The influence of CO2 huff and puff in tight oil reservoirs on pore structure characteristics and oil production from the microscopic scale

Abstract

CO2 huff-n-puff can store CO2 and improve the recovery of tight oil reservoir. The asphaltene precipitation produced by the interaction of CO2 and oil causes damage to tight oil reservoirs. Based on the FE-SEM, CST, Nano-CT, HPMI and NMR, the microscopic classification standard is established. The experiments were carried out on different types of samples to quantitatively evaluate the influence of asphaltenes on pore structure and oil recovery. The results show that the asphaltene precipitation is positively correlated with huff-n-puff rounds and injection pressure. The asphaltene precipitation in type I sample is the highest. The asphaltene precipitation in type II sample is lower. The lowest asphaltene precipitation is in type III sample. In the CO2 immiscible state, asphaltenes are mainly deposited in larger pore throats. In the CO2 miscible state, the CO2 extraction effect is enhanced, the asphaltene deposits in the smaller pore throats. The overall oil recovery of the type II sample is the highest. The asphaltene precipitation, the blockage degree, the oil recovery interact and restrict each other. The oil recovery increases, the asphaltene increases, and the blockage degree of pore throats become more serious. Asphaltene precipitation causes damage to physical properties in reservoir, the porosity and permeability damage rates range from 1.35% to 3.17% and 2.17% to 8.24%, respectively. Asphaltene changes the wettability to lyophilic. The wettability reversal index ranges from 0.86% to 4.92%, which increases the seepage resistance of the oil phase. This research is novel for quantitative evaluation of reservoir microscopic pore throat characteristics and microscopic oil displacement mechanism by asphaltene precipitation formed during CO2 huff and puff in tight oil reservoirs. Also, the research results are of significance for optimizing the field development parameters.