Micro-scale experimental investigations of multiphase flow in oil-wet carbonates. II. Tertiary gas injection and WAG

Abstract

Carbonate reservoirs are typically oil-wet, which can lead to low oil production during the oil recovery processes such as base waterflooding. The residual oil in these reservoirs is the target of many EOR techniques such as tertiary gas injection and Water-Alternating-Gas (WAG) flooding. Currently, there is scarce research investigating the pore-scale displacement physics governing the afore-mentioned EOR techniques in oil-wet carbonates. In this study, three sets of miniature core-flooding experiments were performed in carbonate samples at elevated pressure and temperature conditions using a three-phase core-flooding system integrated with a high-resolution X-ray micro-CT scanner. Pore-scale displacement events governing tertiary gas injection and WAG under weakly oil-wet conditions were examined. The prevailing oil mobilization mechanisms during the tertiary gas injection were further investigated under varying wettability conditions. The results revealed that gas-to-oil-to-brine double displacement is the main displacement chain for oil production by tertiary gas injection. During this process, the greater was the degree of oil-wetness of the rock, the larger became the additional oil recovery that could be achieved. This was due to the increase in the frequency of gas-to-oil-to-brine double displacements and the superior connectivity of the oil phase under stronger oil-wet conditions. Furthermore, WAG flooding significantly increased the displacement efficiency of both gas and brine phases because of the shielding effect of the trapped gas ganglia. In the first WAG cycle, oil was produced through a series of direct and double displacements. Multiple displacements started taking place and further contributed to oil recovery as more WAG cycles were implemented.