Increasing CO2-Storage Efficiency Through a CO2/Brine-Displacement Approach

SummaryPrevious studies have shown that bulk carbon dioxide (CO2) injection in deep saline aquifers supplies insufficient aquifer storage efficiency and causes excessive risk because of aquifer pressurization. To avoid pressurization, we propose to produce the same volume of brine as is injected as CO2 in a CO2/brine displacement. Two approaches to CO2/brine displacement are considered—an external brine-disposal strategy in which brine is disposed of into another formation such as oilfield brine and an internal saturated brine-injection strategy with which the produced brine is desalinated and reinjected into the same formation. The displacement strategies increase the storage efficiency from 0.48% for the bulk-injection case to more than 7%. A conceptual case study with documented aquifer properties of the Woodbine aquifer in Texas indicates that the available volume is sufficient to store all the CO2 being generated by power plants in the vicinity for approximately 20 years only. However, the CO2/brine displacement increases storage efficiency enough to store the CO2 produced for at least 240 years at the current rate of coal-fired electric-power generation. Sensitivity analyses on relative permeability, permeability, and temperature were conducted to see the effects of these reservoir parameters on storage efficiency.For bulk injection, increased permeability resulted in increased storage efficiency, but for the CO2/brine-displacement strategies, decreased permeability increased storage efficiency because this resulted in higher average pressure that increased CO2 storage per unit of pore volume (PV) and increased CO2 viscosity. Also, storage efficiencies for the displacement strategies were highly sensitive to relative permeability. There is an optimal CO2-injection temperature below which the formation-fracturing pressure is lowered and above which CO2 breakthrough occurs for a smaller injection mass. The CO2/brine-displacement approach increased capital expenditures for additional wells and an operating expense for produced-brine disposal, but these additional costs are offset by increased CO2-storage efficiency at least 12 times that achieved by the bulk-injection strategy.