Study on influence of superimposed effect of gas extraction in downhole drilling

In order to better reveal the change of effective extraction radius and the influence of extraction superposition effect on the extraction effect in the process of coal seam gas extraction, a three-dimensional coal seam gas-solid coupling model is established based on the assumption of pore-fracture dual medium. Based on the theoretical model, 2901 working face of Shanxi Yamei Daning Energy Co., Ltd. is taken as the technical background, and numerical simulation of coal seam gas extraction is carried out by using COMSOL software, which analyzes the change rule of effective extraction radius with extraction time, and verifies the accuracy of numerical simulation results by combining with the results of on-site test. The mechanism of the superposition effect of multi-drilling extraction was studied, and the influence of hole spacing and negative pressure on the superposition effect of extraction was analyzed. The results show that: the change amplitude of the effective extraction radius gradually slows down with the extraction time and tends to stabilize, and the two are in a power function relationship, with the functional expression R=0.24t0.59; the average error between the numerical simulation results of the effective extraction radius of coal seam gas and the results of the field test is 7.7% when the field test is carried out by using the gas storage method, which verifies the rationality of the three-dimensional gas-solid coupling model of coal seam gas. The reasonableness of the three-dimensional gas-solid coupling model of coal seam gas is verified. When several downgradient boreholes are arranged in the same area for gas extraction at the same time, there is a superposition effect in the area to form a composite extraction reach area; the smaller the spacing of the boreholes, the more obvious is the superposition effect of the coal seams in the area under the control of the boreholes, and the faster is the decrease of the gas pressure at the same extraction time, and there is almost no influence of the negative pressure on the superposition effect of extraction; With the coefficient k=Pm/Pc as the evaluation index of the superposition effect of extraction, in the case of determining the effective radius R of single-hole extraction, the influencing range of the superposition effect of gas extraction in the well can be up to 2.2R, and the results of the study provide a theoretical basis for the rational design of downhole drilling in coal mines.