Damage-seepage Model of Borehole Coal and Its Application for Pressure Relief in Gas Extraction of Coal Seam

In order to compare and analyze the extraction effect of bedding drilling borehole under the condition of hole protected and unprotected, by considering the dilatancy characteristic of coal, adopting the dynamic mathematic model of porosity and permeability in coal and combining with the Drucker-Prager criterion, the fluid-solid coupling model of borehole coal damage-seepage was established. The damage range of borehole pressure relief and gas extraction was numerical simulated based on the physical parameters of coal seam 2 in Chaohua Coal Mine. The results show that, after the pressure relief damage occurs in the borehole, although the released range of unprotected hole is more than 1.3 times of protected hole, and the porosity and permeability of coal around unprotected hole are bigger than protected hole, but unprotected hole is more likely to collapse and plug. After extracting gas for 90 days, the effective drainage radius of protected hole is more than 1.3 times of unprotected hole. The filed data of gas extraction indicates that the average extraction concentration of protected hole is more than 1.6 times of unprotected hole, the average amount of pure gas extraction of protected hole is more than 1.4 times of unprotected hole, and the borehole extraction time is effectively shortened.