REACTIVE TRANSPORT MODELLING OF DISSOLVED CO2 IN POROUS MEDIA Injection into and leakage from geological reservoirs

Reactive transport modelling of dissolved CO 2 in porous media iii Dedicated to my parents (late) and eldest sister. iv Reactive transport modelling of dissolved CO 2 in porous media v ABSTRACT The geological sequestration of carbon dioxide (CO 2) is seen as one of the options of mitigating the imbalance of greenhouse gas emissions and to avoid global warming. However, leakage of CO 2 from the subsurface storage reservoir is a potential long-term process with consequences for the bio-and hydrosphere. Over longer times, large-scale groundwater motion may displace the brine out of the storage reservoir with associated leakage risk of dissolved CO 2 (CO 2aq) along the conducting pathways. The objectives of this thesis are twofold. First, the modelling study analyzes the leakage of CO 2aq along the conducting pathways in caprock. Second, a relatively safer mode of geological storage is investigated wherein CO 2 dissolved in brine is injected in a carbonate reservoir. A reactive transport model and computer code is developed that accounts for the coupled hydrological transport and the geochemical reactions of CO 2aq in the subsurface porous media. Advection, dispersion, diffusion into the rock matrix, sorption and geochemical reactions are considered along the leakage pathway. The study provides a quantitative assessment of the impact of advection, dispersion, diffusion, sorption, geochemical reactions, temperature, and heat transport on the fate of CO 2aq leaking from the reservoir. The mass exchange between the principal conducting pathway and the rock matrix through matrix diffusion plays an important role in providing extra space for retention and reactions of leaking CO 2aq along the travel pathway. A significant retention of leaking CO 2aq is caused by its mass stored in aqueous and adsorbed states and its consumption in reactions in the rock matrix along the leakage pathway. Under some plausible conditions for clay-rich caprock, the results demonstrate that advection can cause a significant leakage of CO 2aq directly from the reservoir through the matrix in comparison to the diffusion alone in the rock matrix and advection along a highly conducting, but thin fracture. The analysis also demonstrated how geochemical reactions successively changed the porosity and permeability of the medium along the leakage pathway. Hence, the combination of advection, dispersion, diffusion, sorption, geochemical reactions, temperature and heat transported by leaking brine along the transport pathway are found key for understanding the fate of leaking CO 2aq from the reservoir. Injection of CO …