The swelling of clay minerals on contact with an aqueous solution can produce strong adverse effects in the exploration and production of gas and oil. Molecular dynamics and Monte Carlo simulations were used to study the mechanism of swelling of sodium-montmorillonite. The simulations showed that the abundant clay mineral has four stable states at basal spacings of 9.7, 12.0, 15.5, and 18.3 angstroms, respectively. The amount of swelling and the locations of the stable states of sodium-montmorillonite are in good quantitative agreement with the experimental data.

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Summary 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 water-based 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. Introduction Borehole instability in shales is the prime technical problem area in oil and gas well drilling, with lost-time and trouble costs for the drilling industry conservatively estimated at $500 million/year. Moreover, the industry is currently facing new technical and environmental challenges associated with drilling increasingly difficult wells (e.g., horizontal multilaterals and extended reach wells) and replacement of poorly biodegradable oil-based muds (OBM's) that are technically superior but environmentally unacceptable. Waterbased muds (WBM's) are attractive replacements from a direct cost point-of-view, but conventional WBM systems have shown poor shale drilling performance in the past and in general have failed to meet other performance criteria associated with ROP, bit- and stabilizer balling, lubricity, filter cake quality, and thermal stability. Shale stability has greatly suffered from a lack of understanding of shale/drilling fluid interactions. Drilling problems have too often been approached on a trial-and-error basis, going through a costly multiwell learning curve before arriving at satisfying solutions. With the arrival of sophisticated new shale test techniques, new understanding of shale instability has emerged, enabling a more proactive approach to the design and application of water-based shale drilling fluids.