The Study Of Diffusion Behavior Of CO₂ And Influential Factors In Tight Reservoirs

Due to the increasing energy requirement,tight oil has been hot topic in recent years.Tight reservoirs are ultra-low permeable,thus,common development techniques are inefficient in such oil reservoirs.CO₂ is the ideal injection fluid to tight reservoirs for its low viscosity and oil upgrading ability.Moreover,injection of CO₂ into reservoirs can decrease the emission of the greenhouse gas.For above reasons,CO₂ enhanced oil recovery has been main technology in tight reservoirs development.Molecular diffusion is the principle way for CO₂ to transport in reservoirs.However,most studys focus on diffusion phenomenon in bulk phase,and some mechanisms of CO₂ diffusion in porous media are still unclear.Therefore,studying the diffusion behavior of CO₂ in tight reservoirs and effects of various factors are necessary and significant.In this study,the CO₂ diffusion in tight reservoirs has been studied systemically by combining experiment and theoretical computation.A new model has been proposed by coupling Fick's diffusion equation and Peng-Robinson equation of state to describe the diffusion process in oil-saturated porous media.The effects of various factors on CO₂ diffusion coefficient have been studied,and inner mechanisms have been analyzed.Besides,a prediction model have been built on the basis of above researches,which can predict the CO₂ diffusion front location as well as the concentration profile in the diffusion region.According to the result of present study,the diffusion process of CO₂ in oil-saturated porous media can be devided into two stages,the initial stage with a high diffusion coefficient and the later stage with a lower one.Under the experimental conditions in this work,CO₂diffusion coefficients in the oil-saturated tight core lie in the range from 10×10^-10 m²/s to 100×10^-10 m²/s,whose value change with experimental conditions.The CO₂ diffusion coefficient increases with increased temperature or pressure,while the increase tendency slows down gradually.The increase of viscosity and heavy components of oil can enlarge the diffusion resistance,which decreases CO₂ diffusion coefficient greatly.The CO₂ diffusion coefficient increases with the permeability of the porous media until it exceeds 10×10^-3?m².The diffusion coefficient is not sensitive to permeability after it surpasses the threshold.According to the simulation result of LBM,the diffiusion process of CO₂ in porous media is influenced by the pore-throat ratio,the shape of transition region,and the connection way of throats.A prediction model has been proposed on the basis of above research,to predict CO₂diffusion process in reservoirs.The model can determine the location of diffusion front and the CO₂ concentration profile in the diffusion region.Moreover,the properties of oil in the diffusion region can be predicted by combining CO₂ concentration and oil property computation models.The prediction model can evaluate the effect of CO₂ on enhancing oil recovery and oil property improvement,and it can also provide theoretical guidance to modify field techniques.