Asphaltene Deposition And Prediction During Carbon Dioxide Injection In Shale Oil Reservoirs

Shale oil reservoirs are extremely tight,and it is difficult for conventional development techniques to achieve expected economic benefits.As a potential effective development technology,CO₂ is increasingly used in shale oil.However,in the process of CO₂ injection,the compositional change of the crude oil in the formation may cause the deposition of asphaltenes,which may lead to the blockage of the formation pore throats and seriously affect the development effect.First,based on the cubic plus associative equation of state（CPA-EOS）considering the association of polar molecules,a mathematical model for predicting the deposition pressure of asphaltenes was established,and the model was solved and the module was compiled to realize the prediction of deposition pressure during CO₂ injection in shale oil.Secondly,a numerical model of shale oil is constructed for typical blocks,and the probability of asphaltene deposition in different areas（near production wells,intermediate zones and far-well zones）and different development stages（initial,middle and terminal stages of development）.The results show that the asphaltene deposition occurs in the early development stage of shale oil depletion,and the asphaltene deposition occurs later and later from the near-well to the far-well,but the duration becomes longer.Then,by constructing a numerical model of CO₂ injection in shale oil,the possibility of asphaltene deposition in different areas（near production wells,intermediate zones and far-well zones）and development stages（initial,middle and terminal stages of development）of CO₂ injection in shale oil is predicted.The results show that CO₂injection will expand the pressure range of asphaltene deposition and increase the possibility of crude oil asphaltene deposition.Injection pressure and fracture permeability mainly affect the deposition of asphaltenes by affecting the CO₂ content in the fluid,formation pressure and gas channeling time.Finally,the results of the established asphaltene deposition pressure prediction model are compared with the numerical simulation results.It is found that the initial deposition pressure of asphaltenes calculated by the mathematical model of asphaltene depositional pressure prediction is consistent with the upper limit of the asphaltene depositional pressure simulated by the software,which can be used to predict the possibility of asphaltene deposition during CO₂ injection in shale oil.The prediction model of asphaltene deposition possibility during CO₂ injection in shale oil established in this paper has certain theoretical reference significance for shale oil gas injection development.