Drilling in mudrocks: rock behavior issues

Borehole stability analyses use different analytical models to evaluate stresses, pressures, and temperatures around boreholes. Mudrocks do not possess spatio-temporally constant material properties: mechanical properties are time-dependent because of the continuous change in pore pressure, moisture content, and temperature near the borehole. Mudrocks with high specific surface areas also exhibit high sensitivity to changes in physical and chemical properties of their pore fluids. Pressure and concentration diffusion during drilling continuously change the ionic composition of pore fluids and, accordingly, the engineering properties of those fine-grained argillaccous materials. Such fine-grained materials are considered "reactive"; it is shown that a "reactivity coefficient" can be used to assess the physico-chemical sensitivity of mudrocks. Coupling between chemical and mechanical properties may develop in reactive mudrocks, leading to further changes in their mechanical parameters. Experimental data from triaxial and oedometer tests are presented for several materials to illustrate their time-dependent mechano-chemical properties in borehole environments. A discussion of coupled diffusivity processes in mudrocks is included, and the concept of large micro-scale strains that can lead to deterioration of properties is introduced. Finally, we speculate on the degree of complexity required for constitutive modeling of borehole stability analysis of shales in practical situations. (C) 2003 Published by Elsevier Science B.V.