Reactive transport modelling of geologic CO2 sequestration in saline aquifers: The influence of pure CO2 and of mixtures of CO2 with CH4 on the sealing capacity of cap rock at 37 °C and 100 bar

•Co-injection of CO2 with CH4 provides safer storage and reducing separation costs.•CH4 decreases fugacity of CO2 and makes CO2 less acidic.•Less acidic CO2 causes less reaction and the acidic brine front moves slower.•Calcite dissolution dominates the short-term reactions (0–100 years).•Dawsonite precipitation dominates the long term reactions (100–10,000 years).

The costs of CO2 separation for carbon capture and storage can be reduced through capturing less pure CO2. The presence of impurities such as methane (CH4) in the CO2 gas stream, however, affects the geochemical and geophysical processes in the subsurface. The dissolved CO2 in the brine decreases the pH which dissolves minerals such as calcite and albite. The dissolution of these minerals increases the amount of Ca2 + and Na+ in the brine. The presence of these ions leads the precipitation of the secondary solid carbonates calcite and dawsonite. To test this process, we developed a kinetic batch and a one-dimensional reactive transport model using PHREEQC 2.15.0, to predict mineral alteration induced in the cap rock by penetration of brine containing dissolved CO2 from the underlying aquifer over a period of 10,000 years. The chemical composition of the Nordland shale formation water that overlies the Utsira sand in the Sleipner field was used as a model case in this study. The model was run for pure CO2 and for mixtures with CH4 (1–4 (w/w)%) in the injected gas stream at a temperature of 37 °C and at a pressure of 100 bar. The simulations suggest that a mixture of CO2 and CH4 suppresses an anticipated increase in the porosity of the cap rock. Thus, our results suggest that injection of a CO2–CH4 mixture inhibits cap rock dissolution and helps maintain the sealing capacity of the cap rock, while reducing separation costs.