Experimental study on characteristics of replacement and displacement of CH4 at different CO2 injection temperatures

The gas injection temperature is the key technical parameter in the field of enhanced coalbed methane drainage engineering by gas injection. In order to study the influence of different CO2 injection temperature on CH4 displacement,the self-developed CO2 replacement-displacement CH4 experimental system was used to carry out displacement CH4 experiments at 40,50 and 60 ℃ for CO2 injection temperature,to study the changing rules of coal seam temperature,gas concentration,displacement efficiency,CO2 storage volume and CH4 desorption volume in the process of CO2 replacement-displacement CH4. The results show that gas injection temperature is a key factor affecting the amount of CH4 desorption and CO2 storage. Increasing the gas injection temperature can displace more CH4 and store more CO2. The higher the CO2 injection temperature,the shorter the breakthrough time of CO2(107,73.8 and 57.5min,respectively),the shorter the time required for the outlet CO2 concentration to reach 99.99%,and the larger the volume of CH4 desorption and the CO2storage in the same gas injection time. When the temperature of injection rises from 40℃ to 60℃,the cumulative amount of CH4 desorption in coal seam is 6.43,6.57 and 6.71cm3/g,respectively,and the CH4 production rate increases by 2.1%～4.3%. The cumulative volume of CO2 storage in coal seam is 17.39,17.99 and 17.41cm3/g,respectively. In the process of gas injection displacement for CH4,CO2 adsorption in coal seams plays a major role. With the increase of gas injection time,the temperature of coal seams increases,which can be divided into stages of slow temperature increase,rapid temperature increase,and slow temperature decrease. And the slow temperature decrease stage is obviously "lagged" with the increase of CO2 injection temperature,which shows the effect of "gradual displacement"，and the maximum change of coal seam temperature is 10.5,11.4 and 11.3℃.The migration rate increased with the increase of CO2 injection temperature(0.006 53 cm/s,0.007 81 cm/s and 0.008 86 cm/s,respectively),and the migration rate increased by 19.6%-35.7%.Under the same injection temperature,the displacement efficiency gradually increases with the increase of gas injection time,and the higher the injection temperature,the more obvious the displacement effect is in the same gas injection time,which are 88.49%,90.93% and 91.7% at the end of the experiment,respectively,and the displacement efficiency can be increased by 2.44%～3.21%.