Fracture characterisation of coal and its implication on coalbed methane development

Network of naturally occurring fractures provides an important parameter for fluid flow modelling and coalbed methane (CBM) production potential prediction. This paper presents a detailed study of coal fractures identified in coal samples recovered from five coal mines in Weibei Coalfield, China. Endogenetic fractures, including condensed fracture and static pressure fracture, and exogenetic fractures, including tensile, compression, shear and relaxation fracture, were found based on scanning electron microscopy (SEM). The heterogeneity of coal fractures reflected by X-ray computed tomography (CT) numbers was calculated, and the three-dimensional view shows that fractures resemble 'K' type. The nuclear magnetic resonance (NMR) test was conducted to understand the connectivity. Three types of transverse relaxation (T2) time spectrum were classified, corresponding to micropores, mesopores and microfractures. The third spectrum P3, representing microfractures in trimodal T2 spectrum type disappeared after being centrifuged and indicated the significant role of fractures in coal permeability. Results, using the same source samples, show the fracture planar morphology and its origin by SEM, the stereoscopic shape by CT, and the fluid flow ability by NMR. [Received: July 8, 2014; Accepted: February 3, 2015]