Exploring the effect of interfacial tension, viscosity, and flow rate on the effectiveness of capillary trapping of CO2

Typical geologic carbon sequestration scenarios involve the injection of supercritical CO2 into an aquifer while displacing brine in what is equivalent to a water drainage process. Due to density differences, the CO2 will tend to buoyantly migrate upward until an impermeable barrier is encountered. This buoyant CO2 plume may persist for hundreds to thousands of years posing a challenge to ensuring the long-term integrity of CO2 storage. However, a potentially large fraction of the CO2 may be immobilized by one of several trapping mechanisms. Once injection stops, buoyant forces will continue to propel the supercritical CO2 upwards, while brine reoccupies the pore space in a water imbibition process, resulting in capillary trapping of some of the CO2. In addition, supercritical CO2 is partially miscible in brine and will dissolve in the brine forming a negatively buoyant phase (dissolution trapping), and will subsequently dissolve minerals to form bicarbonate and carbonate ions (ionic trapping) and may ultimately precipitate as mineral carbonates (mineral trapping). In the capillary trapping process, supercritical CO2 is immobilized by the imbibition process where capillary interactions lock isolated CO2 bubbles within unconnected rock pore space. These bubbles have significantly increased surface area with respect to brine and rock compared to bulk storage, and may facilitate enhanced dissolution of gaseous CO2 into the brine as well as chemical reactions. Thus, one approach to demonstrating storage security is the quantification of the rate and extent of these trapping mechanisms. We present experimental results based on computed x-ray microtomography (CMT) for quantifying capillary trapping mechanisms as a function of fluid properties using several pairs of analog fluids to span a range of potential supercritical CO2-brine conditions. Our experiments are conducted in a core-flood apparatus using synthetic porous media and we investigate capillary trapping by measuring trapped non-wetting phase area as a function of varying interfacial tension, viscosity, and wetting flow rate. Experiments are repeated for a single sintered glass bead core using three different non-wetting phase fluids and varying concentrations of surfactants to explore and separate the effects of interfacial tension, viscosity, and fluid flow rate. Analysis of the data demonstrates distinct and consistent differences in the amount of initial (i.e. following CO2 injection) and residual (i.e. following flood or WAG scheme) nonwetting phase occupancy as a function of fluid properties and flow rate. Further experimentation and analyses is needed, but these preliminary results indicate trends that can guide design of injection scenarios such that both initial and residual trapped gas occupancy is optimized.