The relative permeability behaviours of gas and water in coal are primary factors in the productivity of a coal seam gas reservoir, and it is dependent on many factors including fluid saturations and pressure, cleat geometry and network, and wettability (surface chemistry). In this study, we performed steady-state relative permeability measurements using X-ray CT scanner on packed beds of coal particles to allow systematic investigation of coal surface chemistry on permeability behaviour. Packed bed approach provides a homogeneous, isotropic coal sample with controllable coal surface properties; pore size and channel geometry (by control of particle size), and also removes the natural cleat geometry effects from the experimental measurement.
In this paper, we compare packed cores made with coals from two different locations within the Bowen Basin, Queensland, Australia. Both samples have the same rank (1% random reflectance), but with different maceral composition. The sample from Broadmeadow (BRDM) is vitrinite rich (76% mineral-matter free basis) and the sample from Isaac Plains (IP) is inertinite rich (61% mmfb) – both petrographic and proximate analyses were carried out. Samples were constructed using coal particles with size between 53 – 212 μm, resulting in a packed bed with porosity around 19%.
Core flooding experiments were conducted at a constant effective pressure of 22 bar followed by a sensitivity analysis using a hypothetical coal seam gas reservoir simulation model. The CT images were used to calculate water saturation based on the grey-scale without relying on the dead-end volumes of the core flooding system. The relative permeability curves suggest that the vitrinite rich coal (BRDM) had a more water-wet behaviour than the inertinite rich coal (IP), where the crossover point occurs at (krw=krg=0.45; Sw = 0.62), for the BRDM, and at (krw=krg=0.32; Sw = 0.44) for the IP. Hypothetical simulation using Eclipse shows that the cumulative water production for the IP can be 40% higher than the BRDM over a period of 30 years of gas production, due to its water wet behaviour.
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