A clear description of the wettability of carbonate rocks poses numerous challenges due to its non-uniformity and uncertainty caused by complex pore systems and weak fluid/rock interactions. Aging in the crude oil and spontaneous brine imbibition of core samples in the laboratory constitute essential steps of wettability restoration, wettability measurement, and oil mobilization. Various aging durations are used to restore the carbonate wettability by researchers. The imbibed oil/water quantities in the Amott test are sometimes in the range of several percentages, and it is not clear if the fluids contact the whole core. This study investigates how the wettability evolves during again and how the imbibition develops in reservoir carbonate rocks using nuclear magnetic resonance imaging techniques.
The carbonate core samples with typical multiple pore systems were first saturated with crude oil at different irreducible saturation by centrifuge and aged for different durations. Then they were immersed in deuterium oxide (D2O) with two ends open (TEO) at an elevated temperature for spontaneous imbibition. The aging and spontaneous imbibition processes were monitored by a 1D magnetic resonance imager either by a standard probe or a high temperature and high pressure core holder. A set of nuclear magnetic resonance measurements were conducted for any wettability alteration and oil/brine imbibition front propagations.
NMR measurements during oil saturation and aging of three carbonate reservoir core samples indicate that the significant wettability change occurs after the oil flood while further aging continues to alter the wettability on a smaller scale. Low oil saturation has less of an effect on the wettability than high oil saturation. The brine imbibition fronts were found to develop from both ends or one end only, or there could be no apparent fronts. After the fronts propagate across the samples, a significant amount of brine continues to imbibe globally. The high permeability macro pore dominated samples appear to have significantly higher total brine imbibition than the low permeability micro pore dominated samples from current experimental results. Lower initial brine saturation impedes the imbibition with negligible effects on the final amount. Aging does not have appreciable effects on the brine imbibition for the samples studied.
The direct observations of wettability evolution during aging and brine spatial propagation during brine imbibition by NMR under reservoir conditions provide novel information about carbonate wettability, which assists in better wettability characterization and enhanced oil mobilization by wettability alteration.