MODELING CO2-WATER-BASALT REACTIONS EFFECT TO FRACTURE ALTERATION IN A BASALT FRACTURE NETWORK

Anthropogenic CO₂emissions are strongly implicated in increasing global temperatures and other serious environmental issues. Carbon capture and sequestration (CCS) is an engineering-based approach designed to reduce the total mass of atmospheric CO₂ releases. Recently, basalt formations have been shown to accommodate rapid mineral trapping of injected CO₂. In basalt reservoirs, CO₂ dissolution in formation water releases H⁺ to drive mineral dissolution of olivine and pyroxene, which releases divalent cations (Ca²⁺, Mg²⁺, Fe²⁺) that react with bicarbonate to form secondary mineral phase, e.g., calcite, magnesite, siderite. Research investigating multiphase CO₂-brine flows in the fracture basalt formations shows that CO₂ tends to accumulate at fracture intersections before diverging into branching fractures, where CO₂ relative permeability and mobility are low. This later study implies the following hypotheses: (1) secondary mineral precipitation focuses at the branching fracture as a result of CO₂accumulation in fracture intersections and (2) secondary alteration at fracture intersections inhibits the vertical migration of gas phase CO₂ resulting from fracture permeability decrease. In order to test these hypotheses, five numerical simulations with different permeability alteration effects are implemented in a 2.5 m × 2.5 m basalt fracture system with one single fracture in the middle. The aperture of this fracture is 5 cm, and it diverges into two branching fractures with 90^o at the height 2/5 of the model. Results from this study show that (1) carbonate and clay minerals focus at the branching fractures, where the system is dominated by diffusion and (2) gas phase CO₂ vertical migration is inhibited by mineral precipitation, which causes fracture permeability decrease. In addition, a mass balance is used to test the carbonation efficiency in the fracture system with different permeability alteration exponents. It shows a large carbon mineralization ratio with permeability alteration, and this provides support that the fracture system could turn into a self-sealing system resulting from secondary mineral precipitation. In conclusion, permeability alteration resulting from carbon mineralization should be considered when modeling the CO₂ sequestration in basalt reservoirs.