Water flow simulation in coal for various cleat structures

Abstract

Gas drainage is an important process in underground coal mining. As coal matures and increases in rank, methane gas is generated and exists adsorbed onto the matrix micro pores held by pore water pressure. In order to remove the gas from the coal, the water pressure must first be reduced which is achieved in practice by dewatering.The permeability of a coal changes during gas drainage in two phenomena. As water is drained, the effective stress of the coal increases resulting in cleat closure and decreased permeability. As gas is drained from the coal micro pores, the matrix shrinks in volume and permeability increases (Moore, 2012). These opposing permeability phenomena have been the focus of research for many decades. The transition from analytical to numerical models has been a recent advancement in coal behavioural simulation. However, the coupling of phenomena is still an issue with many software packages.This research project aimed to investigate the behaviour of water flow in coal, particularly investigating the hypothesis that an increase in cleat aperture results in a decrease in velocity of water exiting those cleats. Focus of the project scope on dewatering and simulation of cleat aperture whilst neglecting gas flow resulted in achieving the aim of the study.A numerical model was constructed based on coal parameters from published experimental work that yielded values for Young’s Modulus and Poisson’s ratio for Sydney Basin coal (Pan, Connell and Camilleri, 2009). Other parameters for water and cleats were gathered from further literature research. From a 10 mm square coal block, two models were built: one featuring a single cleat and one with five evenly spaced cleats. Simulations of the single and multiple cleat models both showed that the velocity of water exiting the cleats decreases as aperture increases, proving the hypothesis correct.While the simulations showed that the analytical theory could be modelled numerically, it is noted that the project results represent an educational exercise. Due to the limitation of the applicability of the data, neither field nor experimental validation could not be performed. However, the research project highlights the numerous phenomena related to the gas drainage of coal and the potential of multi-physics numerical modelling to improve drainage techniques in the future.