Closing the gap between analytical and measured coal permeability

摘要

Coal porosity and permeability are key petrophysical parameters, pivotal in facilitating coalbed methane extraction and CO2 sequestration. Distribution of coal fractures, their association with coal stratigraphy, and their depth of occurrence primarily control coal porosity and permeability. The primary coal fracture networks, made of cleats, are the fundamental pathways for fluid flow through coal seams as matrix porosity and permeability are negligible. This paper documents the distribution of cleats and interrelationships amongst their attributes (aperture, spacing) as a function of depth and their role in controlling the porosity-permeability distribution of the Gondwana coals (Raniganj Formation) from India.The analytically computed effective permeability (based on effective aperture) estimations using the measured cleat parameters are more than two orders of magnitude higher than previously reported permeability values derived from geophysical log analysis, laboratory measurements and field tests. Such gaps between analytical results and measured data are also reported from multiple coal-bearing basins around the world. We, therefore, carry out an uncertainty analysis of measured aperture and used reported limits of cleat compressibility to estimate the range of possible permeabilities. We infer that the analytical solution combining the lower bounds of aperture distribution and cleat compressibility can be used as the closest approximation of effective permeability at pre-drill exploration stage of a coalbed methane project.