Investigation on Asphaltene Deposition Mechanisms during CO2 Flooding Processes in Porous Media: A Novel Experimental Study and a Modified Model Based on Multilayer Theory for Asphaltene Adsorption

In this paper, oil recovery and permeability reduction of a tight sandstone core sample in miscible CO2 flooding processes due to asphaltene deposition were studied using an Iranian bottom hole live oil sample in order to distinguish between the mechanical plugging and adsorption mechanisms of asphaltene involved in the interfacial interaction of the asphaltene/mineral rock system. A novel experimental method was designed and proposed to measure the amount of deposited asphaltene due to different mechanisms using the cyclohexane or toluene reverse flooding and spectrophotometer. In this work, the bottom hole live oil sample was injected first to a long core and then CO2 injection was performed which is close to reservoir conditions, whereas in the majority of previous works, the mixture of recombined oil (mixture dead oil and associated gas) and CO2 was injected in a short core sample which is far from reservoir conditions. Then, the cyclohexane and toluene reverse flooding was performed, and the amount of deposited asphaltene was measured by spectrophotometer. It was found that by increasing the flow rate of injected CO2, pressure drop across the core increased significantly and then decreased. These significant increases in pressure drops indicate more asphaltene deposition and consequently more permeability reduction. Also, it has been found that 20-40% permeability reduction by asphaltene deposition was caused by adsorption mechanism in the CO2 flooding process during a slow process, whereas 60-80% of formation damage is due to a mechanical plugging mechanism and takes place in a short time. Also, a modified model based on multilayer adsorption theory and four material balance equations (oil, asphaltene, light components, and water phase) was developed to account asphaltene adsorption in core sample during CO2 flooding and the model was verified using experimental data obtained in this work. The results show that the developed model based on multilayer adsorption theory and four material balance equations is more accurate than those obtained from the monolayer adsorption theory and two material balance equations (the existing models) and is in good agreement with the experimental data reported in this work.