Abstract
Based on the engineering background of 11,211 working face in Kekegai Coal Mine, this paper systematically studies the influence of key parameters on hydraulic fracturing effect in deep water-rich fractured thick coal seam and its optimal selection method. In this study, the true triaxial physical experiment and numerical simulation are combined, and the control effects of in-situ stress conditions, fracture pressure and interlayer interface on crack propagation behavior are analyzed emphatically. By constructing a three-dimensional stress field model and tracking the dynamic development process of cracks in real time with the help of acoustic emission monitoring technology, it is revealed that rock heterogeneity and stress distribution have a significant impact on fracture morphology: cracks in homogeneous rocks tend to develop smoothly and symmetrically, while complex branch cracks are easy to form in heterogeneous rocks. It is determined that 31.50–36.00 MPa is the optimal fracturing pressure range, which can ensure the fracturing effect and maintain the stability of surrounding rock structure. At the same time, it is found that moderate water injection rate (7.5–10.0 mL/min) is most beneficial to enhance interlayer connectivity and promote the formation of complex fracture network. The numerical simulation based on ABAQUS platform further verifies the stress redistribution law and crack propagation mode, which is consistent with the physical test results. This study provides important theoretical support and engineering guidance for the optimization of hydraulic fracturing parameters and the safe and efficient mining of coal seam under complex geological conditions.
1 Introduction
Rock fragmentation and other dynamic hazards induced by hard roof strata have emerged as critical challenges in the safe and efficient exploitation of deep, thick coal seams in China (Cunhan et al., 2025; Yanan et al., 2022; Rong et al., 2025). During such events, a considerable amount of elastic energy accumulated within the hard roof strata is suddenly released (Liu et al., 2024; Chungui et al., 2021; Pengjun et al., 2024), frequently resulting in violent failure of coal and rock masses, damage to hydraulic supports, and roadway blockages. These occurrences pose severe threats to underground personnel and lead to substantial economic losses (Yang et al., 2025; Anthony et al., 2023).
A representative case is the 11,211 working face of the Kekegai Coal Mine, which spans 400 m and features a roof composed of 18.98 m of medium-grained ar-kose and a 2.9-m-thick silty mudstone floor (in et al., 2025; Xu et al., 2025; Wang et al., 2014). This “stiff-over-soft” composite strata structure is prone to stress concentration under mining-induced loading, thereby increasing the risk of rock fragmentation (L and M, 2023; Numerical Modeling, 2018). While hydraulic fracturing (HF) is widely recognized as an effective method for pressure relief and hard roof weakening (Sainoki et al., 2017), its application in water-rich fractured strata remains limited due to challenges such as imprecise horizon selection and unpredictable fracture propagation paths (Wang et al., 2025; Pengjin et al., 2024; Abdellatif et al., 2025).
Previous studies have identified the in-situ stress state, fracturing pressure, and interlayer interface characteristics as key factors influencing fracture evolution (Li et al., 2015). However, a comprehensive understanding of the coupled mechanisms among these factors remains limited. This gap poses significant challenges for optimizing hydraulic fracturing in thick coal seams with water-rich fractures. To address these challenges, this study aims to systematically investigate the fracture propagation mechanism and permeability enhancement effects in such seams. The methodology involves a combined approach of true triaxial hydraulic fracturing physical simulation and real-time acoustic emission (AE) monitoring. Through this approach, the study seeks to reveal the mechanical mechanisms governing fracture initiation and propagation under variable in-situ stress fields, quantify the influence of fracturing pressure parameters on fracture network morphology, and clarify the limiting role of interlayer interfaces in cross-strata fracture development. The results show that (insert key quantitative findings, e.g., a specific percentage increase in fracture network density or a defined reduction in fracturing pressure). These findings provide valuable insights for optimizing on-site fracturing parameters and enhancing the effectiveness of hydraulic fracturing operations.
By constructing an experimental model that closely mimics real geological conditions, this research not only generates essential data for optimizing fracturing parameters but also offers a novel approach to understanding fracture propagation in water-rich coal seams. This study innovatively integrates true triaxial hydraulic fracturing physical simulation with real-time acoustic emission (AE) monitoring, providing a comprehensive framework to analyze the complex interactions between in-situ stress, fracturing pressure, and interlayer interfaces. Our findings offer valuable insights for improving pressure relief strategies in complex underground environments, thereby making a unique contribution to the field of hydraulic fracturing in water-rich coal seams. The following sections detail the experimental design and implementation methodology.
2 True triaxial hydraulic fracturing experiment
Using a self-developed true triaxial testing platform combined with an acoustic emission (AE) monitoring system, this study constructs a physical model to simulate hydraulic fracturing under heterogeneous in-situ stress conditions. The experiment employs similar-material casting techniques and pre-embedded polyvinyl chloride (PVC) pipes to accurately replicate borehole structures, effectively reducing costs and improving experimental efficiency. This setup enables precise control of mechanical responses, offering a reliable framework to study fracture propagation in complex three-dimensional stress fields. The research focuses on analyzing fracture behavior around boreholes, tracking AE signals as indicators of rock weakening and rupture, identifying distinct stages of fracture development, and quantitatively evaluating stress thresholds and path deflection mechanisms (Tianjun et al., 2023). The findings provide essential parameters for hydraulic fracturing simulations and offer theoretical guidance for optimizing fracturing designs and reducing engineering risks in complex geological environments. Overall, this work contributes to the development of more efficient and safer resource extraction technologies.
2.1 Sample preparation
To simulate the general patterns of crack propagation in rock-like materials, Huang Bingxiang (HUANG, 2009) investigated the mechanical properties of cement mortar specimens with various mix ratios through a series of uniaxial compression tests. It was determined that the mix ratio of cement to fine sand = 1:3.5 produced a complete stress–strain curve most closely resembling that of natural rock materials in key aspects such as the elastic stage, plastic yielding near peak strength, and post-peak softening behavior. This ratio also yielded compressive strength, elastic modulus, and failure modes representative of common natural rocks like sandstone or limestone. Based on these findings, the study adopted a cement-to-fine sand ratio of 1:3.5 to ensure the specimens closely simulated the mechanical characteristics of natural rock masses in subsequent experiments. This study is based on similarity theory and employs similar material samples as substitutes for natural rocks. The substitution is achieved by adjusting the mix proportions, modifying the sample dimensions, and optimizing the molding process, while maintaining consistency in key mechanical properties.
Figure 1 illustrates the three-dimensional structure and positioning details of the sample. To meet the size constraints of the testing machine’s loading chamber, cubic cement-mortar samples with 300 mm edge length were fabricated. Each sample includes a centrally positioned through-hole measuring 6 mm in diameter and 12 mm in length. PVC pipes were embedded during casting to ensure the dimensional and positional accuracy of the injection hole.
FIGURE 1
2.2 Experimental system
This study explores fracture propagation under true triaxial stress conditions using a self-developed hydraulic fracturing system. Figure 2 presents the experimental setup. At the core of the system is an 80 cm cubic, thick-walled steel chamber, designed to maintain pressure integrity. A bolted cover plate with rubber gaskets and multi-stage sealing assemblies around the embedded loading rams ensure effective sealing. The system is equipped with cartridge heaters and fluid-line interfaces and is capable of sustaining confining pressures up to 50 MPa. Hydraulic fracturing is simulated by precisely injecting fluid into the wellbore through a high-pressure fluid injection module. The robust chamber structure and advanced sealing design guarantee both operational safety and leak-tight performance during high-pressure experiments.
FIGURE 2
The hydraulic workstation is an integrated system designed for applying true triaxial stress and injecting high-pressure fracturing fluid. It enables the establishment of a controlled triaxial stress environment, execution of hydraulic fracturing, and maintenance of high-pressure sealing integrity.
The confining pressure loading system, supported by a four-column structural frame, applies horizontal stresses to the sample via dual lateral hydraulic rams. At the same time, the minimum principal stress is applied vertically through hydrostatic pressure, allowing for independent control of all three principal stress directions. True triaxial stress states are achieved through the coordinated application of hydraulic pressures and , along with hydrostatic confining pressure .
The loading rams deliver bidirectional pressure transmission, while the pressure vessel accommodates unidirectional stress transfer. The entire process is controlled via dedicated software, integrating hydraulic control with AE win-based acoustic emission monitoring. This allows for precise load application and real-time analysis of fracture dynamics during the hydraulic fracturing process.
2.3 Experimental scheme
The hydraulic fracturing experimental protocol in this study consists of four core components: acoustic emission (AE) monitoring, true triaxial confining pressure control, water injection flow regulation, and AE signal acquisition. Based on in-situ stress data from the 11,211 working face of the Kekegai Coal Mine, the experiment closely replicates actual underground stress conditions and simulates various stress states. Figure 3 illustrates the configuration of the AE sensors. In this setup, σH and σh represent the maximum and minimum horizontal principal stresses, respectively, while σv denotes the vertical stress. Fluid injection was performed at a constant, controlled flow rate.
FIGURE 3
The rock-like samples used in the true triaxial tests were designed to simulate the mechanical behavior of natural rock. Given the difficulty of processing deep, hard roof strata into full-scale test samples, a physical simulation approach was adopted. This method enables the controlled analysis of hydraulic fracturing effects on fracture propagation and the spatial distribution of AE energy within a laboratory setting.
The sample body is subjected to hydrostatic pressure from all directions, achieved by superimposing the confining pressure and the external cylinder loads. However, the overlapping intermediate portion-specifically, the reaction force from the rigid loading rod-must be deducted. Therefore, the calculation formula for the reaction force of the rigid loading rod is:
Where: is the reaction force of the rigid loading rod, in N; and are the servo cylinder loads in the and directions, respectively, in N; is the side length of the specimen, in mm. 0.011304 is the empirical coefficient for rigid loading in this experiment.
Based on calculation, the servo cylinder load setting parameters corresponding to the stress states of the triaxial confining pressure schemes are presented in Table 1. Accordingly, the confining pressure loading for specimens in the triaxial rock mechanics experiments was performed.
TABLE 1
| Servo cylinder load | Scheme 1 | Scheme 2 | Scheme 3 | Scheme 1 |
|---|---|---|---|---|
| TH、Th (kN | 933.36,168.36 | 711.96,168.36 | 1251.96,348.36 | 933.36,168.36 |
Settings for servo cylinder loads under various stress states.
Triaxial hydraulic fracturing experiments, which simulate in-situ stress conditions, are essential for understanding fracture propagation mechanisms and their coupled effects. These tests reveal that while stress conditions primarily control fracture initiation and growth, rock heterogeneity significantly influences fracture path deviation. By quantifying the influence of stress on fracture network morphology, the experiments offer valuable insights for optimizing fracturing parameters. Pressure and injection flow rate are identified as key diagnostic indicators that determine fracture initiation, propagation capacity, and geometry. In composite rock strata, fracture behavior varies distinctly due to differences in interface properties and material heterogeneity. Such experiments provide a unique opportunity to explore the fundamental mechanical interactions at lithological interfaces.
This physical similarity simulation experiment employed an acoustic emission (AE) monitoring system. The system utilized a MICRO-II EXPRESS acoustic emission monitor, developed by Physical Acoustics Corporation (PAC) of the United States, to monitor the energy released and the intensity of failure during specimen fracturing. The AE system comprised AEwin software, developed by Physical Acoustics Corporation (PAC), USA, and hardware components including AE sensors, preamplifiers, and an AE acquisition unit. The primary AE monitoring parameters configured for this experiment are listed in Table 2 below.
TABLE 2
| Parameter | Acquisition frequency (MHz) | Hit definition time (us) | Peak definition time (us) | Waveform threshold (dB) | Parameter threshold (dB) | Filter frequency (KHz) |
|---|---|---|---|---|---|---|
| Value | 10 | 2000 | 1000 | 45 | 40 | 20–100 |
Settings of acoustic emission (AE) monitoring parameters.
3 True triaxial hydraulic fracturing tests incorporating in-situ stress conditions
Prior to experimentation, ultrasonic velocity testing was conducted on the specimens. Specimens exhibiting uniform and comparable wave velocities were selected for subsequent testing. In the first set of experiments, the applied stress conditions were = 20.94 MPa, = 11.90 MPa, and = 11.47 MPa. Figure 4 presents the resulting failure pattern of Rock Sample No. 1. The small difference between the maximum horizontal stress () and the intermediate principal stress () facilitated the development of branched fractures. This stress configuration, combined with stress interaction effects, led to complex and irregular crack propagation paths within the sample.
FIGURE 4
Figure 5 illustrates the asymmetric fracture morphology observed in Sample No. 1, characterized primarily by unidirectional propagation. Hydraulic fractures extended preferentially along paths of least resistance, showing a lateral bias toward areas with lower mechanical impedance. The limited dimensions of the sample facilitated rapid fluid migration to the specimen boundary, while on the opposite side, incomplete fracture development occurred due to early fluid dissipation. This imbalance ultimately resulted in the observed morphological asymmetry.
FIGURE 5
In the second set of experiments, the applied stresses were = 17.94 MPa, = 11.90 MPa, and = 11.47 MPa. Figure 6 presents the fracture surface morphology of Rock Sample No. 2, where cracks primarily developed within the horizontal plane and propagated in a radial pattern. Additionally, Figure 5 shows that the fractures exhibited a characteristic double-wing shape, which was nearly symmetric in the horizontal direction. The crack paths were smooth and continuous, with no apparent inflection points.
FIGURE 6
In the third group of experiments, the loading stresses were increased to = 23.94 MPa, = 13.90 MPa, and = 11.47 MPa. Figure 6 displays the fracture morphology of Rock Sample No. 3, where fractures developed perpendicular to and parallel to , forming a predominantly elliptical pattern. The fracture morphology of Sample No. Three closely resembles that of Sample No. 2, indicating that in highly homogeneous rock samples, crack propagation tends to be smooth and stable.
In-situ stress plays a critical role in controlling both fracture propagation patterns and pressure evolution during hydraulic fracturing. Under conditions of high stress anisotropy, fractures tend to extend in a preferential, unidirectional manner. In contrast, lower stress differentials encourage the formation of more complex fracture networks. A detailed analysis of the geostress distribution is therefore essential for optimizing perforation placement strategies (Lv et al., 2024).
In this experiment, helium gas was injected at a pressure of 0.5 MPa. As shown in Figure 7, the inlet pressure increased steadily during the initial stages of hydraulic fracturing, while the outlet pressure remained constant. With the introduction of water, pore pressure rose, although the outlet pressure increased only slightly at first. Once fractures interconnected, the inlet pressure peaked and then declined, while the outlet pressure rose sharply. The pressures eventually stabilized and equalized, indicating the successful completion of the hydraulic fracturing process (Tian et al., 2024).
FIGURE 7
Based on the diffusion pattern of the fracturing fluid, it is observed that the fluid enters the rock sample along the direction of the fracturing string and disperses in an elliptical shape toward the direction of the maximum principal stress.
The temporal variation of acoustic emission (AE) signal parameters during hydraulic fracturing is analyzed to understand the fracture evolution process (Shuaifeng et al., 2019). Under different confining pressure conditions, changes in crack propagation characteristics in cement mortar samples are examined to explore their correlation with AE responses.
Acoustic emission technology evaluates the internal state of materials by capturing acoustic wave signals generated during microcrack activity. The number of AE events reflects the frequency of internal damage, while signal amplitude indicates the intensity of energy release. Together, these parameters provide a quantitative basis for assessing the degree and evolution of material damage.
Figure 8A captures the critical moment of rock fracturing, characterized by intensified acoustic emission (AE) activity, elevated high-frequency signals, and a sharp increase in event counts. The AE amplitude peaked at 83.2 dB, with 4,165 detectable events. Following fracture initiation, fluid pressure dropped abruptly and then stabilized as the crack propagated through the sample.
FIGURE 8
Figure 8B shows a rise in AE events to 3,698 and a peak amplitude of 94.5 dB, indicating increased damage intensity and greater energy release from the fracture source. This reflects a stronger AE response resulting from more active internal cracking.
Figure 8C demonstrates that with increased initiation pressure, both the AE event count and amplitude rise significantly, reaching 4,702 events and a maximum amplitude of 121.1 dB. This highlights a clear positive correlation between injection pressure and AE signal strength, confirming that higher pressures lead to more intense and extensive fracturing.
Across all three experiments, high-amplitude acoustic emission (AE) signals are primarily concentrated at the moment of rock fracture. A comparison between Sample No. Two and Sample No. Three shows that increasing the confining pressure results in a 27.14% increase in the total number of AE events and a 28.15% increase in peak amplitude. This indicates that higher confining pressure intensifies internal fracturing activity and energy release.
Figure 9 presents the acoustic emission (AE) positioning map of the fracture in Sample No. 2, revealing a double-wing, approximately symmetrical, and smooth crack pattern. The AE positioning data reflect the progressive evolution of internal micro-fractures leading up to the formation of a macroscopic crack (Zborowski, 2019).
FIGURE 9
Figure 10 further confirms this pattern in Sample No. 2, again displaying a double-wing, symmetrical fracture morphology. The consistency between positioning results reinforces the observed micro-fracture development process prior to full fracture propagation (Science-Geoscience, 2018; School of Mining Engineering et al., 2018).
FIGURE 10
Figure 11 shows the AE positioning diagram for Sample No. 3, which exhibits a fracture pattern similar to that of Sample No. 2. The spatial distribution of AE events is largely consistent across both samples. By integrating the AE data throughout the fracturing process with the final macroscopic fracture geometry, it is evident that Sample No. Three experienced intense AE activity, with numerous events concentrated along the water pressure–dominated fracture surface.
FIGURE 11
4 Numerical simulation
Underground hydraulic fracturing typically employs high-pressure water at approximately 25 MPa to fracture coal and rock, with the goal of redistributing and homogenizing the in-situ stress field during mining. The process generally consists of three stages. In the first stage, high-pressure water weakens the coal and rock around the borehole, initiating primary fractures. In the second stage, the main fracture rapidly propagates as the rock mass is split. In the third stage, water slowly permeates through the fracture network, potentially generating secondary fractures.
Numerical simulations assess stress redistribution and displacement after main fracture formation, assuming fluid pressure acts on fracture surfaces while neglecting matrix infiltration. This approach evaluates how fracturing alters the stress field and helps mitigate dynamic hazards like coal and gas outbursts. Laboratory experiments under controlled conditions provide fracture pressures and morphology data to calibrate simulation parameters, such as cohesive zone properties and in-situ stress boundaries. Although simplified in fluid flow, the model shows good agreement with experiments in stress and displacement predictions, supporting its use in outburst prevention assessment (In situ; Hossein Talebi et al., 2015; Zhu et al., 2024; Wang et al., 2018).
4.1 ABAQUS: software overview and advantages
ABAQUS enables advanced simulation of hydraulic fracturing through fully coupled thermo-hydro-mechanical models, capturing fracture propagation via XFEM/cohesive methods and supporting complex heterogeneity, natural fractures, and adaptive meshing. It facilitates large-scale 3D modeling with high-performance computing and provides detailed post-processing of fractures and stresses for optimizing multi-stage stimulation. With superior nonlinear convergence and multiphysics integration over alternatives like FLAC3D or ANSYS, it is widely preferred for research and design of deep reservoir fracturing.
4.2 Material model determination
Fracture simulation approaches such as the dynamic lattice element method used in (Rizvi et al., 2020) offer an alternative micro-mechanical framework to model stress redistribution in heterogeneous rock.
As shown in Figure 12, laboratory hydraulic fracturing samples were modeled in the ABAQUS visualization module to simulate the roof rock specimens. The objective was to reproduce the modal response and stress characteristics observed in the physical experiments through numerical simulation.
FIGURE 12
To validate the material model, two-dimensional numerical simulations were performed using the same dimensions as the physical samples (300 mm × 300 mm). To improve accuracy near critical regions such as the fracture hole and corners, a refined mesh was applied, with element sizes gradually increasing away from the hole surface. Additionally, nodal singular elements were introduced at the crack tips to better capture stress concentration and enable accurate analysis of crack propagation paths.
The numerical simulations were conducted using the same physical parameters, Darcy-law seepage model, and loading conditions as in the laboratory experiments. To replicate observed failure modes, identical injection holes and pre-existing fracture paths were incorporated into the simulation setup. As shown in Figure 13, the simulated fracture propagation and failure patterns closely resembled those from the experiments, confirming the effectiveness of the Darcy model in reproducing key fracture behaviors.
FIGURE 13
However, natural rocks contain numerous non-uniform microfractures that are difficult to replicate numerically. In physical experiments, these microcracks undergo compaction, contributing to nonlinear deformation. In contrast, the numerical models-despite incorporating preset fractures-cannot fully capture this behavior. As a result, simulated deformations tend to be more linear and less pronounced, leading to an underestimation of actual specimen deformation.
4.3 Underground model establishment
Numerical simulation is employed to analyze stress redistribution following hydraulic fracturing, thereby evaluating its effectiveness. Based on the geological conditions of a 400 m extra-long working face, a simplified numerical model was established, as shown in Figure 14. The model, measuring 500 m × 50 m, features compressive stress boundaries. The upper 45 m represents the roof strata, while the lower 5 m corresponds to the coal seam. As illustrated in Figure 15, water is injected in segmented stages from designated injection points to promote fracture propagation.
FIGURE 14
FIGURE 15
4.4 Numerical simulation analysis of fracturing state
To better visualize stress distribution during hydraulic fracturing, stress contour plots were generated and analyzed. Figure 16 compares the simulated stress distribution around fractures during water injection with the corresponding experimental results (Rizvi et al., 2020).
FIGURE 16
As shown in Figure 17, high-pressure water injection creates unloading zones at the fracture tips, leading to localized stress reduction and displacement in the surrounding coal and rock. This induces plastic deformation near the fracture tips, thereby weakening the local structure. Simultaneously, stress concentrations develop on both sides of the fracture due to the pressure exerted by the injected fluid, although the resulting stress levels remain lower than the injection pressure. This tip unloading effect promotes fracture propagation along pre-existing weak planes.
FIGURE 17
Ideally, fractures continue to extend until fluid injection stops. However, in this simulation, the assumption of a homogeneous rock mass limited control over fracture direction—highlighting the challenges of modeling fracture propagation in geologically complex conditions.
This stress redistribution and pore connectivity enhancement are also evident in thermal stress field studies conducted on underground power systems, as shown in (Ahmad et al., 2025a; Ahmad et al., 2021).
In addition, stress concentrations on both sides of the fracture during hydraulic fracturing introduce complexity to the overall stress distribution. If fracture propagation is not properly controlled, elevated stress levels near the working face may increase operational risks. Therefore, appropriate personal protection measures should be implemented, and, where feasible, real-time monitoring of fracture propagation and stress variations is recommended.
Although the duration of fracturing is relatively short, the induced stress changes primarily occur within the coal and rock mass. While these changes generally have limited immediate impact on resource recovery, the altered post-fracturing stress field can significantly influence subsequent mining operations.
To further evaluate these effects, stress variation profiles ahead of the roadway and geostress contour plots following cessation of water injection were generated. These analyses provide deeper insights into the redistribution of stress within the coal and rock mass and its potential implications for mine stability and safety.
Figure 18 illustrates the stress reduction in the coal and rock mass ahead of the roadway, with stress levels decreasing as they approach the roadway and gradually returning to their original state with increasing distance. This redistribution causes the surrounding rock to move toward the roadway, resulting in greater displacement near the roadway and lesser displacement further away. After hydraulic fracturing, the previously elevated stress on both sides of the fracture is significantly reduced, leading to a substantial drop in stress and localized plastic deformation.
FIGURE 18
These results confirm that hydraulic fracturing is an effective technique for pressure relief. The formation of unloading zones reduces the strain energy stored in the coal and contributes to the mitigation of coal and gas outburst risks. Additionally, the reduction in ground stress facilitates fracture development, allowing gas to migrate along fracture paths and be released into roadways, thereby lowering gas pressure within the coal seam.
However, hydraulic fracturing can also introduce new stress concentrations at fracture tips, creating localized high-stress zones that may pose safety risks during subsequent recovery operations. If such stress cannot be adequately controlled, the application of hydraulic fracturing may be limited.
At the early stage of fracturing, the surge in water pressure quickly generates a strain concentration zone, reflecting the transition of pore wall microfractures from elastic deformation to rupture. As pressure begins to fluctuate or decline, the strain field disperses radially, indicating the initiation of primary fractures and branching of secondary cracks. Once pressure stabilizes during the holding phase, the strain field becomes more uniform, with primary fractures extending along the maximum principal stress direction and secondary fractures forming a low-strain extension zone.
In homogeneous rock layers with poorly developed primary fractures, minor pressure fluctuations lead to a continuous and smooth expansion of the strain field. This results in a gentle strain gradient at the leading edge of the primary fracture and stable crack propagation. In contrast, rock layers with significant permeability variations or distinct structural surfaces exhibit asymmetric strain field distributions under fluctuating pressure. In such cases, abrupt jumps or bifurcations occur within high-strain zones, indicating that fractures either change direction upon encountering barriers or follow weaker structural planes. These zones often feature strain concentration and clustering of secondary cracks, closely influenced by local geological heterogeneity.
Following multiple fracturing events within a single borehole, the strain field evolves into a multi-core, radiating, and interwoven network. The interaction between primary and secondary cracks forms a globally weakened zone, reflecting the combined effects of water-induced softening and crack expansion. During the pressure-holding stage, constant-flow water injection facilitates deeper strain penetration, further reducing rock mass strength through fluid–solid coupling. After support installation, near-surface strain is redistributed, and the strain gradient near the borehole decreases.
To prevent premature shallow crack closure, dynamic monitoring of stress redistribution is necessary. Overall, the observed strain characteristics confirm the effectiveness of hydraulic fracturing in generating a controllable fracture network within the roof rock layer.
5 Integrated discussion of hydraulic fracturing
This study integrates true-triaxial physical modeling, numerical simulation, and field experiments to investigate the dynamic fracture propagation mechanisms and control principles of hydraulic fracturing in thick, water-rich coal seams with hard roofs. The findings provide both theoretical guidance and practical strategies for the prevention and control of roof-related hazards in deep mining environments.
5.1 Horizontal stress difference: fracture growth and pressure response control
The horizontal principal stress difference () plays a critical role in determining fracture geometry and propagation behavior during hydraulic fracturing. When Δσ exceeds 9 MPa, fractures tend to grow asymmetrically at angles of 65°–75° relative to the maximum horizontal stress (). In contrast, when is approximately 6 MPa, fractures exhibit more symmetrical, wing-shaped patterns. This trend was confirmed in the high-stress environment of the 11,211 working face at the Kekuai Coal Mine ( > 9 MPa), where the average fracturing pressure for A/B-type boreholes (oriented perpendicular to ) was significantly higher than that of -type boreholes (parallel to σH). A larger suppresses secondary fracture branching, guiding fracture propagation along the direction, and elevates the required injection pressure by 17%–22% due to increased resistance from rock tensile strength. These findings highlight the necessity of accurate stress field assessment in fracturing design.
In-situ acoustic emission (AE) monitoring further reveals that the confining pressure () significantly influences damage evolution in roof rock masses. When σ3 increases from 11.47 MPa to 14.90 MPa-simulating deeper geological conditions-the number of AE events increases by 28.15%, and cumulative energy release rises by 31%. This indicates that under higher confining pressure, micro-fracture nucleation, propagation, and coalescence are accelerated. Although fracture opening is constrained, energy is released more intensely through concentrated micro-fracturing.
Field borehole observations provide spatial validation: post-fracturing, newly formed fracture networks were identified at depths between 12 and 42 m. Shallow sections (12–20 m) exhibited dense, hydraulically connected fractures, while deeper zones (20–42 m) showed stable main-fracture extension. This zonal pattern aligns with the radial distribution of AE events, indicating a depth-dependent damage mechanism: shallow regions are governed by shear–tensile hybrid failure along weak planes, whereas deeper regions are dominated by tensile-driven fracture propagation.
5.2 Fluid - solid coupling reveals stress redistribution and engineering parameter optimization
A fluid–solid coupling numerical model developed using ABAQUS reveals the dual stress redistribution effects induced by hydraulic fracturing. Specifically, it highlights the formation of unloading zones at fracture tips and stress concentration zones on both sides of the fractures. These findings have two key engineering implications.
First, The in-situ monitoring at borehole B7 critically validates the efficacy of the high-flow injection scheme. The detection of 15 L/min instantaneous flow in an adjacent anchor-bolt hole at 6-m depth confirms successful hydraulic connectivity and provides field-scale confirmation of the laboratory-derived similitude principles for fracture penetration rate. This significant flow not only indicates the achievement of a fracture radius exceeding 10 m but also demonstrates the creation of a highly conductive flow pathway within the stimulated fracture network. The results validate that the 160 L/min injection protocol effectively overcame permeability barriers, enhanced subsurface fluid migration, and corroborated the predictive models for controlled fracture propagation.
Second, the results inform improvements in fracturing strategy. To minimize the risk of stress-induced dynamic events, a segmented fracturing approach with 3-m intervals was implemented. This method effectively reduced localized stress concentrations. During the pressure-holding phase, pressure fluctuations remained within 5%, indicating quasi-static fracture propagation and a lowered risk of impact-related ground pressure.
Similar thermally-driven fluid–solid coupling behavior in subsurface materials has been reported in (Ahma et al., 2019), where cyclic thermal loading significantly influenced subsurface stress fields and fracture evolution in granular backfills.
5.3 Lithological interface heterogeneity active control strategies technology chain construction future prospects
This study suggests that lithological interface heterogeneity may play a crucial role in determining fracturing efficiency. Field observations from -type boreholes appeared to exhibit low-pressure fracturing characteristics, potentially indicating a natural weakening effect at the interface between the immediate roof siltstone and the underlying sandstone. These observations seem to align with numerical simulation results derived from interface cohesion parameters, implying that weak interlayer bonding could significantly reduce the pressures required for both fracture initiation and propagation.
In light of these findings, the research team explored the optimization of borehole geometry—including dip angle, azimuth, and penetration depth—to leverage the inferred interface weakening effect and potentially guide fracture propagation along targeted strata or interfaces. This approach, rooted in geomechanical analysis, may offer a pathway toward layered, sequential, and controlled collapse of hard roof strata, which could improve both the efficiency and safety of fracturing-based weakening interventions.
Looking forward, the study underscores a closed-loop technical framework integrating multi-physics monitoring, numerical simulation, and real-time field control. The process spans from micro-damage evolution to macro stress-field reconstruction, culminating in dynamic parameter adjustment during testing. This integrated approach offers an adaptive, science-based strategy for managing fracturing in complex geological conditions. Such a multi-scale and multi-physics methodology might not only help verify the effectiveness of hydraulic fracturing in challenging roof conditions but could also provide a adaptable and reproducible strategy for future engineering applications.
Future AE monitoring systems could be augmented with AI-driven signal classification methods, similar to those used in subsurface corrosion detection shown in (Chungui et al., 2021).
6 Conclusion
This study focuses on the hydraulic fracturing pressure relief strategy for the hard roof of the 11,211 working face at the Kekegai Coal Mine. Through true triaxial simulation experiments, the effects of in-situ stress state, fracturing pressure, and interlayer interface characteristics in composite strata on fracture propagation were systematically analyzed.
Experimental results demonstrate that hydraulic fractures propagate in an elliptical pattern within the horizontal plane, with their geometry exhibiting high sensitivity to both rock heterogeneity and stress distribution. The pressure curve initially rises gradually, followed by a rapid decline upon the onset of fracture formation, and subsequently stabilizes as the fracture extends through the sample surface.
Acoustic emission (AE) monitoring reveals weak and scattered activity prior to rock failure. Once fracturing begins, AE events increase significantly, especially near the borehole. Higher confining pressure enhances AE intensity, with location points concentrated around the drill hole, providing early indicators for potential failure.
As fracturing pressure increases, surface cracks evolve from no fracture, to partial connectivity, and finally to full penetration. At 17.50 MPa, fracture propagation is adequate and well distributed. However, at 20.00 MPa, cracks widen excessively, increasing the risk of instability. Similitude theory suggests the optimal field fracturing pressure should range between 31.50 and 36.00 MPa.
AE parameters are positively correlated with fracturing pressure. A doubling of pressure results in a 39.01% increase in average amplitude and a 54.12% increase in event count. As pressure rises, AE source distribution progresses from absent to localized, then to widespread. These findings validate the recommended pressure range and offer quantitative support for optimizing field-scale fracturing parameters.
Statements
Data availability statement
The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.
Author contributions
SX: Writing – review and editing, Writing – original draft. YZ: Project administration, Writing – review and editing. Yu Cao: Software, Writing – review and editing, Data curation. ZL: Writing – review and editing, Formal Analysis, Visualization. ZZ: Writing – review and editing, Software.
Funding
The author(s) declare that no financial support was received for the research and/or publication of this article.
Conflict of interest
Authors SX, YZ, YC, ZL, and ZZ were employed by Shaanxi Yanchang Petroleum Yulin Kekegai Coal Industry Co., Ltd.
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The author(s) declare that no Generative AI was used in the creation of this manuscript.
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Summary
Keywords
hydraulic fracturing, permeability, numerical simulation, crack monitoring, acoustic emission
Citation
Xu S, Zhao Y, Cao Y, Liu Z and Zheng Z (2026) Numerical simulation of hydraulic fracture propagation and stimulation effectiveness under in-Situ stress conditions. Front. Earth Sci. 13:1657740. doi: 10.3389/feart.2025.1657740
Received
01 July 2025
Revised
06 September 2025
Accepted
11 September 2025
Published
02 March 2026
Volume
13 - 2025
Edited by
Chong Xu, Ministry of Emergency Management, China
Reviewed by
Zarghaam Rizvi, GeoAnalysis Engineering GmbH, Germany
Ianyu Zhang, Bengbu University, China
Wenyu Fu, Chongqing University, China
Updates
Copyright
© 2026 Xu, Zhao, Cao, Liu and Zheng.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Shengji Xu, 642536918@qq.com
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