Experimental investigation on influencing factors of CO2 huff and puff under fractured low-permeability conditions

The fracture system is a vital component of fractured low-permeability reservoirs. The presence of fractures can improve reservoir flow capacity and injected carbon dioxide (CO2) utilization, thus leading to higher oil recovery. In this study, the effects of the presence of fracture, fracture morphology, soaking time, and CO2 injection volume on CO2 huff and puff were investigated through 11 low-permeability cores with different properties. The experimental results indicated that the presence of fractures enhanced cyclic oil recovery and increased the effective cycle numbers during CO2 huff and puff in low-permeability cores. Moreover, compared with low-permeability cores without fractures, ultimate oil recovery of CO2 huff and puff was risen up by ~11% in fractured cores. Longer soaking time was conducive to enhancing ultimate oil recovery of CO2 huff and puff in fractured low-permeability cores, but the excessive soaking time had little effect on ultimate oil recovery. Meanwhile, excessive CO2 injection volume did not significantly improve the performance of CO2 huff and puff, but it did reduce the CO2 utilization. Moreover, gravity caused the produced oil to deposit on the bottom surface of the blowout end of the fracture, which made oil recovery of the core with a horizontal fracture slightly higher (~7%) than that of the core with a vertical fracture. In addition, variation in the intersection angle of fractures had little effect on ultimate oil recovery of CO2 huff and puff in fractured low-permeability cores. It, however, did change the conductivity of the entire core, thus affecting oil recovery during the first two cycles remarkably.