Dynamics of coalbed methane emission from coal cores under various stress paths and its application in gas extraction in mining-disturbed coal seam

Abstract

Uncontrolled and excessive gas emission may affect normal production and even result in serious disasters such as gas explosion and coal and gas outburst. To find out the essence of abnormal gas emission under mining induced stress, in this work, we conducted experiments on gas emitting from coal cores under mining-induced stress paths, and systematically discussed the controlling mechanism of stress change on gas emission. The results show that at the initial period, gas emission rate decreases regardless of the stress path. In this period, stress has little effect on gas emission, and it is mainly controlled by the attenuation of gas pressure gradient. In the middle period, the gas emission rate fluctuates with time. The tipping points of the gas emission rate in this stage is mainly caused by expansion of pores, opening of primary fractures and newly exposed surface induced by shear slip of the primary isolated fractures. With the further increase of the stress difference, failure of the coal core may occur, which creates a large amount of new exposure surfaces, and significantly improves the gas emission rate. Comparing with the minimum value before failure, the gas emission rate of PDS coal after failure increases by 82.35%–990.91%, and that of the JZS coal increases by 75.0%–175.0%. To verify the laboratory test results, in a recently completed research project, we implemented the technique of stress-relieved gas re-extraction in Shoushan coal mine in China. The results show that mining activity can affect the gas extraction in coal seams within 50 m from the coal wall of the working face. The average gas concentration of the boreholes drilled in the mining-disturbed zone is above 40%, and the average net gas flowrate of single borehole reaches 30 L/min. The gas extraction amount in the mining-disturbed zone, with borehole number less than 1% of the total, accounts for 55.1% of the total gas extraction amount in the roadway. This practice could be used as a valuable example for other coal mines with similar geological conditions.