Experimental Investigation of Countercurrent Spontaneous Imbibition in Tight Sandstone Using Nuclear Magnetic Resonance

Tight oil reservoirs are an unconventional hydrocarbon resource with great potential to help meet energy demands. Horizontal drilling and hydraulic fracturing has been extensively used for the exploitation of these unconventional resources, and fracturing fluids absorbed into formations by spontaneous imbibition (SI) is an important mechanism of oil production. In this paper, imbibition experiments combined with nuclear magnetic resonance were conducted to study the characteristics of fluid displacement in an oil/water system for tight sandstone. In addition, the relative contribution to oil recovery of different types of pores, effects of boundary conditions, and different surfactants on imbibition recovery was determined via the T-2 spectra of each sample. The results show that the tight sandstone features a multiscale pore structure, which is dominated by micropores and small mesopores. As the imbibition process begins, white oil is preferentially displaced from these relatively small pores by water and a large amount of oil production comes from the micropores. Boundary conditions are shown to have a significant impact on imbibition rate and ultimate recovery. Both are higher as the areas available for water imbibition increase. Deionized water with low concentrations of surfactants altered the wettability of the samples, from weakly water-wet to a strongly water-wet on the rock surfaces, while lowering interfacial tension (IFT) at the oil-water interface. Therefore, a higher oil recovery could be obtained to some extent, but enough IFT is still needed to ensure a large capillary force. Because conventional scaling equations do not account for the effect of wettability alteration, such as the addition of surfactants to a system, they cannot be employed to scale imbibition data well. This research demonstrates the imbibition characteristics of tight sandstone and several relevant affecting factors, providing crucial theory foundations for the development of tight oil formations.