Construction of a 3D Geomechanical Model for Development of a Shale Gas Reservoir in Sichuan Basin

摘要

Currently, there is large-scale shale gas exploration and development in the Sichuan basin, western China. Due to high tectonic stress and presence of fracture systems at various scales in the lower Silurian Longmaxi reservoir formation, hydraulic fracturing in shale gas reservoirs in the Sichuan basin has encountered many difficulties, such as difficulty in placing sufficient proppant, poor production performance for some wells, and ambiguity as to the factors controlling production of the reservoir. It has been recognized that lack of geomechanical understanding of the shale gas reservoirs places a major obstacle to effectively addressing these difficulties. A 3D full field geomechanics model was constructed for Changning shale gas reservoir in Sichuan basin through integrating seismic, geological structure, log, and core data by following a newly formulated integrated workflow. The 3D geomechanical model includes 3D anisotropic mechanical properties, 3D pore pressure, and the 3D in-situ stress field. Through leveraging measurements from an advanced sonic tool and core data, the anisotropy of the formation was captured at wellbores and propagated to 3D space guided by prestack seismic inversion data. 3D pore pressure prediction was conducted using seismic data and calibrated against pressure measurements, mud logging data, and flowback data. A discrete fracture network model, which represents multiscale natural fracture systems, was integrated into the 3D geomechanical model during stress modeling to reflect the disturbance on the in-situ stress field by the presence of the natural fracture systems. The 3D pore pressure model was used to calculate more-reliable estimates of gas in place in the shale gas reservoir, and the geomechanical model was used to reveal the root cause of difficulties of proppant placement in this tectonically active and unevenly fractured shale gas reservoir. The paper presents the highlights and innovations in constructing the 3D geomechanical model for the shale gas reservoir and explains how the 3D geomechanical model is used to understand the root cause of poor proppant placement encountered during hydraulic fracturing and events such as mud losses during drilling. Hence, the modeling provides a critical opportunity to improve reservoir stimulation in the shale gas reservoir.