On the physical and chemical stability of shales

The stability of clay-rich shales is profoundly affected by their complex physical and chemical interactions with drilling fluids. In this paper, an attempt is made to clarify the intricate links between transport processes (e.g. hydraulic flow, osmosis, diffusion of ions and pressure), physical change (e.g. loss of hydraulic overbalance due to mud pressure penetration) and chemical change (e.g. ion exchange, alteration of shale water content, changes in swelling pressure) that govern shale stability. It is shown that shale–fluid interactions can be manipulated to enhance cuttings and wellbore stabilization as well as improving hole-making ability in shale formations. The mode of shale-stabilizing action of a wide variety of water-based fluid additives is discussed and the merits of various mud systems are ranked. It is shown that shale stabilization normally achieved using oil-based/synthetic-based muds is now becoming achievable with economical and environmentally friendly water-based drilling fluids.