Disposal of carbon dioxide (CO2) in permeable, porous subsurface rock formations (i.e., geological sequestration) has been identified as a viable option for reducing greenhouse gas emissions into the Earth's atmosphere. Potential subsurface systems considered for geological sequestration include depleted oil and gas reservoirs, coalbed methane and shale gas reservoirs, and deep aquifers. Though each of these disposal systems has their advantages, deep aquifers (mostly filled with non-potable or brackish waters) have the greatest potential for large CO2 sequestration programs primarily because of their relative abundance in most sedimentary basins and their large effective capacities.
Successful selection of potential of CO2 deep aquifer sequestration sites, however, requires an understanding of all physical and chemical trapping mechanisms by which CO2 may be retained. Principle retention mechanisms in aquifers include structural/stratigraphic (CO2 immobilization or trapping below an impermeable confining layer), residual fluid (trapped as immobile fluid phase in aquifer pore spaces), solubility (immobilized as fluid phase dissolved in in-situ water), mineral
(immobilized as solid carbonate minerals formed from reaction with aquifer rock), and hydrodynamic (CO2 dissolved in slow-moving water) trapping. While all of these mechanisms contribute to CO2 sequestration, the structural/stratigraphic and residual fluid mechanisms have the largest and most immediate impact on trapping or retaining CO2 in aquifers.
The effectiveness of both structural/stratigraphic and residual fluid trapping mechanisms is dependent on the capillary pressure characteristics of the aquifer seal and formation, respectively. And, the capillary pressure characteristics are strong functions of the interfacial tension (IFT) properties of the carbon dioxide-water (CO2-H2O) system. Unfortunately, there is a general lack of understanding of the CO2-H2O IFTs, particularly at high-pressure/high-temperature (HP/HT) conditions typical of many potential deep aquifer sites. The vast majority of published CO2-H2O IFT data were obtained at pressures less than 10,000 psia and temperature less than about 250oF. Additionally, there are often inconsistencies among the existing data published in the literature, thereby making it difficult to create predictive models.
To address these inadequacies in the existing technical literature data base, we conducted laboratory studies to measure CO2- H2O IFTs using a pendant drop method at pressures between 1,000 and 18,000 psia and temperatures up to 400 oF. Rather than relying on correlations or previously published data, we also measured water-vapor-saturated CO2 as well as CO2- saturated water densities directly at each pressure and temperature. General observations from our laboratory study include:
CO2-H2O IFTs demonstrated a strong dependence on temperature (decreasing with increasing temperature);
For a given temperature, CO2-H2O IFTs were relatively insensitive to pressure with values between 10 to 23 dynes/cm; values never fell below 10 dynes/cm for all temperatures up to 400oF;
Full miscibility between CO2 and H2O never occurred at any pressure and temperature evaluated in the study;
CO2-saturated water densities showed a strong dependence on pressure and temperature, while water-vapor-saturated CO2 densities showed little change from the CO2 density with no vapor content.