The influence of negative pressure during gas extraction in low-permeability coal seams on the effect of gas extraction

In order to improve the extraction method of gas extraction in coal seams with low permeability, the quadratic polynomial formula between the amount of extracted pure gas and negative pressure during gas extraction (NPGE) can be deduced based on the basic theory of oil and gas seepage mechanics in porous media and the pressure function theory of distribution of gas pressure. By using the measurement of orifice flowmeter, the formula of the amount of extracted mixed gas can be acquired and then the expression between the gas extraction concentration (GEC) and NPGE is deduced. Thus, the equation set concerning gas extraction effect is composed of three equations: the equation between the amount of extracted pure gas and NPGE, the formula for calculating the amount of extracted mixed gas, and the formula of relationship between the GEC and NPGE. According to the equation set, the GEC and the amount of extracted pure gas are calculated to further draw the scatter diagram and trend line of function. Afterwards, by analyzing the calculation result and trend line of functions, it can be seen that the increase amplitude of GEC is limited by improving the NPGE. By observing the on-site NPGE of the 20109 Wangjialing Coal mine (Shanxi Province, China), it can be speculated that with increasing NPGE, the increase trends of the GEC and the amount of extracted pure gas were consistent with that deduced theoretical formula for calculating the amount of extracted pure gas. It was basically shown as a quadratic polynomial relation in which the amount of extracted pure gas insignificantly increased. Therefore, it is infeasible to improve the gas extraction in coal seams with low permeability only relying on increasing the NPGE, and other technology methods are necessary.