Shale oil recovery by CO2 injection in Jiyang Depression, Bohai Bay Basin, East China

Laboratory experiments, numerical simulations and fracturing technology were combined to address the problems in shale oil recovery by CO₂ injection. The laboratory experiments were conducted to investigate the displacement mechanisms of shale oil extraction by CO₂ injection, and the influences of CO₂ pre-pad on shale mechanical properties. Numerical simulations were performed about influences of CO₂ pre-pad fracturing and puff-n-huff for energy replenishment on the recovery efficiency. The findings obtained were applied to the field tests of CO₂ pre-pad fracturing and single well puff-n-huff. The results show that the efficiency of CO₂ puff-n-huff is affected by micro- and nano-scale effect, kerogen, adsorbed oil and so on, and a longer soaking time in a reasonable range leads to a higher exploitation degree of shale oil. In the “injection + soaking” stage, the exploitation degree of heavy hydrocarbons is enhanced by CO₂ through its effects of solubility-diffusion and mass-transfer. In the “huff” stage, crude oil in large pores is displaced by CO₂ to surrounding larger pores or bedding fractures and finally flows to the production well. The injection of CO₂ pre-pad is conducive to keeping the rock brittle and reducing the fracture breakdown pressure, and the CO₂ is liable to filter along the bedding surface, thereby creating a more complex fracture. Increasing the volume of CO₂ pre-pad can improve the energizing effect, and enhance the replenishment of formation energy. Moreover, the oil recovery is more enhanced by CO₂ huff-n-puff with the lower shale matrix permeability, the lower formation pressure, and the larger heavy hydrocarbon content. The field tests demonstrate a good performance with the pressure maintained well after CO₂ pre-pad fracturing, the formation energy replenished effectively after CO₂ huff-n-puff in a single well, and the well productivity improved.