Does the Presence of Pseudovitrinite Indicate Gas-saturated Coals? Some Interesting Observations from the Gething Coals in Canada

Abstract

The presence of pseudovitrinite at a depth of 1,000 m in the very gassy (up to 862 scf/ton total gas content, as-received basis) but low absolute permeability (0.5 mD), low-volatile bituminous Gething coals in the Canadian Foothills has been documented. Because oxidation is unlikely to occur at such depth, it is reasonable to expect that pseudovitrinite formed as a result of desiccation in a gas-saturated environment prior to the coals being uplifted to their present day depth. This raises the possibility that a coal that contains pseudovitrinite may have moisture content that is below its equilibrium moisture, which leads to higher methane adsorptive capacity compared with the same coal that has “normal” vitrinite (collotelinite). The presence of inertinite macerals in the coal, derived from wood fibers and charred remnants, has aided in the development and preservation of phyteral porosity and in the formation of interconnected microcavities, which should result in higher micropermeability and aid the flow of gas locally within the coal seam and surrounding strata. The Gething coals in the Highhat corehole share some of these characteristics, which may have important implications on the dynamics of coal-bed methane production. Volumetric strain (matrix shrinkage) of these gassy coals during production is conservatively estimated to be 0.5–0.75%, which may result in an absolute permeability increase of between 5 to 12 times, based on studies on coals of similar rank and gas content in United States basins. Although observations made in this preliminary study do not constitute a proof, they leave open the possibility of using pseudovitrinite, under certain circumstances, as an indicator of improved gas sorptive capacity and enhanced permeability in deep coals.

Keywords:

• adsorption
• Gething coals
• matrix shrinkage
• pseudovitrinite

Notes

^a 2.6%

^b 2% pseudovitrinite.