Simulation Study on the Spontaneous Imbibition Mechanism and Influencing Factors of Laminated Shale Oil Reservoirs in Jiyang Depression, Eastern China

Spontaneous imbibition has an important influence on the well production and water cut in shale oil development, but the imbibition mechanism in the pore structure and the influence of different factors, such as wettability, are still unclear. In this study, the representative pore structure was extracted based on the scanning electron microscopy image of natural samples taken from the laminated shale oil reservoir in Jiyang Depression, eastern China, and a two-dimensional model of the pore structure was established. On this basis, the dynamic changes of pore pressure, fluid distribution, and recovery during spontaneous imbibition in the natural pore structure were simulated by combining the Navier–Stokes equation and the phase field method. Furthermore, the influence of the pore structure, fracture network, and wettability on imbibition was compared by changing the simulation schemes. The research results demonstrated that water-wet was the basis of spontaneous imbibition, pressure gradient was the key to fluid flow, and cocurrent imbibition was more efficient than counter-current imbibition. Due to the influence of capillary force, fracturing fluid preferentially entered from the pores with smaller radii, driving shale oil out from the pores with larger radii during cocurrent imbibition. In the complex fracture network, the fracturing fluid might be preferentially imbibed into the bedding fracture, resulting in a high imbibition efficiency of calcareous lamina with strong pore connectivity and multiple imbibition paths. In addition, wettability was one of the main factors controlling the imbibition paths, the spontaneous imbibition recovery decreased with the decrease of water-wet, and the influence of mixed wetting was closely related to the distribution and proportion of water-wet pores. Therefore, the synergistic effects of water-wet, developed laminated characteristic, and complex fracture network lead to high spontaneous imbibition efficiency in the Jiyang laminated shale oil reservoir, making horizontal well development characterized by a high production rate, low water cut, and low flowback rate.