Polyols (e.g. glycols, glycerols, polyalkylene glycols) are now established as effective shale inhibitors in water-based muds (WBM). These water-soluble compounds are typically used at concentrations between 3 and 10% in WBM and are generally found to reduce wellbore problems associated with reactive shales, increase drilling rates and reduce the environmental impact of the drilling operation. The mechanisms by which these molecules provide shale inhibition are not well understood and the aim of this paper is to provide some insights into these mechanisms.

The adsorption of three polyols on to a swelling clay mineral (montmorillonite) was studied. The adsorption experiments were carried out from distilled water and from potassium chloride solutions. The resulting clay complexes were characterised by X-ray diffraction and infra-red spectroscopy techniques. Shale inhibition measurements were also made and the results correlated with the adsorption of the polyols.

The polyols are shown to be strongly adsorbed by the clay. During the adsorption process, water is displaced from the clay surface and ordered structures of polyol are formed. The nature of these structures and their stability in aqueous fluids is strongly controlled by the presence or absence of potassium cations:

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    Under most conditions, a single polyol layer forms on the clay in the presence of potassium and the resulting complexes are stable in water.

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    A complex containing 2 polyol layers is formed when potassium is absent. This complex is less stable in water.

The interaction between potassium ions and polyols at the clay surface is proposed as the critical factor in the provision of shale inhibition. This interaction appears to account for all the observed inhibition in the case of two of the polyols studied. For a 3rd polyol, an additional (minor) contribution may come from interactions between polyol molecules at the clay surface.

The selection of the most appropriate polyol for inhibitive muds is discussed. This is a complex issue that must consider cost and environmental acceptability as well as technical performance.

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