Key points are not available for this paper at this time.
Summary To investigate the mechanisms of the asphaltene precipitation in oil caused by CO2, the sandstone core oil displacement experiments and asphaltene structure observation experiments are designed in this work. The oil displacement experiments create CO2 flooding conditions under different pressures in heavy oil reservoirs and analyze the produced oil components and precipitated asphaltene proportions. Meanwhile, nuclear magnetic resonance (NMR) analysis is conducted on the sandstone cores to discuss the precipitation characteristics of asphaltene in the reservoir pores. The observation experiments analyze the microstructure of precipitated asphaltene after interactions between oil and CO2. The results show that the increasing pressure promotes the precipitation of asphaltene from oil by enhancing the dissolution and component extraction of CO2 in oil, which reduces oil viscosity and promotes reservoir development efficiency. This process also leads to an increase in CO2 sequestration in the reservoir. However, the precipitated asphaltene reduces reservoir permeability, hindering the optimization of the oil recovery rate. During the process of increasing pressure, the rate of increase in oil recovery decreases. In reservoirs containing oil with high asphaltene proportion, the oil recovery rate even decreases under high pressure. Additionally, in-situ targeted precipitation and retention of asphaltenes in large pores can reduce the distribution differences of pores with different sizes in the reservoir, weakening the above negative effects and enhancing oil recovery by regulating gas channeling. Moreover, the ratio of resin in oil affects the asphaltene precipitation form, and CO2 can promote the association of asphaltenes by weakening the steric stabilization effect of resin on asphaltene in oil, which makes the microstructure of precipitated asphaltenes dense and regular and promotes asphaltene precipitation and oil recovery increasing. This work aims to verify the advantages of CO2-induced asphaltene precipitation in improving the efficient and environmentally friendly development of heavy oil reservoirs, while exploring the significance of CO2 flooding in promoting carbon sequestration.
Gu et al. (Sun,) studied this question.