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Effects of coal composition and fabric on porosity, sorption capacity and gas flow properties in Western Canada Sedimentary Basin coals

University of British Columbia

Porosity, methane sorption capacity, diffusivity and permeability of a suite of vitrinite-rich coals from the Horseshoe Canyon and Mannville formations of the Western Canada Sedimentary Basin were investigated. Coal rank ranges from subbituminous to medium volatile bituminous, equilibrium moisture is between 2.32%-23.75%, and ash is up to 72% although < 20% on average. Total coal porosity estimated using mercury porosimetry and helium pycnometry is between 4.4% and 18%. Helium pycnometry porosity is higher than mercury porosimetry porosity because the smaller molecular diameter of helium allows it to access coal pores which are inaccessible to mercury at test pressures. Greater vitrinite content is generally correlated with higher coal total pore area due to the abundant microporosity in vitrinite. Coal methane sorption capacity is up to 23.5 cc/g on a moisture equilibrated basis and is up to 40.4 cc/g for dry coals. Moisture equilibrated and dry coals sorb differently due to competition for adsorption sites in coal between methane and moisture. No relationship is observed between sorption capacity and coal rank or between maceral content and sorption capacity because of the narrow rank and maceral composition of the samples studied. Permeability was investigated on crushed coals and plugs with crushed permeability not exceeding 1.79∙10⁻² md while plug permeability is up to 0.9 md. Average diffusivity is estimated to be around 10⁻¹¹ to 10⁻¹² m²/s. Coal matrix properties influence crushed permeability. Inertinite-rich coals have higher matrix permeability and diffusivity because of the greater macro- and meso- porosity of inertinite. Plug permeability is dependent on coal matrix properties and the presence of fractures on tested plugs. Coals with better developed fractures are more permeable than coals with poorly developed fractures at the same effective stresses. Probe gas type influences plug permeability. Helium permeability measurements are higher than permeability measured with methane or nitrogen. Permeability difference with probe gas is attributed to a combination of different probe gas molecule size, relative swelling effects of probe gas on coal and associated changes at in-situ stress during tests. Understanding the reasons for permeability variations in coals will help in more focused coal bed methane exploration and development.

Text

2011-02-09

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/31137

Geological Sciences

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/