EXPLORATION FOR OPTIMAL VALUE INTERVAL OF MINIMUM PORE RADIUS OF SANDSTONE BASED ON FRACTAL THEORY

Permeability is one of the most important parameters for evaluating the water conductivity characteristics of porous media, which is the macroscopic property of micro pore structure in coal reservoir. To obtain the microscopic pore geometry parameters, we performed scanning electron microscopy (SEM) on sandstone samples collected from Xingtai coal mines in Northern China. The image processing techniques were also applied for estimating different statistical parameters (e.g., fractal dimensions) and geometrical parameters. Then, fractal theory were introduced to investigate the influence of micro pore structure on permeability, with the hypothesis of pore-space geometry possessing fractal characteristics. At the same time, the minimum pore radius r_min interval acting on the seepage was obtained throughout the constraint of measured permeability values. This interval can be used as the optimal value interval of the minimum pore radius r_min when predicting the permeability, and we can obtain the corresponding cumulative percentage interval of pore radius. The results show that: 1) the value of minimum pore radius r_min of sandstone has a great influence on permeability. With the increase of minimum pore radius, its permeability value decreases sharply at first, then tends to be stable gradually, which is finally accompanied by an upward trend. 2) In compact sandstone, when the theoretical permeability value is equivalent to measured ones, the minimum pore radius r_min falls within a cumulative percentage interval of 15% - 25% of the pore radius, and the corresponding pore radius interval can be used as the optimal value interval of the minimum pore radius r_min when we calculate the theoretical permeability of sandstone. Meanwhile, the bigger permeability of sandstone, and the bigger cumulative percentage of pore radius that corresponds.