Pore-scale investigation of CO2/oil exsolution in CO2 huff-n-puff for enhanced oil recovery

Abstract

A pore-scale high-pressure visualization experimental system is used to investigate CO2 exsolution during the CO2 huff-n-puff process for enhanced oil recovery and geological CO2 storage. Eighteen different experimental cases are examined to investigate the mechanisms by which depressurization-induced CO2 exsolution is affected by a near-miscible vs an immiscible CO2/oil initial state, by the depressurization rate, and by the presence of a water phase under different wettability conditions. CO2 exsolution is divided into three processes: nucleation, growth and coalescence, and migration. Visual observations and statistical results indicate that a near-miscible CO2/oil initial state causes intense and instantaneous CO2 nucleation. The presence of water effectively hinders the coalescence and migration of CO2 ganglia, reducing the generation and rapid departure of large CO2 ganglia in both water-wet and oil-wet cases. The amount of residual CO2 ganglia increases significantly in the presence of water, and the volumes of most of the increased residual CO2 ganglia are small. Hindrance by the presence of water is predominantly due to contact angle hysteresis rather than the Jamin effect, since it is found that the triple-phase contact lines do not move under most conditions. The residual CO2 saturation increases substantially in the presence of water, regardless of the wettability (oil vs water), with the improvement reaching 95%.