Modeling CO2 leakage through faults and fractures from subsurface storage sites
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Due to the concerns about the effect of greenhouse gases on the climate, geologic CO₂ storage is a very active area of research. Storage will take place in specifically selected target formations to achieve permanent containment. The biggest risk associated with geological storage is the possibility of leakage. The motivation for this research was the need to have a better understanding of potential leakage scenarios, leakage behavior, factors controlling leakage and other essential information about potential leakage. Possible leakage pathways include faults/fractures and leaky wells. Multiphase flow is likely because spatial gradients in pressure and temperature will occur as the CO₂ flows toward the surface. Below the CO₂ saturation pressure, liquid condensation of the CO₂ may occur. At even lower temperatures and pressures and in the presence of water, hydrate formation may occur. As a consequence, the fluid properties will change and affect leakage mass flux. The main purpose of this dissertation research was to develop and test models needed to estimate the leakage mass flux for different scenarios taking thermodynamic phase changes into account. A numerical model with coupled mass and energy balances was developed 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 was required. The multi-parameter Span-Wagner technical equation of state for CO₂ was used to achieve this. The numerical model allows for CO₂ to exist in gas, liquid and hydrate phases. Heat flux from the surroundings plays an important role because of its effect on the phase behavior. Example calculations indicate a cyclical nature of the leakage mass flux under certain conditions. Hydrate formation results in partial to complete blockage of the fault until melted. The effect of factors such as constant and varying reservoir pressure at the bottom of the fault, permeability and fault effective width were quantified with numerical simulations. A steady-state flow model was also developed for quick estimation of leakage mass fluxes through faults and fractures. The model was highly simplified and was intended for inclusion in risk assessment studies at the site-selection phase for geologic storage. The model was motivated by geological, non-isothermal properties and multiphase flow considerations. The model will estimate leakage mass fluxes for two different temperature conditions, namely, 1) non-isothermal conditions and 2) adiabatic temperature conditions. The resulting estimates act as the lower (multiphase coexistence and hydrates) and upper (non-isothermal nature) bounds for possible leakage mass flux for a particular set of physical properties of the pathway and surrounding geology. The effects of multiphase coexistence and hydrates on leakage mass flux were quantified. The effects of factors such as reservoir pressure and temperature, depth and permeability that affect multiphase coexistence and leakage mass flux were quantified with a sensitivity analysis.