Permeability-Increasing Theory Of Array Crossing Boreholes Using Blowout-Inducing And Its Application On Gas Drainage In Outburst Coal Seams

Based on theories of the elasticity mechanics, damage mechanics, seepage mechanics and coal gas control, using theory analysis, field test, numerical Simulation and field application, the paper studies on mechanical mechanism of borehole blowout and its control and inducement technology,stress distribution and fracture development of borehole cavern,permeability-increasing theory and technology of array blowout-inducing boreholes,and gas flow model in the permeability-increasing area. The research provides a new way for gas drainage in low permeability coal seams.Boreholes blowout belongs to coal and gas outburst. Coal with high pressure gas exposes because of borehole drilling rapidly or instability coal throwing-out from rupture zone. Ground stress and gas pressure make coal failure, grinding, throwing-out, and outburst starts. The rapid expansion of outburst gas makes the outburst coal and the water running out borehole with high speed, which is borehole blowout. Blowout can be controlled by setting safety rock pillar over 10m, installing blowout-leading system in orifice and improving drilling process. Blowout can be induced by increasing water pressure in drilling(10~15MPa) and enlarging the borehole diameter(90～100mm).The displacement and stress distribution law around the blowout borehole is analyzed. The surrounding coal of blowout cavern moving toward to the cavern centre, and the displacement value becomes smaller with the increasing of distance to the hole. A relief area is formed surrounding blowout cavern, which size is proportional to the radius of cavern, the embedding depth and thickness of coal seams. The relief mass ratio, which is mass ratio of blowout coal and the coal within relief area, becomes larger with the increasing of the blowout cavern.Based on the original layout plan of array crossing borehole, a permeability-increasing method using array blowout-inducing boreholes is proposed. Using blowout control and inducement technology, several borehole blowout was induced and a few coal was blow out from borehole. The remaining coal around array boreholes creeps, unloads press and expands, fractures sprout, extend, coalesce and dilate, and coal permeability can increase nearly 300 times. Based on this, a permeability-increasing technology using array blowout-inducing boreholes combining hydraulic fracturing is also proposed. In this technology, borehole blowout provides free surface, hydraulic fracturing forces coal fracture open, extend and coalesce and forms great cracks to connect boreholes. Water in fractures is discharged, fractures can keep open state because of stress reducing and coal blowout, and coal permeability increases. Furthermore, some supplement boreholes are needed for gas drainage. It is estimated that this technology can reduce more than 50% drilling work quantities and coal permeability can improve sever hundred times.The gas flow model of coal with array blowout-inducing boreholes changes fundamentally, fractures just likes extending boreholes and they keep negative pressure in gas drainage. So, gas in the coal flows into these extending boreholes and is sucked to real borehole. Permeability increasing and gas flow model changing improves gas flow rate. By the permeability-increasing and gas drainage test of the technology in Huaibei Qinan coal mine, coal permeability coefficient increased 55.6 to 292.7 times and 152.6 times in average, gas flow increased 4 times, gas drainage time shortened to 4 months, borehole drilling efficiency improved more than one times and the driving speed in coal entray increased to 200m per month.