and the rock mass deform together; the optical fiber is connected to a TKS-8108 distributed optical fiber shape monitoring and analysis instrument, which adopts the optical frequency domain reflectance (OFDR) working mode, with a spatial resolution of 1 mm and a strain measurement accuracy of ±1 με, thereby enabling continuous strain monitoring and crack location along the entire length of the optical fiber.

[0051] In some embodiments, the static stress-strain testing subsystem is configured as follows: preferably, BX-120-3AA type resistance strain gauges are used, with a grid length of 3mm and a sensitivity coefficient of 2.0±1%; strain gauges are attached to key stress concentration locations such as the crack tip, fault plane center, and both sides of the lithological interface, using a half-bridge connection and temperature compensation plates to eliminate the influence of temperature drift; the strain gauges are connected to the H3816N static stress-strain testing and analysis system, with a sampling frequency of 5Hz, enabling 72-channel synchronous acquisition with a resolution of up to 0.1με, accurately capturing local stress abrupt changes and stress transmission timing.

[0052] S3. By first performing mechanical compression tests and then performing fluid fracturing tests, uniaxial compression fracturing experiments and hydraulic fracturing fracturing experiments are carried out respectively, which can more realistically restore the stress state of the underground reservoir under the combined action of far-field stress and internal fluid pressure.

[0053] In some embodiments, a uniaxial compression fracture test is performed, specifically as follows: First, the specimen is placed at the center of the loading stage of a TAW-2000 microcomputer-controlled triaxial testing machine, and a preload of 1 MPa is applied to ensure that the indenter, pad, and specimen end face are in complete contact. Then, using a displacement control mode, the loading rate is kept constant at 0.05 mm / min, and loading is continued until the specimen completely fractures macroscopically. Throughout the loading process, axial load, axial displacement, strain gauge timing signals, acoustic emission parameters, and a full-field image sequence are simultaneously acquired. The acoustic emission parameters specifically include acoustic emission ringing count parameters, energy parameters, and RA-AF parameters.

[0054] In some embodiments, a hydraulic fracturing experiment is performed, specifically as follows: First, a Φ11mm injection hole with a depth of 50mm is drilled along the central axis of the sample, a stainless steel injection tube is inserted, a 10mm open hole section is left at the bottom, and the tube opening is sealed with epoxy resin to prevent fracturing fluid leakage; then, a constant axial pressure of 4MPa is applied to the sample to simulate the minimum principal stress state in the far field of the formation; next, clean water is pumped in at a constant rate of 3mL / min using a horizontal flow pump, and the pump pressure-time curve is collected in real time; during the experiment, pump pressure, acoustic emission signal, and fiber optic distributed strain signal are collected simultaneously, and the pump is stopped immediately when the sample rupture pressure suddenly drops, and all data are saved.

[0055] S4. The monitoring data of digital image-related full-field deformation monitoring, strain gauge local stress monitoring, acoustic emission internal micro-damage monitoring, and distributed optical fiber full-domain strain monitoring are fused under a unified time axis and spatial coordinate system to establish a four-in-one quantitative characterization method of mechanical response-damage evolution-deformation field-stress transmission.

[0056] In detail, a unified clock and synchronous trigger control are implemented. Specifically, all subsystems and the loading system achieve zero-delay linkage through synchronous triggers. The loading start signal simultaneously triggers the DIC, strain gauges, acoustic emission, and fiber optic acquisition, ensuring that all signals are strictly aligned on the time axis, thus providing a foundation for subsequent multi-source data fusion.

[0057] In some embodiments, a timestamp alignment algorithm is used to unify the time sequence of sampling data from each subsystem, a spatial interpolation algorithm is used to map monitoring data of different resolutions to a unified spatial coordinate system, and a multi-scale data fusion algorithm is used to achieve data association.

[0058] S5. Perform analysis based on multi-source data fusion.

[0059] In some embodiments, when performing acoustic emission data processing, the specific steps are as follows: extracting ring count, cumulative energy, amplitude, duration, and rise time; calculating RA value and AF value; classifying tensile failure, shear failure, and combined tensile-shear failure; and determining the crack initiation time, propagation stage, and instability time.

[0060] In some embodiments, when performing DIC data processing, the specific steps are as follows: reconstructing the full-field horizontal displacement U, vertical displacement V, and principal strain E1 cloud map through a relevant matching algorithm, and locating the strain concentration area, crack initiation point, propagation direction, penetration path, and slip zone.

[0061] In some embodiments, when performing strain gauge data processing, the specific steps include: plotting local stress-time curves, identifying the stress concentration initiation point, stress abrupt change time, stress transmission direction, and stress attenuation characteristics.

[0062] In some embodiments, when performing fiber optic data processing, the specific steps include: generating strain distribution curves along the fiber optic path, and calibrating the location of high-strain regions, crack propagation length, stress transmission range, and interface blocking effect.

[0063] In some embodiments, when performing multi-source joint time series analysis, the specific steps are as follows: align the stress-time curve, pump pressure curve, acoustic emission time series, DIC cloud map, and fiber strain curve to determine key parameters such as crack initiation stress, crack initiation angle, rupture pressure, crack propagation speed, stress transmission range, and discontinuous medium interference intensity, and finally reveal the coupling mechanism of crack propagation and stress transmission under multi-medium conditions.

[0064] It should be noted that this embodiment adopts a compression-hydraulic fracturing synergistic loading method, which can more realistically reproduce the stress state of underground reservoir under the combined action of geostress and fluid pressure, and the experimental results are more consistent with the field conditions. This embodiment establishes a multi-source data fusion analysis method, which can accurately and quantitatively characterize the initiation stress, fracturing pressure, failure mode, fracture propagation path and stress transmission range. The testing method provided in this embodiment has high experimental system stability, good repeatability and reliable data, and can be directly used for reservoir fracturing parameter optimization, inter-well interference mechanism analysis and engineering risk prevention and control.

[0065] Experimental Example 1

[0066] Please refer to Figure 6 In this experimental example, crack propagation and stress transmission tests were conducted on a 45° pre-existing crack specimen under uniaxial compression. The specific process is as follows:

[0067] S1. Sample preparation: PC42.5R cement, standard sand, and water were mixed in a ratio of 2:1:0.7, with 0.5% water-reducing agent added. After thorough mixing, Φ50mm×100mm standard cylindrical samples were poured and cured for 28 days. A 45° through-crack was fabricated using water jet cutting, with a crack length of 20mm and a crack width of 2mm. The crack center coincided with the geometric center of the sample. The crack was not filled, simulating the open-type natural fracture of the reservoir.

[0068] S2. Deployment of the four-dimensional integrated monitoring system: The digital image correlation full-field deformation monitoring subsystem uses speckle stickers affixed to the front of the sample, with dual cameras orthogonally arranged and calibrated, sampling at a frequency of 20 frames / s. The strain gauge local stress monitoring subsystem uses one strain gauge each at the upper and lower tips of the pre-fabricated crack, connected in a half-bridge configuration, and equipped with temperature compensation gauges. The acoustic emission internal micro-damage monitoring subsystem uses four sensors evenly arranged circumferentially on the sample, with good coupling, a gain of 40dB, and a threshold of 40dB. The distributed fiber optic full-domain strain monitoring subsystem uses spiral grooves on the sample surface, embedding single-mode optical fibers and securing them firmly. Finally, all subsystems are connected to a synchronous trigger to achieve clock synchronization.

[0069] S3. Perform a uniaxial compression fracture test: Place the specimen in the center of the loading stage and apply a preload of 1 MPa to ensure close contact; use displacement-controlled loading at a loading rate of 0.05 mm / min; continuously load until the specimen undergoes macroscopic penetrating fracture without interruption or disturbance.

[0070] S4. Perform fusion of monitoring data from digital image-related full-field deformation monitoring, strain gauge local stress monitoring, acoustic emission internal micro-damage monitoring, and distributed fiber optic full-domain strain monitoring under a unified time axis and spatial coordinate system.

[0071] S5. Execute analysis based on multi-source data fusion, specifically:

[0072] (1) Mechanical response characteristics analysis: The peak stress of the sample is 25.57 MPa, the peak strain is 0.23%, and the elastic modulus is 8.7 GPa. The stress-strain curve is divided into compaction stage, elastic stage, yielding stage and failure stage. Multiple step-like stress drops appear before the peak, indicating that microcracks continue to propagate at the crack tip.

[0073] (2) Analysis of acoustic emission damage evolution characteristics: In the early stage of loading (0–60% peak stress), the ring count is low and the cumulative energy is gradual, mainly due to the compaction of the original pores; in the middle stage (60%–90% peak stress), the ring count increases intermittently and the cumulative energy increases stepwise, and the microcracks expand stably; before and after the peak, the ring count and cumulative energy increase sharply, and the specimen becomes macroscopically unstable; RA–AF analysis shows that tension cracks account for 87.18% and shear cracks account for 12.82%, and the failure is mainly of the tension type.

[0074] (3) Analysis of the deformation and strain characteristics of DIC: In the early stage of loading, strain concentration first appears at the tips of both ends of the precast crack; in the middle stage, airfoil tensile cracks appear at the tips and extend along the direction of the maximum principal stress; in the later stage, the cracks continue to expand and penetrate, forming a through main crack; the principal strain cloud map clearly shows that the crack propagation path and the stress concentration area are highly coincident.

[0075] (4) Analysis of local stress evolution characteristics of strain gauges: The strain gauge at the crack tip shows obvious tensile stress abrupt change in the middle of loading; the stress rise rate is significantly accelerated, marking the formal crack initiation; the stress abrupt change time is completely consistent with the first active moment of acoustic emission and the starting moment of DIC strain concentration.

[0076] (5) Analysis of crack propagation and stress transmission mechanism: 45° precast crack significantly changes the stress transmission path, and the stress deflects along the crack surface and concentrates at the tip; the crack initiation position is strictly controlled at the tip of the precast crack; the stress transmission shows discontinuous and localized characteristics, forming an obvious stress shadow area; the final crack propagation mode is the tension-controlled airfoil crack propagation mode.

[0077] Experiment Example 2

[0078] Please refer to Figure 7 In this experimental example, crack propagation and stress transmission tests were conducted on a 30° dip fault specimen under uniaxial compression. The specific process is as follows:

[0079] S1. Sample preparation: After the standard cylindrical sample has been cured, it is cut into two sections at a 30° angle; the cut surface is polished flat, and epoxy resin is used to bond and reposition the sample, which is then cured for 24 hours; the sample size is Φ50mm×100mm, and the end face accuracy meets the standard.

[0080] S2. Deployment of the four-dimensional integrated monitoring system: The digital image correlation (DIC) full-field deformation monitoring subsystem uses a DIC camera directly facing the fault plane to ensure clear imaging of the entire fault area; the strain gauge local stress monitoring subsystem uses strain gauges attached to the center of the upper and lower disks of the fault plane to monitor stress differences on both sides of the fault; the acoustic emission internal micro-damage monitoring subsystem uses four acoustic emission channels arranged in a ring to ensure no fault slip signals are missed; and the distributed fiber optic full-domain strain monitoring subsystem uses spiral grooves on the sample surface to embed single-mode optical fibers, ensuring secure fixation. Finally, all subsystems are connected to a synchronous trigger to achieve clock synchronization.

[0081] S3. Perform a uniaxial compression fracture test: Place the specimen in the center and apply a preload of 1 MPa to ensure contact stability; use the displacement control mode and load at a uniform rate of 0.05 mm / min until the specimen exhibits obvious slippage and macroscopic damage.

[0082] S4. Perform fusion of monitoring data from digital image-related full-field deformation monitoring, strain gauge local stress monitoring, acoustic emission internal micro-damage monitoring, and distributed fiber optic full-domain strain monitoring under a unified time axis and spatial coordinate system.

[0083] S5. Execute analysis based on multi-source data fusion, specifically:

[0084] (1) Mechanical response characteristics analysis: The peak stress of the specimen is 39.5 MPa, which is lower than that of the intact matrix specimen. The stress-strain curve shows significant nonlinear characteristics before the peak, indicating slip-controlled failure.

[0085] (2) Acoustic emission characteristics analysis: Continuous signal fluctuations appear in the early stage of loading, and the cumulative energy increases in a stepwise manner, reflecting the closure, slippage and local stress redistribution of the fault plane.

[0086] (3) DIC deformation characteristics analysis: The specimen shows obvious relative displacement along the fault plane, the main strain concentration zone is distributed along the fault direction, and the cracks preferentially extend along the fault.

[0087] (4) Stress characteristics analysis of strain gauges: The upper plate is dominated by tensile stress, while the lower plate is dominated by compressive stress. The stress difference increases with loading, indicating that the stress differentiation on both sides of the fault is obvious.

[0088] (5) Stress transmission mechanism analysis: As a weak surface, the fault has the function of absorbing and redistributing stress. The stress transmission path changes from continuous transmission to transmission along the fault, and the failure mode exhibits typical tensile-shear composite characteristics.

[0089] Experimental Example 3

[0090] Please refer to Figure 8 In this experimental example, tests were conducted on multi-lithological B–C stratified (hard upper layer, soft lower layer) samples under hydraulic fracturing. The specific procedure is as follows:

[0091] S1. Sample preparation: The upper layer uses PC42.5R cement (strength grade B), and the lower layer uses PC32.5R cement (strength grade C); the samples are poured in layers with horizontal interfaces, an initial setting interval of 0.5h, and standard curing for 28d; a 50mm deep injection hole is drilled in the center of the sample, with a 10mm open section, and the pipe opening is sealed properly.

[0092] S2. Deployment of the four-dimensional integrated monitoring system: The digital image correlation (DIC) full-field deformation monitoring subsystem uses a DIC camera directly facing the fault plane to ensure clear imaging of the fault area throughout. The strain gauge local stress monitoring subsystem is attached to the strong rock layer above the interface, at the interface, and the weak rock layer below the interface. The acoustic emission internal micro-damage monitoring subsystem uses four sensors evenly arranged in a circumferential pattern, covering the upper and lower rock layers and the interface area. The distributed fiber optic full-domain strain monitoring subsystem is spirally deployed along the sample surface, fully covering the interface and weak rock layer. Finally, the pumping system, loading system, and monitoring system are triggered uniformly.

[0093] S3. Perform hydraulic fracturing test: Apply a constant axial pressure of 4 MPa to the sample; pump fracturing fluid at a constant rate of 3 mL / min; when the pump pressure rises steadily to 4.6 MPa and then suddenly drops, the sample is determined to have fractured. Immediately stop pumping and save the data.

[0094] S4. Perform fusion of monitoring data from digital image-related full-field deformation monitoring, strain gauge local stress monitoring, acoustic emission internal micro-damage monitoring, and distributed fiber optic full-domain strain monitoring under a unified time axis and spatial coordinate system.

[0095] S5. Execute analysis based on multi-source data fusion, specifically:

[0096] (1) Pump pressure curve characteristics analysis: It shows a steady rise-rapid fracture-rapid decline pattern. The fracture has obvious brittle characteristics, and the fracture pressure is controlled by the strength of the underlying weak rock layer.

[0097] (2) Acoustic emission characteristics analysis: The signal is calm in the early stage of pumping, and the ringing count and energy burst at the moment of rupture. The proportion of shear cracks increases significantly, and the failure type changes to tensile-shear composite failure.

[0098] (3) Analysis of fiber strain characteristics: The high strain zone first appears in the lower weak rock layer. The strain at the lithological interface shows obvious obstruction and abrupt change, indicating that the interface has a blocking effect on stress transmission.

[0099] (4) Analysis of crack propagation morphology: Hydraulic cracks preferentially initiate and rapidly propagate in the lower weak rock layer. When they propagate upward to the lithological interface, they deflect and stop, making it difficult to penetrate the strong rock layer.

[0100] (5) Stress transmission law analysis: stress preferentially accumulates and is transmitted to low-strength rock layers. The interface causes discontinuous stress transmission and forms upper and lower stress zones. Strong rock layers have a significant blocking and limiting effect on crack propagation and stress transmission.

[0101] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. It will be apparent to those skilled in the art that the invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered illustrative and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the scope of the invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0102] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A multi-source monitoring and testing method for crack propagation and stress transmission in multi-medium rock mass, characterized in that, Includes the following steps: S1. Based on the mechanical parameters of unconventional reservoir rocks, standard samples were prepared using cement-based similar materials, and a multi-discontinuous medium structure was constructed. S2. Construct a four-dimensional integrated monitoring system that integrates a digital image correlation full-field deformation monitoring subsystem, a strain gauge local stress monitoring subsystem, an acoustic emission internal micro-damage monitoring subsystem, and a distributed optical fiber full-domain strain monitoring subsystem. S3. The method of first performing mechanical compression test and then performing fluid fracturing test is adopted to perform uniaxial compression fracture test and hydraulic fracturing fracturing test respectively; S4. Perform multi-source data fusion on the monitoring data of digital image-related full-field deformation monitoring, strain gauge local stress monitoring, acoustic emission internal micro-damage monitoring, and distributed optical fiber full-domain strain monitoring under a unified time axis and spatial coordinate system. S5. Perform analysis based on multi-source data fusion.

2. The method according to claim 1, characterized in that, In step S1, the cement-based similar material specifically includes: composite silicate cement, quartz sand, water, and water-reducing agent, with the ratio of cement:quartz sand:water = 2:1:0.7, and 0.5% water-reducing agent added.

3. The method according to claim 2, characterized in that, In step S1, the multiple discontinuous medium structure includes: matrix sample, pre-fabricated fracture sample, fault sample, and multi-lithological layered sample. The matrix sample is a uniformly cast whole and serves as a reference control sample; the precast crack sample is made by processing a through-type straight crack at the geometric center of the sample using water jet cutting technology; the fault sample is made by cutting a standard cylindrical sample into two segments at a set inclination angle, and the cut surfaces are bonded and repositioned with high-strength epoxy resin adhesive; the multi-lithological layered sample is made by casting in layers with cement of different strength grades to form a layered structure.

4. The method according to claim 1, characterized in that, The digital image correlation full-field deformation monitoring subsystem in step S2 adopts a dual high-speed camera orthogonally arranged, with a matte speckle sticker pasted on the sample observation surface, and the sampling frequency is set to 20 frames per second.

5. The method according to claim 4, characterized in that, In step S2, the camera performs internal and external parameter calibration, and the calibration error does not exceed 0.01 pixels.

6. The method according to claim 1, characterized in that, The acoustic emission internal micro-damage monitoring subsystem in step S2 includes: multiple resonant acoustic emission sensors evenly spaced in the circumferential direction of the sample, wherein the contact surface between the resonant acoustic emission sensors and the sample is coated with a coupling agent.

7. The method according to claim 6, characterized in that, The acoustic emission internal micro-damage monitoring subsystem in step S2 uses a multi-channel acoustic emission acquisition instrument, and is set with a preamplifier gain of 40dB, a threshold value of 40dB, and a sampling rate of 10MHz.

8. The method according to claim 1, characterized in that, The distributed optical fiber global strain monitoring subsystem in step S2 involves processing a spiral groove on the sample surface, embedding a single-mode optical fiber into the groove, and fixing it with quick-drying epoxy resin. The optical fiber is then connected to a distributed optical fiber shape monitoring and analysis instrument and operates in optical frequency domain reflection mode.

9. The method according to claim 1, characterized in that, The uniaxial compression fracture loading test in step S3 adopts a displacement control mode, with the loading rate set to 0.05 mm / min. The axial load, axial displacement, strain gauge timing signals, acoustic emission parameters, and full-field image sequence are collected synchronously throughout the loading process.

10. The method according to claim 1, characterized in that, In step S3, the hydraulic fracturing loading test involves drilling an injection hole along the central axis of the sample, inserting a stainless steel injection tube, applying a constant axial pressure to the sample, and using a horizontal flow pump to pump clean water at a constant speed. The pump pressure-time curve is collected in real time, and pump pressure, acoustic emission signals, and fiber optic distributed strain signals are collected simultaneously.

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