When in contact with water-based muds, shales can absorb water and ions that causes serious wellbore stability problems. An efficient stability model should combine together mechanical and chemical effects, but to run such a model, shale mechanical properties and hydration tendency as function of mud composition have to be identified. Characteristics would be determined while shale is under hydration and stress states representative of downhole conditions. A laboratory methodology has been developed to restore initial rock hydration state. Modelling of such experiments points out the existence of a suction pressure that can be wrongly assimilated to a swelling pressure. This paper will contribute to specify the representative experimental conditions and show how rock behaviour can be affected if such conditions are not respected.

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