Analysis of the effects of coal facture shape factor on water seepage based on computerized tomography (CT) 3D reconstructed artificial fractures

Abstract

Seepage during coal seam water injection occurs in coal fractures, and thus the properties of the fractures directly affect the water injection effect. Yet, the current research on the influence of fractures with different shapes on seepage is mostly based on the construction of idealized models, which is different from the actual coal structure. Therefore, a structural model containing five types of fractures, spherical fracture, tubular fracture, slot-shaped fracture, slit-shaped fracture, and flat facture, is conducted in CT images of real coal samples based on computerized tomography (CT) scan, 3D reconstruction and artificial fracture. The water injection seepage in the model was simulated using the Navier-Stokes equation, and the relationships between the fracture parameters (equivalent diameter, volume, coordination number, and maximum Feret diameter) and shape factor were calculated. The results show that the seepage velocity distributions in the fractures of different shapes are different. The seepage velocity increases with the increase of shape factor with the spherical fracture excluded. Compared with those of the model without the artificial fracture, the cross-sectional seepage velocity and permeability of the model with artificial fracture added increase by 2.9893–11.9923% and 0.8058–3.0267%, respectively. The number of fractures gradually decreases with the increase of shape factor, but other parameters including equivalent diameter, maximum Feret diameter, and coordination number tend to increase. The increases in the maximum Feret diameter and coordination number can improve the model connectivity, which is conducive to the continuous advance of seepage. The research results are helpful to evaluate the contribution of different shape fractures to coal seepage and provide theoretical guidance for coal seam water injection engineering.