Prediction of In-Situ Stresses and Pore Pressure in a Shale Gas Reservoir Subject to Finite Fault Slip

Abstract

AbstractThe shale reservoirs in Sichuan Basin, west China, are located in a geological region plagued by frequent seismic activities. Fault slips can induce significant time-dependent stress and pore pressure changes in those reservoirs. In order to properly drill a stable wellbore or perform hydraulic fracturing, it is desired to evaluate the state of in-situ stresses and pore pressure after slip of faults at relatively close distances to the wellbore. The surrounding solid matrix was assumed to be linear elastic and fluid infiltrated. First, the in-situ stresses of a horizontal rectangular fault subject to shear dislocations along its edges were analyzed based on poroelasticity. Secondly, the solutions were projected to a fault having specific strike and dip angles. The direction and magnitude of fault slip were recorded from the three-dimensional seismic profile. The pore pressure was calculated based on the predicted stresses using Skempton's coefficients. Finally, the complete stress and pressure fields were derived from superposition of the solutions with the static analysis results from using the finite element (FE) method on the continuum reservoir. It was found that the mean normal stress σ and octahedral shear stress τ induced by a finite fault slip exhibit antisymmetric patterns about each fault edge if the dip angle φ is zero, and decrease rapidly with distance to the fault plane. The contours gradually distort with an increase of φ while being much less sensitive to the strike angle θ. In addition, the magnitude and extent of the induced stress or pressure enhance with slip magnitudes. Implementing the poroelastic approach to a shale reservoir in Weiyuan region of Sichuan province in southwest China disclosed that the predicted maximum and minimum stress curves match the measured data satisfactorily. Meanwhile, the induced pore pressure is significantly dependent on the Skempton's B coefficients of the shale matrix. In summary, this study provides a poroelastic means to quantitatively predict the 3D in-situ stress and pore pressure fields in a shale reservoir with finite faults, which have reservoir stratigraphy as well as fault dimension and slip obtained from interpretation of the seismic profile.