一种基于分子扩散理论的储层迂曲度测量方法

An Approach to Measure Tortuosity Based on Molecular Diffusion Theory

迂曲度是描述流体在多孔介质中渗流路径弯曲程度的重要参数,也是在实际测量中较难准确测定的参数。在迂曲度实验测量方法中电导率法与分子扩散法应用最为普遍,但电导率法测量的结果存在一定的争议,分子扩散法则更适用于气田开发实际。分子扩散法测试理论认为,迂曲度体现了多孔介质结构的复杂特征,而气体自扩散系数与有效扩散系数的比值表征多孔介质对气体扩散的阻碍效应,二者存在密切联系。以国内外参考文献中测量扩散系数的压力衰减法为基础,改进其计算公式,采用饱和原油的油砂模拟砂岩储层,通过测量甲烷在原油中的扩散系数和不同孔隙度、渗透率的油砂中的有效扩散系数,分析扩散系数、有效扩散系数、孔隙度和迂曲度之间的关系,计算出油砂的迂曲度。经对比分析,发现甲烷在油砂中的有效扩散系数与油砂孔隙度、渗透率呈正相关关系;油砂迂曲度与孔隙度、渗透率呈负相关关系。

Tortuosity is an important parameter to describe a bending deformation degree of seepage path flowing in porous media for fluids. However, it's more difficult to accurately determine this parameter. Two methods of both electrical conductivity and molecular diffusion are widely used for tortuosity calculation in lab.Molecular diffusion is more suitable for development of gasfield because tortuosity obtained from electrical conductivity method is a debatable point. For molecular diffusion, the testing theory points out that tortuosity reflects complex attributes of porous media structure whereas the ratio of self-diffusion coefficient vs. effective diffusion coefficient characterizes an obstruction effect of porous media on gas diffusion; and tortuosity has a necessarily close connection with the ratio. In this study, based on a method of pressure attenuation to calculate diffusion coefficient from reference, and we improved this calculation equation, such as adopting oil sand saturated with crude oil to simulate sandstone reservoirs at first, and then measuring the diffusion coefficient of CH4 in oil and the effective diffusion coefficient of oil sand with different permeability andporosity, thirdly analyzing the relationship among diffusion coefficient, effective diffusion coefficient, porosity, and tortuosity, finally calculating tortuosity of oil sand. Results show that the effective coefficient of CH4 positively correlates with both porosity and permeability whereas tortuosity negatively correlates with permeability and porosity.