Study On Mechanism And Characteristics Laws Of Displacement Coalbed Methane By Underground Gas Injection

Inspired by the successful test of Enhanced Coal Bed Methane (ECBM) by means of gas injection, the gas injection technology was firstly applied to accelerate coal seam methane emission and drainage in underground coal mine. The adsorption– desorption laws of N2, CO2, CH4 and their mixture on coal have been researched; and a series of lab experiment of displacement adsorption and competition adsorption have be made. Before underground testing, based on Darcy's law, Fick's law and Langmuir's law, a set of numerical models were established under multi-physics field coupling conditions, and coal seam gas injection was simulated make use of The Comsol Multiphysics software. On this basis, a set of technical study about injection gas in coal seam to displace coal methane underground has been made, and the displacement effect of"gas-injection as methane releasing"and"gas-injection as drainage"in roadway head, road wall drill shack and roadside hole alone coal seam has been researched. Finally, the mechanism of displace methane by gas injection has been proposed.A special experimental equipment for big quantity coal saples adsorption and desorption to multicomponent gas has been developed, and the problem, which the gas phase component change continually in the sampling process has been solved by separate sampling method used a gas sample storage vessel. Based on the equipment, two experimental methods of competition adsorption on multicomponent gas mixture and displacement adsorption for gases displacing methane in coal has been established. The results show that the adsorption constant a values of coal sample to CO2, CH4 and N2 weae 63.6943 m3/t, 41.1523 m3/t and 16.5837 m3/t, the absorbability descending sequence of the three gas is CO2, CH4 and N2. Two experiments showed the seam laws between displacement adsorption and competition adsorption. It indicated that the gases adsorption has nothing to do with the order of adsorption of every component, it only relates to start and end condition of adsorption and component of mixture.Displacement and replacement of coal seam gas incection is a complex movement with mass exchange under the multi-physics field coupling conditions of permeation, diffusion, and displacement adsorptiom-desorptiom. Based on Darcy's law, Fick's law and multicomponent gas Langmuir's law, a series of multi-physics field coupling mathematical model has been established, the gas injection numerical simulation has been made. The results were indicated that methane flow quality in bore hole made a significant increase after gas injection, which the increase multiple of N2 injection is 29.5 times, and that of CO2 injection is 37.54 times. With the gas drainage, the increase multiple of N2 injection is 8.34 times, and that of CO2 injection is 10.10 times.The two testing of gas injection-nature emission and gas injection-drainage has been made in head-on of coal drift face, coal roadside wall and drill shack. The site test result indicated that the effective influence radius of gas injection is 1.2m to 2.0m for the testing coal seam, and the radius was influenced greatly by pressure and flow of gas injection. With compared to nature methane emission, the pure methane flow quality of bore hole is increase to 31.56~140.93 times under air injection and nature emission in tunneling face head-on. Correspondingly, the methane flow quality is increase to 1.63~2.06 times under air injection and drainage. With the same drainage methane quality,the drainage time of drainage only is 19 days, that of air injection and drainage only need 9.2~11.6 days. The pure methane flow quality is increase 4.54~24.57 times on the condition of air injection and drainage in roadside hole alone coal seam. When injected pure N2 and CO2 into coalbed use high-pressure cylinders, the methane emission effect of pure gas injection is better than that of air injection as viewed from methane quality of coal sample. But from the macro point of view, pure gas injection effect is no better than that of air injection because of the limit of quality and period of injection.The mechanism of gas injection to accelerating methane emission and drainage are the action of displacement adsorption of injection gas, the action of carrying of injection gas to coal seam methane, the action of dilution and diffuse, and the action of expansion and increased permeability. The action of carry and drive of injection gas is main role, and the action of competition and replacement adsorption is played secondary role.