ABSTRACT
The main operational parameters controlling borehole stability in drilling shales with environmentally acceptable water-based drilling fluids are the density and the chemical composition of the fluid. An electro-chemo-poro-elastic model for saturated shale is presented, which describes the interaction between the drilling fluid composition and the mechanical behaviour of the shale. Based on the theory of mixtures, the model for the shale distinguishes four constituents: a compressible charged porous solid, compressible fluid, cations and anions. Each constituent is described by its individual balance equation, which allows for the necessary interaction between the constituents. The model includes osmotic swelling of the charged porous matrix and the flows of the fluid, ions and the current through the matrix are all fully coupled. Exact analytical solutions for the electro-chemical loading of a confined cylindrical shale sample are presented for the simplified linear case.
