Search Results (85)

Deep coal mining encounters substantial challenges associated with diminished permeability and elevated gas pressure under multi-physics coupling conditions. This paper introduces an innovative methodology that integrates the staged high-pressure hydraulic fracturing of roof key strata with the static expansion fracturing of coal seams using solid expansive materials. The proposed technique facilitates the coordinated regulation of the stress and fracture fields, enabling efficient stress relief, long-term anti-reflection performance, and low-damage seam modification. By employing theoretical modeling based on Hamilton’s canonical equations, conducting experimental impact tests, and performing field validation, this research demonstrates that the synergistic strategy markedly alleviates stress concentration by more than 25%, increases permeability by a factor of 3.7, and enhances gas extraction efficiency while preserving fracture network stability. The results indicate sustainable gas drainage characterized by reduced decay coefficients, offering a reliable and efficient approach for strata pressure management and gas disaster mitigation in deep mining operations. This integrated solution contributes to both safety enhancement and resource recovery optimization under complex geo-mechanical conditions. Full article

The development of deep coalbed methane is hindered by the strong heterogeneity of coal mechanical properties and complex hydraulic fracturing behavior. To identify the key factors controlling fracture geometry and permeability enhancement, this study developed a thermo-hydro-mechanical-damage coupled model within a COMSOL Multiphysics 6.3-MATLAB R2022b co-simulation framework, incorporating a Weibull random field to characterize mechanical heterogeneity. Sensitivity analysis demonstrates that tensile strength is the predominant factor governing both the fracturing damage zone and permeability-enhanced area, with its damage area extreme difference (10.094) and coefficient of variation (0.85) significantly surpassing those of other parameters. Poisson’s ratio and elastic modulus emerge as key secondary parameters, while compressive strength shows the lowest sensitivity. The parametric influences exhibit distinct patterns: tensile strength shows a strong negative correlation with damage and permeability-enhanced areas (up to 85% reduction), whereas the maximum permeability enhancement rate follows a non-monotonic trend, peaking at 215 when tensile strength reaches 3.33 MPa. Compressive strength minimally affects the damage area (~15%) but steadily improves the maximum permeability enhancement rate (7.5% increase). Elastic modulus exhibits an optimal value (8.93 GPa) for maximizing damage area, while negatively correlating with maximum permeability enhancement rate (9.1% decrease). Fracture morphology is differentially controlled by multiple parameters: low compressive strength promotes fracture deflection and branching, elastic modulus regulates fracture network complexity, and low Poisson’s ratio enhances coal brittleness to effectively activate natural fractures, thereby facilitating complex fracture network formation. Full article

Addressing the issues of low permeability, stress sensitivity in CBM reservoirs, and severe reservoir damage from traditional fracturing fluids, we prepared a Gemini surfactant (designated GEM-CBM) for CBM development using ethanolamine, epichlorohydrin, and alkylamidopropyl dimethylamine as feedstocks. On this basis, we further developed a clean fracturing fluid system. The synthesis process of GEM-CBM was optimized via single-factor and orthogonal experiments. The surface activity of GEM-CBM was assessed through surface tension measurements, whereas the sand-carrying capacity, the rheological properties, gel-breaking performance, and reservoir compatibility were comprehensively examined. The optimal conditions for GEM-CBM are listed as follows: the molar ratio of intermediate to alkylamidopropyl dimethylamine being 1:2.2, reacted at 80 °C for 20 h, with a conversion rate of 96.5%. FTIR verified the existence of characteristic functional groups, and EA results matched the theoretical molecular composition. GEM-CBM has good performance, with a critical micelle concentration (CMC) of 19.0 μmol/L and a surface tension at CMC (γCMC) of 37.44 mN/m. The optimized clean fracturing fluid (formulation: 2.3% GEM-CBM + 0.3% Tween-80 + simulated formation water with 150,000 mg/L mineralization) exhibited a viscosity of 82 mPa·s (66.7% viscosity retention rate) after being subjected to 100 min of shearing at 90 °C and 170 s⁻¹. At 90 °C, the proppant settlement velocity was less than 0.15 mm/s, and complete gel breaking was achieved within 30 min without residues. For coal cores from the Qinshui Basin, the permeability recovery rate reached 78.6%. The permeability recovery rate of coal cores from the Qinshui Basin reached 78.6%. This fracturing fluid realizes viscosity enhancement and sand carrying via the worm-like micellar network formed by GEM-CBM, inducing minimal damage to CBM reservoirs and offering technical support for efficient CBM extraction. Full article

Reservoir stimulation is a critical technique for the efficient development of coalbed methane (CBM), playing a significant role in improving permeability. Controlled shock wave fracturing, as an emerging stimulation method, offers advantages such as safety and high energy utilization, making it a promising candidate for CBM reservoir enhancement. Due to the substantial potential of deep CBM reservoirs, conventional physical simulations and field experiments are limited in accurately analyzing the fracturing effects. Research on the fracture propagation and damage evolution of coal rock under the influence of different geological and engineering parameters is limited, hindering the determination of key operational parameters. In this study, a coupled mathematical model of solid mechanics and damage continuum mechanics is established using the finite element method, alongside a geometric model, to investigate fracture propagation characteristics under the influence of geological and engineering factors. The core contribution of this work is a systematic numerical analysis that clarifies the controlling effects of key parameters. The main conclusions are as follows: (1) a high stress contrast (≥6 MPa) favors fracture extension along the direction of the maximum principal stress while inhibiting the expansion of the damage area; (2) the increase in the orientation of natural fissures and the angle of horizontal stress inhibits the propagation of fractures and the growth of damage area; (3) engineering parameters exert a considerable effect on fracture propagation and multiple shock cycles (≥2 times) and high peak pressure (≥250 MPa) are conducive to fracture formation; and (4) a key distinguishing feature is the formation of radioactive fractures induced by high-energy shock waves, which are beneficial for enhancing communication between rock layers and natural fractures. Compared to hydraulic fracturing, the shock wave method achieves distinctly faster fracture extension in a shorter time, highlighting its unique advantage for improving coalbed permeability and porosity. This study extends the numerical simulation research on controlled shock waves in deep coal seams, elucidates the dynamic response of fracture propagation and damage evolution under the control of geological and engineering parameters, reveals the sensitivity of key parameters to fracture extension, and provides a critical basis for the selection and optimization of operational parameters in field applications of shock wave fracturing. Full article

This study examines the seepage and damage behavior of coal under cyclic loading and unloading, typical in multi-layer coal seam mining. Four stress paths were designed: isobaric, stepwise, incrementally increasing, and cross-cyclic, based on real-time stress monitoring in protected coal strata. Seepage tests on gas-bearing coal were conducted using a fluid–solid coupled triaxial apparatus. The results show that axial compression most significantly affects axial strain, followed by volumetric strain, with minimal impact on radial strain. Permeability variation closely follows the stress–strain curve. Under isobaric cyclic loading (below specimen failure strength), specimens with higher initial damage (0.6) exhibit a sharp permeability decrease (75.47%) after the first cycle, with gradual recovery in subsequent cycles. In contrast, samples with lower initial damage (0.05) show higher permeability during loading, which eventually reverses, with unloading permeability surpassing loading permeability. Across all paths, a significant increase in residual deformation and permeability recovery exceeding 100% indicate the onset of instability. Continued cyclic loading increases damage accumulation, with different evolution patterns based on initial damage levels. These findings provide valuable insights into the pressure-relief permeability enhancement mechanism in coal seam mining and inform optimal gas drainage borehole design. Full article

With the gradual increase in coal production capacity, the problem of water damage from the coal seam roof is becoming more and more prominent. Neogene loose strata overlie coal seams in eastern China, and pressurized aquifers commonly lie at the bottom of the loose strata. The aquifers are mainly composed of unconsolidated sand, gravel, and weakly consolidated marl, which has strong permeability and an extremely unfavorable impact on safe production. Identifying the target area to prevent and control roof water damage can reduce the likelihood of water damage accidents in mines. This study takes the 8₅ mining district of Wobei mine as an engineering case. The discriminant indexes are selected for aquifer thickness, gradation coefficient, marlstone thickness, permeability, grouting quantity, and grouting termination pressure. A model integrating the newly proposed Crowned Porcupine Optimization (CPO, 2024), Convolutional Neural Network (CNN), and Bidirectional Long Short-Term Memory (BiLSTM) was constructed to predict unit water influx. A zonal map was generated based on the expected unit water influx of the fourth aquifer after grouting. In addition, the prediction results are compared with those from other models. Results indicate that the CPO-CNN-BiLSTM model achieves a higher accuracy and fewer errors in water abundance prediction, with an RMSE of 2.58 × 10⁻⁵ and an R² of 0.982 for the testing dataset. According to the prediction result, the fourth aquifer after grouting in the 8₅ mining district is divided into five water abundance zones. The strong and medium–strong water abundance zones are mainly distributed in the study area’s eastern region. A small portion of them is distributed in the northwestern and northern areas. This study provides a new insight for predicting the water abundance of thick loose aquifers and a theoretical basis for safe mining under thick loose aquifers. Full article

Permeability evolution is one of the key parameters influencing the efficient exploitation of deep unconventional energy resources, as it reflects the dynamic development of pore-fracture structures under complex engineering effects. Using fractal geometry to describe the pore-fracture system, rock permeability enhancement can be quantitatively evaluated. In this study, fractured coal specimens were analyzed under simulated mining-induced stress relief and CH₄ release conditions based on fractal geometry theory. The permeability-enhancement rate was derived and verified through CT (Computed Tomography) characterization of the pore-fracture network. The fractal dimension of the fracture aperture distribution and the tortuosity of fracture paths were determined to establish a fractal permeability-enhancement model, and its sensitivity was analyzed. The results indicate that permeability evolution undergoes four distinct stages: a stable stage, a slow-growth stage, a rapid-growth stage, and a stable or declining stage. The mining-induced stress relief and gas desorption effects significantly accelerate permeability enhancement, providing new insights into the mechanisms governing gas flow and pressure relief in deep coal seams. The proposed model, highly sensitive to the fracture aperture ratio (λ_min/λ_max), reveals that a smaller aperture span leads to greater permeability enhancement during the damage and fracture stage. These findings offer practical guidance for predicting permeability evolution, optimizing gas drainage design, and enhancing the safety and efficiency of coal mining and methane extraction operations. Full article

The extraction of multiple coal seams not only increases the risk of water inrush disasters in mines but also exacerbates the long-term depletion of groundwater, posing challenges for sustainable resource management in ecologically sensitive areas. This study utilizes the plastic damage–permeability coupling model in Abaqus CAE to analyze the impact of coal seam thickness and pillar layout on the evolution of the plastic zone and groundwater loss in the Shen Dong mining area, specifically at the Buertai coal mine. The results indicate that coal seam thickness is a strong driving factor for aquifer depletion: the water inflow under a 10 m thick coal seam is 1.56 times that under a 4 m thick coal seam. In contrast, the optimized staggered pillar layout alters stress distribution and reduces the water inflow under deeper coal seams by approximately 38%, demonstrating excellent water-saving potential. To translate these findings into a sustainability framework, this study proposes three new indicators: the Groundwater Loss Index (GLI) to quantify depletion intensity, the Aquifer Protection Efficiency (APE) to assess protection benefits, and the Sustainability Trade-off Index (STI) to balance coal recovery, safety, and groundwater protection. These metrics establish a dual-objective optimization approach that ensures safe mining and the sustainability of the aquifer. This study provides practical benchmarks for environmental impact assessment and aligns with the global sustainable development agenda, particularly the United Nations Sustainable Development Goals concerning clean water (SDG 6), responsible consumption (SDG 12), and terrestrial ecosystems (SDG 15). By incorporating groundwater protection into the design of the Buertai coal mine, this study advances the transition of multi-seam mining at Buertai from disaster prevention to sustainability orientation. Full article

Modeling the pre-extraction of coalbed methane presents a significant mathematical challenge due to the complex interplay of multiple physical fields. This paper presents a robust mathematical model based on a thermo-hydro-mechanical damage (THMD) framework to describe this process. The model is formulated as a system of coupled, non-linear partial differential equations (PDEs) that integrate governing equations for heat transfer, fluid seepage, and solid mechanics with a damage evolution law derived from continuum damage mechanics. A key contribution of this work is the integration of this multi-physics model, solved numerically using the Finite Element Method (FEM), with a statistical modeling approach using Response Surface Methodology (RSM) and Analysis of Variance (ANOVA). This integrated framework allows for a systematic analysis of the model’s parameter space and a rigorous quantification of sensitivities. The ANOVA results reveal that the model’s damage output is most sensitive to the borehole diameter (F = 2531.51), while the effective extraction radius is predominantly governed by the initial permeability (F = 4219.59). This work demonstrates the power of combining a PDE-based multi-physics model with statistical metamodeling to provide deep, quantitative insights for optimizing gas extraction strategies in deep, low-permeability coal seams. Full article

To overcome wellbore instability problems in deep coalbed gas reservoirs in the Ordos Basin, drilling fluid additives were evaluated and a drilling fluid system was designed. According to the SEM and CT analysis results, there were not only face and butt cleats in the coal rock but also bedding and layered fractures. Potassium chloride (KCl) and Potassium formate (HCOOK) drilling fluid systems were formulated. The recovery rate of shale and coal rock cuttings reached 99%, and the linear swelling rates for coal rock in both types of drilling fluid were less than 0.18%. Measured with a servo-controlled compression frame at a loading rate of 1 mm/min, the uniaxial compression strength of coal rock was 11.74 MPa, and it was 9.13 MPa and 10.35 MPa after immersion in KCl and HCOOK drilling fluid, respectively. This indicates that both systems have good inhibition properties. The invasion depth in packed sand was 15.5 mm for KCl drilling fluid and 8 mm for HCOOK drilling fluid, demonstrating good sealing performance by the systems. Compared to KCl drilling fluid, the HCOOK system exhibited better inhibition and sealing performance. After the removal of the 10 mm deep invasion section of drilling fluid, the permeability of the coal rock recovered by more than 90%, and the drilling fluid caused minimum damage to the reservoir. The optimized drilling fluid exhibits excellent sealing and inhibition capabilities, making it highly effective in addressing wellbore stability challenges in carbonaceous mudstone formations at 4000 m in depth in the deep coalbed methane reservoirs of the Ordos Basin. Full article

To investigate hydraulic fracture propagation in multi-layered porous media such as sand–coal interbedded formations, we present a new phase-field-based model. In this formulation, a diffuse fracture is activated only when the local element strain exceeds the rock’s critical strain, and the fracture width is represented by orthogonal components in the x and y directions. Unlike common PFM approaches that map the permeability directly from the damage field, our scheme triggers fractures only beyond a critical strain. It then builds anisotropy via a width-to-element-size weighting with parallel mixing along and series mixing across the fracture. At the element scale, the permeability is constructed as a weighted sum of the initial rock permeability and the fracture permeability, with the weighting coefficients defined as functions of the local width and the element size. Using this model, we examined how the in situ stress contrast, interface strength, Young’s modulus, Poisson’s ratio, and injection rate influence the hydraulic fracture growth in sand–coal interbedded formations. The results indicate that a larger stress contrast, stronger interfaces, a greater stiffness, and higher injection rates increase the likelihood that a hydraulic fracture will cross the interface and penetrate the barrier layer. When propagation is constrained to the interface, the width within the interface segment is markedly smaller than that within the coal-seam segment, and interface-guided growth elevates the fluid pressure inside the fracture. Full article

Deep coalbed methane (CBM) resources possess significant potential. However, their development is challenged by geological characteristics such as high in situ stress and low permeability. Furthermore, existing production strategies often prove inadequate. In order to achieve long-term stable production of deep coalbed methane reservoirs and increase their final recoverable reserves, it is urgent to construct a scientific and reasonable drainage system. This study focuses on the deep CBM reservoir in the Daning-Jixian Block of the Ordos Basin. First, a thermal–hydraulic–mechanical (THM) multi-physics coupling mathematical model was constructed and validated against historical well production data. Then, the model was used to forecast production. Finally, key control measures for enhancing well productivity were identified through production strategy adjustment. The results indicate that controlling the bottom-hole flowing pressure drop rate at 1.5 times the current pressure drop rate accelerates the early-stage pressure drop, enabling gas wells to reach the peak gas production earlier. The optimized pressure drop rates for each stage are as follows: 0.15 MPa/d during the dewatering stage, 0.057 MPa/d during the gas production rise stage, 0.035 MPa/d during the stable production stage, and 0.01 MPa/d during the production decline stage. This strategy increases peak daily gas production by 15.90% and cumulative production by 3.68%. It also avoids excessive pressure drop, which can cause premature production decline during the stable phase. Consequently, the approach maximizes production over the entire life cycle of the well. Mechanistically, the 1.5× flowing pressure drop offers multiple advantages. Firstly, it significantly shortens the dewatering and production ramp-up periods. This acceleration promotes efficient gas desorption, increasing the desorbed gas volume by 1.9%, and enhances diffusion, yielding a 39.2% higher peak diffusion rate, all while preserving reservoir properties. Additionally, this strategy synergistically optimizes the water saturation and temperature fields, which mitigates the water-blocking effect. Furthermore, by enhancing coal matrix shrinkage, it rebounds permeability to 88.9%, thus avoiding stress-induced damage from aggressive extraction. Full article

Deep coal seams have low permeability and poor wettability, making gas extraction difficult. This study presents a zero-energy consumption pulsating water hammer fracturing technique that uses the gravitational potential energy of high-elevation water and the pulsating pressure waves from the water hammer effect to induce fatigue damage in coal, creating an interconnected network of cracks. The research included experiments on water hammer pressure waves, multi-physics field coupling simulations at different flow rates, and discrete element simulations to analyze the fracture behavior of underwater hammer pressure. Results showed that initial flow velocity impacts the water hammer pressure’s intensity, range, and duration. Pressure shock waves propagate as expansion and compression waves, with peaks rising from 4.99 to 19.91 MPa within a 2–12 m/s flow rate range. Water hammer pressure reduced fracture initiation pressure by 23% compared to static pressure loading and increased fracture numbers by 13.4%. With pressure amplitudes between 2–18 MPa, fractures tripled, and the damaged area grew from 2.2 to 11%. A variable frequency combination loading strategy, starting with low frequency and then high frequency, was more effective for fracture propagation. This study offers a theoretical foundation for applying this technology to enhance coal seam permeability and gas pumping efficiency. Full article

The proportion of neutral and weakly alkaline high-sulfate mine water in China is over 50%, resulting in the problem of high treatment costs. Low-cost, sustainable, and non-secondary pollution remediation technologies for in situ application in underground coal mines have rarely been reported. Here, the mixed packed and layered packed SRB-PRB (sulfate-reducing bacteria-permeable reactive barrier) column experiments at a flow speed of 300 mL/d using low-cost corncob as a carbon source were conducted to simulate sulfate in situ remediation in goafs. The column experiments utilized the simulated weakly alkaline mine water, with an initial sulfate concentration of 1027.45 mg/L. The results showed that during the 40 d operation, the SO₄²⁻ removal kinetics included three stages: rapid reduction (0–6 d), stable reduction (6–16 d), and reduction attenuation (16–40 d). Corncob could provide a relatively long-term carbon source supply, with the maximum average removal efficiency of 65.5% for the mixed packed column and 56.6% for the layered packed column. A large number of complex organic-degrading bacteria were detected in both the effluent water samples and the solid packed media, while SRB became dominant only in the solid packed media. However, the low-abundance SRB could still maintain a high-efficiency SO₄²⁻ reduction, due to the supply of readily utilizable carbon sources provided by hydrolytic and fermentative bacteria. This indicated that the synergistic effect between SRB and these organic matter-degrading bacteria was the critical limiting factor for SO₄²⁻ removal. The microscopic characterizations of SEM-EDS (scanning electron microscopy and energy-dispersive spectroscopy) and FTIR (Fourier transform infrared spectroscopy) confirmed the damage of functional groups in corncobs and the generation of SO₄²⁻ removal products (i.e., FeS). The engineering application schemes of the SRB-PRB under both in-production and abandoned mining scenarios were proposed. Additionally, the material cost estimate results showed that the SRB-PRB could achieve in situ low-cost remediation (0.2–1.55 USD/m³) of the characteristic pollutant SO₄²⁻. These findings would benefit the engineering application of in situ microbial remediation technology for high-sulfate mine water. Full article

This study focuses on investigating the damage characteristics and mechanisms of Slickwo clean fracturing fluid to the reservoir by using the deep coal seam in the Yan’an gas field as the research subject. During the experiment, fracturing fluids with varying A content were employed to displace coal and rock cores. The impact of these fluids on the permeability and pore structure of coal and rock was analyzed using a combination of nuclear magnetic resonance and high-pressure mercury injection technology. The findings indicate that the permeability damage rates of cores Y-1 and Y-2 post-displacement are 48.4% and 53.6% correspondingly, with the damage worsening as the agent A content increases. NMR data reveals that the fracturing fluid exhibits the highest retention in small pores, followed by medium-sized pores, and the least in large pores. The rise in agent A content enhanced the retention degree in individual pore throats and overall, increasing from 62.24% to 68.74%. The escalation in agent A content results in higher macromolecular residues, causing seepage channel blockages and enhancing the adsorption properties between fracturing fluid and coal rock. This phenomenon leads to inadequate backflow, primarily in smaller apertures. Simultaneously, the interaction between the gel breaker and clay minerals triggers particle migration, blockage, and expansion, consequently diminishing the permeability of coal and rock and inducing specific damages. Full article