The impact of cyclic injection cycles on capillary trapping: comparison of ambient and reservoir condition experiments

The success and safety of geologic CO2 sequestration operations relies on accurate predictions and modelling of buoyant CO2 plume migration after the CO2 has been injected into a subsurface reservoir. This requires understanding and characterization of several interlinked physical and chemical mechanisms: hydrodynamic trapping beneath low permeability layers (caprocks), dissolution and mineralization processes, and capillary trapping of small CO2 ganglia within the pores of the geologic formation. Capillary trapping can immobilize a significant fraction of injected CO2, thus reducing the reliance on caprock integrity (necessary for hydrodynamic trapping), and also increases the surface area of the CO2 phase, enhancing subsequent dissolution and mineralization processes. Accurate estimates of CO2 capillary trapping capacity are crucial to the design of safe and efficient CO2 sequestration operations; and numerous recent studies have investigated capillary trapping of supercritical CO2 in geologic formations. However, there are still significant uncertainties in the long-term stability and fate of capillary trapped CO2; and in particular, there have been conflicting reports regarding capillary trapping levels when multiple cycles of CO2 and brine are injected in alternating patterns, or when the solid matrix is exposed to CO2 over long periods of time. We present new analysis of previously reported experimental studies [1,2], and provide new measurements of characteristics of capillary trapped (residual) nonwetting phase (ambient condition air and supercritical CO2) over multiple multi-cycle drainage-imbibition experiments in Bentheimer sandstone. We focus on a comparison of ambient condition (air-brine) studies and those conducted with supercritical CO2 and brine under high pressure, high temperature conditions relevant to storage reservoirs (“reservoir condition”). All experiments were characterized with X-ray microtomography, a nondestructive imaging technique that provides full three dimensional information on the structure of the porous material and fluid phases residing within the pore space of the sandstone. Ganglia size distributions were measured, and we provide additional characterization of the topology and geometry of the residual nonwetting phase via the image analysis program Diamorse. We investigate nonwetting phase characteristics from both global (whole column) and local (on the scale of individual pore bodies/ganglia) perspectives to investigate the causes of the anomalous capillary trapping trends observed in supercritical condition experiments. The results are consistent with the theory that progressive surface property modification occurs due to CO2 adhesion on quartz surfaces, a finding with implications to prediction of CO2 capillary trapping and long-term CO2 stability in geologic formations.