Semi-analytical solution for thermo-poro-elastic stresses in a wellbore cement plug and implications for cement properties that minimize risk of failure

Abstract

In high temperature and high pressure environments, inducing pore pressure changes in low permeability cement can be problematic. Despite the typical design goal of achieving very low permeability in cement plugs for better isolation, this work reveals through a new semi-analytical TPE stress solution that lower permeability is not always ideal. In fact, such materials are unable to rapidly drain excess pore pressure compared to the rate at which it builds due to thermal changes may lead to the generation of internal tensile stress and increased risk of damage. This work identifies the parameter groups linked to the “permeability penalty” by utilizing dimensional and pairwise bivariate analysis through Monte Carlo sampling. It proposes two dimensionless parameter groups that divide into three distinct regions based on the likelihood of producing tensile effective stresses in a plug. This work provides a new wellbore cement design concept focuses on achieving the lowest permeability without risking failure from pore pressure buildup, rather than simply achieving the lowest possible permeability. Furthermore, this work for the first time highlights the important role of cement's specific heat in mitigating pore pressure buildup, paving a new path for enhancing this property in future cement design.