Abstract
Hydraulic fracturing can increase the fracture of coal seams, improve the permeability in the coal seam, and reduce the risk of coal and gas outburst. Most of the existing experimental specimens are homogeneous, and the influence of the roof and floor on hydraulic fracture expansion is not considered. Therefore, the hydraulic fracturing test of the simulated combination of the coal seam and the roof and floor under different stress conditions was carried out using the self-developed true triaxial coal mine dynamic disaster large-scale simulation test rig. The results show that (1) under the condition of triaxial unequal pressure, the hydraulic fractures are vertical in the coal seam, and the extension direction of hydraulic fractures in the coal seam will be deflected, with the increase of the ratio of the horizontal maximum principal stress to the horizontal minimum principal stress. The angle between the extension direction of the hydraulic fracture and the horizontal maximum principal stress decreases. (2) Under the condition of triaxial equal confining pressure, the extension of hydraulic fractures in the coal seam are random, and the hydraulic fracture will expand along the dominant fracture surface and form a unilateral expansion fracture when a crack is formed. (3) When the pressure in one direction is unloaded under the condition of the triaxial unequal pressure, the hydraulic fractures in the coal seam will reorientate, and the cracks will expand in the direction of the decreased confining pressure, forming almost mutually perpendicular turning cracks.
Highlights
China is poor in oil and gas but abundant in coal resources
(3) When the pressure in one direction is unloaded under the condition of the triaxial unequal pressure, the hydraulic fractures in the coal seam will reorientate, and the cracks will expand in the direction of the decreased confining pressure, forming almost mutually perpendicular turning cracks
The water injection rate increases gradually, and the injection pressure starts to increase after the opening of the fracturing water pump
Summary
China is poor in oil and gas but abundant in coal resources. Coal accounts for 94.0% of the explored reserves of primary energy and always plays an important position in China’s energy production [1]. As an effective method of improving the permeability of the coal seam, hydraulic fracturing has been applied many mines. There is less control analysis of rock mechanical properties of the coal seam roof and floor on hydraulic fracturing. The difference in mechanical properties of the coal seam, roof, and floor rocks is an important factor to control hydraulic fracturing of the coal seam. The difference in mechanical properties of the coal seam roof and floor will affect the distribution of the ground stress field in the coal seam and directly determines the hydraulic fracturing effect of the coal seam. The control effect of rock mechanics properties of the coal seam and its roof and floor on hydraulic fracturing fracture propagation is further analyzed. The research results have great significance for studying the extension mechanism and species distribution of the cracks
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