The Permeability of Coal to Gas and Water

Abstract

Abstract Air and water permeabilities of a large number of samples from the Pittsburghand Pocahontas coals were measured at various overburden and mean flowpressures. A wide variation (< 0.0 1 to > 100 md) in the air and waterpermeabilities was obtained for each type of coal, and flow was primarilythrough microfractures. Overburden pressure has the most significant effect onthe single-phase permeability. Considerable hysteresis was observed for bothair and water permeabilities. Gas permeabilities are affected to a lesserdegree by mean flow pressure above atmospheric. However, at subatmospheric meanpressures, appreciable increase in permeability occurs for low-permeabilitysamples. This was attributed to gaseous molecules desorbed at pore necks. At highoverburden pressure (~> 400 psig) water permeabilities are smaller than orequal to air permeabilities measured at the same pressures. Introduction The permeability of coal to gas and water is of interest to engineers in boththe mining and the petroleum industries. Much of the interest of the miningengineer stems from concern for the health and safety of the coal miner becausethe flow of methane into coal mines is one of the major causes of mine disasters in thiscountry. Some deep mines produce 10 to 15 MMscf/D of methane and require thecirculation of as much as 10 to 15 tons of air per ton of coal mined in orderto clear the gas from the mine. If this little-known source of natural gascould be produced from the coal before the coal is mined, it would help relievethe gas shortage as we11 as protect the safety of the coal miner. In recent years a number of petroleum companies have shown an interest in coalas a primary energy material that can be converted into electrical energy orinto gaseous and liquid products. Both above- and below-ground processes arebeing studied intensively. Knowledge of the basic permeability and relativepermeability of coal to gas and water should be very useful to petroleumengineers contemplating these new processes for the conversion of coal intoenergy forms suitable for the consumer. Although some studies of the permeability of coal to gas and to water have beenconducted, no gas/water relative permeability studies have been reported eventhough the presence of significant quantities of water is known to have amarked effect on the flow of methane from the coal seam. In this and a relatedpaper, we present the results of gas/water relative permeability studies onPittsburgh and Pocahontas coal samples.