Asphaltene Precipitation Characteristics and Reservoir Damage When Applying CO2 Injection in Deep Reservoirs
Authors: Li Lei , Gao Xiaogang , Su Yuliang , Wang Mingjian , Wang Wendong , Tu Jiawei , Wang Xinhao 
Published: October 2023

Abstract
Asphaltenes are heavy aromatic hydrocarbon compounds contained in reservoir fluids and may precipitate when reservoir pressure is reduced by production or when gas is injected into the reservoir, and then further deposit on pore-throat surfaces causing reservoir damage. At present, the research on asphaltene precipitation and reservoir damage is carried out in conventional reservoirs, and the influence of CO2 injection under high-temperature high-pressure conditions has not yet been clearly understood.
In this work, the perturbed-chain statistical associating fluid theory (PC-SAFT) is applied to construct the asphaltene precipitation envelope of crude oil under the conditions of depletion development and CO2 injection development, and the precipitation conditions of asphaltenes during CO2 injection are studied. Based on the Nghiem solid phase model, the phase state simulation of asphaltene precipitation was carried out, the characteristics of asphaltene precipitation were quantitatively described. Further, through the asphaltene deposition core experiment during the CO2 injection process, the damage degree of the reservoir after asphaltene deposition and reservoir permeability levels was evaluated. Finally, the numerical simulation study was conducted after correcting the parameters to the experimental results. The asphaltene deposition distribution in the reservoir and its influence on the productivity were evaluated.
The results show that with the increase in the proportion of CO2 injection, the precipitation envelope of asphaltene expands, resulting in the earlier precipitation. While there is a crossover temperature near 140 ℃, when the temperature is lower than the crossover temperature, CO2 acts as an inhibitor. The precipitation of asphaltenes in the process of CO2 injection is the desorption of colloid-asphaltene inclusions caused by gas molecules, and then to the mutual polymerization process between dispersed asphaltene molecules. CO2 injection will increase the amount of precipitation and move the precipitation curve to the right side. The degree of permeability reduction caused by the deposition of asphaltenes in the core is 12.87% - 37.54%; the deposition of asphaltenes in the reservoir is mainly around the injection-production well and along the injected gas profile. Considering asphaltenes, the oil recovery degree is reduced by 1.5%, and the injection rate is reduced by 17%. The reservoir pressure, temperature and reservoir physical properties have a strong correlation with the degree of reservoir damage, while the initial asphaltene content has a low correlation.
The main contribution of this work lies in the combination of PC-SAFT calculation, experinments, phase state simulation, and numerical simulation to predict the asphatene precipitation with different pressure, temperature, and the amount of injected gas, and clarify the influence on reservoir permeability and oil production when using CO2 injection. This work will be of great interest to operators seeking to enhance oil recovery by CO2 injection in deep reservoirs.