| A potential strategy to mitigate global climate change is to capture the carbon dioxide (CO2) that would normally be emitted from fossil fuel combustion and sequester it in geological formations. Deep offshore saline aquifers are promising options for long-term safe storage of large amounts of CO2, whereby CO2 is injected directly into the aquifer. Sequestration is achieved by the combination of three principal mechanisms, namely: (1) hydrodynamic trapping, (2) solubility trapping, and (3) mineral trapping. Storage capacity, fluid distribution, and the potential for rapid leakage through the reservoir seal or cap-rock are largely controlled by the brine/CO 2 interfacial tension (IFT). In this research, a comprehensive review of experimental studies on the IFT, solubility, and density of CO2/H 2O, H2O/CO2, and CO2/H2O/NaCl systems has been undertaken. Brines/CO2 IFT also have been measured for five pressures and temperatures, and several salt (NaCl) concentrations from zero (pure water) to 6M (the saturation point at 90°C and 33 MPa). Measurements were made in a high-pressure pendant drop apparatus under typical conditions of pressure and temperature corresponding to the offshore Scotian Shelf basin aquifers at depths from 2000 to 3000 m. The axi-symmetric drop shape analysis (ADSA) method for the pendant drop makes it possible to determine the IFT and monitor the interfacial interactions under practical reservoir conditions. A 4000-second video enabled observation of shrinking, swelling, and salt precipitation phenomena and the effect of the change in the volume and area of the pendant drop on the IFT. A linear relationship was developed which can predict IFT as a function of depth. Cap-rock sealing capacity for CO2 storage was estimated in terms of mass per unit surface of the aquifer, using the measured brine/CO2 IFT values and the density of both the brine and pure CO2 using the equations reported by Chiquet et al. (2007). This research shows that the storage capacity decreases with both increasing brine salinity and depth of the reservoir; this is in contrast with the many other researchers, but is consistent with the recent model predictions of Eccles et al. (2009). Values for minimum miscibility based on brine in CO2 systems were obtained by extrapolation of the measured IFT values to zero as a function of depth (pressure and temperature), from which minimum miscible depth and pressure were estimated. |
