Diffusion, mass transfer and oil mobilization mechanisms of CO2 huff-n-puff in Gulong shale oil reservoirs

To reveal the complex crude oil-CO 2 interaction mechanism and oil mobilization behavior during CO 2 huff-n-puff in shale-type shale oil reservoirs, CO 2 huff-n-puff experiments with on-line nuclear magnetic resonance monitoring were conducted on Gulong shale cores, combined with the prediction model of CO 2 dynamic diffusion coefficient, the flow mechanism and factors influencing oil mobilization during CO 2 huff and puff in Gulong shale oil reservoirs are studied, and the diffusion and mass transfer behavior of CO 2 in shale is investigated. The results show that at the injection stage, CO 2 invades into macropores near the injection end, and drives part of the crude oil to micropores in the deep part of the core. At the shut-in stage, the crude oil gradually reflows to macropores near the injection end and is redistributed in the core. At the production stage, the oil mobilization zone is gradually expanded from the production end (injection end) to the deep part of the core. The contribution ratio of produced oil from macropores and micropores is about 8 : 3 after production. The diffusion coefficient of CO 2 in shale porous media gradually decreases with the advance of diffusion front at shut-in stage. The better the porosity and permeability of core samples, the higher the CO 2 concentration at diffusion front, the greater the CO 2 diffusion coefficient, and the slower the diffusion decline rate is. Increasing the huff and puff cycles could effectively enhance oil displacement efficiency, though its impact on the crude oil mobilization zone remains insignificant. The crude oil in small pores of the small layer with undeveloped laminae is difficult to be produced during CO 2 huff and puff, and the oil recovery is only 12.72 %. The crude oil in macropores and micropores of the small layer with developed laminae can be effectively mobilized during CO 2 huff and puff, and the oil recovery can reach 39.11%.