碳酸水-原油体系中CO2分子的扩散行为

Diffusion of CO molecules in the carbonated water-crude oil system

CO2分子在油水相间的扩散传质行为对于致密油藏注CO2提高采收率具有重要意义。通过融合实验测定数据和理论扩散模型,利用试错法同时确定了碳酸水-原油二元体系中CO2分子在油、水两相中的扩散系数。重点研究了油水混合体系的初始压力对扩散系数的影响规律,深入探讨了扩散过程中油水相密度、CO2浓度、油水界面移动以及CO2在油相中扩散前缘位置的变化过程。研究结果表明:(1)随着CO2从水相扩散进入油相,碳酸水-原油体系压力增大,60℃下,扩散10 h,初始压力分别为15.39 MPa、19.25 MPa和22.82 MPa(实验1—实验3)体系,压力增加24%~31%;(2)在两相界面附近,水相密度及水相中CO2浓度逐渐降低,而油相密度及油相中的CO2浓度逐渐升高,导致油相体积膨胀,水相体积收缩,两相界面向水相方向移动;(3)初始压力越高,扩散系数越大。实验1—实验3水相和油相中CO2扩散系数的增幅分别为52.0%和9.2%,表明水相中CO2扩散系数对初始压力更敏感;(4)初始压力越高,相同扩散时间内CO2在油相中的扩散前缘运移越远,扩散100 h后,实验1—实验3前缘位置运移距离分别为4.11 cm、4.32 cm和4.43 cm。

The mass transfer by diffusion of CO2 molecules between oil and aqueous phases is of great significance to the enhanced oil recovery by CO2 flooding in tight reservoirs. Through combining experimental measurement data with theoretical diffusion model, this paper simultaneously determines the diffusion coefficients of CO2 molecules in oil and aqueous phases in the carbonated water-crude oil binary system using the trial-and-error method. This study focuses on the influence of initial pressure of the oil-water mixing system on the diffusion coefficient, and deeply explores the variations of the density of oil-aqueous phases, CO2 concentration, oil-water interface movement, and CO2 diffusion front position in the oil phase during diffusion. The results show that:(1)As CO2 diffuses from the aqueous phase into the oil phase, the pressure of the carbonated water-crude oil system increases. After 10 hours of diffusion at 60 ℃, the pressure is increased by 24% to 31% for the initial pressures of 15.39,19.25 and 22.82 MPa(Experiments 1 to 3).(2)Near the two-phase interface, the density of the aqueous phase and CO2 concentration in the aqueous phase gradually decrease, while the density of the oil phase and CO2 concentration in the oil phase gradually increases, causing the volume of the oil phase to expand and that of the aqueous phase to shrink, and the two-phase interface moves toward the aqueous phase.(3)The higher the initial pressure, the greater the diffusion coefficient. The CO2 diffusion coefficient in the aqueous and oil phase in experiments 1 to 3 is increased by 52% and 9.2%, respectively, indicating that the CO2 diffusion coefficient in the aqueous phase is more sensitive to the initial pressure.(4)The higher the initial pressure, the farther the CO2 diffusion front migrates in the oil phase within the same time period. After 100 hours of diffusion, the migrating distances of the front positions in experiment 1 to 3 are 4.11 cm, 4.32 cm and 4.43 cm, respectively.