Volumetrics of CO2 Storage in Deep Saline Formations

Concern about the role of greenhouse gases in global climate change has generated interest in sequestering CO2 from fossil-fuel combustion in deep saline formations. Pore space in these formations is initially filled with brine, and space to accommodate injected CO2 must be generated by displacing brine, and to a lesser extent by compression of brine and rock. The formation volume required to store a given mass of CO2 depends on the storage mechanism. We compare the equilibrium volumetric requirements of three end-member processes: CO2 stored as a supercritical fluid (structural or stratigraphic trapping); CO2 dissolved in pre-existing brine (solubility trapping); and CO2 solubility enhanced by dissolution of calcite. For typical storage conditions, storing CO2 by solubility trapping reduces the volume required to store the same amount of CO2 by structural or stratigraphic trapping by about 50%. Accessibility of CO2 to brine determines which storage mechanism (structural/stratigraphic versus solubility) dominates at a given time, which is a critical factor in evaluating CO2 volumetric requirements and long-term storage security.