Microscopic production characteristics of crude oil in nanopores of shale oil reservoirs during CO 2 huff and puff

The parameters such as pore size distribution, specific surface area and pore volume of shale rock samples are analyzed by low-temperature nitrogen adsorption experiment, and then the conversion coefficient between relaxation time (T2) and pore size is calibrated. On this basis, NMR experiments of CO2 huff and puff in shale samples are carried out to study the effects of gas injection pressure, soaking time and fractures on the oil production characteristics of shale pores from the micro scale. The the recovery degrees of small pores (less than or equal to 50 nm) and large pores (greater than 50 nm) are quantitatively evaluated. The experimental results show that the recovery degree of crude oil in large pores increases rapidly with the increase of injection pressure under non-miscible conditions, and the effect of injection pressure rise on recovery degree of large pores decreases under miscible conditions; whether miscible or not, the recovery degree of crude oil in small pores basically maintains a linear increase with the increase of injection pressure, and the lower size limit of pores in which oil can be recovered by CO2 decreases with the increase of gas injection pressure; with the increase of soaking time, the recovery degree of crude oil in large pores decreases gradually, while the recovery degree of crude oil in small pores increases first and then decreases, and the best soaking time from the experiments is about 10 h; the existence of fractures can enhance the recovery degrees of crude oil in small pores and large pores noticeably.