Huff-puff by water has been conducted to enhance oil recovery after hydraulic fracturing in tight/shale oil reservoirs. However, the microscopic mechanism behind this approach is still unclear, which significantly limits the efficient development of tight oil. In order to reveal the oil and water transport law in nano scale during huff-puff by water, the whole process of pressurizing, high-pressure soaking, and depressurizing was studied by molecular simulation technology. The micro mechanism of crude oil transport at each stage was analyzed, and the effects of reservoir temperature, soaking pressure and soaking time on the transport characteristics of crude oil are investigated. The results show that the crude oil in the pore moves along the positive direction of pressure gradient, and the velocity increases first and then decreases in the pressurizing stage. In the soaking stage, the boundary layer oil molecules move along the positive direction of pressure gradient, while the bulk oil moves along the negative direction of pressure gradient under imbibition. In the depressurizing stage, the crude oil velocity increases rapidly first and then gradually flattens; The same direction of huff and puff system (TSDS) is more beneficial to improve tight oil forced imbibition recovery than the opposite direction of huff and puff system (TODS), and the recovery of TSDS is about 4 times that of TODS; Increasing temperature has a significant effect on each stage of forced imbibition, and the recovery increases about 2% for every 20℃ increase in temperature. The pressure increase only has a significant effect on the depressurizing process. For every 10MPa increase in pressure, the recovery increases by 2%; Prolonged soaking time can significantly improve the recovery in the opposite direction of huff and puff system. This work further reveals the micro mechanism of fluid transport in nano-pores during the high-pressure soaking and drainage process of tight oil, providing theoretical guidance for efficiently adjusting the pressurizing, soaking, and depressurizing system to improve tight oil recovery.
Keywords Tight/shale oil, Forced Imbibition, Huff-puff, Molecular Simulation, Transportation