Spontaneous Imbibition Microscopic Pore Space Fluid Occurrence Characteristics of Shale Reservoir

Spontaneous imbibition plays a key role in the development of shale oil. Previous research mainly analyzed the relationship between the quality of shale self-priming liquid or T2 spectrum and time, but failed to comprehensively describe the fluid occurrence in the core during imbibition and the distribution of fluid occurrence in plane and longitudinal direction. Therefore, based on the nuclear magnetic resonance (NMR) T2 spectrum and one-dimensional frequency coding spectrum, the occurrence amount of fluid (water) in the core at different stages and the distribution law of the occurrence state in the plane and longitudinal direction were studied by carrying out the spontaneous imbibition experiment of shale core. The results showed that the pore size of the main occurrence space of fluid (water) in shale was mainly distributed in the range of 1–100 nm. Spontaneous imbibition of shale could be divided into three stages: in the initial stage, the signal quantity of small holes increased rapidly and the imbibition efficiency was higher; in the middle stage of imbibition, the signal quantity of large and small holes increased slowly, and the imbibition efficiency slowed down; in the later stage of imbibition, the increase of the signal quantity of the large and small holes was not obvious, the imbibition efficiency was slow, the duration was long, and the imbibition was stable. The content of water along the Y-direction (parallel bedding) in different stages of spontaneous imbibition gradually decreases with the increase of the distance from the spontaneous imbibition end face. The content of water along the Z-direction (vertical bedding) was concentrated in the middle of the core (50 mm), and the signal on both sides (40–50 and 50–60 mm) was low. This new experiment provides a new method and idea for the study of the distribution of fluid occurrence in the pore space of shale during spontaneous imbibition.