On the evolution of cement sheath stress in wells during curing

Portland Cement in the well construction provides zonal isolation, preventing unwanted fluid migration to shallow aquifers or the surface. Despite its widespread application for well sealing, the evolution of cement sheath stress state during curing is not well-known. The radial stress in the cement sheath is the key parameter behind the formation of microannuli, a continuous fracture-like feature at the cement sheath's interface that can become a leakage pathway. In this work, we developed an experimental setup to measure the evolution of stress and pore pressure of a cement sheath. The experiments simulate a cement sheath placed next to an impermeable formation, such as a caprock. Two different axial boundary conditions of plane stress and plane strain were considered. The results show that pore pressure loss due to water consumption by cement clinker hydration occurs at a faster rate for the plane strain condition than the plane stress. Due to the poromechanical properties of cement, the drop in pore pressure also reduces the stress at the inner and outer interfaces of a cement sheath. An analytical model is proposed to confirm the stress evolution and to explain the influence of the formation stiffness on the stress drop. The impact of hydration on cement stress is significant and must be taken into account in well integrity and leakage assessment studies.