The Effect of Wettability on Waterflood Oil Recovery in Carbonate Rock Samples: A Systematic Multi-scale Experimental Investigation

摘要

The effect of wettability on waterflooding oil recovery and the associated pore-scale displacement mechanisms are systematically investigated during flow processes in limestone core samples with a broad spectrum of wettability. Using a miniature core-flooding setup integrated with micro-computed tomography (CT) imaging apparatus, we, for the first time, characterize in situ equilibrium wettability states and demonstrate capillary interactions of the flowing phases in strongly water-wet (SWW), intermediate-wet (IW), weakly oil-wet (WOW), and strongly oil-wet (SOW) systems. The microscale observations were then employed to explain the recovery results obtained from replicate, macroscale experiments. The waterflooding parameters, such as experimental temperature, brine composition, defending and invading phase properties, and the initial water saturation, were maintained nearly identical in all of the waterflooding tests, thereby ensuring that the wettability state was the only factor that controlled the variations in waterflood oil recoveries. We provide pore-scale evidences of various pore-scale displacement mechanisms and the consequent fluid configurations in systems with different wettability states. These findings are then linked to the recovery trends of macroscale experiments. The production from IW, WOW, and SOW cases portrayed a prolonged oil recovery owing to the gradual invasion of brine into small and intermediate-sized oil-wet pores. The IW case showed the highest oil recovery among all cases. Moreover, when the injection flow rate was increased, the oil recovery was gradually increased for the IW, WOW, and SOW systems, whereas no significant additional production was observed in the case of SWW.