Heterogeneity effects on the storage and production of gas from coal seams

摘要

This paper presents the results of the experiments conducted on two structurally different coals from Turkey to investigate the effect of different heterogeneities on gas flow and storage in coal beds. Gas flow experiments were conducted on 3.8 cm diameter core plugs of these coals using xenon as the gas phase. Dual energy CT imaging technique was used to visualize the flow and storage of gas along the coal core to get qualitative and quantitative data from the images. After storage experiments end, nitrogen injection was performed to recover the stored gas from coal to represent an enhanced gas recovery operation. An analytical solution to this phase of one specific experiment was proposed to be able to calculate the diffusion coefficients from the gas recovery data calculated from CT images. The results of the experiments were analyzed to show the storage capacities of these two coals. The effect of different coal structures on the gas transport and storage and the matrix/fracture interactions were discussed by making a pixel based analysis and coupling it with the microstructure of coal investigated with scanning electron microscopy (SEM). Results show that, if the coal is structurally heterogeneous in the sense that there will be different layers of mineral and organic matrix, there will be different zones that affect the storage and transport of gas. Due to these heterogeneities, gas recovery by nitrogen injection is not efficient at some regions. On the other hand, the coal that is more homogeneous has a higher storage capacity due to its microporous structure. In this coal, even if there are mineral fillings in the micropores, such mineral inclusions do not have an adverse effect on gas stroage and transport as evidenced from CT images.