CO_(2)在水—原油体系中的分子扩散规律及仿真模拟

Molecular diffusion of CO_(2) in water-crude oil system and its simulation

CO_(2)扩散具有促进原油膨胀降黏的特点,对提高CO_(2)驱油效果起到重要作用。然而,在常用的CO_(2)—水气交替驱中,目前对于CO_(2)传质扩散的研究未考虑水段塞对CO_(2)在水—原油体系中传质扩散的影响。针对该问题,本文开展了CO_(2)在水—原油体系中的扩散实验,考虑超临界状态的CO_(2)动态压缩因子,建立了CO_(2)扩散系数计算新模型。同时,研究了CO_(2)在水—原油中的扩散规律,并重点分析了水段塞、不同开始时间和对流对CO_(2)在水—原油体系中扩散规律的影响。结果表明:校正后的CO_(2)在原油和水中的扩散系数分别为1.17×10^(-9)m^(2)/s和0.44×10^(-9)m^(2)/s。CO_(2)在水—原油体系中扩散分为在原油中扩散和在水段塞中扩散。在原油中扩散时,分子扩散初期,短时间内受对流的影响,造成压力下降速度大,随后主要受浓度扩散的影响,压力下降缓慢;在水段塞中扩散时,分子扩散不受对流的影响,CO_(2)浓度差小,且CO_(2)在水中的溶解度小于在原油中的溶解度,使得压力下降慢,压力变化速率变小;不同开始时间,压力下降速度不同,CO_(2)的扩散传质速度随之发生改变。

CO_(2)diffusion can promote the expansion and reduce viscosity of crude oil, which plays an important role in improving the effect of CO_(2)flooding. However, the effect of water slug on mass transfer and diffusion of CO_(2)in water-crude oil system is not considered in the current study in CO_(2)water-alternating-gas flooding. To solve this problem, the diffusion experiments of CO_(2)in water-crude oil system were carried out. The dynamic compression factor of CO_(2)considering supercritical state was proposed, and a new model for CO_(2)diffusion coefficient calculation was established. The diffusion law of CO_(2)in water-crude oil system was studied also, and the influence of water slug, different starting time and convection on the diffusion law of CO_(2)in water-crude oil system were analyzed. The results show that the revised diffusion coefficients of CO_(2)in crude oil and formation water are 1.17×10^(-9)m^(2)/s and 0.44×10^(-9)m^(2)/s respectively. The diffusion of CO_(2)in the water-crude oil system can be divided into crude oil diffusion and water slug diffusion. In the crude oil diffusion, at the initial stage of molecular diffusion, it is affected by convection in a short time, resulting in a high pressure drop rate. After that it is mainly affected by concentration diffusion, resulting in a slow pressure drop rate. In water slug, the molecular diffusion is not affected by convection, the difference of CO_(2)concentration is smaller, and the solubility of CO_(2)in water is smaller than that in crude oil. So the pressure drops slowly and the pressure change rate becomes smaller. At different starting time, the pressure drop rate is different and the diffusion mass transfer rate of CO_(2)is different accordingly. To eliminate the influence of convection on the diffusion and mass transfer of CO_(2)in the initial diffusion stage, a reasonable start time should be selected.