Impact Of Injected Water On Kinetics Characteristics Of Methane Desorption In Coal

Injecting water into coal seam can increase the additional moisture of coal, and then themethane desorption characteristics of the coal will change. To research the change rule of coalmethane desorption properties after injecting water, based on the analysis of moisture effecton methane desorption, a device was developed for injecting water and the measurement ofdesorption under the high-pressure adsorption state. The device could stir the coal sampleswhen injecting water into coal, and it improved the wet effect of the coal sample.Through simulation for CH4and H2O molecules adsorption on the surface of coal andestimation for forces of CH4and H2O molecules on coal, the paper obtained the results whichthe adsorption ability of H2O on coal was bigger than the adsorption ability of CH4on coal.Coal desorption methane types were divided into cooling desorption, step-down desorption,replacement desorption and mixed desorption, and replacement desorption and step-downdesorption occured in the process of injecting water.Based on study the reasonable size of1-3mm for injecting water experiment, throughthe homemade device of high-pressure adsorption-injecting water-desorption,the methanedesorption of Yonghong coal (YH coal) and Gaojiazhuang coal (GJZ coal), which exhibited ahigh metamorphic degree, and Qinan coal (QN coal) and Dalong coal (DL coal), whichexhibited a moderate and a low metamorphic degree, were tested after the injection of water.The experimental temperature was30℃, and the experimental adsorption pressures were0.5MPa,0.84MPa,1.5MPa, and2.5MPa.The results show that injected water can displaceadsorbed methane, and the replacement amount and replacement ratio increase with theamount of injected water. the maximum replacement amount and replacement ratio areobtained from the YH coal, moderate replacement amount and replacement ratio are obtainedfrom the DL and the QN coal, and the lowest replacement amount and replacement ratio isobtained from the GJZ coal. Injected waters have the impacts on methane desorption quantity,desorption velocity, gas desorption index of drill cuttings, methane diffusion coefficient,desorption rate and methane initial diffusion velocity after the pressure relief,and the methanedesorption quantity,desorption velocity, gas desorption index of drill cuttings, methanediffusion coefficient, desorption rate and methane initial diffusion velocity are graduallydecreased with the increase of injected water. In the first40minutes, the impact of theinjected water on the methane desorption velocity is obvious; however, after40minutes, thiseffect can be ignored.The influence of the injected water on the methane desorption quantityand gas desorption index of drill cuttings are different for coal samples with differentmetamorphic degrees.The maximum effect is obtained from the DL coal methane desorption quantity, moderate effects are obtained from the QN and the YH coal methane desorptionquantity, and the lowest effect is obtained from the GJZ coal methane desorption quantity.The main reason of the difference is the total surface area of the macropores, mesopores, andpores in the different coal samples. The influence of the injected water on the gas desorptionindex of drill cuttings is maximal from YH coal. In contrast, the influences of the injectedwater on the gas desorption index of drill cuttings are moderate from the QN and the DL coals,and the influence of the injected water on the gas desorption index of drill cuttings is lowestfrom the GJZ coal. These differences are due to the ratio of the pore volume to the totalvolume.The comprehensive effects of injected water on methane desorption for different coalsare inconsistent from the experimental datas, and injected waters have promoting effect onmethane desorption for the high and moderate metamorphic degree coal, but injected watershave inhibition effect on methane desorption for the low metamorphic degree coal.Injected water can promote and inhibit methane desorption of coal seam from thenumerical simulation and engineering practice. The combined effects of injecting water intocoal seam on methane desorption have something to do with the changes of coal seampermeability and replacement adsorbed methane effect of water. When the decrease degree ofcoal seam permeability is smaller and replacement adsorbed methane effect of water is largerafter injecting water, the injected water will promote the coalbed methane desorption,conversely, it will inhibit coal seam methane desorption.