Numerical Study on the 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 favorable. 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. A numerical model with coupled mass and energy balances was developed and used to estimate the flux as a function of time. Due to wide temperature and pressure changes over the course of the simulation, an accurate fluid properties model is required to reduce fluid properties related errors. Multi-parameter Span-Wagner equation of state for CO₂ is used to achieve this. The numerical model allows for CO₂ to exist in gas, liquid and hydrate phases. Hydrate formation was modelled using the van der Waals-Platteeuw model. Heat flux from the surroundings play an important role in effecting a phase change. Example calculations indicate a cyclical nature of the leakage flux under certain conditions. These calculations indicate the importance pressure of the leakage source (aquifer), the amount of water in the pathway (fracture/faults), the geothermal gradient and the surface temperature besides the thickness and the permeability of the fracture.