Study On Mechanism Of Supercritical Carbon Dioxide Enhancing Heavy Oil Recovery

For many years,the development of deep low-permeability heavy oil reservoirs has always been a difficulty and hot spot in the development of unconventional reservoirs.Compared with conventional oil reservoirs,heavy oil is difficult to develop economically and effectively because of its higher content of colloids and asphaltenes,high viscosity,easy precipitation,and poor fluidity.For these unmoved and difficult to move reservoirs,a variety of non-thermal development technologies and methods have been developed and explored at home and abroad.CO₂ flooding has become the fastest-growing method to enhance oil recovery.At the same time,it is not affected by reservoir depth and permeability.Due to the influence of other properties,CO₂ flooding can realize the effective development of deep-seated heavy oil.However,the characteristics and laws of CO₂ swelling and extraction of heavy oil are still unclear.Aiming at the micro-mechanism of CO₂ flooding heavy oil,this paper uses molecular dynamics simulation methods to reveal the structure and dynamics of heavy oil molecules and clarify the mechanism of CO₂ recovery of heavy oil.The main conclusions are as follows:Due to the presence of asphaltene self-aggregates and asphaltene-colloidal associations in the heavy oil system,when the temperature and pressure changes,relatively small aromatic hydrocarbon molecules affect the structure of the aggregates and associations,thereby affecting the viscosity of the heavy oil And liquidity.When CO₂ swells heavy oil,it not only occupies the gaps between oil molecules,but also separates the originally tightly bound molecules to form a new volume space,which makes the system expand.For heavy oil in a pure system,the order of CO₂ extraction is saturated hydrocarbons>aromatic hydrocarbons>gums/asphaltenes.Due to the association of asphaltenes and gums,asphaltenes and gums cannot be extracted by CO₂.When there is a sandstone surface,the interaction between asphaltene and the wall is greatest,but due to the mutual adhesion with other components,the distribution of asphaltene near the wall is not as good as the smallest aromatic hydrocarbon molecules.Saturated hydrocarbons have the least polarity and are distributed at the outermost periphery.For the heavy oil on the sandstone surface,the order of CO₂ extraction is saturated hydrocarbons>aromatic hydrocarbons>colloid/asphaltene,which cannot be extracted due to the association of asphaltenes and gums and the interaction between asphaltenes and walls.When CO₂ at different rates displaces the heavy oil on the sandstone surface,it has both dissolution and extraction effects and driving effects.At low rates,dissolution dominates.At high rates,driving dominates.When the content of the components in the heavy oil system is different,the yield stress is different.