The Effects of Water Within the Coal Matrix on Gas Drainage

Gas drainage from coal is a complex process whereby gas, initially adsorbed within the coal matrix, becomes a free phase and migrates through the matrix into the cleat system and eventually into a drainage borehole. The effect of water in the cleat system on the gas migration process is well known and represented through the relative permeability relationship. However, the effect of water within the coal matrix on the gas flow process is very poorly understood, but it will potentially play a key role in the desorption and migration of gas. Furthermore, the water in the matrix may change the coal's geomechanical properties, which are important inputs for modelling gas drainage and outburst.

The objectives of this project were to investigate the effects of water in the coal matrix on gas desorption rate, coal shrinkage and coal's Young's Modulus. Laboratory studies were performed on a coal sample from the Bulli seam, Sydney basin , with the adsorption/desorption rate measured for both CH₄ and CO₂ at different water content levels in the coal sample. In addition, the swelling/shrinkage strains and Young's modulus of the coal sample were measured at the water content levels.

The experimental measurements of gas adsorption/desorption rates show that (1) water content in the matrix has a significant impact on the gas adsorption/desorption rate; (2) The impact of water content on gas adsorption/desorption rate is stronger for CH₄ than CO₂ on the coal sample studied. The experimental results also show that the coal matrix swells as the water content increases. Moreover the swelling strain shows a linear relationship with respect to the matrix water content before reaching the equilibrium water content level. Furthermore, the Young's modulus of the coal sample underwent a significant increase with decreasing water content , which may mean that the coal is hardening with the loss of matrix water. Coal hardening and softening is one of the indicators of zones prone to coal outburst and is a key parameter in modelling coal outbursts.

Modelling results show that gas adsorption/desorption on the core samples with respect to water content follows a bidisperse approach, which includes a fast adsorption/desorption step in the macropores and a slow adsorption/desorption step in the micropores. The results show that both the macro and micro diffusivities increase with decreasing matrix water content. The diffusivities for CO₂ are larger than those for CH₄ on the coal sample studied.

The results of this project show that water content has a significant impact on gas diffusion rates, coal swelling and Young's modulus. The next step with this work, although outside the scope of the current project, is to study the impact of water content with respect to coal rank to complete this research. The results will lead to the development of a model for gas drainage behaviour that allows for varying matrix water content, and thus improve the simulation of gas outbursts and gas drainage practices, leading to improved gas drainage strategies.