A COUPLED POROCHEMOTHERMOELASTIC MODEL FOR A BOREHOLE IN SHALES

The evolution of borehole stresses and borehole stability in chemically active shales is profoundly affected by their complex physical and chemical interactions with drilling fluids (e.g. hydraulic flow, diffusion of ions, heat transport, thermo-osmosis, electro-osmosis, chemo-osmosis, ion exchange, and changes in swelling pressure). In this paper, a coupled porochemothermoelastic model for chemically active rocks is presented to evaluate these complex intricate links and their impact on the evolution of borehole stresses and pore pressure, which incorporates cross-coupled fluid flow equation, energy conservation equation, ion diffusion equation and mechanical equilibrium equation with many cross-coupling terms. A 2-D coupled porochemothermoelastic example of drilling in shales is solved by using a COMSOL-based simulator, which is the first engineering tool that performs partial differential equation-based multiphysics modelling in an interactive environment. The evolutions of borehole stresses, pore pressure, temperature and solute concentration in the vicinity of the borehole due to cooling and the difference of ion concentration between formulation fluid and drilling mud are presented to demonstrate the significant thermo-chemical effects on stress and pore pressure distributions.