The role of flow rates on flow patterns and saturation in high-permeability porous media

This study aimed to investigate dynamic CO2 drainage using high-resolution magnetic resonance imaging (MRI) technology. Gaseous and supercritical CO2 were injected downward in brine-saturated porous media at different flow rates at 40 °C/6 MPa and 40 °C/8 MPa. These flow rates (0.015, 0.03 and 0.1 mL/min, under 10 Mt/year), as reflected in, were chosen according to the distance (1 km-100 m) from the injection well. Three stages were found from the change of signal intensity during CO2 drainage: before the CO2 front reached the field of view (FOV), breakthrough, and steady state. Channelling or drainage fronts immediately established through the large pores, and CO2 travelled vertically through these channels until breakthrough. The breakthrough time decreased with increasing flow rates and was longer for ScCO2 than gCO2 at the same flow rate, resulting in a longer residence time for ScCO2 in the sample. At low flow rates, the fingers first established along the larger pore spaces (especially at 0.015 mL/min) and then gradually extended into adjacent regions, resulting in a relatively flat interface. However, at high flow rates, the front moved along the larger pore spaces until breakthrough. The flow patterns for ScCO2 drainage were more uniform than those for gCO2 drainage. The pore volume fraction occupied by CO2, as a quantitative parameter of the flow pattern, reflected that the sweep efficiency and pore space utilization were optimized at 0.03 mL/min (Ca = 4.35 × 10−9 for gCO2 and Ca = 1.06 × 10-8 for ScCO2). The effect of the flow rate on the CO2 saturation and distribution was analysed. At low flow rates, the saturation gradient along the porous media gradually reduced, but trend to be stable at high flow rates. Additionally, the saturation at breakthrough and steady state were observed to be linearly related to the maximum rate of change in saturation during CO2 injection. Overly fast drainage results in relatively low saturation and an inhomogeneous distribution. The results can be applied to provide information for enhancing pore space utilization and improving sweep efficiencies during field storage.