Pore-scale Evolution of Supercritical CO2 within Bentheimer Sandstone during Multiple Drainage-Imbibition Cycles

Geologic CO₂ sequestration has been proposed as a climate change mitigation strategy to limit emissions of CO₂ to the atmosphere from large fossil-fuel burning CO₂ point sources; however, there are concerns associated with the long-term stability of a mobile subsurface CO₂ plume. The large-scale movement of subsurface supercritical CO₂ (scCO₂) can be prevented via capillary trapping, wherein scCO₂ is immobilized in the subsurface by capillary interactions between the solid surface, resident brine, and scCO₂. Capillary trapping occurs in two steps: first, the porous medium undergoes drainage as scCO₂ is injected into the system; then, wetting fluid re-enters the medium in an imbibition process, isolating small bubbles of scCO2 in the pore bodies of the medium. There are many empirical models which predict capillary trapping for a single drainage-imbibition cycle; however, in an engineered CO₂ sequestration project, it is possible to implement cyclic scCO₂-water injections in a water-alternating-gas (WAG) injection scheme in which the system may undergo multiple CO₂ injections to potentially increase the trapping efficiency of scCO₂.

We present experimental results of multiple drainage-imbibition cycles of scCO₂ and 1:6 by mass potassium iodide (KI) brine within Bentheimer sandstone. Capillary (differential) pressure and absolute pressures for each phase were continuously measured throughout each flow process, which is a unique feature of our experimental system. Experiments were conducted at a working pressure of 8.3 MPa (1200 PSI) and 40^oC, and synchrotron x-ray computed microtomography (x-ray CMT) images were collected of the drainage and imbibition process endpoints at a resolution of 3.19 µm at the Advanced Photon Source at Argonne National Laboratory.

The evolution of the connectivity, topology, morphology, and capillary trapping of scCO₂ phase is analyzed as a function of capillary pressure, scCO₂ saturation, and sample history. Preliminary results suggest that capillary trapping of scCO2 is hysteretic and that this pore-scale phenomenon and dependence should be considered when designing large scale CO₂ injection processes. These results are discussed in terms of current empirical capillary trapping models as well as from alternative theoretical perspectives.