GAS FLOW THROUGH STRESSED COAL (FLUID FLOW, POROUS MEDIC).

摘要

The main objective of this research undertaking was to establish flow characteristics of gas through coal. To assess the flow behavior, the capacity of coal to hold methane, mechanism and rate of its release were also studied. The influence of applied stress--axial and hydrostatic-- and gas pressure on permeability and pore volume of coal were investigated.; The analysis of the observed flowrates showed that the permeability of coal reduced significantly with respect to applied stress, and that a hysteresis effect occurred during a loading-unloading cycle. This hysteresis effect was greatly diminished when the sample was loaded and unloaded hydrostatically, leading to the conclusion that non-uniform triaxial stress was primarily responsible for the hysteresis. On monitoring the volumetric strain during a loading-unloading cycle, a similar hysteresis was observed, which appeared to provide a good explanation for the variation in permeability. For the permeability-gas pressure study, a negative Klinkenberg effect--increase in permeability with increase in gas pressure--was observed at pressures above 500 kPa (72.5 psi). It was observed that the permeability of coal to methane is lower than that to nitrogen.; One other effect noted was that the process of evacuation to clean the sample of existing gases produced a significant decrease in permeability. A procedure to overcome this phenomenon was developed and used for all the experiments.; From adsorption tests, it was found that the capacity of coal to hold methane varies significantly with gas pressure and that the coefficient of diffusion is not a constant, but varies with gas pressure at which desorption is taking place.; Last of all, the pore configuration of coal was studied using a transmission electron microscope. At a magnification of 50,000, a sponge-like appearance was observed revealing the extensive internal surface area of coal that enables large quantities of gas to be adsorbed. This large pore volume interconnected by bridge like connections might also be the explanation for the reduction in permeability and pore volume with increase in stress--causing fracture of some of the connections.