Effect of thermodynamic phase changes on CO₂ leakage

Due to the concerns about the effect of greenhouse gases on the climate, Geologic CO₂ storage is a very active area of research. One of the biggest risks associated with such projects is the possibility of leakage. Detrimental environmental consequences present a need to study potential leakage scenarios. Stored CO₂ may leak if possible leakage pathways are available and favourable. Pressure and temperature decrease from the leakage source to the surface. Below the CO₂ saturation pressure, liquid condensation of the CO₂ occurs. At even lower temperatures and pressures, in the presence of water, hydrate formation occurs. CO₂-hydrate forms when free water is available and the temperature is below 283 K and pressure is below 647 psi. During leakage, decreases in the temperature and pressure results in a CO₂ phase change that affects the leakage flux. The purpose of this study is to estimate the leakage flux for different scenarios taking thermodynamic phase changes into account. An analytical model was built to predict steady state leakage flux taking the phase transitions into account. Several important limiting assumptions were required to perform these calculations. A numerical model with coupled mass and energy balances was developed and used to estimate the flux under less restrictive assumptions than the analytical model. Hydrate formation was modelled using the Van der Waals-Platteeuw model. Example analytical calculations indicate that for a uniform permeability pathway the CO₂ leakage flux decreases by a factor of about two due to CO₂ condensation and about three when hydrate forms compared with the isothermal leakage rate. These calculations illustrate the importance of the pressure and temperature of the leakage source (aquifer) and the amount of water in the pathway (fault) among other variables. Example numerical calculations indicate a cyclical nature of the leakage flux under certain conditions. Hydrate formation results in partial to complete blockage of the fault until melted.