Dependence of CO2-brine interfacial tension on aquifer pressure, temperature and water salinity

The displacement of brine by CO2 during CO2 injection and migration, and the displacement of CO2 by invading brine in the wake of migrating CO2 depend on the interfacial tension of the CO2-brine system. An extensive laboratory program was conducted for the measurement of the interfacial tension (IFT) between CO2 and water or brine covering the ranges of 2 to 27 MPa pressure, 36 degrees C to 125 degrees C temperature, and 0 to 334,000 mg/l water salinity. The laboratory experiments were conducted using the pendant drop method combined with the solution of the Laplace equation for capillarity for the profile of the brine drop in the CO2-rich environment. The analysis of the results reveals that: 1) for conditions of constant temperature and water salinity, IFT decreases steeply with increasing pressure in the range P<P-c, and mildly for P>P-c, with an asymptotic trend towards a constant value for high pressures; 2) for the same conditions of constant pressure and temperature, IFT increases with increasing water salinity, reflecting decreasing CO2 solubility in brine as salinity increases; 3) IFT increases with increasing temperature for T>T-c, with an asymptotic trend towards a constant value for high temperatures. These results indicate that, in the case of CO2 storage in deep saline aquifers, trapping of CO2 at irreducible saturation, storage efficiency and safety, and the evolution of the plume of injected CO2 depend on the in-situ conditions of pressure, temperature and water salinity through the effects that these primary variables have on the IFT between CO2 and aquifer brine. In addition, it was found that the IFT of CO2-brine systems correlates well with CO2 solubility in brine, which also depends on the same primary variables of pressure, temperature and water salinity. (C) 2008 Elsevier Ltd. All rights reserved