Abstract: In the coal seam gas extraction technology,the distribution of gas pressure around the drainage borehole determines the optimal extraction time and extraction radius.In order to study the law of gas pressure distribution in coal seam around drainage borehole,an expression of gas flow around drainage borehole was worked out by theoretical analysis and numerical simulation,and an analytical expression of gas pressure around drainage borehole was deduced by applying Darcy's seepage law.The rate at which the gas extraction radius changes with the extraction time was taken as the basis for determining the best time for gas extraction,a critical value was obtained,and an engineering application was carried out.The results show that with the increase of distance between the measuring point and the borehole center,the gas pressure of coal seam increases gradually,and finally tends to the original value,with the extension of extraction time,the gas pressure decreases exponentially,the critical value of gas extraction radius changing with extraction time can be tentatively set at 0.47.

Key words: gas drainage, gas flow rate, gas pressure, extraction radius, distribution rule, extraction time