Relative permeability of coal to gas and water is an important variable in coalbed methane reservoir development as it is required for reserve estimation and field production planning. In this study, gas-water relative permeability of two coal samples: Grande Prairie (GP) and Goldsource (GS) were determined by the Johnson-Bossler-Naumann (JBN) method, which is widely used for petroleum reservoir rocks.
An experimental apparatus was designed, built, tested, and commissioned for various two-phase gas-water relative permeability measurements. The gas phases used were helium, methane, and carbon dioxide in this order of adsorbing strength on coal and the water phase used was formation brine. Forty-eight drainage and imbibition displacement runs were carried out on the two coal samples using helium-brine, methane-brine, and carbon dioxide-brine combinations at three different operating pressures (100, 300, 400 psi) at a constant overburden pressure (800 psi), and the results were analyzed and plotted on drainage and imbibition curves.
Imbibition relative permeability showed that the irreducible water saturation correlated with pressure and the absolute permeability of the coal samples changed after each experiment. The GS coal sample showed a greater two-phase flow saturation range than the Grande Prairie coal sample. In both cases, the relative permeability to gas was low and high for water. The coal samples tended to become more water-wet at higher pressures in the case of adsorbing gases, namely methane and carbon dioxide but in the case of non-adsorbing gas helium, higher pressure prevented the inflow of water into the smaller pores leading to a decrease in irreducible water saturation with an increase in pressure. The drainage experiments were highly unstable based on the Instability Number calculated. The resulting relative permeability curves showed oscillation.
From this study, it was concluded that coal relative permeability to gas and water depended on the nature of gas, the operational pressure, and fluid-mineral interactions.
Coalbed methane is gaining increasing importance because of the growing emphasis on clean fuels on one hand and as a source of hydrogen on the other. Historically, natural gas price has been about one-half of the price of oil on the same energy basis primarily because of the difficulty of transporting gas. In Canada, natural gas is particularly important because of the production and upgrading of bitumen from oilsands. Gas consumption is about 3 Mscf/bbl of bitumen.
With the looming natural gas supply shortages, coalbed methane is becoming an important energy resource of Canada, U.S.A., Australia, and other developing countries. In the U.S.A., coalbed methane supplies about 8% of the total gas consumption of about 23 Tcf/year (1 Tcf = 1012 scf) and in Canada, about 1% of natural gas needs. It is believed that this resource would contribute significantly to future natural gas supply as the natural gas reserves dwindle and development of tight gas costs rise.