Present-day stress regime, permeability, and fracture stimulations of coal reservoirs in the Qinshui Basin, northern China

Abstract

Present-day stress, permeability, and hydraulic fracturing of coals at 323- to 1454-m (1060- to 4770-ft) depths were studied from the Qinshui Basin to analyze the influence of variable present-day stress regimes on coalbed methane productivity. The present maximum horizontal stress orientation is primarily northeast-southwest, with some local variations. Stress magnitudes generally increase with depth but with U-shaped variations in stress gradients. A strike-slip fault stress regime is dominant and is consecutively distributed vertically, whereas normal and reverse fault stress regimes are distributed mainly in specific depth intervals. Permeability (0.004–13.18 md) and stress regime present high variability, with changes in structural trends; structural lows result in a strike-slip fault stress regime and extremely low permeability (<0.1 md), and structural highs create relatively higher permeability regions with lower horizontal stress differential. Fracture stimulation designs should consider transitions in depth- and structural trends-related stress regimes and preexisting fractures, instead, using the current uniform schemes. Hydraulic fracture geometries are influenced by both stress and preexisting fractures in structural highs, occurring at multiple orientations with larger angles and allowing for greater reservoir stimulation volumes. Conversely, in structural lows, the higher differential stresses direct both major fracture and branches propagation along the maximum horizontal orientation. Most deep seams located in syncline axis, fault troughs, and subsags within the basin require a better proppant-supported profile, whereas small-scale fracturing has shown limited adaptability. Higher pumped rates and treating pressures are necessary in deep structural highs to reduce fracture complexity and improve proppant filling effectiveness.