Nanopores-to-microfractures flow mechanism and remaining distribution of shale oil during dynamic water spontaneous imbibition studied by NMR

Abstract

The mechanism by which spontaneous imbibition affects shale oil mobility is still unclear. In this study, three types of lithofacies shales was selected, and an integrated approach such as nuclear magnetic resonance (NMR), N2 adsorption (NP), mercury injection porosimetry (MICP) and rock pyrolysis, was used to investigate the flow mechanism and remaining distribution of shale oil during dynamic water spontaneous imbibition. The results show that the micropores in Type I shale are mainly ink bottle shaped with weakly water-wet, while mesopores and macropores exhibit weakly oil-wet; The micropores of type II shales are mainly narrow slit shaped, and this type shales exhibit weakly water-wet except for macropores, while the micropores of type III shale are mainly parallel plate-like pores, and all pores exhibit weakly water-wet and mixed-wet. The contribution of water (MnCl2 solution) imbibition to shale oil recovery is significant, ranging from 27.5% to 55.5%, mainly due to the contribution of micropores. And the utilization rates of micropores and macropores are relatively high (average of 79% and 86%, respectively), while the utilization rate of mesopores is the lowest. The recovery rate of shale oil in micropores is positively correlated with quartz, feldspar, and clay minerals, and is proportional to the pore volume controlled by pore throats less than 10 nm. In addition, micropores with weakly water-wet or mixed-wet have a significant impact on shale recovery. Based on the above factors, the flow mechanism of shale oil under spontaneous imbibition is proposed: MnCl2 solution enters hydrophilic micropores and drives the oil in the micropores to enter lipophilic mesopores. This part of the oil can be transmitted to the microfractures through the mesopores by the capillary pressure difference on both sides of the micropores and discharged; And the distribution pattern of remaining oil after water imbibition of Type I shales is summarized, providing a theoretical support for the exploration and development of lacustrine shale oil.