Vertically Integrated Models for CO2 Storage with Coupled Thermal Processes
Abstract:CO2 storage involves coupled processes that affect the migration and ultimate fate of injected CO2 over multiple length and time scales. Coupled thermal and mechanical processes may have implications for storage security, including thermally induced fracturing and loss of caprock integrity near the wellbore. This may occur when CO2 is injected at a different temperature from reservoir conditions, e.g. Snøhvit injection, potentially leading to large temperature, density and volume changes over space and time. In addition, thermally induced density changes impact plume buoyancy that may affect large-scale migration patterns in gravity-driven systems, e.g. Sleipner injection. This interaction becomes particularly important near the critical point. Therefore, thermal processes should be considered in order to correctly capture plume migration within the reservoir.
A practical modeling approach for CO2 storage at the field scale is the vertical-equilibrium (VE) model, which solves partially integrated conservation equations for flow in two lateral dimensions. This class of models is well suited for strongly segregated flows. We extend the classical VE model to nonisothermal systems by integrating the heat transport equations, focusing on thermal processes that most impact the CO2 plume. The model allows for heating/cooling of the CO2 plume through heat exchange with the surrounding environment. The upscaling procedure assumes vertically constant temperature across the plume thickness for relatively thin plumes. Conduction across the plume boundaries, into the caprock above and brine below, is modeled by an analytical heat transfer function.
As a starting point, we investigate the validity of the simplifying assumptions and heat transfer boundary conditions for relatively simple systems. We find that the upscaled model compares well for systems where heat advection in the plume is the dominant heat transport mechanism. For high CO2 flux, improvements to the model can be made with more complex heat transfer functions. For lower CO2 flux, conduction within the surroundings becomes an important process that requires additional consideration. The results of this work demonstrate the potential for reduced models to advance our understanding of the impact of thermal processes in realistic storage systems.