Coal permeability prediction method based on the microscopic pore-fracture dual-porosity structure

Abstract

To better understand the effect of the pore-fracture dual-porosity structure on the coal permeability, the methods for calculating the pore size distribution as well as the fracture width distribution based on the Euclidean distance map function were given first. On this basis, a permeability estimation model considering pore-fracture structure was derived. Then, the pore-fracture structure and permeability of coal samples were quantitatively analyzed by scanning electron microscopy (SEM). By considering the pore size distribution and fracture width distribution characteristics separately, the complex pore structure of the coal sample can be more clearly defined. We found that both pores and fractures have their own distribution characteristics, but due to the effect of resolution, there is some influence in quantifying the minimum size distribution, especially in quantifying the pore size distribution. In predicting the permeability, the direct calculation using the pore and fracture size distribution is significantly larger than the experimental results (1.18✕10−14 m2 vs. 6.9✕10−16 m2 (measured) for the A1 and 1.22✕10−14 m2 vs. 8.1 ✕10−16 m2 (measured) for the A2 at ✕1000 magnification). However, by further considering the effect of tortuosity, more accurate prediction results can be obtained (1.14✕10−15 m2 for the A1 and 1.22✕10−15 m2 for the A2 at ✕1000 magnification). Fractures occupy less space, but they would cause higher permeability. The method proposed in the current study can be further applied to quantify the pore-fracture structure and permeability characteristics in microgeology.