Shear behavior of the interface between drilling equipments and shale rocks

Abstract

Drilling operations in a borehole can lead to problems when the friction between the drilling equipments and the host rock are not well identified. This paper reports an experimental program carried out to characterize the shear behavior of an interface between a shale rock and the steel parts of the drilling equipment (bottom hole assembly and casing). Direct shear tests were performed with an advanced shear box, on interfaces made up of shale specimens and steel plates both in natural and water re-saturated conditions. The shear tests were performed at different normal stress levels from 2 to 12 MPa and the results show that the shear stress increases when the normal stress is increased. While the cohesion strength between the steel plate and shale is almost zero in a dry condition, this parameter reaches about 1 MPa in the presence of water. The average friction coefficient is determined to be 0.45 in the dry condition and it decreases to 0.21 when water is present. These findings should allow engineers involved in drilling operations and casing installation to assess more accurately the forces required to pull tools out of the borehole when problems arise. Therefore, this information could help to mitigate the risks encountered during well drilling.