Complex in situ stress states in a deep shale gas reservoir in the southern Sichuan Basin, China: From field stress measurements to in situ stress modeling

Abstract

The deep shale gas reservoir (3,500 ∼ 5,000 m) of the Silurian Longmaxi Formation in the southern Sichuan Basin of China has great potential for exploration and development. Complicated by the fault zones and other geostructures, the in situ stress state of shale gas formations in the southern Sichuan Basin is poorly understood, and field in situ stress measurement data are rare in these deep formations. This study provides vital insights into the in situ stress state based on in situ stress measurements and into the development of a 3D in situ stress model. To accurately model the distributions of in situ stress in the Longmaxi shale gas formation, multiple diagnostic fracturing injection tests (DFIT) were conducted on a 310-km2 shale gas development block. The minimum horizontal principal stress (σhmin) and pore pressure were interpreted from the DFIT data by using pressure transient analysis (PTA) methods. The maximum horizontal principal stress (σhmax) was estimated from multiple sources. The 3D in situ stress model was constructed by using the finite element (FE) method based on a site-specific 3D geological model. The stress model was calibrated and constrained by the results from field DFIT stress measurements.The in situ stress modeling revealed a complex in situ stress state in this deep shale gas formation. The presence of faults changes both the magnitudes and orientations of in situ stress. There is a general decrease in the angle between the σhmax direction and the fault strike in the vicinity of the faults. The stress magnitudes changed significantly near some faults, and the stress state transitioned from a strike-slip faulting stress regime to a reversed faulting stress regime. The reservoir is highly overpressured, with pore pressure gradients ranging from 17 to 20 kPa/m within the reservoir, which causes some faults to have a higher risk of slipping. Based on the 3D stress model, the fault stability and pore pressures that are required to slip the faults were estimated. Because many previous studies about in situ stress modeling have been based on well log-interpreted stresses, this study emphasizes the importance of constructing in situ stress models based on direct field stress measurements and of constraining the stresses through multiple sources and methods.