Advanced Experimental Analysis of Gas Flow in Coal

Coal is considered as a dual-porosity rock that is highly heterogeneous and hosts complex and unique gas flow mechanisms. Coalbed methane can be stored in coal as free gas and in adsorbed state. The gas flow mechanisms with deformability of coal due to the change of stress during production from coalbed methane reservoirs have an extensive impact on reservoir characteristics. Because of the complexity of coupled pressure change and sorption phenomena, the understanding of the coal structure and gas flow in it is still limited. In this Thesis, X-ray micro-computed tomography (micro-CT) and gas flooding experiments are conducted to study flow mechanisms in the complex structure of coal. The properties that are discussed include fracture aperture, permeability and effective gas diffusivity using methane, helium and krypton. Our results show how imposing stress affects fracture aperture and coal permeability. There exists a difference between helium and methane permeability due to the adsorptive nature of methane gas. Effective diffusivity is analysed at different scales: pore-scale (micrometres) from the micro-CT images, particle-scale (millimetres) in crushed coal adsorption experiments and core-scale (centimetres) in bulk diffusion experiments. The results demonstrate that the diffusivities obtained at different scales are in the range of 10-9 to 10-11 m2/s, with the particle-scale diffusivity being the highest and pore-scale diffusivity being the lowest. The mechanisms associated with diffusion at these scales are discussed and compared. Overall, results of this Thesis provide insights into the complex gas flow mechanisms occurring in coalbed methane reservoirs specifically in terms of impact of sorption, gas pressure, stress and gas type on fracture aperture, permeability and effective diffusivity