Transport in shales and the design of improved water-based shale drilling fluids

Transport of water and ions in shales and its impact on shale stability were studied to facilitate the improvement of water-based muds as shale drilling fluids. Transport parameters associated with flows driven by gradients in pressure and chemical potential were quantified in key laboratory and full-scale experiments. The experimental results show that the low-permeability matrices of intact, clay-rich shales can act as imperfect or ''leaky'' membranes that will sustain osmotic flow of water. Moreover, the ability of shales to act as osmotic membranes is shown to provide a powerful new means for stabilizing these rocks when exposed to water-based drilling fluids. Guidelines are presented for effective exploitation of shale membrane action and induced osmotic flows through optimized waterbased drilling fluid formulation. In addition, special attention is given to induced electro-osmotic water flow in shales driven by electric potential gradients, which may provide an exciting, new, environmentally benign means for stabilizing shale formations.