Research on theory and application of hydraulic fracture weakening for coal-rock mass

Based on the engineering background of coal-rock mass strength weakening(structure reformation),researched on theory and technology of hydraulic fracture weakening for coal-rock mass systematically with research methods including experimental research,theoretical analysis,numerical simulation and field test.The main achievements were obtained as follows.True triaxial stimulation experiment system for rockseam hydraulic fracturing with 4 000 kN force was developed,which can complete the simulation experiment with the greatest size 500 mm × 500 mm × 500 mm under the condition of three principal stresses.And oil-water loading converter with pressurization function was designed.Applying true triaxial stimulation experiment system for rockseam hydraulic fracturing and theoretical study,research on crack propagation law of hydraulic fracturing for coal-rock mass with large size was carried out.Crack front shape of hydraulic fracturing and the relationship between hydraulic crack on the hole-bottom and principal stress were researched,and then the law of hydraulic crack propagation on the hole-bottom affected by intermediate principal stress was found.The characteristic of bifurcate hydraulic crack propagation direction was analyzed,and then structural failure characteristic of original crack affected by osmotic water pressure was got.Based on pore-water pressure(hydraulic gradient) caused by osmosis filtration,water pressure parameters of hydraulic crack propagation in fissured coal-rock mass were obtained.According to experimental results,the crack frontier of hydraulic fracturing is oval shape with the centre of borehole fracturing section.Besides,the ratio of long axis and short axis keeps unchanged.After bifurcation,the trend of hydraulic fracturing failure surface is vertical to both σ3 and σ2.The roughness of hydraulic fracturing failure surface is significantly greater than general rock material.Research on structural failure of fissured rock mass under the condition of solid-liquid coupling was made.Based on the distribution law of coalseam fissures,the minimum fissure-water pressure which starts hydraulic fissures in coalrock mass and the extended length of hydraulic airfoil branching crack were got according to fracture mechanics.Structural transformation shape of hydraulic fracture weakening was simulated by RFPA software under the condition of into-groups primary cracks.Based on energy dissipation injury,the relationship between microscopic structural failure of fissured coal-rock mass and macroscopic mechanical property weakening was obtained.Research on control technology of hydraulic fracture weakening for coal-rock mass was carried out.With the advantages of hydraulic slotting and hydraulic fracturing,directional fracturing technology with beforehand hydraulic slotting was put forward.Based on superpositional effects of both drilling and drilling water pressure,the propagation direction of hydraulic cracks was controlled by the combination of multiple drillings with liquid injection methods.With the advantages of both water pressure blasting and hydraulic fracturing,the weakening and increasing permeability technology of hydraulic blasting fracturing was proposed.Hydraulic fracturing after water pressure control blasting can form multiple hydraulic main cracks and fracturing crack band along hydraulic main failure surface in the coal-rock mass.Experimental results prove that it is an efficient way to increase the number and range of hydraulic cracks.Research on temporal and spatial relationship of hydraulic fracture weakening for mining coal-rock mass was conducted.The relationship between natural water absorption of coalseam and time was studied by experiment.The water absorption differences between dry coal sample and natural coal sample were analyzed.According to the initial hydraulic gradient of seepage and the attenuation law of radial hydraulic gradient of hydraulic fracturing,penetration radius of hydraulic fracture weakening injection was got.Hydraulic main crack propagation stage of hydraulic fracture weakening must be completed ahead of abutment pressure peak zone.Field tests of gassy hard-thick top coal show that hydraulic fracturing weakens the whole mechanical property of coal seam and reduces top coal fragmentation significantly,and then increases recovery rate of workface by 23%.It releases gas in advance to reduce gas concentration in workface and dust concentration decreases by 21%,which inhab-its the spontaneous combustion of coal.