The density of the drilling fluid and its chemical composition are the main operational parameters controlling the stability of wellbores drilled in shales. To quantify their impact on the stress evolution around wellbores in pervious shales, we have developed a model that accounts for the physico-chemical interaction between the invading drilling fluid and a shale formation. The model is based on the principles of non-equilibrium thermodynamics and provides generalisations of Biot's classical poro-elastic theory and Darcy's flow equation. We demonstrate that our model can reproduce the salient features of experimental work on the fluid/shale interaction and use a finite-element scheme to numerically calculate the stress and strain distribution around a wellbore when interactions occur. We show that these effects promote tensile failure of the wellbore wall.

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