Optimization Strategy to Reduce Asphaltene Deposition-Associated Damage During CO2 Huff-n-Puff Injection in Shale

CO2 huff-n-puff injection has been proved an efficient and applicable method to enhance oil recovery in shale. However, the asphaltene precipitation and deposition in such injection process have not been considered in previous studies. In this paper, a numerical reservoir simulation method was applied to mimic CO2 huff-n-puff injection process and CO2-associated asphaltene precipitation and deposition in a hydraulically fractured shale oil reservoir. Effects of CO2 huff-n-puff injection operational parameters including huff pressure, puff pressure, huff time, and puff time on asphaltene deposition and associated oil production loss were examined in detail. The numerical reservoir simulation modeling study provides a better understanding of the physical mechanisms and key parameters affecting the asphaltene deposition and the oil production loss during CO2 huff-n-puff injection in hydraulically fractured shale formation. The results show that the asphaltene precipitation and deposition behaviors in the rock matrix and fracture network are different, resulting in a difference in permeability reduction. In the fracture network, most of the asphaltene precipitation and deposition is formed during the puff period, while in the rock matrix, the asphaltene precipitation and deposition is formed during both the huff period and the puff period. It was found that optimization strategies such as reducing huff time and increasing the puff time are favorable for decreasing the reduction in oil recovery caused by asphaltene deposition; although higher huff pressure causes more reduction in oil recovery by asphaltene deposition, the oil recovery by higher pressure is still higher; thus, a higher huff pressure is preferred; similarly, although a lower pressure causes higher reduction in oil recovery by asphaltene deposition, the oil recovery by lower puff pressure is still higher; thus, a lower puff pressure is preferred. In summary, the simulation results show that the optimized principles proposed by Sheng (Petroleum 3:431-437, 2017) in the huff-n-puff CO2 injection are not changed by including asphaltene deposition. In the simulated reservoir case, the huff time and puff time should be 100days and 200days, respectively.