Several instability issues such as pipe sticking, washout and borehole collapse can be experienced during drilling in chemically active formations. The conventional approaches towards the geomechanical modeling of such formations fall short of describing complex behavior of chemically active rocks, such as the swelling shales. The effect of crystalline swelling and chemical osmosis on elastic deformation have been adequately addressed by many authors, however, the effect of these changes on plastic deformation of rocks under three principle stress conditions have not been investigated. In this paper the effect of hydration/dehydration due to thermal and chemical osmosis on plastic deformation of chemically reactive shales is studied.

The 3D-model is developed to evaluate borehole stability problems in drilling a troublesome shale formation. From the results of this study it was revealed that most reactive shales at typical drill depth (2500m to 3500m) undergo plastic deformation. Hydration/dehydration due to crystalline swelling and osmosis (chemical and thermal) further aggravates the mechanical properties of shales. The numerical experiments also show that the vertical stress plays a significant rule in formation of plastic zone around the wellbore thus, making the wellbore prone to collapse failure.

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