含甲烷水合物的石英砂渗透率实验和分形模型对比研究

含水合物的多孔介质渗透率是影响水合物开采的关键参数,多孔介质渗透率与水合物的饱和度密切相关。定量研究多孔介质渗透率随水合物饱和度的变化,对自然界中天然气水合物藏内渗流场的研究具有重要的理论价值。本文以平均粒径为139.612 μm的石英砂为多孔介质,采用稳态注水法测量在不同甲烷水合物饱和度（0 ~ 28.56%）下的石英砂渗透率,将实验数据与两种不同水合物赋存形式（颗粒包裹、孔隙填充）下的石英砂渗透率二维分形模型进行了对比。结果表明,石英砂渗透率比K_r随甲烷水合物饱和度S_h的增大呈现指数减小的趋势。当水合物饱和度低于11.83%时,渗透率比下降缓慢。而当水合物饱和度高于11.83%时,渗透率比下降迅速;当饱和度指数n = 12时,渗透率分形模型与实验数据吻合良好。通过分形模型与实验数据对比,发现当水合物饱和度低于11.83%时,甲烷水合物的赋存形式为颗粒包裹型。在11.83% ~ 28.56%水合物饱和度范围内,甲烷水合物的赋存形式为孔隙填充型。本研究成果量化了石英砂渗透率与甲烷水合物饱和度的关系,确定了含甲烷水合物的石英砂的渗透率分形模型的参数取值。

Comparative Study of Permeability Experiment and Fractal Model of Quartz Sands with Methane Hydrate

The permeability of porous media containing hydrate is a key parameter affecting hydrate production. The permeability of porous media is significantly affected by the hydrate saturation. The quantitative study on the change of the permeability of porous media with the hydrate saturation is of great significance to the fluid flow in the natural gas hydrate reservoir. In this study, quartz sands with the average particle size of 139.612 μm were used as the porous media, and the permeability of quartz sands under different methane hydrate saturations (0-28.56%) was measured by the steady state water injection. The two-dimension fractal model of the permeability of quartz sands with two hydrate occurrence forms (grain-coating, pore-filling) were compared with the experimental data. The experimental results indicated that the ratio of permeability of quartz sands K_r decreased exponentially with the increase of methane hydrate saturation S_h. When hydrate saturation was less than 11.83%, the ratio of permeability decreased slowly. However, when hydrate saturation was more than 11.83%, the ratio of permeability decreased sharply. When the saturation exponent had a value of n = 12, the fractal model of permeability fitted the experimental data well. Comparison between the fractal model and experimental data showed that the occurrence form of methane hydrate was grain-coating when hydrate saturation was lower than 11.83%. When the hydrate saturation was in the range of 11.83%-28.56%, the occurrence form of methane hydrate was pore-filling. It is concluded that this study quantifies the relationship between the permeability of quartz sands and the hydrate saturation, and determines the parameters’ values of the fractal permeability model of quartz sands containing methane hydrate.