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 mud pressure penetration. Mud pressure penetration can either be prevented by generating an isolation membrane on the borehole wall or be reduced by minimizing hydraulic diffusivity. To help ensure adequate outflow and/or minimize hydraulic diffusivity with water-based drilling fluids, an efficient semi-permeable membrane should be generated within the shale. Several membrane generation mechanisms with varying degrees of effectiveness, depending on shale properties, are described in this paper. Mechanisms described involve chemical reactions between the drilling fluid and shale pore fluid, generation of electrical charges on exposed shale surface to selectively restrict ion movement and modification of clay structure by chemical and mechanical means.

Laboratory experiments on shale samples under realistic downhole conditions exposed to a range of water-based drilling fluids are presented to emphasize the time-dependent nature of membrane generation. The experiments showed that maintenance of shale stability with a new generation of water-based drilling fluids can be achieved through significant increase in membrane efficiencies (greater than 80%) within relatively short exposure times.

A fundamental understanding of the osmotic membrane generation in shales and the application of experimental data have resulted in the development of a new generation of water-based drilling fluids designed to successfully achieve drilling objectives through shale stabilization. Guidelines presented here for drilling of shales should help to significantly reduce shale stability-related non-productive time during drilling.

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