In situ stress distribution and its impact on CBM reservoir properties in the Zhengzhuang area, southern Qinshui Basin, North China

In situ stress is crucial for hydraulic fracturing during enhanced coalbed methane (CBM) recovery. The study is an attempt to get a better idea of fine evaluation of the stress distribution, and to clarify the stress distribution near the fault zone. The in situ stresses and formation pore pressure of coal seams at depths of 300–1300 m in the Zhengzhuang area of the southern Qinshui Basin were systematically analysed using well test data. The research area was divided into three partitions based on formation pore pressure gradient and regional geological structure. The three partitions present various petrophysical properties. Moreover, a 3D simulation was conducted to evaluate the effects of faulting on the stress state. Excellent relations exist among the pore pressure, minimum horizontal stress (P_o and σ_h) and depth of the target coal seam, which can be used to predict the distribution of in situ stresses in the research area where few well test data exist. A lower lateral stress coefficient (κ) suggests a higher permeability in the extensional southern Qinshui Basin. Lower horizontal tectonic stress coefficients and relative stress factors suggest a higher permeability area. The simulation and microseismic fracture monitoring results show that the horizontal principal stress direction obviously changes near the fault zone, suggesting the existence of a complex in situ stress state. Faulting has a great influence on σ_H orientation. The stress simulation could be a means to detect faults and predict the direction and magnitude of σ_H for areas without adequate well test data. Therefore, these results may have significant implications for the permeability evaluation of coal seams during safety mining and CBM production.