Geometrical and topological characterization of Micro-fractures in coal seams for flow modelling

Natural micro-fractures in ultra-tight formations, such as shales and coal seams, provide key information about in-situ states of stress, geo-chemical reaction and fluid flow in past and present. By characterising micro-fractures in core samples, in terms of their connectivity, geometry and surface properties, we can therefore develop appropriate models to gain better understanding of the roles of open micro-fractures on the drainage of hydrocarbons from matrix pores and to design field treatments for mineral-filled fractures by forward simulations.

In this work, we developed a new characterization method to identify fractures in CT images and to characterize their geometric properties and surface roughness. First we improve the medial surface-based fracture representation by introducing a new set of skeleton points, especially including surface points with Euclidean distance and edge points with connectivity information. Ideal and real fracture models are used to verify our algorithm, and all results show good agreement between the topological structures of the fractures and flow path obtained from flow simulations. By combining Euclidean distance and geometric transformation, the width, orientation, and inclination angle of micro-fractures can be computed. Connectivity information at each and every edge point is obtained to enable the simulation of flow interaction between a fracture and the matrix appropriately. Moreover, as surface roughness could cause methane gas to be trapped in fractures, cross section of the edge points’ 26 neighboring voxels was introduced to quantify surface roughness of the micro-fracture. Overall, our new method improves micro-fracture characterization and the accuracy and precision of the pore-space reconstruction for gas flow modelling.