Analysis of the tempo-spatial effects of hydraulic fracturing by drilling through underground coal mine strata on desorption characteristics

Abstract

To elucidate the tempo-spatial effect and the corresponding mechanism of hydraulic fracturing on gas desorption, the experiments of the hydraulic fracturing in the field and the gas adsorption-desorption in the laboratory were carried out, respectively. The difference of the effects on the total amount of gas desorption, desorption proportion, moisture content, and in situ gas drainage concentration of the original and fracturing coal samples were studied. The results show that coal moisture increase in the 29 m area of hydraulic fracturing, the greater moisture content and the smaller amount of gas desorption, and the 9 m coal samples moisture content is 2.77 times greater than that of the original coal samples, and whose total amount of gas desorption for 29.8% of the original coal samples. In addition, the closer to the fracturing borehole, the smaller the desorption velocity, which is the fastest in the initial 5 minutes. The desorption velocity of the original coal samples is 5.6 times greater than that of the 9 m coal samples. The desorption proportion of the coal samples is high in early stages, η 30 min is 64% (η i is the proportion of the instantaneous total of gas desorption to the total gas desorption, %), and for η 6 h-12 h is 10%. The tempo-spatial effects of desorption characteristics can be expressed as the time-delay effect and a space-reduction effect. Furthermore, the upper limit of deformation of the coal seam is 3.3‰ and the ability of gas desorption in coal seam significantly decreases. The time of gas drainage concentration exhibits more than 30% increases from the 10 days in the nonfracturing area to 90 days in the fracturing area. Finally, hydraulic fracturing crushes coal, replaces absorption gas, increases the moisture content of coal, reduces various stresses, makes coal shrink, and ultimately achieves the effects of pressure relief and permeability improvement.