Relative permeability of coal to gas and water is an important variable in coalbed methane (CBM) reservoir development as it is required for reserve estimation and field production planning. In this study, two coal samples (GP and GS) were used to determine the relative permeabilities using three gases (He, CH4 and CO2) at three different operating pressures (100, 300, 400 psi) and a constant overburden pressure of 800 psi. Both drainage and imbibition curves were obtained. Unsteady state displacements were used.
The petrophysical properties of these coal samples were also measured. The very low absolute permeability of the cores (0.07 mD) required special procedures to minimize errors. The experimental results presented here showed that the relative permeabilities were typically very low for all gases and high for water. The irreducible water saturation showed correlation with the type of gas (non-adsorbable He to strongly adsorbable CO2) and pressure.
It is concluded that coal relative permeability to gas and water depended on the nature of gas and the operational pressure. Relative permeability of coal changed also because of fluid-mineral interactions. It is concluded that the GP and GS samples tended to be more water-wet at higher pressures in the case of the adsorbable gases, viz. methane and carbon dioxide. However, in the case of non-adsorbing helium, higher pressure prevented the influx of water into the larger pores leading to smaller irreducible water saturation.