On stability of a deviated wellbore in shale rocks

Shale is an argillaceous rock that contains moisture-sensitive clay minerals. Upon the adsorption of moisture, the rock swells and material property of the rock is degraded. A method was developed for calculating the swelling stress and in-situ stresses around the wellbore. The failure of wellbore was then studied by applying a moisture dependent failure criterion to the stress field. It was demonstrated that the theory provided a reasonable estimation on the stability of deviated wellbore in moisture sensitive shale rocks.; At the presence of a semi-permeable membrane, the water movement between the drilling fluid and shale rock is dependent upon the difference in the ionic concentration between them. A theory was developed for calculating the fictitious force that controlled the water movement in the system. The theory was used to calculate the swelling strain needed in the wellbore stability analysis.