: We present results of tests performed under simulated downhole conditions, where Tertiary shale cores from the North Sea are exposed to different brines. We find a chemically induced strain of the shale, which depends on ion concentration and on ion type, and which is not governed by chemical activity. This contradicts the basic findings of osmotic theory, and can only be explained by incorporating effects of ionic transport and exchange. These results demonstrate the need for a novel approach to modelling of shale - fluid interaction effects on borehole stability.
Borehole collapse during drilling, in particular in shale, leads to tight hole / stuck pipe incidents which are very costly for the oil industry (Darley & Gray, 1988; Fj ær et al., 1992; Bol et al., 1994; Gazianol et al., 1995). Hole collapse is normally caused by shear failure of the rock near the hole, and thus controlled by the stress alteration taking place as a result of drilling. Borehole stability is therefore controlled mainly by the pressure in the borehole, i.e. the mud weight. The mud composition may however be adjusted so that interaction between shale and drilling fluid may help stabilize the borehole. Otherwise, if the mud interacts in a non-favourable way with the formation, chemically triggered borehole collapse may occur.