Search Results (362)

Deep coalbed methane (CBM) reservoirs are characterized by high hydrocarbon content and are considered an important strategic resource. Due to their inherently low permeability and porosity, horizontal well drilling is commonly employed to enhance production, with the length of the horizontal section playing a critical role in determining CBM output. However, during extended horizontal drilling, wellbore instability frequently occurs as a result of drilling fluid invasion into the coal formation, posing significant safety challenges. This instability is primarily caused by the physical intrusion of drilling fluids and their interactions with the coal seam, which alter the mechanical integrity of the formation. To address these challenges, interpenetrating and semi-interpenetrating network (IPN/s-IPN) hydrogels have gained attention due to their superior physicochemical properties. This material offers enhanced sealing and support performance across fracture widths ranging from micrometers to millimeters, making it especially suited for plugging applications in deep CBM reservoirs. A self-degradable interpenetrating double-network hydrogel particle plugging agent (SSG) was developed in this study, using polyacrylamide (PAM) as the primary network and an ionic polymer as the secondary network. The SSG demonstrated excellent thermal stability, remaining intact for at least 40 h in simulated formation water at 120 °C with a degradation rate as high as 90.8%, thereby minimizing potential damage to the reservoir. After thermal aging at 120 °C, the SSG maintained strong plugging performance and favorable viscoelastic properties. A drilling fluid containing 2% SSG achieved an invasion depth of only 2.85 cm in an 80–100 mesh sand bed. The linear viscoelastic region (LVR) ranged from 0.1% to 0.98%, and the elastic modulus reached 2100 Pa, indicating robust mechanical support and deformation resistance. Full article

In comparison to shallow coal seams, deep coal seams exhibit characteristics of high temperature, pressure, and in-situ stress, leading to significant differences in reservoir properties that constrain the effective development of deep coalbed methane (CBM). This study takes the Carboniferous deep 8# coal seam in the Yulin area of Ordos basin as the research subject. Based on the test results from core drilling wells, a comprehensive analysis of the characteristics and variation patterns of coal reservoir properties and a comparative analysis of the exploration and development potential of deep CBM are conducted, aiming to provide guidance for the development of deep CBM in the Ordos basin. The research results indicate that the coal seams are primarily composed of primary structure coal, with semi-bright to bright being the dominant macroscopic coal types. The maximum vitrinite reflectance (R_o,max) ranges between 1.99% and 2.24%, the organic is type III, and the high Vitrinite content provides a substantial material basis for the generation of CBM. Longitudinally, influenced by sedimentary environment and plant types, the lower part of the coal seam exhibits higher Vitrinite content and fixed carbon (FC_ad). The pore morphology is mainly characterized by wedge-shaped/parallel plate-shaped pores and open ventilation pores, with good connectivity, which is favorable for the storage and output of CBM. Micropores (<2 nm) have the highest volume proportion, showing an increasing trend with burial depth, and due to interlayer sliding and capillary condensation, the pore size (<2 nm) distribution follows an N shape. The full-scale pore heterogeneity (fractal dimension) gradually increases with increasing buried depth. Macroscopic fractures are mostly found in bright coal bands, while microscopic fractures are more developed in Vitrinite, showing a positive correlation between fracture density and Vitrinite content. The porosity and permeability conditions of reservoirs are comparable to the Daning–Jixian block, mostly constituting oversaturated gas reservoirs with a critical depth of 2400–2600 m and a high proportion of free gas, exhibiting promising development prospects, and the middle and upper coal seams are favorable intervals. In terms of resource conditions, preservation conditions, and reservoir alterability, the development potential of CBM from the Carboniferous deep 8# coal seam is comparable to the Linxing block but inferior to the Daning–Jixian block and Baijiahai uplift. Full article

Addressing the stability control challenges of roadways with composite roofs in the No. 34 coal seam of Donghai Mine under high-strength mining conditions, this study employed integrated methodologies including laboratory experiments, numerical modeling, and field trials. It investigated the mechanical response characteristics of the composite roof and developed a synergistic control system, validated through industrial application. Key findings indicate significant differences in mechanical behavior and failure mechanisms between individual rock specimens and composite rock masses. A theoretical “elastic-plastic-fractured” zoning model for the composite roof was established based on the theory of surrounding rock deterioration, elucidating the mechanical mechanism where the cohesive strength of hard rock governs the load-bearing capacity of the outer shell, while the cohesive strength of soft rock controls plastic flow. The influence of in situ stress and support resistance on the evolution of the surrounding rock zone radii was quantitatively determined. The FLAC^3D strain-softening model accurately simulated the post-peak behavior of the surrounding rock. Analysis demonstrated specific inherent patterns in the magnitude, ratio, and orientation of principal stresses within the composite roof under mining influence. A high differential stress zone (σ₁/σ₃ = 6–7) formed within 20 m of the working face, accompanied by a deflection of the maximum principal stress direction by 53, triggering the expansion of a butterfly-shaped plastic zone. Based on these insights, we proposed and implemented a synergistic control system integrating high-pressure grouting, pre-stressed cables, and energy-absorbing bolts. Field tests demonstrated significant improvements: roof-to-floor convergence reduced by 48.4%, rib-to-rib convergence decreased by 39.3%, microseismic events declined by 61%, and the self-stabilization period of the surrounding rock shortened by 11%. Consequently, this research establishes a holistic “theoretical modeling-evolution diagnosis-synergistic control” solution chain, providing a validated theoretical foundation and engineering paradigm for composite roof support design. Full article

In the development of deep coalbed methane (CBM) resources, the adsorption behavior of supercritical methane is a key factor restricting reserve evaluation and development efficiency. This study integrates scanning electron microscopy (SEM), low-temperature CO₂ adsorption (LTCO₂A), mercury intrusion porosimetry (MIP), high-temperature and high-pressure CH₄ adsorption experiments (HTHP-CH₄A), and theoretical models to reveal the pore–fracture structure of deep coal seams and the adsorption characteristics of supercritical methane. Based on a predictive model for supercritical methane adsorption capacity, the adsorption capacity of deep methane was predicted. Results show that micro-pores are well-developed in deep coal rocks, but pore connectivity is generally poor, predominantly consisting of fine bottleneck pores and semi-closed pores, with a certain proportion of open pores. The fractal dimension (D_m) of micro-pore structures in deep coal samples ranges from 2.0447 to 2.2439, indicating high micro-pore surface roughness and a large specific surface area, which provide favorable sites for methane adsorption. Pores larger than 100 nm exhibit fractal values between 2.6459 and 2.8833, suggesting that the pore surfaces in deep coal seams approach a three-dimensional pore space with rough surfaces and complex pore structures. As temperature and pressure enter the supercritical region, the adsorption capacity shows an abnormal trend of “first increasing and then decreasing” with increasing pressure. The deep coal rock–supercritical methane adsorption system exhibits two scenarios in low-pressure and high-pressure regions, corresponding to self-adsorption driven by strong methane adsorption potential and external force adsorption or overpressure micro-pore adsorption, respectively. The supercritical adsorption prediction model considering temperature and methane adsorption phase density has extremely low deviation (1.11–1.25%) and high accuracy. The average dispersion between predicted and actual values ranges from 0.44 cm³/g to 0.48 cm³/g, with small error fluctuations and no significant deviation. This study provides theoretical support for the recoverability evaluation and efficient development of deep CBM resources. Full article

A quantitative characterization of the change in coal molecular structures with different cyclical microwave modification parameters and a better understanding of the reaction mechanism of the modification are of great significance for the commercial extraction of coal bed methane (CBM). Therefore, long-flame coal samples obtained from the Ordos Basin, China, were modified by microwave radiation with different times, and the long-flame coal molecular structure parameters were determined by solid-state ¹³C nuclear magnetic resonance (ss¹³C NMR), Fourier transform infrared (FTIR) spectrometry, and X-ray photoelectron spectrometry (XPS). Atomistic representations of the raw long-flame coal molecular model and modified long-flame coal molecular models were established. The temperature rise, pore volume increase, mineral removal, and functional group changes after the modification have a negative effect on methane adsorption. After the modification, the decrease in surface area of the micropores reduced the adsorption site of methane in coal. As a result, the methane adsorption amount decreased linearly with the decreasing surface area. The CH₄ adsorption isotherms of the long-flame models were dynamically simulated and analyzed. The results of this study can prove that after multiple cycles of microwave modifications, the functional groups in long-flame coal were fractured, and the number of micropores was reduced, which effectively decreased the methane adsorption performance in long-flame coal seams, thereby promoting methane extraction. Microwave modification is a promising method for enhancing CBM recovery. Full article

The efficiency of coalbed methane (CBM) extraction is closely related to the drilling response of coal seams, which is significantly influenced by natural fracture development of coal seams. This work investigated 11 coal samples from the Baode, Xinyuan, and Huolinhe mines, employing quantitative fracture characterization, acoustic wave testing, drilling experiments, and cuttings analysis to systematically reveal the relationships and mechanisms between fracture parameters and coal drilling response characteristics. The result found that acoustic parameters (average wave velocity v and drilling surface wave velocity v₀) exhibit significant negative correlations with fracture line density (ρ₁) and area ratio (ρ₂) (|r| > 0.7), while the geological strength index (GSI) positively correlates with acoustic parameters, confirming their utility as indirect indicators of fracture development. Fracture area ratio (ρ₂) strongly correlates with drilling cuttings rate q (r = 0.82), whereas GSI negatively correlates with drilling rate w, indicating that highly fractured coal is more friable but structural stability constrains drilling efficiency, while fracture parameters show limited influence on drill cuttings quantity Q. Cuttings characteristics vary with fracture types and density. Type I coal (low-density coexisting exogenous fractures and cleats) produces cuttings dominated by fine particles with concentrated size distribution (average particle size d ≈ 0.52 mm, crushability index n = 0.46–0.61). Type II coal (exogenous-fracture-dominant) exhibits coarser particle sizes in cuttings (d ≈ 0.8 mm, n = 0.43–0.53). Type III coal (dense-cleat-dominant) drill cuttings are mainly coarse particles and are concentrated in distribution (d ≈ 1.53 mm, n = 0.72–0.98). Additionally, drilling response differences are governed by the coupling effects of vitrinite reflectance (R_o), density, and firmness coefficient (f), with Huolinhe coal being easier to drill due to its lower R_o, f, and density. This study elucidates the mechanism by which fracture development affects coal drilling response through multi-parameter correlation analysis, while also providing novel insights into the optimization of fracturing sweet spot selection for CBM development. Full article

Transitional shale gas represents a critical frontier for China’s oil and gas exploration, characterized by extensive distribution and substantial resource potential. However, its frequent interbedding with coal seams and tight sandstones results in a complex reservoir architecture, significantly increasing extraction challenges. Hydraulic fracturing remains the primary method for effectively stimulating production in such reservoirs. Nevertheless, due to the complex stacking patterns of coal, shale, and tight sandstone layers, fracturing often generates complex fracture networks, leading to pronounced stress-sensitive effects and fracture interference during production. Moreover, the development of transitional shale gas reservoirs typically employs multi-well pad fracturing (“factory-mode” drilling) with tight well spacing, intensifying the well interference and its impact on well group productivity. These factors collectively complicate post-fracturing production forecasting. Existing productivity models predominantly focus on single-lithology reservoirs with idealized fracture networks, neglecting critical factors such as the fracture interference, well interference, and stress sensitivity. To address this gap, this study targets the Ordos Basin’s transitional shale gas reservoirs. By integrating the multi-lithology, multi-layer stacked reservoir characteristics, we developed a productivity model for infill wells in such reservoirs. Using a semi-analytical approach, we analyzed post-fracturing production behavior in horizontal wells, optimized key development parameters, and provided a scientific basis for the efficient development of these reservoirs. Full article

This study systematically investigates the pore–fracture architecture of deep coal seams in the JiaTan (JT) block of the Ordos Basin using an integrated suite of advanced techniques, including nuclear magnetic resonance (NMR), high-pressure mercury intrusion, low-temperature nitrogen adsorption, low-pressure carbon dioxide adsorption, and micro-computed tomography (micro-CT). These complementary methods enable a quantitative assessment of pore structures spanning nano- to microscale dimensions. The results reveal a pore system overwhelmingly dominated by micropores—accounting for more than 98% of the total pore volume—which play a central role in coalbed methane (CBM) storage. Microfractures, although limited in volumetric proportion, markedly enhance permeability by forming critical flow pathways. Together, these features establish a dual-porosity system that governs methane transport and recovery in deep coal reservoirs. The multiscale characterization employed here proves essential for resolving reservoir heterogeneity and designing effective stimulation strategies. Notably, enhancing methane desorption in micropore-rich matrices and improving fracture connectivity are identified as key levers for optimizing deep CBM extraction. These insights offer a valuable foundation for the development of deep coalbed methane (DCBM) resources in the Ordos Basin and similar geological settings. Full article

The Jurassic aquifer in Northwest China is the key aquifer for mine water filling, which is significant due to its loose structure, large porosity, strong rock permeability, and fracture development characteristics. In addition, the water richness in space is extremely uneven, and many coal mine roof water inrush events are closely related to it. A case of evaluation of water-richness of the roof sandstone in the 3-1 coal seam of the Bayangaole minefield was analyzed in depth, and the evaluation index system is established based on lithology and structural characteristics. Specifically, the evaluation indexes are under the influence of the influencing factors of lithology, the density of fault intersection endpoints, and the density of fault scale and the strength of folds as the influencing factors of structure. On this basis, the set pair analysis-variable fuzzy set coupling evaluation method is introduced to form a targeted water-rich evaluation model of a roof sandstone aquifer. By using the coupling method of set pair analysis and variable fuzzy set, a targeted evaluation model is formed to realize the organic integration of indicators. Through the comprehensive analysis of the relative zoning of water abundance and the data from the borehole pumping (drainage) test, the distribution of water abundance grade in the study area is clarified. Full article

Sudden coal and gas outbursts pose a significant hazard in deep-seated coal seam extraction, necessitating reliable risk assessment methods. Traditionally, assessments focus on gas-dynamic parameters, but experience shows they must be supplemented with tectonic factors such as fault-related disturbances, weak interlayers, and increased fracturing. Even minor faults in the Karaganda Basin can weaken the coal massif and trigger outbursts. The integration of 3D modeling enhances risk evaluation by incorporating both dynamic (gas-related) and static (tectonic) parameters. Based on exploratory drilling and geophysical studies, these models map coal seam geometry, fault positioning, and high-risk structural zones. In weakened coal areas, stress distribution changes can lead to avalanche-like gas releases, even under normal gas-dynamic conditions. An expert scoring system was used to convert geological and gas-dynamic data into a comprehensive risk index guiding preventive measures. An analysis of Karaganda Basin incidents (1959–2021) shows all outbursts occurred in geological disturbance zones, with 43% linked to fault proximity, 30% to minor tectonic shifts, and 21% to sudden coal seam changes. Advancing 3D modeling, geomechanical analysis, and microseismic monitoring will improve predictive accuracy, ensuring safer coal mining operations. Full article

The acidification modification treatment of coal is a key technical means to improve the permeability of coal seams and enhance the efficiency of coalbed methane extraction. Yet, current acidic fracturing fluids are highly corrosive, corroding downhole pipelines and contaminating groundwater. By compounding environmentally friendly and non-polluting acidic fracturing fluids and combining fractal theory and the Frenkel–Halsey–Hill (FHH) model, this paper systematically investigates their effects on the pore structure, permeability, and mechanical properties of coal bodies. It was found that the complex acid treatment significantly reduced the surface fractal dimension D₁ and spatial fractal dimension D₂ of the coal samples and optimized pore connectivity, thus improving gas transport efficiency. Meanwhile, a static splitting test and digital image analysis showed that the fracture evolution pattern of the treated coal samples changed from a centralized strain extension of the original coal to a discrete distribution, peak stress and strain were significantly reduced, and permeability was significantly increased. These findings can offer dramatic support for the optimal optimization of acidic fracturing fluids. Full article

The layout of directional high-level long boreholes in fracture zones for extracting pressure-relieved gas from goaf is a key technology to address the gas concentration exceedance in upper corners. To solve the gas exceedance issue at Fengtai Mine’s upper corner, this study established a gas–solid coupling model using COMSOL Multiphysics 6.3 based on actual mine parameters. The research investigated the influence patterns of different extraction parameters (negative pressure, borehole diameter, and extraction duration) on coal seam gas pressure and effective extraction radius (critical gas pressure 0.74 MPa). The results demonstrate the following. The effective extraction radius shows positive correlations with extraction negative pressure, borehole diameter, and extraction time while exhibiting a negative correlation with gas pressure. When borehole diameter exceeds 203 mm, extraction negative pressure surpasses 25 kPa, and extraction duration extends beyond 90 days; the effective extraction radius stabilizes, with the gas pressure influence range ceasing to decrease significantly with further parameter increases. Field validation results showed that, during the observation period, both pure gas extraction volume and mixed gas volume exceeded those of high-level drainage galleries. The gas concentrations in both the upper corner and return airflow remained within safe limits, effectively resolving the gas exceedance issue. This achievement not only established an efficient gas control system for the Fengtai Mine but also provides valuable parameter optimization methods and engineering experience for similar coal mines. The implementation has significantly enhanced gas control efficiency and safety production levels in the mining area. Full article

With the increasing depth of coal mining operations, the repeated extraction of multiple coal seams has led to serious safety threats to mines, including secondary roof fracturing, interlayer separation-induced water hazards, and intense mine pressure. Due to the limited research available on the roof failure laws of gently inclined coal seam groups, this study focuses on the Yindonggou Coal Mine and employs a comprehensive approach combining theoretical analysis, numerical simulation, and field measurement. Theoretical calculations indicate that after the mining of Seam 1, the caving zone height ranges from 6.69 to 11.09 m, and the height of the water-conducting fracture zone ranges from 29.59 to 40.79 m. After Coal Seam 2 is mined, the caving zone extends 24.05–33.47 m above the roof of Coal Seam 1, and the fracture zone develops for up to 74.10–94.94 m. Following the mining of Seam 4, the caving zone expands to 30.73–40.15 m above the roof of Coal Seam 1, and the fracture zone reaches 92.26–113.10 m. The numerical simulation results show that after mining Seam 1, the caving zone height is 8.4 m, and the fracture zone reaches 36 m. After Seam 2 is mined, the caving zone extends to 27 m above the roof of Coal Seam 1 and the fracture zone extends to 89 m. After Seam 4 is mined, the caving zone expands to 40 m above the roof of Coal Seam 1 and the fracture zone develops to 112.6 m. The field measurements validate the following findings: a loss of flushing fluid during drilling indicates that after Coal Seam 4 is mined, the fracture zone develops up to 110.5 m above the roof of Coal Seam 1, and the caving zone reaches 47.5 m. Optical imaging logging shows the fracture zone developing to 114.5 m and the caving zone extending to 48.1 m above the roof of Coal Seam 1. The results demonstrate good consistency among these theoretical calculations, numerical simulations, and field measurements. This study reveals a progressive development pattern of roof failure during the repeated mining of gently inclined coal seam groups, providing a theoretical foundation for water hazard prevention and mine pressure control in deep multi-seam mining operations. Full article

The hanging of hard roofs in coal seams poses a significant threat to the safe mining of coal. Hydraulic fracturing is an important method to achieve the pre-weakening of coal seam roofs. Clarifying the scope of hydraulic fracturing in coal seam roofs and its influencing factors is a prerequisite for ensuring the effectiveness of the pre-weakening process. In this paper, we developed a fluid–structure coupling numerical simulation model for hydraulic fracturing based on the element damage theory, and have systematically examined the effects of both engineering parameters and geological factors on the hydraulic fracture propagation behavior of the segmented fracturing of coal seam roofs. Results indicate that increasing the injection rate can significantly enhance fracture propagation length. A larger stress difference directs fractures along the maximum principal stress direction and effectively extends their length. Additionally, increasing the spacing between fracture stages reduces stress interference between clusters, leading to a transition from asymmetric to uniform fracture propagation. To validate the numerical simulation results, we conducted a field test on the hydraulic fracturing of the coal seam roof, and monitored the affected area by using transient electromagnetic and microseismic monitoring techniques. Monitoring results indicated that the effective impact range of field hydraulic fracturing was consistent with the numerical simulation results. Through the systematic monitoring of support resistance and coal body stress, the supporting resistance in the fractured zone decreased by 25.10%, and the coal seam stress in the fractured zone exhibited a 1 MPa reduction. Observations demonstrate the significant effectiveness of hydraulic fracturing in regional control of the coal seam roof. This study combines numerical simulation with engineering practice to investigate hydraulic fracturing performance under varying operational conditions, with the findings providing robust technical support for safe and efficient mining production. Full article

In coal seam mining operations, the presence of overlying water bodies presents persistent challenges, particularly during multi-seam extraction, where water accumulation in upper seam goafs requires careful management. This study examined the Lingzhida Coal Mine, focusing on the geological conditions of the 3# seam (upper) and the 15# seam (lower), as well as the distribution of water accumulation in the corresponding goafs. The mechanism of water inrush from the upper goaf was studied, and the role of the water-resisting belt (WRB) is suggested. By utilizing empirical equations and field measurements, a method for calculating the floor fracture depth of the 3# seam and the roof fracture height of the 15# seam was derived through multi-linear regression analysis. Based on the relationship between the thickness of the WRB (H_w) and the protective layer (H_p), a classification criterion for the water-inrush risk (the likelihood of water entering the lower seam from the upper goaf) is proposed. The mining area was divided into four risk zones: high-risk (H_w < 0), medium-risk (0 ≤ H_w < 0.5H_p), low-risk (0.5H_p ≤ H_w < H_p), and safe (H_w ≥ H_p). Then, an adaptive zoning approach for water-preserved mining was introduced, considering the spatial distribution of goaf water. This approach incorporates water-preserved mining technologies, including the staggered layout of working faces, reduction in mining height, and the transfer–storage of water resources. These research findings provide crucial insights for ensuring the safe and efficient extraction of the multi-seam. Full article