Sealing failure of a cement sheath under temperature variation

Abstract

The sealing failure of cement sheaths caused by temperature variations in wellbores seriously threatens the safe production of oil and gas wells. The finite element method (FEM) is usually employed to analyze this complex engineering problem. However, the current FEM models solved this problem are based on the constant temperature change value assumption, which ignore the damage of cement sheaths during the evolution of temperature field and the temperature change rate. This work established a FEM model based on the transient heat conduction theory and concrete damage plastic (CDP) model. The proposed numerical model is implemented in the commercial finite element software Abaqus and its accuracy has been verified through experiments. The damage states of the cement sheath during temperature changes were analyzed. The results indicate the variety of failure types of the cement sheath caused by temperature variation in the wellbore. An increase in temperature by 70 °C in the wellbore leads to both compression and tensile damages of the cement sheath, while a decrease in temperature by more than 46 °C will lead to the interface failure between casing and cement sheath. The variation rate of temperature in the wellbore has a significant influence on the sealing integrity of the cement sheath, and damage to the cement sheath increases with the temperature change rate increasing. Therefore, the cement sheath will be protected to reduce its damage by lowering the variation rate of temperature, such as by controlling the production rate of oil and gas wells. These research results provide theoretical supports for production allocation measures in oil and gas exploitation.