Migration mode of brine and supercritical CO2 during steady-state relative permeability measurements at very slow fluid flow velocity

Abstract

Characteristics of two-phase flow for a brine–CO2 system in Berea sandstone were investigated to understand flow mechanisms on the basis of ‘ganglion dynamics’ and ‘connected-pathway flow’, both recently established through studies using synchrotron-beam micro-X-ray CT (μCT). Relative permeability curves (RPCs) in the brine–CO2 system were measured under steady-state conditions by calculating CO2 saturation from medical X-ray CT images. Fluid flow velocity was less than 10–5 m s−1, which represents the scenario of fluid movements near CO2 diffusion fronts in reservoirs. The flow direction was perpendicular to sedimentary layers during the injections in the experiment. The differential pressure across the core at each steady-flow state generally decreases with increasing CO2 content. This suggests a CO2 flow regime transition from one where ganglion dynamics dominated to one where connected-pathway-flow dominated. Randomly distributed pathways appear in each layer, resulting in a critical CO2 saturation (35–40 per cent) for achieving steady flow. The critical saturation value corresponds to the critical values reported in electrical resistivity in sandstones where the resistivity deviates from Archie's law. In contrast, for a flow direction parallel to sedimentary layers, steady-flow states are achieved when CO2 percolation clusters (channels) appear at lower CO2 saturation values (0–10 per cent). The flow perpendicular to bedding showed high CO2 saturation (ca. 40 per cent) at the brine-rich endpoint of RPC. Then, a rapid increase in RPC appeared at the CO2 saturation values 40–50 per cent. This suggests that the steady flow between inlet and outlet of sample ends is achieved through percolation pathways, and that the percolation clusters grow rapidly above critical saturation, as was indicated by the classical percolation theory. The relationship between sedimentary layers and flow direction seems to control the shapes of RPC, but the crucial factor is which type of CO2 pathway dominates: channeled pathway or randomly distributed pathway.