Study on CO2 seepage characteristics and permeability enhancement effect of coal seam under supercritical CO2 condition

In order to study the permeability enhancement effect of supercritical CO2 on coal seam,a self-developed adsorption seepage test device is used to carr y out CO2 seepage and adsorption tests in coal seam considering the influence of injection pressure and temperature. The longitudinal wave velocity changes and the p ermeability changes of the specimens before and after the test are compared to analyze the anti-reflection effect of supercritical CO2 on raw coal specimens. The test r esults show that when the volume stress of the specimen is 36 MPa,the permeability decreases with the increase of temperature and increases with the increase of por e pressure. After the injection of supercritical CO2,the permeability of the raw coal specimen increases by an average of 93% than that of the injected CO2. The modifie d D-R model is used to fit the adsorption data,and it is found that the maximum adsorption capacity of coal on supercritical CO2 is around 8 MPa,and then decreased wi th the increase of pressure. When the temperature increased by 10。C,the adsorption capacity decreased by an average of 8.3%. When CO2 does not reach the supercr itical state ,the absolute adsorption amount of CO2 in coal is close to the excess adsorption amount and the change trend is consistent. When CO2 reaches the supercrit ical state,the excess adsorption capacity decreases with the increase of pressure,and the difference between absolute adsorption capacity and excess adsorption capa city increases with the increase of temperature. After the supercritical CO2 is applied,the propagation velocity of longitudinal wave in the specimen decreases significan tly,and the permeability is increased by 53.34% on average. It shows that supercritical CO2 can effectively promote the development of pores and fractures in the coal s ample,and has obvious anti-reflection effect on coal seam,with the best anti-reflection effect at35 °C.