Analysis of Stress and Pore Pressure in Naturally Fractured Shale Formations: A Finite Element Based Chemo-Thermo-Poroplastic Model

摘要

Most shale formations are naturally fractured and wellbore instability is a common problem in these formations. In previous wellbore stability analysis shale formations have been treated as a homogeneous rock which underestimates drilling mud parameters. The aim of this paper is to study the transient change in pore pressure and stress state around a wellbore drilled in a chemically active fractured shale formation. For this purpose a finite element model for coupled chemo-thermo-poro- plasticity is developed. A discrete fracture network is generated based on an object-based hybrid neuro-stochastic simulation. Permeability tensors of the fracture network were calculated by using boundary element method which is based on periodic boundary conditions around the grid block and interface boundary conditions around the fracture edges. In order to solve the plasticity problem a single step backward Euler algorithm including a yield surface correction scheme is used to integrate the plastic stress-strain relation and an initial stress method is employed to solve the non-linearity of the plastic equation. Super convergent patch recovery is used to accurately evaluate the time dependent stress tensor. The solute advection and thermal convection are also considered due to presence of the natural fracture system. From the results of this study it was revealed that the pore pressure decreases around the wellbore due to the backflow of chemical and thermal osmosis. When drilling through fractured shale formations, however, the decrease in pore pressure is lower than that of intact shale formations which is mainly due to solute advection through the fractures. Also it was found that the effective radial and tangential stresses can reach yield strength of the rock in presence of fracture system thus casusing serious instability problems. It is more likely to form a plastic zone near the wellbore wall in the presence of natural fracture than otherwise. In this study the formation of plastic zone leads to wellbore failure by exfoliation.