ABSTRACT:
A petroleum industry trend towards the use of water-based drilling muds is driving research into the chemohydromechanical interaction between the drilling fluid and shale formations, which cause the majority of drilling problems. Examination of solutions for simple geometries lends important insights into shale/mud interaction, provides benchmarks for numerical models, and can give a basis for the development of new laboratory tests. This paper presents a chemoporoelastic solution for a chemically active shale ball that is subjected to both hydraulic and chemical loading at its surface. It is shown that the size of a ball subjected to an increase in surrounding ion concentration eventually decreases, but depending on the chemomechanical and chemo-hydraulic coupling parameters it can follow a path of monotonic contraction, contraction to a minimum size followed by recovery, or initial swelling followed by contraction. These results demonstrate the importance of chemoporoelastic considerations for predicting the response of a shale to changes in drilling mud composition.
1 INTRODUCTION
The petroleum industry has long relied on oil-based drilling fluids to stabilize the wellbore during drilling operations. However, the environmental constraints and associated costs which surround the disposal of oil-based waste products are driving a trend towards the use of water-based drilling fluids. One major technological concern with water-based drilling fluids arises from the interaction of the fluids with shales which, along with mudstones, siltstones, and claystones, comprise 75% of drilled sections in oil and gas wells and cause 90% of the drilling problems related to wellbore instability (Tan et al. 2002). One common means for improving wellbore stability is to add various salts to the drilling fluid. Success of this approach relies on alteration of the (coupled) pore pressure and stress fields in the immediate vicinity of the wellbore, which in turn relies on the shale possessing certain chemo-mechanical properties.
