A study of the relationship between the coefficient of permeability and microstructure of the Pliocene laterite

Pliocene(N₂) laterite is the key aquifer soil to actualize the coal mining with water protection and ecological environment protection in northwest China. Examination of the microstructure of laterite is helpful in analyzing the internal mechanism of the permeability change. In this paper, the SEM images of N₂ laterite with different permeability are preprocessed and binarized by using Matlab. The pore profile is obtained to get the micro-characteristics such as pore size, arrangement, shape, type, etc. The relationship between the coefficient of permeability of laterite and pore microstructure parameters such as the gray entropy, average aperture, oblateness, distribution fractal dimension and probability entropy are analyzed. The results show that there is no obvious relationship between the gray entropy and the coefficient of permeability, and the gray entropy ranges between 0.88 and 0.92. The average aperture is between 2.7 μm and 3.7 μm, and the coefficient of permeability increases exponentially with the increasing average aperture. The pore are classified as four groups of micro pore (< 1 μm), small micro pore (1~4 μm), medium micro pore (4~16 μm) and large micro pore (> 16 μm). It is found that the increase in the coefficient of permeability is mainly related to medium and large pore sizes. The oblateness varies between 0.54 and 0.57, the shape coefficient is between 0.63 and 0.75, and the pores are mainly oblate. The coefficient of permeability decreases exponentially with the increasing flatness, and decreases linearly with the increasing shape coefficient. The distribution fractal dimension of the pores ranges from 1.00 to 1.40. With the increasing distribution fractal dimension, the pores become smaller, and the coefficient of permeability decreases exponentially. The probability entropy ranges from 0.97 to 0.99. Pore arrangement is confusing and lacks obvious directionality.