Numerical study of reservoir permeability effects on gravity changes associated with CO2 geological storage: implications for gravimetric monitoring feasibility

Gravimetric methods are used for monitoring the migration of CO2 stored in the reservoir. Reservoir permeability is crucial for injectivity and potentially important for gravity changes associated with CO2 storage. Nevertheless, its effects on gravity changes have not been investigated systematically. After conducting numerical simulations of CO2 injection and storage using models with varying permeability conditions (50–1000 mD), we calculated surface gravity changes. Results show that two mechanisms affect gravity decrease, related to differences in CO2 density and in CO2 spatial patterns. Under low‐permeability conditions, gravity decrease is minimal during injection because of increases in pressure and the resulting relative high density of CO2. Gravity decrease is enhanced after injection ceases because of the pressure release and resulting CO2 expansion. These effects are strong at shallow depths, at which CO2 is near the critical point and highly compressible. Under higher permeability conditions, rapid spreading of the CO2 plume causes smaller gravity reduction rate after some decades of injection. After injection ceases, upward CO2 migration because of buoyancy and plume spreading increase gravity above the injection zone. The time required for gravity change detection depends on permeability. Detection during injection can be made earlier under higher permeability conditions and under lower permeability conditions after injection ceases. These points suggest that permeability affects the feasibility of gravimetric methods. Moreover, designing monitoring programs based on permeability and depth conditions is necessary for successful monitoring. © 2020 Society of Chemical Industry and John Wiley & Sons, Ltd.