Prediction Method And Application Of Heavy Organic Matter Deposition During CO₂ Injection In Shale Oil Reservoirs

As a critical technology for the efficient development of shale oil reservoirs,CO₂-enhanced oil recovery technology is crucial to promoting CO₂ utilization and geological storage and achieving the national strategic goal of"carbon neutrality."Temperature,pressure,and oil composition changes will disturb the phase equilibrium of shale oil during CO₂ injection,which may induce heavy organic matter（asphaltene）deposition.It might impair the flow ability of shale oil and reduce well productivity.Therefore,it is urgent to investigate the mechanism of heavy organic matter deposition,establish the prediction method and characterize the flow parameters of shale oil during CO₂ injection.Representative organic and inorganic pore models were constructed based on the structural characteristics of shale matrix nanopores.Molecular simulation methods were adopted to reveal the mechanism of hydrocarbon components storage and heavy organic matter deposition during CO₂ injection in shale nanopores.The stable structure of colloid-asphaltene was destroyed by CO₂ extraction of light components in shale oil.Heavy organic matter molecules were associated by theπ-πstacking effect between aromatic cores,deposited,and adsorbed in nanopore walls of shale matrix.In addition,CO₂injection can replace part of the adsorbed methane and ethane,and the microporous space in the kerogen matrix is the main space for CO₂ geological storage.A modified Soave-Redlich-Kwong（SRK）equation of state（EOS）model was proposed to accurately characterize the phase behavior of nanoconfined fluid by considering the confinement effect,capillary pressure effect and the association between polar molecules in shale matrix nanopores.A prediction model for heavy organic matter deposition was proposed,the characteristic parameters and model were soluted,and a prediction method suitable for heavy organic matter deposition during CO₂ injection in shale oil reservoirs was established.When the pore radius was less than 100 nm,the confinement and capillary pressure effect on the fluid phase behavior must be considered.There is a critical pore radius around 2 nm that the molar volume of nanoconfined fluids changes sharply.Based on selecting shale oil samples from typical blocks to predict the risk of heavy organic matter deposition,the change of phase boundary and deposition amount of heavy organic matter induced by the association,confinement effect,and coupling effect was explored.The risk of heavy organic matter deposition in shale oil reservoirs during CO₂injection was quantitatively analyzed.The typical oil sample had the most significant heavy organic matter deposition risk by considering the coupling effect.When the CO₂injection volume exceeds ten mol%,more than 60%of heavy organic matter in shale oil will be deposited.Controlling CO₂ injection volume and avoiding rapid reduction of reservoir pressure were effective means to avoid the risk of heavy organic matter deposition.An apparent permeability model considering the boundary slippage effect was adopted.The mechanism of heavy organic matter deposition in different stages was considered to characterize the flow parameters in shale matrix nanopores based on flow enhancement.Heavy organic matter deposition’s effect on shale oil’s flow characteristics was determined.The intrinsic permeability was about 4-6 times higher than the apparent permeability in the pore size distribution of inorganic pores by considering the comprehensive effect of heavy organic matter deposition.At the same time,it was less than a third of the apparent permeability in the pore size distribution of organic pores.This paper reveals the microscopic mechanism of heavy organic matter deposition during CO₂ injection in shale oil,and the main space for CO₂ geological storage in shale reservoirs is clarified.A prediction method for heavy organic matter deposition in shale oil reservoirs during CO₂ injection is established.It can quantificationally predict the risk of heavy organic matter deposition in shale oil reservoirs and provide a theoretical basis for optimizing and evaluating the development program for CO₂ injection and CO₂geological storage in shale oil reservoirs.