测量天然气在稠油中扩散系数的新方法

A new measurement method for diffusion coefficient of natural gas in heavy oil

可靠的扩散系数测量方法对于稠油开发中气体注入方案设计、页岩气开发中流动机理研究和天然气成藏保存过程中气体运移方式的确定等都具有重要的意义。为此,提出了一种基于压力脉冲技术在稠油中测量扩散系数的新方法。首先通过压力脉冲实验来记录PVT容器空腔内的甲烷气体向稠油扩散过程中的压力变化,然后根据Fick第二定律和油气界面的动态边界条件建立数学模型,利用模型所得到的解析解对实验数据进行拟合得到相应的扩散系数。通过对数学模型的分析可以发现:在任一时刻空腔内还未流入油相中的气体物质的量与扩散达到平衡时流入油相中总的物质的量的比值,在时间较长时与无因次时间呈线性关系;与其他文献进行比较的结果表明,所建立方法的测量结果合理,并可推测出扩散系数随油相黏度的增加而逐渐减小;动态边界数学扩散模型符合实际气体扩散的物理过程,能准确地测量气体在稠油等有机质中扩散系数的大小。同时,所得到的新扩散系数测量方法还具有实验简便、操作简单、不用分析油相组分的优点。

Reliable diffusion coefficient measurement methods are significant in the design of gas injection program for heavy oil de- velopment, flow mechanism study in shale gas development and gas transfer mode determination in gas reservoir formation and pres- ervation. Therefore, a new method based on pressure pulse technology was proposed in this paper for measuring diffusion coefficient of gas in heavy oil. Firstly, based on pressure pulse experiments, the pressure variation was recorded while the methane diffuses to heavy oil in the chambers of PVT containers. Secondly, a mathematical model was established on the basis of Fick Second Law and oil-gas contact dynamic boundary conditions. And finally, the corresponding diffusion coefficients were obtained by fitting the analyt- ical solutions got from the mathematical model with the experimental data. It is indicated by the analysis on the mathematical model that in a longer period there is a linear relationship between the ratio （that of the amount of the gas that has not flown into the oil phase in the empty chamber at any moment and the total amount of the material that has flown into the oil phase the moment the dif- fusion is balanced） and the dimensionless time. Based on the comparative analysis with other literatures, the measurements got by the established method in this paper are reasonable. It can also be inferred that diffusion coefficient increases with the gradual de- crease of oil viscosity, and the diffusion coefficients of gas in organic matters （e.g. heavy oil） can be measured accurately by using dynamic-boundary mathematical diffusion model, which is in accordance with the physical process of real gas diffusion. Moreover, this method is so convenient and concise that it can be used to measure diffusion coefficients without analyzing oil compositions.