| With the increase of mining depth, the risk of coal and gas outburst increases. Howto reduce coal and gas outburst risk effectively is a major safety problem and letter to beresolved for low permeability and gas bursting coal seam. Hydraulic fracturing ofcoal-rock mass is an effective technical approach for gas drainage of low permeabilityand gas bursting coal seam, especially for a single coal seam. Crack propagation ofhydraulic fracturing for coal-rock mass and gas migration are major factors deciding theeffect of hydraulic fracturing. Thus, this paper took crack propagation of hydraulicfracturing for coal-rock mass and gas migration as main study object, crack propagationof hydraulic fracturing for coal-rock mass and its influence on gas migration weresystematically and thoroughly studied by combining theoretical analysis with numericalsimulation and field investigation, which had important theoretical significance andengineering application prospect. The main research results and conclusions are asfollows:Firstly, mathematical model for seepage and damage coupling of hydraulicfracturing was established, which used statistical approach to represent heterogeneity ofcoal-rock mass parameters, involved coupling effect between seepage filed and stressfield, contained three constitutive relations, namely elastic-brittle, elastic-soften andelastic-weaken, considered the evolutionary process of damage and its influence onpermeability and other physical parameters; the coupling iterative algorithm for thismodel was worked out and numerically solved by finite element programming usingmatlab software, and the results of which were compared with business software andtheoretical analsysis, which verified the validity of mathematical model and itsnumerical method. Designed programme of numerical calculations were used to studyheterogeneity of coal-rock mass, in-situ stress field, diameter and shape of boreholewhose influences on crack and propagation of hydraulic fracturing for coal-rock mass.Hydraulic fracturing technology of hydraulic cutting in advance and multiboreholecontrol fracturing were studied. The above research results provided theoretical basisfor parameter design and borehole layout for field hydraulic fracturing.Secondly, gas migration influenced by hydraulic fracturing was studied from threeaspects, namely methane driven effect, gas flow feature changed and gas adsorption anddesorption influenced. Based on this, the unified mathematical model for hydraulic fracturing and gas drainage was established, and then was applied to numericalsimulation of gas drainage after fracturing, which studied the gas migration afterfracturing. The results proved that gas pressure near damage zone decreasedsignificantly, which illustrated that coal seam permeability near hydraulic fracture wasgreatly improved and new gas migration channel was formed near hydraulic fracture,which could significantly improve gas drainage effect. The above results providedguiding significance for the field borehole location optimization of gas drainage.Thirdly, based on deformation model of coal-rock mass, fluid pressure drop modelin hydraulic fracture, crack model and criterion for hydraulic fracture propagation,mathematical model of single crack propagation for low permeability coal-rock masswas established, whose numerical algorithm was studied and implemented using thesecondary development language APDL of ANSYS. The crack propagationcharacteristics, by changing injection pressure, elastic modulue, and in-situ stress fieldand fracturing fluid viscosity respectively were studied. The numerical simulationresults were consistent with the field observation results, which showed that thesimulation results could provide guidance for the choice of field construction parameterof hydraulic fracturing.Fourthly, based on six discriminant index of water infusion difficulty degree forcoal seam, Fisher discrimination model and extension discrimination model for coalseam water infusion difficulty degree after hydraulic fracturing were built. Thediscrimination results of engineering cases showed that the two above methods wereconsistent or better than the results of engineering investigation, fuzzy clusteringmethod and neural network method, which achieved the scientific evaluation andclassification of water infusion difficulty degree after hydraulic fracturing, and providedreference for the feasibility judgement of static pressure water infusion measure afterhydraulic fracturing to prevent coal and gas outburst.Fifthly, aiming at different coal structure types and considering constructionconditions underground coal mine and constraint factors of gas drainage, hydraulicfracturing process and technology was proposed, and then was applied in guiding thedesign of field hydraulic fracturing. The field hydraulic fracturing observation resultsshowed that the crack propagation had went through process of closure, expansion andnew crack generated. With the increase of distance between borehole and fractureleading edge, the water pressure decayed along distance and the intrinsic crack reopened,the water pressure needed for crack propagation was increased gradually. The gas concentration in inspection boreholes and fracturing boreholes after hydraulic fracturingwer all increased, which verified the methane driven effect of hydraulic fracturing. Thefiled investigation results of gas drainage and outburst prediction index showed that thegas drainage scalar quantity was high and kept smooth and steady during initial gasdrainage stage, followed by a short reduction period, and then gas drainage scalarquantity increased again later, a persistent gas production period emerged; besides, theoutburst prediction index in the influence range of hydraulic fracturing was lower thanthe critical index, which verified gas migration influenced by hydraulic fracturing,namely, hydraulic fracturing of coal-rock mass could increase permeability of coal-rockmass effectively, enhance gas drainage efficiency and eliminate the risk of coal and gasoutburst, it had obvious technical and economic benefit. |
PDF Full Text Request