RETRACTED: A new method for layout layer optimization of long horizontal borehole for gas extraction in overlying strata: a case study in Guhanshan coalmine, China

Abstract

The large emission of pressure-relief gas in coal mining will cause atmospheric greenhouse effect. Arranging long horizontal borehole (LHB) to extract the gas is an effective solution. However, the determination of LHB layout layer in strata has a decisive effect on efficient gas extraction. A "zone-block" theoretical method for determining the LHB layout layer, in present study, was proposed by combining physical simulation test, theoretical analysis and engineering application. Three processes of the method were presented. Firstly, stable fractured subzone (SFSZ) was the optimum zone of the LHB layout based on the analysis of mining-induced fractures distribution and the borehole stability, and spatial location boundaries of the SFSZ in overlying strata were defined. Secondly, the SFSZ was divided into nine-grid blocks, the LHB layout suitability rate of each block were determined according to borehole stability rate, fracture permeability rate and gas accumulation rate. Finally, the LHB drilling could be conducted sequentially according to the layout suitability rate of each block in SFSZ. Field application results show that: the maximum and average amount of pure gas extracted through single borehole arranged in block I can reach up to 5.52 and 2.43 m3·min-1, respectively; and the pure amount in the entire extraction stage of the borehole is 2.53 and 6.69 times of boreholes arranged in blocks II and III, respectively. The proposed method can effectively determine the LHB layout layer in strata, so as to improve the gas extraction efficiency and ensure safe and green mining.