Dependence of fluid flow on cleat aperture distribution and aperture\u2013length scaling: a case study from Gondwana coal seams of Raniganj Formation, Eastern India

Abstract

Dimensions and mutual relationships amongst fracture (cleat) parameters such as maximum aperture width, average aperture width, length, spacing etc. control the connectivity within a fracture network and the fluid flow in a coal seam as the matrix permeability, here, is negligible. In this paper, we document cleat size distributions and investigate length–aperture relationships from coals of Raniganj coalfield in Eastern India. This coalfield has a proven extractable reserve of six billion tons of coal and holds immense potential to be one of the largest coal bed methane fields serving India’s growing energy needs. Here, cleat length (L) correlates with corresponding maximum aperture width (Dmax) in a power-law function with an exponent of 0.84 (Dmax α L0.84) instead of the commonly observed exponents of 1 or 0.5 applicable for other natural ‘opening-mode’ fractures. The conventional wisdom pertains that laminar fluid flow (Q) through an isolated, smooth-walled, parallel-plate fracture, embedded in an impermeable matrix, is directly proportional to the cube of its aperture width (b, equivalent to Davg; cubic law: Q α b3). This assumes a linear relationship between length and fracture aperture. However, the modified relationship between cleat length and average aperture width changes the cubic law applicable for Raniganj coal seam and now fluid flow correlates with aperture width in a power-law function with an exponent of 4.25 (Q α b4.25) instead of 3 (cube). Such simplifications will come handy for the modeling and estimation of fluid flow as it will reduce the effort of cleat length measurement which is anyway difficult and can be misleading due to the risk of undersampling.