| Cyclic Solvent Injection(CSI)is an important successor way in the late stage of cold heavy oil production with sand(CHOPS)during the heavy oil recovery.Viscosity reduction and foamy oil effect caused by solvent are the main recovery mechanisms.In this study,the flow mechanism of foamy oil in CSI is investigated through experimental observation,theoretical modeling,and numerical simulation.Firstly,a sand-packed visualization experiment was performed to directly observe the macroscopic flow characteristics of foamy oil with near-reservoir conditions.Secondly,aiming to provide a theoretical explanation for the experimental observation,a mathematical model of foamy oil flow was established and solved based on the production period of CSI process.The model couples pressure field and concentration field through viscosity and velocity,in which a pseudo-chemical reaction is used to describe the release of gas,and meanwhile the effects of foamy oil on pressure and mass transfer processes are considered.Finally,based on more reasonable assumptions,a mathematical model of four-component was established and solved by numerical discrete and fully implicit solutions.Observed phenomenon shows that the foamy oil in solvent huff and puff exhibits multi-wave flow characteristics in real reservoirs.Following conclusions are obtained by the analysis of simulation results.First,the non-equilibrium release of solvent in the transition zone causes the formation of foamy oil.As the solvent continues to release,the concentration decreases and the viscosity increases,forming a high-viscosity "hard shell" at the leading edge of the transition zone,which binds the escape of bubbles and slows down the diffusion of the solvent.Second,the presence of bubbles contributes to the buildup of local pressure gradient and flow velocity.When the threshold velocity is reached,the bubbles coalesce rapidly to form free gas,pushing the oil in the foamy region forward to the production well,thereby inducing a new round of foamy-oil flow.Third,the total waves of foamy-oil flow are generally 2 to 4,depending on the initial solvent concentration,oil viscosity,and pressure depletion rate.Higher initial solvent concentration,faster pressure depletion,and lighter original heavy oil lead to the easier induction of foamy-oil flow.Fourth,the numerical simulation is in good comparison with the experimental recorded data,which proves the accuracy of the model.Through the oil production analysis,the oil production of depleted production is divided into three stages:elastic driving period;foamy oil period;gas channeling period.The faster pressure depletion rate,larger critical gas saturation,slower rate of the free gas’ s formation result in higher cumulative oil production.The oil production of CSI presents a step-like shape,reflecting the multi-wave flow characteristics of foamy oil in porous media.The larger the diffusion coefficient,the larger the initial solvent concentration,the slower the formation of free gas,the more waves of foamy-oil flow,and the larger cumulative oil production. |
