Jilin Wang

Particle Flow Code (PFC) numerical simulations were adopted to simulate the mining process and the process of goaf collapse and to predict the residual subsidence of abandoned goafs in steeply inclined multi-seam coal mines, taking the No. 101 Coal Mine in the Xishan Mining Area of Urumqi, China, as an example. Scaled physical simulations were also employed to simulate the evolution of voids in the coal–rock mixture in the goaf. The results show that after mining, the roof of shallow coal seams becomes unstable and collapses in the anti-dip direction, causing the materials within the unconsolidated layer to fall and backfill the goaf, which further leads to ground subsidence. The mining of deep coal seams is also accompanied by the overall movement of overlying strata along the dip direction of the coal seams and surface subsidence. The content of voids within the broken coal–rock mass in the goaf tends to decrease with increasing pressure, showing a negative exponential correlation. Based on the observed relationship between displacement and void content obtained from the simulation experiments, it is inferred that the residual displacement under the current conditions of the study area accounts for approximately 10.5% of the total displacement. Combining the results of the PFC simulation and the evolution law of void content, the residual subsidence of the goaf in the study area since mine closure is predicted to range from 0 to 1 m, with a high-value zone distributed in the northeastern part of the study area. Deep goafs within the B₇–B_11–12 and B₁₄–B₁₈ coal seam groups mainly contribute to the residual subsidence. The distribution of goaf collapse pits, as revealed by field investigation, also verifies the reliability of the prediction results.

To address the synergistic stability evaluation of coal pillars and caving deposits in room-and-pillar mining of nearly vertical coal seams, this study takes the 101 Coal Mine (104th Regiment, Xishan Area, Urumqi, Xinjiang) as the engineering background. It combines physical similarity simulation and theoretical analysis to explore the synergistic bearing mechanism of coal pillars and caved deposits. Based on limit equilibrium theory, a combined instability criterion considering roof mudstone’s bending-toppling and shear-sliding is established; the Rankine earth pressure theory is modified, and a stability coefficient K_s (reflecting synergistic bearing effect) is proposed to realize quantitative evaluation of goaf stability. A model experiment simulates the mining of a 73° nearly vertical coal seam. Results show the roof instability mode (under coal pillars and caved deposits) is equivalent to anti-dip slope’s toppling-sliding composite failure. Experimental and theoretical results agree well, verifying the model’s rationality and applicability. This study provides a theoretical basis and analytical method for calculating the synergistic stability of coal pillars and caving deposits in nearly vertical coal seams.

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.

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.

In order to improve the CO2 injection and CH4 production efficiencies during the CO2-ECBM process, it is necessary to clarify the relationship among the complexity of pore and fracture structures, the typicality of the fluid migration path, and the heterogeneity of reservoir permeability. In this study, crushed soft coal with low permeability from Huainan and Huaibei coalfields of China was taken as the research object. First, the three-dimensional (3D) visualization reconstruction of pore and fracture structures was realized. Second, the equivalent pore and fracture network model was constructed. Finally, the permeability evolution and its anisotropy of the coal reservoir were dynamically demonstrated. In this study, the implication of surface porosity on the heterogeneity of pore and fracture structures was first discussed, followed by the implication of coordination number on the anisotropy of fluid flow, and finally, the influence of the anisotropy of fluid flow on the CO2-ECBM process was discussed. The results show that the equivalent pore and fracture network models of the reservoir structure can be constructed based on the digital rock physics technology. The analysis results of porosity, interconnected porosity, typical path of fluid migration, absolute permeability, and surface porosity of each sample have good consistency in characterizing the complexity of pore and fracture structures and the heterogeneity of permeability. The average coordination numbers of RL and LZ samples are 5.99 and 5.78, respectively, and the number of pores and throats is well-balanced, which indicates that LZ and RL collieries are suitable for the development of CO2-ECBM industrial tests. When the interconnected pores and fractures are mainly developed vertically and horizontally, the construction of drilling technology of the CO2-ECBM process should be mainly designed for vertical wells and horizontal wells, respectively. This study has important theoretical and practical significance for the industrial testing and commercialization of CO2-ECBM technology in crushed soft coal with low permeability.

This paper presents the on-site monitoring of a medium–long highway tunnel constructed above a goaf of a multi-layer coal seam, in order to evaluate and maintain safety during operation. The case study of the Tianzimiao medium–long highway tunnel in Shanxi province was conducted above a goaf of a multi-layer coal seam with typical geological and engineering conditions in China, where a total of four coal seams (seam no. 3, 8, 12, and 15) were mined out with a total thickness of up to 11 m. Methods including data collection, engineering geological survey, drilling, geophysical prospecting, testing, and on-site monitoring were adopted, and a geo-mechanical model was established to conduct the research. Stratified monitoring was applied to investigate the individual settlement and deformation of the four layers of the goaf below, and a prediction of the possible deformation in tunnel floor ground was made based on the stratified measurements. The settlement of the tunnel sidewall, the internal stress in the fractured zone of the surrounding rock, and the deformation above the tunnel entrance were also monitored, and the monitoring data were compared with the numerical simulation results for the safety evaluation of the tunnel. The results show that the current tunnel deformation values and trend are both within the safety scope of the evaluation and prediction. The stability evaluation method for the multi-layer goaf used in this paper and the long-term on-site monitoring and timely feedback during operation is helpful to ensure the safe use of the tunnels above the goaves of multi-layer thick coal seams.

Knowledge of in situ stress distribution is fundamental for coalbed methane production; however, it is poorly understood in the eastern Yunnan region, South China. In this study, the horizontal maximum (SHmax) and minimum (Shmin) principal stress and vertical stress (Sv) were systematically analyzed for the first time. The results indicated that the magnitudes of SHmax, Shmin, and Sv showed positive correlations with burial depth. In general, three types of in situ stress fields were determined: (1) SHmax > Sv > Shmin in shallow layers with burial depths less than approximately 600 m (∼1970 ft) below ground level (bgl), indicating a dominant strike-slip faulting stress regime; (2) in medium layers approximately 600–800 m (∼1970–2625 ft) bgl, the in situ stress state followed multiple relationships, suggesting that the in situ stress regime was transformed; and (3) Sv > SHmax >Shmin in deep layers with burial depths greater than approximately 800 m (∼2625 ft) bgl, indicating a dominant normal faulting stress regime. Coal permeabilities obtained from injection–falloff well tests showed that they were widely distributed, and no obvious relationships were found between coal permeability and effective in situ stress magnitude. In the study area, the development and orientation of previously generated natural fractures combined with the present-day in situ stress distribution controlled the permeability in coal reservoirs. Differential stress and presence of natural fractures significantly affected the geometry and pattern of hydraulic fractures. In addition, in the eastern Yunnan region, locations with relatively deep depths in vertical wells and approximately west–northwest/east–southeast-trending horizontal wells suffered high potential of borehole instability because of the high differential stress.

In order to predict in advance the coal and gas outburst risks in the No. 3 coal seam in the non-mining area of the Xinjing mine, the strata were divided into seven rock assemblage types based on the lithologic characteristics of strata in the research area, and eight geological profile models were constructed. The finite element methods were used to simulate the ground stress field of the No. 3 seam floor. Based on the log curve, the coal structural type of the No. 3 coal seam was identified, with the thicknesses of the coal body under different types of damage being marked off in each borehole. A damage index of the coal structure (DV) was also proposed, and the DV indexes for all boreholes were calculated. The coal and gas outburst risks in the research area were comprehensively evaluated and predicted through a superposition analysis of the spatial distribution states of three indexes, namely: ground stress, coal structure damage degree, and coal seam gas content. The results show that the equivalent stress of the No. 3 coal seam floor is usually within the range of 9–26 MPa. The high-stress zone presents a strip distribution along the northeast–southwest (NE–SW) direction. The distribution of ground stress is mainly subject to the folds and buried depth of the coal seam. The distribution range where the damage degree of the coal structure falls under Types II and III is DV ≥ 22. The gas outburst risk in the mid-southern and northeastern parts of the research area is high, whereas that in the mid-western part is low. The zones with a high and low degrees of gas outburst risks are all mainly present as strips in the NE–SW direction. The gas outburst risk in the northwest and southeast is moderate. The research results can provide guidance for gas control in non-mining areas.