| As a big country in coal production and consumption,China's CBM resource is very rich,but the gas is also the chief killer of coal mine production.In addition,the gas discharging cause great damage to the ecological environment.In recent years,the hydraulic fracturing used to increase the permeability of hydrocarbon reservoirs was introduced into gas drainage of seam in underground coal mines.However,because of the differences between the coal seam and the hydrocarbon reservoirs,the hydraulic fracturing theory and equipment for hydrocarbon reservoirs can not be used directly in underground coal mine.The cracks expansion irregular,cracks closure prematurely,and the instability of safety pillar often occur in engineering.Which have greatly reduced the enhancement effect and gas drainage efficiency,and also brought hidden danger to fracturing and mining operation,which seriously hindered the application of hydraulic fracturing in underground coal mines.Therefore,this thesis proposes a new method to delay the closure of fracturing cracks and promote the production of gas migration by perforating-guided and progressive drainage.The key to extract gas efficiently by this method lies in the crack orderly expansion and the reasonable connection between the drainage holes.The main research contents and achievements are as follows:(1)Based on the analysis of the characteristics of hydraulic fracturing in coal mines,considering the ground stress field,gas pressure,water pressure in the borehole,and fluid loss from the high pressure water to the coal seam,the crack initiation pressure,initiation angle mechanical model and calculation steps of perforating-guided fracturing cracks are constructed by superposition principle and maximum tensile stress criterion.The critical hydraulic pressure and crack extension angle mechanical model and calculation steps for crack propagation are established by shear-shear composite failure criterion and maximum circumferential stress theory.The correctness of the model is verified by engineering example calculation and numerical simulation.(2)The crack initiation and propagation law of hydraulic cracks under perforation guidance are revealed.The cracking pressure of coal guided by perforation is 30%lower than that of conventional hydraulic fracturing.The perforation parameters have different effects on the cracking pressure:the perforation pore size has little effect on it,and the cracking pressure decreases rapidly with the perforation depth increasing from 0 to 1.5 times the diameter of the hole,while the perforation azimuth has a significant stage effect.Perforation fracturing undergoes four stages of stress accumulation,crack initiation and expansion along the perforation direction,crack propagation and crack propagation along the direction of the maximum principal stress.The crack initiation position and the extended shape depend on the difference between the two principal stresses(??)perpendicular to the maximum principal stress plane and the perforation azimuth angle(?).There is a critical perforation azimuth(?0)in any combination.When ?<?0,the crack starts at the tip of the perforation and expands in the direction of the perforation,but gradually turns to the direction of the maximum principal stress,and the steering distance increases with ? increases.When ?>?0,the crack cracks on the wall of the borehole and expands along the direction of the maximum principal stress to form a regular double-wing straight crack.When ?= ?0,the crack simultaneously cracks at the tip of the perforating hole and the wall surface of the borehole,and a single-wing curved crack and a single-wing straight crack extending along the direction of the maximum principal stress are respectively formed on both sides of the borehole.The larger ?? is,the smaller ?0 is,the more easily to forms a multi-crack morphology.(3)The perforating-guided fracturing forms an elliptical effective influence range including a macro-fracture zone,a micro-fracture penetration zone and a micro-fracture zone in the coal seam.The law of gas migration in this space is analyzed:the high-pressure water enters the coal matrix to increase the moisture content of the coal seam during the water injection process.The drill cuttings gas desorption index K1 and adsorption constant a decrease logarithmically,and the adsorption gas desorption is inhibited.Moreover,the longer the water injection time,the more obvious the inhibition effect.At the same time,the high-pressure water drives the free gas to the deep coal seam,forming a gas enrichment zone in the range of 33?45m from the fracturing hole.At the same time,the high-pressure water drives away the dissociative gas to the deep coal seam,forming a gas enrichment zone between the annular effective space line of hydraulic fracturing and push it 5m out.During the drainage process,the pressure of the coal seam is continuously decreasing.But the pressure drop along the main crack direction is most effective,and priority reduce to the critical gas desorption pressure to initiate desorption,and the gas permeability increase is most obvious.On this basis,a method to delay the crack closure and promote gas migration output by progressively arranging multi-stage drilling along the direction of the main crack in accordance with the pressure drop transmission law is proposed.(4)The perforating-guided fracturing and progressive drainage system equipment include wheeled vehicle positioning drilling rigs,high pressure pump sets,hydraulic perforators and gas jet pump drainage devices suitable for underground coal mines is developed.The minimum safe thickness model of hydraulic fracturing rock column and the optimal placement distance model of fracturing hole are constructed.A high pressure capsule packer and a pressure quick sealing device are respectively developed for the fracturing hole and the drainage hole.Optimized process parameters suah as sealing,perforating,fracturing,crack monitoring and drainage,and the safety prevention and control system for fracturing area geological prediction,rock column grouting reinforcement,orifice anti-backlash and safety monitoring was constructed.(5)The field test shows that compared with the conventional process,the cracking pressure of the coal under the perforation guidance is significantly reduced,the crack initiation and expansion are significantly guided by the perforation,and the damage of the high pressure water to the surrounding rock are effectively weakened,and the effective influence range is expanded more than 35%.In addition,the gas drainage concentration increased by 15?20%the average single-hole gas extraction smelter increased by 2.73 times,the coal seam gas drainage rate increased by 14.2%,and the drilling engineering volume saved by 40%.The gas extraction time has been greatly shortened,and put an end to the phenomenon of gas blasting over limit.The technology was applied to over 30 mines and gas tunnels in the southwestern region,which ensured the safe production of coal mines,improved the development and utilization rate of coalbed methane,reduced the damage to the ecological environment,and produced good economic and social benefits. |
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