Abstract
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.
Highlights
Restricted by permeability and natural conditions, efficiency of gas extraction in China seems low
The quadratic polynomial relation between the amount of extracted pure gas and negative pressure during gas extraction (NPGE) was deduced based on the fundamental theory of the mechanics of oil and gas seepage in porous media and the pressure function theory of distribution of gas pressure in strata
The three equations were combined to be one equation set concerning the gas seepage during extraction which shows the influence of NPGE on the effect of gas extraction
Summary
Restricted by permeability and natural conditions, efficiency of gas extraction in China seems low. Increasing of negative pressure during gas extraction (NPGE) in boreholes can improve the amount of gas extraction from boreholes after using effective hole-sealing technology. The influence of improving NPGE after employing effective hole sealing technology on the extraction effect of the coal seam was intensively analyzed. It is necessary to investigate the influence of NPGE on the GEC and the amount of extracted gas based on the basic theory of seepage mechanics. Li et al[6] investigated the law of gas seepage in soft tectonic coal under negative pressure extraction: gas permeability increases with rising negative pressure in gas extraction. Cheng et al[8] investigated the action mechanism of NPGE: with increasing time of gas extraction, the effect of negative pressure gradually lowers and decreasing NPGE can effectively improve the GEC. The influence of NPGE in low-permeability coal seams during gas extraction was analyzed on site
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