Abstract:

Deep mountain shale gas play in Yichang, Hubei Province is located in the northern margin of Jianghan Basin in Mid-Yangtze area and has the characteristics of shale gas in Longmaxi formation such as large buried depth, complex geological condition, and large horizontal stress difference. The study of geomechanics is helpful to deepen the understanding of the influence of complex structure, fault and natural fracture system on in-situ stress field, providing basic data for wellbore stability analysis, drilling risk prediction, fracturing design optimization and post-fracturing evaluation of shale gas horizontal wells. The establishment of 3D geomechanical model depends on fine structural interpretation, shale gas geology and multi-scale natural fracture model. Therefore, one for the whole area and development platform is established. The platform scale model has more fine resolution. It can accurately depict the ground stress field around the wells and provides better service for the optimization design of drilling and fracturing. The results show that the stress mechanism in Yichang exploration area is mainly extrusion strike-slip type, the horizontal two-direction stress difference is larger than that in shale gas area of Sichuan Basin, and the risk of natural fracture activation is greater. Meanwhile, the safe window of drilling fluid is narrow and the stability of wellbore is poor. The graded fracturing of horizontal wells is affected by the large horizontal stress difference and the high risk of natural fracture activation, so the hydraulic fracture pattern is relatively simple. Based on the platform model, the 4D geomechanical simulation after fracturing is carried out, and the change of in-situ stress field after pressure is analyzed. It is showed that the minimum principal stress around the fracture net after fracturing increases significantly, the stress mechanism transitions to the reverse impulse type, and the influence range of stress shadow is 20 ~ 40 m. Making full use of geomechanical results plays an important role in improving the efficiency and development benefit of drilling engineering. This study is the first time in China to establish a fine 3D geomechanical model for the deep mountain shale gas in the complex structural area outside the Sichuan Basin. The research results have important demonstration guidance and reference significance for the large-scale development of shale gas in deep layer.

Key words: shale gas in deep layer, mountainous shale gas, geomechanics, geological-engineering integration, pore pressure, stress shadow, mud weight window, Yichang area