Thermoporoelastic model for the cement sheath failure in a cased and cemented wellbore

Abstract

In the paper, we analyze factors that can lead to a failure of the cement sheath of a cased wellbore. By using a fully coupled linear thermo-poro-elastic model, we describe mechanical compression of the casing by fluid pressure and inhomogeneous geological stresses together with heating or cooling of the casing relative to the reservoir temperature. Due to the characteristic small ratio of the well diameter to its length, we observe the 2D problem in the section perpendicular to the well axis by adopting the plane strain approximation. We model the construction of single- or double-casings of the well, at that, both casings can be non-centered relative to the well axis. The cement sheath can be loaded by a prestress appeared during the cement hydration. The model is implemented numerically using the finite element method (FEM). Although many articles consider individual factors of the cement sheath failure, in our paper all these factors are modeled in a complex. Among the triggering effects for the failure we distinguish the strong contrast of geological stresses, the prestress in the cement sheath coupled with the action of the temperature variation. We also found that the thermo-poro-elastic coupling may lead to the significant growth of the pore pressure due to temperature changes when the pore pressure cannot dissolve either in the case of the very low permeability of cement or for the double-cased wellbore. In many situations, it is a combination of various factors that causes the failure of the cement sheath of the well, and not each factor separately.