Shale oil mobility and pore size-associated wettability under capillary pressures

The capillary force shows great potential to improve the recovery of shale oil and gas reservoirs through spontaneous imbibition. However, the mechanism of capillary force on shale oil migration and its controlling factors are still unclear. By NMR, low-temperature nitrogen adsorption, high-pressure mercury injection and other experimental means, this work attempts to investigate the role of capillary force in improving shale oil recovery. The results show that the nuclear magnetic resonance T₂ spectra obtained through spontaneous imbibition can be divided into three types, and the shale oil recovery can reach 38.72% - 65.52%, which is mainly contributed by the first peak (P1). The water imbibition and oil imbibition experiments were carried out on samples of the same size, and the dynamic wettability index of the samples with the spontaneous imbibition time was calculated. It was found that type 1 shale is mainly lipophilic, type 2 and type 3 samples are mainly hydrophilic, the P1 of three types of shale is hydrophilic to neutral, and the water imbibition volume of the three samples was greater than the oil imbibition volume. In addition, by comparing the relationship between pore throats and pores and combining the structural characteristics of samples, three typical types of pore throats are summarized. Finally, through a comprehensive study on the wettability, pore structure of shale and shale oil recovery , it is concluded that water can drive oil droplets in micropores or pore throats (P1) to enter the mesopore (P2), and then the mesopore (P2) transmits the oil to the fractures by transfering pressure difference, and the oil-water distribution pattern before and after spontaneous imbibition under the effect of capillary force is summarized to provide theoretical basis for shale oil exploration. and development.