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Implications of the pore pressure and in situ stress for the coalbed methane exploration in the southern Junggar Basin, China

Li, Guoqing, Yan, Detian, Zhuang, Xinguo, Zhang, Zheng, Fu, Haijiao
Engineering geology 2019
basins, coal, coalbed methane, hydraulic fracturing, mathematical models, permeability, prediction, regression analysis, strength (mechanics), zoning, China
The southern Junggar Basin hosts abundant coalbed methane (CBM) resources and is expected to be an important CBM production base in China. The pore pressure and in situ stress are key factors that influence the exploration and development of CBM; however, these factors are poorly understood in this area. Based on extended leak-off test data, mathematical models were established for pore pressure and in situ stress prediction using regression analysis. Subsequently, the correlations between the pore pressure, in situ stress, and permeability were analyzed, and the stress ratios were investigated. Finally, the forming mechanism of stress zonation in the study area was discussed. The results show that CBM reservoirs are generally underpressured and partly overpressured at depths greater than 800 m in the study area. The study area is characterized by notable stress zoning. The stress regime varies from reverse to strike-slip to normal faulting with increasing depth. Significantly positive correlations exist between the pore pressure, maximum principal stress, minimum principal stress, vertical stress, and principal stress difference. The minimum horizontal stress is approximately 70% of the vertical stress in the study area. A mud weight window, varying from the pore pressure to the most likely breakdown pressure, is suggested. At depths greater than 600 m, a relatively high principal stress difference is observed, which indicates that deep coals have a relatively high mechanical strength and their permeability can be enhanced by hydraulic fracturing. The depth-dependent stress zonation is mainly due to the influence of thrusting on the in situ stress, which diminishes with increasing depth. The compressional stress regime in the shallow area benefits the sealing of faults and the preservation of CBM resources in the study area. The deep CBM resources have a development potential. The outcomes of this study contribute to the CBM exploration and development in the study area.