An equivalent time-dependent analysis for predicting horizontal wellbore instability in a reactive shale formation using the imbibition/diffusion principle

Abstract

Osmotic pressure generation, ion migration, shale swelling, and strength reduction are some problems engineers face when drilling through reactive shale. Mody and Hale's model for evaluating the osmotic pressure is not explicitly time-dependent, whereas diffusion is. Moreover, it does not consider membrane destruction with time. Thus, it becomes difficult to apply the model during the analysis of time-dependent instability without needing the conventional thermo-chemo-mechanical coupling process. This process is costly, rigorous, and time-consuming and does not consider the membrane's destruction with time. To this end, the authors analyzed wellbore instability in reactive shale by exploiting Kirsch's stresses. The authors derived some equations for strength reduction, membrane destruction with time, and the time-dependent osmotic pressure based on empirical analysis. Then, they added these equations to the constitutive stresses and applied the Mogi-Coulomb criterion. The study area is located in the passive continental margin of the Tertiary Niger Delta, which has shale diapirs and many growth faults near the deep Atlantic. The results show that the wellbore instability increases with time in an open face mainly because of diffusion and strength reduction. Wellbore instability affected the fracture pressure more than the collapse pressure. In addition, the increase in the Cation Exchange Capacity (CEC) or clay content causes the mud window to reduce with time.