Hydraulic fracturing after water pressure control blasting for increased fracturing

Traditional hydraulic fracturing techniques generally form main hydraulic cracks and airfoil branch fissures, but main hydraulic cracks are relatively few in number. Hydraulic fracturing after water pressure control blasting can transform the structure of coal and rock mass. Experiments prove that it is an effective method for increasing the number and range of hydraulic cracks, as well as for improving the permeability of coal seams. The technical principle is as follows. First, a hole is drilled in the coal seam and is injected with a gel explosive (a mining water-proof explosive). Then, water is injected into the hole to seal it, at low enough pressure to prevent cracks from forming. Third, water pressure blasting is done by detonating the explosive. The water shock waves and bubble pulsations produced by the explosion cause a high strain rate in the rock wall surrounding the hole. When the stress imposed on the rock wall surrounding the hole exceeds its dynamic critical fracture strength, the surrounding rock breaks and numerous circumferential and radial fractures propagate outward. Lastly, water injection processes, such as general injection, pulse injection, and/or cyclic injection, are carried out to promote hydraulic fracturing. Depending on the fissure water pressure, detonation fissures continue to expand and additional hydraulic fractures with a wider range are formed. Under the effect of detonation pressure, joints and fissures in the coal mass open and propagate, leading to reduced adhesive forces on structural surfaces and thereby enhancing coal cutting. Therefore, this method improves the permeability of the coal seam, effectively weakens the strength of the coal and rock mass, and reduces the surrounding rock stress of the weakened area, effectively solving the problem of having a small number of big cracks. It is a useful technical approach for improving top coal caving, preventing rock burst, preventing coal and gas outbursts, and raising the gas extraction efficiency in colliery. (C) 2011 Elsevier Ltd. All rights reserved.