3D Investigation of Thermal Stresses Around Wellbore Area During CO2 Injection

Abstract

Examining the stability of the wellbore plays a crucial role in CO2 injection projects, primarily utilized for geologic sequestration and storage applications by injecting a cold fluid into the reservoir. The thermal stress variation around the wellbore when the injected CO2 temperature is much lower than that of the formation section can risk the sealing ability of well barriers. To investigate the impact of the imposed thermal risk, a 3D Finite Element-based model is employed which accounts for induced thermal stresses in wellbore area by coupling various physical modules including pipeline flow, heat transfer, and poroelasticity in COMSOL Multiphysics software.

In the current work, we investigate thermal diffusion and subsequent stress changes during non-isothermal cold CO2 injection into a 2 km depth well. The analysis includes an assessment of potential cement damage and debonding at cement-casing interface during CO2 injection. A conservation of mass and momentum equation is employed to calculate pressure and velocity inside the tubing. The modelling encompasses the pre-drilling stresses, post-drilling mechanical stress distribution, and stresses after cementing and casing procedures. These steps establish a realistic initial stress and strain condition before injection. The final stage involves initiating the CO2 injection process to measure stress and temperature variation. For a typical onshore 2 Km injection well, our findings indicated a significant reduction in the mean stress at the casing when subjected to CO2 injection, attributed to the influence of the cold fluid. Additionally, it was noted that the mean stress at the cement and formation near the wellbore area also experienced a decrease. Our findings suggest that, at depths near the bottom hole, the temperature remains unchanged at specific distances from the well’s azimuthal axis. The extent of these areas is highly dependent on the injection time, velocity, and friction inside the tubing. Importantly, the equivalent plastic strain remains minimal, suggesting a low likelihood of fractures occurring and subsequent cement damage after the injection of cold CO2.

The sensitivity analysis scenarios encompass intermittent injection scenario, different geothermal gradients of the field, and existence of multiple rock layers. The results indicate that changes in extent of frictional dissipation, and injection scenario can impact temperature diffusion pattern near the wellbore. Moreover, system variables such as material properties can make an enormous impact on temperature and stress profiles. The results of this study suggest that failure may occur at the casing-cement boundary. Additionally, tensile failure in the form of debonding is highly likely at the bottom of the well in models with the greatest temperature variation at the bottomhole.

The established model allows for the evaluation of wellbore stability during CO2 injection process, providing crucial insights with substantial implications for Carbon Capture and Storage (CCS) projects. This research specifically tackles the challenges associated with subsurface fluid injection, addressing both geological and operational uncertainties. It aims to deliver an effective and reliable solution for well design and the application of CO2 injection.

Keywords: CO2 injection; Thermal stress; geomechanics; Finite element; 3D simulation; Heat transport; Wellbore stability