Experimental study of core size effect on CH4 huff-n-puff enhanced oil recovery in liquid-rich shale reservoirs

Gas huff-n-puff injection, which avoids the viscous fingering phenomenon or gas channeling between connected wells, is a potential method to efficiently enhance oil recovery (EOR) in liquid-rich shale oil cores. In this paper, an experimental study of core size effect on the gas huff-n-puff EOR was implemented by using two groups of cores from the Wolfcamp field in West Texas. The first group contained core plugs with a length of 2", but different diameters varying from 1" to 4". The second group of core plugs had a diameter of 1.5", but differed in length from 1" to 3.5". CH4 huff-n-puff EOR experiments with an injection pressure of 2000 psi were conducted to analyze the size effects. CT scan was also implemented to check the oil saturation and cumulative oil recovery for each cycle. The results show that after eight huff-n-puff cycles, the oil recovery of cores with 1", 1.5", 2", 3", 3.5" and 4" in diameter are 49.65%, 48.57%, 47.76%, 46.15%, 43.55%, and 42.64%, respectively. The core plug with a larger diameter yields a relatively smaller oil recovery due to its lower apparent surface-to-volume ratio (AS/V) and smaller pressure gradient (Delta P/Delta r). The core plugs with 1", 2", 2.75", and 3.5" achieve almost the same oil recovery during the huff-n-puff process. It is demonstrated that core length has no effect on final cumulative oil recovery. The result also shows that the oil recovered in one cycle decreases as the huff-n-puff cycle number increases. The first cycle gives the peak incremental oil recovery. The obtained CT scan images together with the analysis of CT number reveal the density and oil saturation change inside the core and reflect the cumulative oil recovery during the gas huff-n-puff process. (C) 2016 Elsevier B.V. All rights reserved.