An NMR Study on the Coupled Effect of IFT Reduction and Wettability Alteration on Oil Recovery

Spontaneous imbibition is of crucial importance in the residual oil extraction from strongly heterogeneous reservoirs, such as in the naturally fractured formations. The relative importance of the two major mechanisms, IFT reduction and wettability alteration, in the early stages of spontaneous imbibition constitutes the focus of this work. In this study, two selected surfactants, SS885 and GS2, had opposite strengths and weaknesses in the two mechanisms, as was confirmed with spinning drop and pendant drop IFT measurements and static contact angle measurements. Four Indiana limestone core plugs of two permeability levels, ~30 mD and ~150 mD, were divided into two groups. The effect of oil-aging was confirmed with NMR T2 distribution changes. After oil-aging, the two plugs in each group were soaked in SS885 and GS2 solutions, respectively. The oil recovery was recorded and compared to support the understanding of the relative contribution in the early stages of spontaneous imbibition in different permeability conditions. Results showed that wettability alteration has more contribution to the oil recovery than IFT reduction in the early stages of the spontaneous imbibition regardless of the permeability difference in the range of 24~167 mD. A higher permeability promotes a faster oil recovery when only IFT reduction occurs. A lower permeability promotes a faster oil recovery when the rock is altered towards water-wet conditions.