Drilling of shale has long been a challenge due to its strong potential for wellbore instability. Designing of drilling fluids which minimize the interaction with shale is critical in the success of these drilling practices.
Shale instability is mainly related to its abundance of clay content, distribution of reactive clays such as smectite, bedding and thin laminae (fissility). In many situations, small faults and fractures may intersect laminated shale, which causes the rock to crack in various directions. Though shale has many common properties, each shale formation has specific features in clay minerals, rock structures, and deformation properties; consequently their response to drilling fluids varies. As a result, our goal is to understand the petrologic and deformation features of each specific shale formation and their potential interactions with various drilling fluids.
The traditional laboratory methods such as dispersion test, bulk hardness test, and swelling test cannot fully reflect the impacts of rock structure on fracture development and rock failing. This can be compensated by immersion test which is designed to directly observe the rock-fluid interactions and fracture development. Our laboratory test results indicate that the composition and concentration of chemical additives in drilling fluids have significant impact on controlling and reducing the interaction between shale and fluids. Based on laboratory studies, we have designed custom drilling fluids for many shale formations around the world. The custom drilling fluids reduce the inherent wellbore instability found in drilling shales.