To predict and cure borehole instabilities during drilling, it is important to have a profound understanding of the effects that the drilling fluid (in particular water-based muds) may have on shale behaviour. This paper focuses on the consequences of KCl exposure on smectite-rich shales. These effects have been studied by various experimental techniques, both under atmospheric and simulated downhole conditions. This has identified several particular effects as a consequence of potassium intruding the shale. One effect is that the shale shrinks, primarily due to cation exchange. This process increases the permeability and changes the deformability and possibly also the strength of the shale. Simulations of the shrinkage effect around a borehole show that the compressive stresses will decrease with increasing KCl concentration, and this improves the stability. Tensile stresses may eventually be generated in the formation at high concentrations. This enhances the possibility of borehole instability problems and also increases the rate of potentially damaging effects (pore pressure diffusion, ion transport). An important conclusion is that an optimum KCl concentration appears to exist.
Shale-fluid interactions were found to depend strongly on the test methods and procedures, calling for a careful review of methods used for optimum drill fluid design. The testing also revealed that the effect of potassium is fundamentally different from other cations upon exposure to shale.