Prevention and Control of Coal and Gas Outburst by Directional Hydraulic Fracturing through Seams and Its Application.

Abstract

The regionalized pressure relief, permeability enhancement, and outburst prevention of "three high and one low" coal seams with high gas, high ground stress, high outburst risk, and low permeability have become important problems to be solved urgently in realizing the sustainable development of coal mines. In this study, a combination of theoretical research, RFPA2D-Flow numerical simulation, and field test was used to study the initiation mechanism and propagation law of directional hydraulic fracturing fractures through the seam. The results show that fracture initiation depends on the axial and radial horizontal stress of the fracture hole, the physical and mechanical properties of coal and rock, and the inside of the weakest layer. Single-hole hydraulic fracturing can achieve a pressure relief radius of 7-8 m, but there is a stress concentration zone outside, which is not conducive to regional pressure relief and permeability enhancement. Directional hydraulic fracturing with multiple holes produces an approximate cylindrical compression and crushing ring and a penetrating fracture surface along the center line of the pressure crack hole and the directional hole, which better eliminates the phenomenon of stress concentration in nondirectional hydraulic fracturing. The technology was applied to the 2238 auxiliary roadway of Chengzhuang Mine of Jinmei Group, and the field implementation results showed that field implementation results showed that directional hydraulic fracturing through the seam reduced the gas content in the coal seam to a great extent, and the coal seam gas content was reduced by about 42.3%, indicating that the technology can effectively reduce the risk of coal and gas outbursts.