数值弥散和物理弥散对CO_2混相驱替效率的影响

Influence of numerical dispersion and physical dispersion on displacement efficiency of CO_2 miscible flooding

为了明确CO2多次接触混相驱数值模拟过程中数值弥散与物理弥散对驱替效率的影响及其影响机理,建立了二维的平面多组分数值模型,并结合对流-扩散差分方程的数值弥散相关理论对计算结果进行分析。结果表明,数值弥散与物理弥散对CO2混相驱驱替效率的影响具有相似性,二者均能降低微观驱油效率、增加体积波及效率,但二者对驱替效率的影响机理不同。数值弥散使驱替前缘"模糊化",增大体积波及效率,同时降低CO2与原油的混相性,增大残余油饱和度;在小规模非均质地层中,对流弥散效应会扩大油气两相区,造成驱替前缘参差不齐,增大体积波及效率,同时两相区扩大也会降低CO2与原油混相性,降低微观驱油效率。

In order to understand the influence of physical dispersion and numerical dispersion on the efficiency of CO2 multi-contact miscible( MCM) displacement,2D multi-component numerical model is set up based on the laboratory experiments of actual reservoir fluid- CO2- crude oil system and the numerical dispersion theory of convection-diffusion differential equation. The calculation results show that the effects of physical dispersion and numerical dispersion on the efficiency of CO2 multi-contact miscible( MCM) displacement are similar: they all decrease the microscopic oil displacement efficiency and meanwhile increase the volumetric sweep efficiency. But they have different influencing mechanism: the numerical dispersion makes displacing front become fuzzy and therefore increases the volumetric sweep efficiency,at the same time,it decreases the miscibility of CO2with crude oil and therefore increases residual oil saturation; in small scale heterogeneous reservoir,the physical dispersion will enlarge the oil-water region and make the displacing front become irregular,and therefore increase the volumetric sweep efficiency,at the same time,the enlarging of the oil-water twophase region will decrease the miscibility of CO2 with crude oil and therefore reduce the microscopic oil displacement efficiency.