The migration mechanism of oil under natural depletion development for shale oil in Ordos Basin

The primary driving energies during the natural depletion development process in shale oil are rock and fluid elastic energy, as well as dissolved gas energy. The release times of these energies significantly affect the flow feature of oil, gas, and water in the pore throat space, thereby impacting the oil recovery. This study elucidated the microscopic migration mechanisms of oil under various driving energies through physical simulation experiments utilizing self-developed nuclear magnetic resonance (NMR) technology and high-temperature high-pressure micro-visualization methods (HTHPMF). Additionally, the contribution of different driving energies to recovery in the natural depletion development process was quantitatively characterized using the new material balance model and numerical simulation. The findings indicated that: (1) The oil flow in pore space exhibited three distinct stages of exploitation during the natural depletion development., namely the elastic drive stage, the transition stage, and the dissolved gas drive stage, with about 70% oil coming from the macro-pores. (2) The elastic energy of rock and oil, as well as dissolved gas energy in the transition stage were mainly driving energies during the nature depletion development process. Continuous dissolved gas drive stage was found to be unfavorable for reservoir development. (3) The reasonable bottomhole flowing pressure should be controlled to be near the bubble point pressure so as to maximize the utilization of various driving energies. This study provided a basis for formulating a reasonable production system for shale reservoirs during the nature depletion development process.