Search Results (16)

The study block is located on the eastern edge of the Ordos Basin and is one of the typical medium coalbed methane blocks in China that have previously been subjected to exploration and development work. The rich CBM resource base and good exploration and development situation in this block mean there is an urgent need to accelerate development efforts, but compared with the current situation for tight sandstone gas where development is in full swing in the area, the production capacity construction of CBM wells in the area shows a phenomenon of lagging to a certain degree. In this study, taking the 4 + 5 coal seam of the YJP block in the Ordos Basin as the research object, we carried out technical research on an integrated program concerning CBM geology and engineering and put forward a comprehensive seismic geology analysis method for the prediction of the CBM content. The study quantitatively assessed the tectonic conditions, depositional environment, and coal seam thickness as potential controlling factors using gray relationship analysis, trend surface analysis, and seismic geological data integration. The results show that tectonic conditions, especially the burial depth, residual deformation, and fault development, are the main controlling factors affecting the coalbed methane content, showing a strong correlation (gray relational value greater than 0.75). The effects of the depositional environment (sand–shale ratio) and coal bed thickness were negligible. A weighted fusion model incorporating seismic attributes and geological parameters was developed to predict the gas content distribution, achieving relative prediction errors of below 15% in validation wells, significantly outperforming traditional interpolation methods. The integrated approach demonstrated enhanced spatial resolution and accuracy in delineating the lateral CBM distribution, particularly in structurally complex zones. However, limitations persist due to the seismic data resolution and logging data reliability. This method provides a robust framework for CBM exploration in heterogeneous coal reservoirs, emphasizing the critical role of tectonic characterization in gas content prediction. Full article

The prediction of structural fractures in concealed coal-bearing strata has always been a complex problem. The purpose of this study was to clarify the tectonic evolution of the study area, i.e., the Tucheng syncline, since the coal-forming period and to predict the development of structural fractures. The tectonic evolution of the study area was divided into three stages using regional tectonic analysis. The paleotectonic stress field of the study area was reconstructed through the field investigation, statistics, and analysis of joints. Based on the tectonic deformation analysis, numerical simulation was used to reveal the stress field characteristics of different tectonic deformation stages, and combined with the Mohr–Coulomb criterion, the degree of structural fracture development in the target layers (No.17^# coal seam) of the study area was predicted. This study concludes the following: (1) The study area underwent two tectonic deformations during the Yanshanian period, transitioning from an ellipsoidal columnar shape to a semi-ellipsoidal and stereotyped form, forming a superimposed short-axis syncline, and then tilting southeastward as a whole, and was locally cut by faults during the Himalayan period. (2) The distribution characteristics of the stress field in different tectonic stages vary. The stress concentration zones in the first and second stages have a more obvious symmetry, and the present-day stress concentration zone is located in the center of the syncline basin. (3) The superimposed rock fracture indices are larger in the edge zone parallel to the long axis of the syncline and at the bottom of the syncline, which also indicates a higher degree of structural fracture development at the corresponding locations. Full article

The study of pore characteristics in tectonic coal is essential for a deeper understanding of gas diffusion, seepage, and other transport processes within coal seams, and plays a crucial role in the development of coalbed methane resources. Based on low-temperature N₂ and CO₂ adsorption experiments, this study investigated the pore structure characteristics of four tectonic coal samples collected from the Hegang and Jixi basins in China. The results show that the mylonitic coal sample exhibits a clear capillary condensation and evaporation phenomenon around a relative pressure (P/P₀) of 0.5. The degree of tectonic deformation in coal has a significant impact on its pore characteristics. As the degree of deformation increases, both the pore volume and specific surface area of the coal gradually increase. The pore volume and specific surface area of micropores are primarily concentrated in pores with diameters of 0.5–0.7 nm and 0.8–0.9 nm, while those of mesopores are mainly distributed in pores with diameters of 2.3–6.2 nm. The proportion of pore volume and specific surface area contributed by micropores is much greater than that of mesopores. The fractal dimension is positively correlated with the degree of tectonic deformation in coal. As the fractal dimension increases, the average pore diameter decreases, closely tied to the destruction and reconstruction of the coal’s pore structure under tectonic stress. These findings will contribute to a deeper understanding of the pore structure characteristics of tectonic coal and effectively advance coalbed methane development. Full article

Deep coal mining is essential for energy use and sustainable development. In a situation where coal–rock–gas dynamic disasters are prone to occur in coal seam variation areas affected by different degrees of roof angle during deep coal seam mining, a disaster energy equation considering the influence of roof elastic energy is established, and the disaster energy criterion considering the influence of roof elastic energy is derived and introduced into COMSOL6.1 software for numerical simulation. The results show that, compared with the simple change of coal thickness and coal strength, the stress concentration degree of a thick coal belt with small structure is higher, and the maximum horizontal stress can reach 47.6 MPa. There is a short rise area of gas pressure in front of the working face, and the maximum gas pressure reaches 0.82 MPa. The plastic deformation of the coal body in a small-structure thick coal belt is the largest, and the maximum value is 18.04 m³. The simulated elastic energy of rock mass is about one third of that of coal mass, and the influence of the elastic energy of roof rock on a disaster cannot be ignored. When the coal seam is excavated from thin to thick with a small-structural thick coal belt, the peak value of the energy criterion in front of the excavation face is the largest, and the maximum value is 1.42, indicating that a dynamic disaster can occur and the harm degree will be the greatest. It is easy to cause a coal and gas outburst accident when the excavation face enters a soft coal seam from a hard coal seam and a small-structural thick coal belt from a thin coal belt. Practice shows that holistic prevention and control measures based on high-pressure water jet slit drilling technology make it possible to increase the average pure volume of gas extracted from the drilled holes by 4.5 times, and the stress peak is shifted to the deeper part of the coal wall. At the same time, the use of encrypted drilling in local small tectonic thick coal zones can effectively attenuate the concentrated stress in the coal seam and reduce the expansion energy of gas. This study enriches our understanding of the mechanism of coal–rock–gas dynamic disaster, provides methods and a basis for the prevention and control of dynamic disaster in deep coal seam variation areas, and promotes the sustainable development of energy. Full article

The structural coal seam drilling process often faces challenges such as shallow drilling depth, low hole formation rate, and the presence of blind areas in gas control. To address these issues, this study proposes a novel high-strength acoustic penetration approach and optimization design method under in situ conditions. Field tests were conducted at the Yunnan Bailongshan Coal Mine and Huainan Xieqiao Coal Mine to evaluate the effectiveness of this technique. The results demonstrate that the coal seam or its roof can act as an acoustic energy converter to generate high-intensity acoustic waves that penetrate the coal seam, and the field test results confirm the efficacy of this method in increasing gas extraction. This study proposes a novel ‘hole replaces seam’ technique, optimizing the extraction process and reducing the risk of explosions and providing a more efficient and safer method for gas control in structural coal seams. Accordingly, a new technical method for replacing the bottom (top) extraction lane is proposed. Full article

In the process of roadway excavation in thick and hard coal seams with a hard roof, the instantaneous release of a large amount of elastic energy accumulated in coal and rock mass causes disasters. Especially under the action of tectonic stress, dynamic disasters of roadway-surrounding rock are extremely strong. Therefore, this paper takes the 110,505 roadway of the Yushuling Coal Mine as the engineering background. Aiming at the serious deformation of roadway-surrounding rock and the problem of strong mine pressure, the deformation mechanism of roadway-surrounding rock is studied by means of theoretical analysis, indoor experimentation, numerical simulation and field testing, and the surrounding rock control technology is proposed. Firstly, the results show that the stress field type of the Yushuling Coal Mine is a ${\sigma }_{Hv}$ type, the azimuth angle of the maximum horizontal principal stress is concentrated in 110.30°~114.12°, the dip angle is −33.04°~−3.43°, and the maximum horizontal principal stress is 1.94~2.76 times of the minimum horizontal principal stress. Secondly, the brittleness index of No. 5 is 0.62; the failure energy release of the surrounding rock compressive energy floor rock sample is up to 150,000 mv * ms. The more the cumulative number of rock samples, the greater the strength, and the more severe the damage. Thirdly, with the increase in tectonic stress, the stress of roadway-surrounding rock is asymmetrically distributed, and the plastic zone develops along the tendency. The maximum range of the plastic zone expands from 4.18 m to 10.19 m. Lastly, according to the deformation characteristics of roadway-surrounding rock, left side > roof > right side > floor, the surrounding rock control technology of ‘asymmetric anchor net cable support + borehole pressure relief’ is proposed, which realizes the effective control of roadway-surrounding rock deformation. Full article

The macroscopic structural fractures (joints) and geostress distribution characteristics of coal reservoirs are important factors affecting the exploitation of coalbed methane (CBM). In this study, the joints in the sedimentary strata of the Dahebian block in Liupanshui area, Guizhou Province were investigated. Directional coal samples were collected for observation and statistical analysis of coal microfractures, the paleotectonic stress fields of the study area were reconstructed, and the tectonic evolution was elucidated. The geostress distribution characteristics of the target coal seam (coal seam No. 11, P₃l) in the study area were analyzed using the finite element numerical simulation method. The results indicate that the structural evolution of the Dahebian syncline in the study area can be divided into two stages. The Late Jurassic–Early Cretaceous stage (Early Yanshanian) is the first stage. Affected by the sinistral strike slip of the Weining–Ziyun–Luodian (WZL) fault zone, the derived stress field in the study area exhibits maximum principal stress (σ₁) in the NEE–SWW direction. The Late Cretaceous stage (Late Yanshanian) is the second stage. Affected by the dextral strike slip of the WZL fault zone, the derived stress field exhibits σ₁ in the NNW–SSE direction. The folds and faults formed in the first stage were modified by the structural deformation in the second stage. The dominant strikes of joints in the sedimentary strata are found to be in the NW–NNW (300°–360°) and NE (30°–60°) directions, with dip angles mostly ranging from 60° to 90°. The dominant strikes of coal microfractures are in the NW (285°–304°) and NE (43°–53°) directions. The distribution of geostress in the study area is characterized by high levels of geostress in the syncline center, decreasing towards the surrounding periphery. The overall trend of the geostress contour line is similar to the shape of the syncline and is influenced by folds and faults. The σ₁ of coal seam No. 11 is vertical stress. The prediction results show that the joint density of coal seam No. 11 in the block is 36–50 joints/m, and the shape of the joint density contour line is also affected by the axial direction of the Dahebian syncline and the surrounding faults. The variation in coal seam joint density and the control effect of geostress on joints opening or closing affects the permeability of coal reservoirs. The study results provide significant guidance for the exploitation of CBM. Full article

The tectonically deformed coal (TDC) reservoirs with abundant gas resources and low permeability are expected to become one of the target coal seams for carbon dioxide geological storage-enhanced coalbed methane recovery (CO₂-ECBM). The pore–fracture structure plays a crucial role in determining the effectiveness of CO₂ storage. Fractal analysis provides a valuable approach to quantitatively describe the complex and heterogeneous pore–fracture structures across various scales in coal matrixes. Accordingly, the TDC samples in the Huainan–Huaibei coalfield and primary-undeformed coal (PUC) samples in the Qinshui Basin were selected for pore–fracture structure parameter tests using the mercury intrusion porosimetry (MIP) and low–temperature nitrogen adsorption (LNA) methods. Their multiscale pore–fracture parameters were analyzed using different fractal methods based on pore diameter. According to the fractal results, a multiscale classification standard for pore–fracture structures was devised in this study that is suitable for the controlling gas migration process. A parameter of 8 nm is set as the separating pore diameter for gas migration and storage. It was observed that the connectivity of migration pores (>8 nm) in TDC samples was stronger compared to PUC samples, reflected in larger pore volumes and smaller fractal dimensions. However, its complex development of seepage pores (150–300 nm) may hinder the flow of CO₂ injection. As for the storage pores (<8 nm), the fractal dimension of the 2–8 nm pores in TDC was found to be similar to that of PUC but with larger pore volumes. The fractal dimension of the filling pores (<2 nm) in TDC samples was relatively lower, which facilitates efficient gas volume filling. Therefore, the pore–fracture structure of the TDC samples is found to be more advantages for CO₂ injection and storage compared to the PUC. This suggests that TDC reservoirs holds promising geological potential for CO₂-ECBM implementation. Full article

Maintaining the stability of the surrounding rock is an important prerequisite in ensuring the safe and efficient construction of underground mines—in particular, the surrounding rock of the cross-layer roadway, which is a combination of different media with different lithologies. Numerical models were established to investigate the effects of the different lateral pressure coefficients (λ), the angle (α) between the roadway and the maximum horizontal principal stress, and typical lithological combinations on the deformation of the surrounding rock of weakly cemented roadways. The main outcomes obtained from our research indicated the following: (1) under the action of tectonic stress, the focus should be on strengthening the roof of the roadway support of the slab, which is conducive to the stability of the surrounding rock; (2) roadway deformation and failure for the cases λ < 1.5 are approximately symmetrically distributed, whereas those for the cases λ > 1.5 are asymmetric; (3) roadway deformation and failure for the cases α < 45° are approximately symmetrically distributed, whereas those for the cases α > 45° are asymmetric; (4) tectonic stress has an important influence on stress redistribution, deformability, and damage in cross-layer roadways; and (5) when excavating cross-layer roadways under the action of tectonic stress, the concentrated stress around the end of the working face (especially the bottom corner) should be reduced. The research results provide insights for the roadway layout through coal seam and cross-layer excavation and deepen the understanding of the deformation mechanism of weakly cemented cross-layer roadway under tectonic stress. Full article

Trapezoidal roadways in large inclination coal seams show asymmetrical tectonic characteristics, while there is still a lack of theoretical results on stress, deformation, and efficient and effective supporting methods on high walls. In this paper, based on the geological characteristics of a large, inclined coal seam roadway, a mechanical model for stress–deformation analysis of trapezoidal section roadway was established. Complex analysis and a comfort map were employed to investigate the stress and deformation distribution on the roadway surface, and a novel yielding prop with high load capacity and constant working resistance was employed to support a high wall side based on analytical results. The results are as follows: (1) The deformation of the high wall is larger than that of the low wall, and the deformation of the roof is larger than that of the floor. The overall deformation of the surrounding rock shows that the rib closure is larger than the roof-to-floor closure. (2) The stress of the surrounding rock shows that both horizontal and vertical stresses are highest in the upper corner, indicating that the broken zone is most likely to occur at this location. (3) A new support employed with a high-yielding prop and a high-strength cable in a large, inclined angle roadway is proposed. On-site experiments were conducted in a large 5-1081 roadway of a coal mine in Shanxi, China. Under the influence of mining disturbance, the deformations at the top corner decreased by 40% compared with before. The test results show that the new support scheme can effectively control the development of roadway deformation and damage during the mining process. The new support also shows friendly environmental support and fast installation. Full article

In order to explore the influences of effective stress change on gas adsorption–desorption behaviors, primary undeformed coal (PUC) and tectonically deformed coal (TDC) from the same coal seam were used for adsorption–desorption experiments under different effective stress conditions. Experimental results showed that gas adsorption and desorption behaviors were controlled by the coal core structure and the pore-fissure connectivity under effective stress. The coal matrixes and fissures were compressed together under effective stress to reduce connectivity, and it was difficult for gas to absorb and desorb as the stress increased in primary undeformed coal. The loose structure of tectonically deformed coal cores can help gas to fully contact with the coal matrix, resulting in higher adsorption gas volumes. The support of coal particles in tectonically deformed coal cores weakens the compression of intergranular pores when effective stress increases, which in this study manifested in the fact that while the volumetric strain of the coal matrix change rapidly under low effective stress, but the adsorbed gas volume did not decrease significantly. The reduction in effective stress induced the rapid elastic recovery of the coal matrix and the expansion of cracks, and increased desorption gas volumes. The stress reduction significantly increased the initial gas volume of the tectonically deformed coal, while promoting slow and continuous gas desorption in primary undeformed coal. Therefore, the promotion effect of the reservoir pressure reduction on gas desorption and coal connectivity enhancement can help to improve coalbed methane recovery in primary undeformed coal and tectonically deformed coal reservoirs. Full article

The coalbed methane (CBM) geology in Guizhou is characterized by a high gas content, pressure and resource abundance, indicating superior CBM resource potential. However, there are also many unfavorable factors, such as complex structure geology, significant regional differences in CBM geology, the widespread development of tectonically deformed coal, and the unclear understanding of the configuration of geological factors for CBM enrichment and high yield, which restrict the increase in CBM production and a large-scale development. Taking the Dahebian Block in Liupanshui coal field and the Dafang Block in Qianbei coal field as examples, this study presented the CBM geological differences between middle- and high-rank coals; their origins were analyzed and the effect of depth on gas content and permeability was discussed. A CBM enrichment and high-yield model was illustrated, and the geologic fitness-related exploration and development methods for Guizhou CBM were finally proposed. The results show that (1) significant differences between the middle- and high-rank coals occur in coal occurrence and distribution, coal qualities, and coal reservoir properties. Compared to Dahebian coal, Dafang coal has a higher coal rank, vitrinite content, and gas content, but a lower number of coal layers and permeability. (2) The sedimentary–tectonic evolution of the Longtan coal-bearing sequence is the fundamental reason for CBM geological differences between the Dadebian Block and Dafang Block, consisting of coal occurrence, qualities, maceral, rank, structure, and their associated reservoir properties. (3) The coordinated variation of gas content and permeability contributes to a greater depth for CBM enrichment and a high yield of the middle-rank coal. It is suggested that the best depths for CBM enrichment and high yield in Guizhou are 600–800 m for the middle-rank coal and 500 m for the high-rank coal, respectively. (4) Considering the bottleneck of inefficient CBM development in Guizhou, we proposed three CBM assessment and development technologies, including the CBM optimization of the classification–hierarchical optimization–analytical hierarchy, multiple coal seams commingling production with the pressure relief of tectonically deformed coal, and surface–underground CBM three-dimensional drainage development. The aim of this study was to provide new insights into the efficient exploration and development of CBM in Guizhou. Full article

Underground coal mining of CH₄ gas-rich tectonic coal seams often induces methane outburst disasters. Investigating gas permeability evolution in pores of the tectonic coal is vital to understanding the mechanism of gas outburst disasters. In this study, the triaxial loading–unloading stresses induced gas permeability evolutions in the briquette tectonic coal samples, which were studied by employing the triaxial-loading–gas-seepage test system. Specifically, effects of loading paths and initial gas pressures on the gas permeability of coal samples were analyzed. The results showed the following: (1) The gas permeability evolution of coal samples was correlated with the volumetric strain change during triaxial compression scenarios. In the initial compaction and elastic deformation stages, pores and cracks in the coal were compacted, resulting in a reduction in gas permeability in the coal body. However, after the yield stage, the gas permeability could be enhanced due to sample failure. (2) The gas permeability of the tectonic coal decreased as a negative exponential function with the increase in initial gas pressure, in which the permeability was decreased by 67.32% as the initial gas pressure increased from 0.3 MPa to 1.5 MPa. (3) Coal samples underwent a period of strain development before they began to fail during confining pressure releasing. After the stress releasing-induced yield stage, the coal sample was deformed and cracked, resulting in a quickly increase in gas permeability. With a further releasing process, failure of the sample occurred, and thus induced rapidly increasing gas permeability. These obtained results could provide foundations for gas outburst prevention in mining gas-rich tectonic coal seams. Full article

In order to study the evolution of the mechanical properties and macromolecular structures in different macerals of tectonically deformed coal (TDC), vitrinite and inertinite samples were handpicked from six block TDCs in the same coal seam with an increasing deformation degree (unaltered, cataclastic, porphyroclast, scaly and powdery coal). The micro mechanical properties were tested by the nanoindentation experiment and the macromolecular structures were measured using ¹³C nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR). The results show that the range of hardness and elastic modulus of inertinite is 0.373–1.517 GPa and 4.339–12.158 GPa, respectively, which is significantly higher than that of vitrinite with values of 0.278–0.456 GPa and 4.857–7.810 GPa, respectively. From unaltered coal to powdery coal, the hardness of vitrinite and inertinite gradually decreases, with the difference between these macerals becomes smaller and the elastic modulus of vitrinite shows an increasing trend, while that of inertinite was more variable. Both the NMR and FITR results reveal that the macromolecular structure of inertinite has similar structural transitions as vitrinite. As the degree of deformation increases, the aliphatic side chains become shorter and the aromaticity is increasing. Macromolecular alterations caused by tectonic stress is expected to produce defects in the TDCs, therefore there should be more interspacing among the macromolecular groups for the extrusion of macromolecules caused by the indenter of the nanoindentation experiment, thereby reducing the hardness. The elastic modulus of coal is believed to be related to intermolecular forces, which are positively correlated to the dipole moment. By calculating the dipole moments of the typical aromatic molecular structures with aliphatic side chains, the detachment of the aliphatic side chains and the growth of benzene rings can both increase the dipole moment, which can promote elastic modulus. In addition, the increasing number of benzene rings can create more π-π bonds between the molecules, which can lead to an increase in the intermolecular forces, further increasing the elastic modulus. Full article

Tectonically deformed coal (TDC) has a significant influence on coalbed methane recovery. Well-logging is a reliable and efficient way to predict the development of TDCs for understanding the role of TDC on the productivity of coalbed methane (CBM) wells. However, there has been insufficient research regarding both the static physical properties and dynamic invasion of mud-filtrate related to TDCs. Therefore, a new TDC-detecting approach using two indicators of the relative physical properties and fluid attributes was proposed. Through the data normalization and optimization of correlation coefficients and factor analysis, five logs were chosen to construct the two indicators, and three types of TDC were recognized: I—undeformed or cataclastic; II—granulated; and III—mylonitized. It was found out that the identification error rate decreased from 30% to 15%. Furthermore, the thickness ratio of a well-preserved coal layer derived from TDC interpretation was adopted to correlate the gas production of a coal seam. An application in the Hancheng block demonstrated that the thickness ratio of 60% is an explicit threshold value to distinguish between high-yield well (>1000 m³/d) and low-yield one (<750 m³/d). The development of granulated and mylonitized coals mainly exerts negative influence on CBM well production. Full article