Theoretical Analysis of the Microannulus in CO2 Injectors: Transient Thermal Effect pre- and post-Shut-in

ABSTRACT: To prevent zonal isolation failure and CO2 leakage, it's essential to account for both mechanical and thermal properties of cement, especially in shallow fields exposed to new conditions. While existing research has focused on designing cement systems with high mechanical strength and flexibility, many studies overlook formation properties and the associated temperature gradients. This paper examines the development of the microannulus at the cement-casing and cement-formation interfaces. In a previous study, Khan et al. (2025) developed an analytical formulation to model the evolution of the microannulus. Khan et al. (2025) used the resistor model to calculate temperature development, which is unable to capture transient conditions such as pre- and post-shut-in periods. The resistor model is widely used in most simulation applications, but its main limitation is that the analytical correlations in the model cannot predict transient behaviors, particularly during pre- and post-shut-in. Since the evolution of the microannulus is primarily concerned with these transient changes, a more accurate model is needed. To capture the formation's transient behavior, the finite-difference method (FDM) was used to model temperature variations. Its derivatives were linked to a resistor network, enabling accurate, stable solutions without iterative instability.