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In recent years, CO2 huff and puff has become one of the most important methods developed for unconventional shale oil reservoirs and has been widely used in all major shale oil fields. However, the microscopic mechanism of CO2 contacting with crude oil is complex, and the change law of the residual oil occurrence after CO2 injection is unclear. In this paper, a micro visualization fluid flow simulation experiment (microfluidic experiment) under high temperatures and high pressure of a shale reservoir was conducted to reveal the micro mechanism of CO2 and crude oil after contact at the microscale. This allows conclusion of more precise results than any experiment conducted in a room environment. Combined with gas–oil two-phase micro flow characteristics, the production mechanisms of crude oil by CO2 huff and puff at the pore scale are clarified, and the change characteristics of the remaining oil occurrence state after CO2 injection are quantified. The results show that CO2 mainly produces crude oil in macropores and microfractures in the injection stage of huff and puff, improves the mobility of crude oil through diffusion dissolution in the soaking stage, and that the driving of dissolved gas is dominant in depressurization production. The major micro-mechanisms for CO2 to improve shale oil are extraction and dissolution expansion, accompanied by a variety of secondary mechanisms, such as the miscibility effect, oil expansion, viscosity reduction and other contact effects, as well as the improvement of crude oil properties. The simulation results of huff and puff development show that soaking is an important stage to enhance oil recovery. With increasing soaking time or the soaking pressure, the recovery degree of crude oil will increase positively.
Lei et al. (Wed,) studied this question.