Pore-scale experimental investigation of the remaining oil formation in water-wet, mixed-wet and oil-wet sandstone samples

The constant injection rate core-flooding experiments were conducted on three sandstone samples with the almost similar physical properties and the wettabilities are water-wet (WW), mixed-wet (MW) and oil-wet (OW) respectively. Using in-situ 4D X-ray micro-tomography, we investigated the impact of wettability on pore-scale distribution of fluids in core plugs under similar flow conditions. We systematically study pore-scale displacement sequences, remaining oil saturation as well as the size distribution and morphology of its disconnected clusters (droplets) to explain the observed remaining oil formation process during brine injection. For the WW sandstone (contact angle θ = 47 ± 8°), the brine will tend to imbibe into any small- or medium-sized pores, leaving disconnected oil droplets in the center of larger pores. The number of oil clusters (droplets) increase in WW sandstone but decrease sharply in MW sandstone during 50 PV of brine flooding. Brine tends to invade the water-wet pore spaces in MW condition (θ = 87 ± 30°), some of the brine-filled pores may be reoccupied by disconnected oil droplets during the subsequent brine injection which resulted in the highest remaining oil saturation. For the case of a OW sandstone (θ = 150 ± 10°), the remaining oil could be trapped in the smaller pore corners or attached to the pore surface as connected thin films. We observed that the ultimate oil recovery in OW sandstone (86.1%) is larger than that obtained in the WW (72.2%) and MW (38.8%) sandstone samples. Finally, the results show that a large amount of water flooding can significantly change the pore fluid distribution, resulting in higher oil recovery.