Intensive permeability enhancement experiment through hydraulic fracturing by way of water-sand-water in kilometer deep well with high gas seam

Hydraulic fracturing is one of the effective methods which increases the permeability of coal and rock. The technology of permeability enhancement through hydraulic fracturing by the way of water-sand-water ( W-S-W) was put forward in order to overcome the problems of hydraulic fracturing in deep mining. The experiments of permeability enhancement through upward hydraulic fracturing by the way of W-S-W and conventional hydraulic fracturing were carried out at a high gas coal seam in a deep mine,and the effect of permeability enhancement was investigated. The results show that the heterogeneity of coal and the non-uniformity of the pore and fracture distribution leads to the asymmetric permeability enhancement. The extension and evolution of crack is progressive-circulating under the effect of hydraulic fracturing and experiences five stages in turn including the slow growth of energy,micro-cracks initiation, local damage and destruction,crack rapid expansion,the cyclic extension and evolution of crack network. The internal structure of coal has been cut by high pressure water and a chain effect of weak surface in fracture,which is constantly expanded driven by high pressure water,has been formed. The proppant (sand) is wedged in the ends of cracks and inhibited their closings,which increases the permeability of coal body. The highest gas extraction volume of one hun-dred holes is up to 1. 2 m3 / min in the permeability enhancement area through W-S-W hydraulic fracturing,the average extraction volume and gas volume fraction of one hundred holes is 0. 77 m3 / min and 52% respectively,which is in-creased 0. 75 times and 0. 68 times compared with the average extraction volume (0. 44 m3 / min) and gas volume fraction (31% ) of one hundred holes in the conventional hydraulic fracturing region,and which is increased 1. 4 times and 1. 2 times compared with the average extraction volume (0. 32 m3 / min) and gas volume fraction(24% ) of one hundred holes in the non-fractured region. There are some distinct peak stages of the extraction volume and gas volume fraction of one hundred holes in the permeability enhancement area through W-S-W hydraulic fracturing. The stable extraction stage can continue over 45 days and the timeliness of gas drainage is obvious,the permeability enhancement in a wide range seam and long-term and efficient drainage gas in high-gas coal seam in kilometer deep well is a-chieved.