CO2 Huff-n-Puff after Surfactant-Assisted Imbibition to Enhance Oil Recovery for Tight Oil Reservoirs

The oil recovery factor in tight oil reservoirs is quite low when compared with conventional reservoirs. CO2 injection and addition of surfactants in fracturing fluids to assist imbibition have been shown to be two effective and practical methods to enhance oil recovery of tight/shale reservoirs, which, for the former, achieves carbon sequestration as well. In this study, a series of imbibition and huff-n-puff (HNF) laboratory experiments were performed on tight oil samples to explore the possible combination of the two enhanced oil recovery (EOR) methods. After evaluation, a hybrid EOR strategy, CO2 HNF after surfactant-assisted imbibition, was proposed to enhance oil recovery in tight reservoirs. In the hybrid method, the imbibition process preferentially displaces the crude oil in relatively small pores, while the CO2 HNF process mostly produces the crude oil in relatively large pores. Imbibition and HNF compensate each other to recover the residual oil in the pore networks, which makes them a perfect combination to enhance oil recovery in tight reservoirs. In addition, the operation parameters including cycle number, injection volume, and soaking time of this hybrid EOR method were also examined, and the results showed that the optimization of these parameters is different from that in the conventional CO2 HNF operation. This work provides a promising hybrid EOR method and can be a good reference to evaluate operational parameters and improve the production performance for tight oil reservoirs.