Simulation and analysis of wellbore stability using FLAC for horizontal well drilling in shale formations

Wellbore stability is of critical importance in all drilling operations. Wellbore instability may cause stuck pipes, lost circulation, and/or collapse of the wellbore, resulting in high drilling cost and significant time loss. In this study, computer simulations using FLAC (Fast Lagrangian Analysis of Continua) were conducted on the stability of horizontal wells in shale formations. Laboratory-tested geomechanical properties of seven shale samples and in-situ stress conditions collected from the literature were used. Computer simulations were carried out to estimate minimum downholc pressures for maintaining wellbore integrity in each type of shale formation under different states of in-situ stresses. The results showed that the minimum downhole pressure to maintain wellbore stability is positively related to stress differentiation and pore pressure, and negatively related to internal frictional angle and cohesion of the surrounding rock. The determination of the minimum downhole pressure from the regression analyses may serve as a basis for engineers to quickly select proper mud density when drilling horizontal wells in potentially problematic rock formations, particularly shale formations.