| During the process of coal mining in China, the hazards related to gas is moreand more serious, which will not only affect the coal mine production safety, but gasemissions into the air will also cause atmospheric pollution, meanwhile gas as a kindof unconventional natural gas energy, reserves of36.8trillion m3. About70%ofChina Coalbed gas have the characteristics of micro-porosity, low permeability andhigh adsorption, resulting the gas disaster-prone. Hydraulic fracturing is an effectivetechnique to improve the efficiency of gas extraction from coal seam, but the existinghydraulic fracturing technology has big equipment and pressure, which is notconducive to the coal mine.We developed pulsating hydraulic fracturing technologybased on the conventional hydraulic fracturing, which can increase seam permeability,reduce stress and increase the amount of gas drainage.In this paper, we analysed the fracture damage mechanism of coal rock massunder the effect of two loads, crustal stress and pulsating stress wave firstly,confirmed that the total stress intensity should be the sum of displacement and stresscomponent of crustal stress and pulsating stress wave acting on the coal rock mass.Total stress intensity factor curve was traced out and pulsating hydraulic fracturingprocess can be divided into three stages. Under repetitive effect of pulsating stresswave, the energy of coal rock mass accumulates continuously which makes it fracturerandomly. The fracture of coal rock mass makes the static stress field changescontinuously, meanwhile, the follow-up dynamic stress field changes randomly.Comprehensive changes of the two stress fields increases the randomness of fatiguedamage of rock mass in turn and then lead to the damage accumulates continuously,mechanical property of rock mass degenerates ceaseless, at last, it makes thecondition of jointed rock changes from crack initiation, expansion to joint wellconnected. Analysis of numerical simulation showed that after hydraulic fracturing,crustal stress of elevated stress area on both sides of cracks reduced to a very lowlevel and plastic deformation was formed in a large area. This indicates that hydraulicfracturing technology can reduce the crustal stress effectively while damaging coalrock mass and reduce the gas pressure of coal seam. At the same time, direction ofcoal crack can be controlled by setting oriented control drill holes.secondly, regarding the spread of pulsating pressure as the propagation of stresswave, the transient flow model was selected for the study of pulse pressure wave propagation, fluctuating pressure propagation equation was used to calculate the valueof fluctuating pressure. The variation of fluctuating pressure before and after thecracks connect with each other was analysed. Due to the superposition of wavevelocity and flow velocity, pressure of pulsating reflected wave increases at the tip ofcrack. Using pulsating hydraulic fracturing experiment platform, different pulsefrequency was tested. At the condition of50Hz, pulsating stress wave increased by50%, reached to1.5MPa. However, at40Hz, pulsating stress wave increased by28.6%, reached to0.9MPa. It can be seen that the damage effect on coal-rock mass isbetter at the condition of50Hz.Thirdly, we analyzed the basic principle of the high pressure sealing in responseto the problems of drilling sealing in pulsating hydraulic fracturing. We determinedthe ratio of expansion agent and water reducing agent through experiments, anddeveloped the cement-based high-pressure sealing material by adding expansion agentand water reducing agent. What’s more, the hydration process of cement paste withexpansion agent and water reducing agent was divided into four stages, through theresearch by low-field NMR equipment in it. It was pointed out that the grouting workshould accomplished during the reaction period and the start time of hydration processwas33~35minutes, providing theoretical guidance for the site construction.Finally, to cope with the application of high pressure pulse hydraulic fracturingtechnology and problems occurred in engineering practice, we manufactured theintegration equipment of high pressure pulsating hydraulic fracturing, proposed drill-fracture-drainage trinity technology for pressure relief and permeability increase inhigh gas, low permeability coal seam. On that basis, the equipment was applied in theChengzhuang Mine of Jinmei Group. Field tests were conducted in three differentareas and relevant parameters were analysed.Field test results show that there is an obvious effect in gas extraction afterpulsating hydraulic fracturing. The average pure flow of pulsation fracturing hole is0.05m3/min,2.5times of normal drainage hole. After fracturing, water content infracturing area increased by2times and permeability coefficient of coal seamincreased by47.8times, reached to41m2/(MPa2d). Pulse and amplitude valuedecreased obviously within the10m range of fracturing hole which reduces the peakvalue of abutment pressure.In addition,7papers about research results were and will be published, in which2has been indexed by EI,2will be published in EI source journals. |
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