CO_2混相与非混相共同驱极限井距计算方法

A computational method of critical well spacing of CO_2 miscible and immiscible concurrent flooding

基于非达西渗流理论,考虑原油黏度、油相及CO_2相启动压力梯度的变化,建立了CO_2混相与非混相共同驱渗流数学模型,用传质-扩散-吸附方程求解混相波及区内CO_2浓度的分布,用特征线法求解非混相波及区内饱和度方程,推导出特低渗透油藏CO_2混相与非混相共同驱极限井距计算方法。以F142和G89两个区块为例,进行极限井距计算,结果表明:(1)极限井距随注气压力的增大而增大,随注气速度的增大而减小;(2)纯CO_2渗流区长度对极限井距的贡献最大,CO_2-原油有效传质区域及非混相波及区长度对极限井距的贡献居中,纯油区的贡献最小,注气速度越大,纯CO_2渗流区与CO_2-原油有效传质区域及非混相波及区长度差距越小,随着注气速度减小,差距越来越大,此规律表现得更加显著;(3)混相波及区与非混相波及区的压力下降梯度有着显著的差异,混相波及区中CO_2-原油有效传质区域的压力下降梯度比非混相波及区大。

Based on the theory of non-Darcy seepage, a mathematical model for CO2 miscible and immiscible concurrent flooding considering changes of oil viscosity and threshold pressure gradient of oil and CO2 is established. A computational method of critical well spacing of CO2 miscible and immiscible concurrent flooding in ultra-low permeability reservoirs is deduced by solving the distribution of CO2 concentration in miscible flooding area with the mass transfer-diffusion-absorption equation and solving the saturation equation of immiscible affected area with characteristic line method. A critical well spacing example is built in the F142 and G89 reservoir blocks and the results show： （1） The critical well spacing increases with gas injection pressure, while decreases with gas injection speed; （2） The contribution of length in pure C02 seepage area to the critical well spacing is the largest, the contribution of length in CO2-Oil effective mass transfer area and immiscible affected area is secondary, the contribution of pure oil area is the least, the gap of length between pure CO2 seepage area and CO2-Oil effective mass transfer area and immiscible affected area decreases with gas injection speed and it increases with the decreasing of gas injection speed, meanwhile, the law is more significant; （3） Pressure drop gradient of miscible affected area is significantly different from that of immiscible affected area and pressure drop gradient of CO2-Oil effective mass transfer area in miscible affected area is bigger than that of immiscible affected area.