Effects of matrix moisture on gas diffusion and flow in coal

Gas production from coal is a complex process whereby gas, initially adsorbed in the coal matrix, desorbs and diffuses through the matrix into the cleat and eventually flows through the cleat system into a production well or a drainage borehole. Hence, the gas production rate is mainly controlled by the gas diffusivity in the matrix and gas permeability in the cleat system. Moisture in the coal matrix has significant impact on gas adsorption capacity and would also play a key role in desorption and migration of gas. However, how moisture affects gas desorption and diffusion in the coal matrix is still poorly understood. In this work, experimental study is performed to investigate effects of moisture on gas sorption rate for an Australian coal. Coal seam gases, CH4 and CO2, are used in the study. The experimental results show that moisture content in the matrix has significant impact on the gas sorption rate and the impact of moisture content on the diffusion rate is stronger for CH4 than CO2. Moreover, the impact of moisture on gas diffusivity in pores with different size is different, suggested from the modelling results using the bidisperse approach. Furthermore, moisture in coal matrix would cause coal swelling/shrinkage and mechanical properties change that could impact on coal permeability under reservoir conditions. Experimental measurements of coal matrix swelling and Young’s modulus on the same coal sample show that matrix moisture content has significant impact on those properties and may have significant implications on coalbed methane recovery and CO2 storage in coal.