| According to the reality of large amount of residual coal and coal bed methane resources abandoned underground caused by the occurrence conditions in coal seams and the mining technology restrictions during the mining process and alleviating the pressure of the CO2emissions, combining with that the changed stress field induced by disturbance of mining residual coal and the complex construction composed of granular material in bulk and solid formed in residual coal have a obvious influence on seepage, diffusion, adsorption and CH4displacement-replacement during injecting CO2into coal seams, and considering temperature influence on CH4/CO2. adsorption-desorption, this paper carried out the non-isothermal seepage experiment of CO2, CH4/CO2single component and binary mixture non-isothermal adsorption experiment, CH4displacement-replacement experiment with CO2and its numerical simulation in coal seams during CO2injected with the combination method of theoretical analysis, experiment research and numerical simulation, the results on the displacement mechanism and migration law of CH4by injecting CO2into residual coal under coupled thermo-mechanical condition are as follows:(1) The CO2and CH4non-isothermal seepage experiment reveals the change rule of CH4/CO2permeability with pore pressure, coal sample volume stress and temperature, namely gas permeability satisfied the positive exponent change rule with the pore pressure change, satisfied the negative exponent change rule with the volume stress, the higher temperature the lower permeability of CO2and CH4, and the seepage rule of CO2and CH4in coal belongs to non-Darcy seepage, the permeability of CO2in coal is ten times larger than that of CH4in the same conditions.(2) The CH4non-isothermal adsorption-desorption experiment verified the adsorption desorption characteristics that in isothermal condition, adsorption and desorption on block briquette is reversible, the adsorption quantity and pressure satisfied the Langmuir equation and lag phenomenon appeared in desorption experiments; in different temperature conditions, adsorption and desorption change rule has regionality. Between10℃to30℃, the higher temperature, the more adsorption quantity gradient declined, adsorption quantity at10℃is4-5times larger than that at30℃when adsorption was balance; Between30℃to50℃, temperature change has little influence on adsorption quantity and desorption quantity. Fitted the CH4adsorption-desorption equation with Langmuir equation in different temperature conditions.(3) CH4/CO2non-isothermal adsorption experiment revealed the change rule of gas mixture adsorption quantity and desorption quantity with the change of pressure, temperature and the component proportion. In the same temperature conditions, gas mixture adsorption quantity change rule with pressure satisfied Langmuir equation, the more CO2concentration in gas mixture, the more CO2quantity, the adsorption ability to CO2is obviously more than that to CH4in gas mixture, the max CO2adsorption quantity is two times than CH4when adsorption is balance. In the same conditions, the adsorption quantity got from the experiment on briquette sample was less than that on coal powder. CO2adsorption rate and CH4desorption rate are concerned with the separation factor of different gas components to coal, the larger CO2separation factor relative to CH4, the more CO2adsorption rate and CH4desorption rate would be. Between30℃to60℃, temperature has less influence on gas mixture adsorption, the max adsorption quantity appeared at40℃in gas mixture with the same component.(4) Gas displacement experiment revealed the CH4displacement quantity influence rule with coal sample volume stress, pore pressure and temperature. In the same temperature conditions, the coal with more separation factor on CO2relative to CH4, the larger injecting CO2pore pressure, the longer injecting time and the less volume stress in coal seams, the better displacement effect; Between20℃-50℃, the displacement quantity change tendency is V20℃> V40℃> V30℃> V50℃under the same conditions, so20℃is the rational displacement temperature.(5) Established the diffusion, seepage, adsorption and desorption mathematical model of CH4displacement by injecting CO2in residual coal under thermo-mechanical condition, and simulated numerically, the results show that:CO2injected improved the CH4extraction rate through the double effects of declining CH4pressure in coal seams and competitive adsorption between CH4and CO2, and then improved the CH4recovery efficiency and realized the purpose of CO2underground storage; Meanwhile, the surrounding temperature change could also influence CH4desorption quantity, the higher temperature, the easier CH4desorbed, the simulation results above is consistent with the experiment. |
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