Experimental And Numerical Simulation Study Of CO₂ Huff-n-puff Process In Low Permeability Oil Reservoirs

CO₂ huff-n-puff process is an effective technique for enhanced oil recovery(EOR),which can be applied to low permeability oil reservoirs faced with poor natural productivity and low water injectivity.In this study,the EOR mechanisms of CO₂ huff-n-puff process and its influencing factors are studied through laboratory experiments and reservoir numerical simulations,and the results will be helpful to field project design and operation optimization.A series of CO₂ huff-n-puff experiments have been conducted utilizing sandpack model and oil samples from target ultra low permeability reservoir,and the effects of different parameters have been investigated,including cyclic gas injection quantity,soaking period,oil propertis and etc.,which can provide some basis for field operation design.The EOR mechanisms of CO₂ huff-n-puff process have been explored based on the oil recovery performance and variation of oil compositions before and after CO₂ injection.A comprehensive PVT model for CO₂ and target crude oil systems was established based on the original compositions of formation oil and the measured PVT properties.Combining with relevant empirical correlations,the interactions between CO₂ and crude oil were investigated,including the variation of phase envelope after CO₂ injection,the oil swelling and viscosity reduction effect and the minimum miscibility pressure(MMP)between CO₂ and crude oil,and the effect of CO₂ diffusion in oil phase was discussed.According to the petrophysical properties of target oil formation,a comprehensive numerical simulation model was established,and a series of numerical simulations were conducted to study the effects of reservoir heterogeneity,reservoir static properties and operations conditions on oil recovery performance.The experimental results demonstrate that,cyclic gas injection quantity,soaking period,formation temperature and oil viscosity all have significant effects on oil production performance.With the increase of cyclic injection quantity,the oil recovery factor increases but the CO₂ utilization factor decreases.With the increase of soaking period,the oil recovery factor and CO₂ utilization factor firstly increase and then decrease,and there is an optimal soaking period.The oil recovery factor and CO₂ utilization factor both increase with the increasing formation temperature.The performance of CO₂ huff-n-puff for light oil are much better than that for heavy oil.The injected CO₂ will extract the light components in crude oil,and thus a vapor recovery unit can be used to capture the lighter stripped hydrocarbons in a light oil CO₂huff-n-puff process.The PVT analyses show that,after CO₂ injection,the dissolution of CO₂ in oil phase can promote oil swelling and viscosity reduction,and the oil and gas two-phase region will increase,illustrating that solution gas drive can contribute to oil production.The simulation results show that,the advantage of CO₂ huff-n-puff is more obvious than primary depletion in heterogeneous model,compared with homogeneous model.The diffusion of CO₂ in crude oil is helpful to improve the oil recovery,and if the CO₂ diffusion in oleic phase is weak,the contribution of diffusion to recovery will be low.Low bottom hole pressure is beneficial to CO₂huff-n-puff.With the increase of cyclic injection quantity,the oil recovery factor increases but the CO₂ utilization factor decreases.There is an optimal soaking time,it is necessary to consider the effects of diffusion of CO₂ and production time,when determining the soaking time.