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Wellbore instability in shales is a major problem costing the petroleum industry US$900 million annually, according to conservative estimates. Water-based drilling fluids are being increasingly used to drill through troublesome shale formations. Major drivers for using water-based drilling fluids over oil-based drilling fluids are their cost-effectiveness and greater environmental acceptability. Despite these incentives to use water-based drilling fluids, improper application of such fluids while drilling sensitive shale formations can lead to costly wellbore instability problems. For correct application of water-based drilling fluids, an in-depth understanding of their physico-chemical interaction with the shale formations being drilled is important.

Shale stabilization with water-based drilling fluids can be achieved through a combination of osmotic outflow of pore fluid (chemical potential mechanism) and minimization of …