Variation of gas flow properties in coal with probe gas, composition and fabric: Examples from western Canadian sedimentary basin

Flow properties (permeability and diffusivity) of subbituminous to high volatile bituminous coals from the Horseshoe Canyon and Mannville formations of the Western Canada Sedimentary Basin have been investigated using both solid coal plugs and samples crushed to between 0.8 mm and 0.6 mm (20–30 mesh). The coals examined have contents of vitrinite between 65%–98%, inertinite up to 31% and rare liptinite macerals.Permeability of crushed coal ranges from 1.46 ∙ 10−5 md to 7.60 ∙ 10−3 md whereas coal plug permeability is between 0.38 md to 0.01 md. Average diffusivity of crushed coal is estimated to be on the order of 10−11 m2/s. This difference of up to four orders of magnitude between crushed and coal plug permeability is attributed to different stress conditions during sample testing and the influence of coal cleat and coal fractures on coal plug permeability. The permeability of crushed coal is influenced by coal matrix properties including maceral content and micro fabric. The coals with greatest amount of inertinite have the greatest matrix permeability and diffusivity due to the greater macro- and meso- porosity of inertinite. Increasing effective stress, with all other factors kept constant, leads to a decrease in coal plug permeability. Coal plug permeability declines exponentially with increasing effective stress which is attributed to the closure of permeability pathways due to compaction of coal at high effective stress levels.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.

► Average diffusivity of crushed coals is on the order of 10−11 m2/s.
► Coal matrix permeability is between 7.18 ∙ 10−5 md to 1.79 ∙ 10−2 md and coal plug permeability is up to 0.3 md.
► Plug permeability declines exponentially with increasing effective stress due to cleat and fracture closure.
► Greater coal inertinite content enhances coal diffusivity and coal matrix permeability measured on crushed coals.
► KHelium > Kmethane > KNitrogen for coal plugs.