| China’s shale gas resource potential is large,according to the Ministry of Land and Resources estimates,the geological resource potential of shale gas in China’s land area is 134.42 trillion m~3,and the recoverable resources of shale gas is about 25 trillion m~3;the technical recoverable reserves as assessed by the U.S.Energy Information Administration is 36.1 trillion m~3.However,due to the majority of no-scale pores in shale,it is an ultra-low permeability reservoir,which leads to low development efficiency of shale gas.Methane in-situ combustion and explosion fracturing is a new method to improve shale gas recovery.In the implementation of combustion and explosion fracturing technology,the high pressure shock wave generated by combustion and explosion in the borehole mainly produces impact damage to shale reservoirs,which in turn produces damage fractures,thus improving the permeability and oil and gas recovery of shale reservoirs.The degree of improvement in permeability and recovery is directly related to the artificial fractures formed in the shale reservoir after detonation,and the core scientific issue is the damage characteristics of the rock body by high-speed detonation shock load and its main control factors,therefore,the study of shale gas detonation propagation process and shock wave attenuation characteristics in the rock fractures can help provide basic theoretical guidance for the exploration of related technologies.In this study,a set of three-dimensional micro-crack internal detonation propagation experimental system was independently developed and designed to study the variation process of methane explosion flame propagation under different working conditions,and to characterize the propagation process and characteristic law of methane explosion flame in rock cracks.Four kinds of oscillating glass beads with different particle sizes were selected to be placed in the middle of the explosion tube and the combination of two to form different crack channels.A series of explosion experiments were carried out in a small closed pipe with a circular section of 0.5m in length and 40mm in diameter using methane/air mixture with five concentrations.The effects of methane gas concentration,crack equivalent diameter,crack zone length and pore network formed by different crack sizes combination on the explosion propagation characteristics of methane combustible gas in the closed pipe were studied.The propagation law of high temperature flame and high pressure shock wave of methane premixed gas in rock mass fracture is studied by numerical simulation.The main findings are as follows:(1)Comparing the flame propagation process of methane explosion under the condition of empty pipe and under the condition of different crack equivalent diameters in a certain length crack zone,when the crack equivalent diameters are 2.74mm,2.19mm and 1.64mm,Tulip flame appears after the flame passes through the crack zone,and the depression degree of the tulip flame front gradually increases with the decrease of crack size.The existence of fracture zone significantly shortens the propagation time of detonation flame,accelerates the propagation speed of flame,and the flame surface is stretched,leading to the intensification of turbulent flow effect.In addition,the existence of pore network shortens the time for the explosion overpressure to reach its peak,and the time is shorter with the increase of crack equivalent diameter.The smaller the crack size,the decrease trend with the peak of explosion pressure,and the attenuation degree of explosion overpressure gradually increases.When the crack equivalent diameter is 1.64mm and 1.10mm respectively,the pore network shows the strongest weakening effect on methane explosion propagation,especially when the flame propagates into the pore network,it completely changes into a slow combustion state.The explosion pressure was also significantly reduced.(2)After the flame passes through the fractured zone,the change of the length of different fractured zones speeds up the transformation of the flame from laminar flow to turbulent flow to some extent due to the extrusion and stretching of pore network.When the crack equivalent diameter of the crack zone is 2.74mm,with the decrease of the crack zone length,the indented degree of the middle flame front gradually deepens,the flame propagation velocity peak in the pore network decreases with the increase of the length,and the time for the flame front to reach the entrance of the crack zone also increases with the increase of the crack zone length.The propagation velocity of methane flame and the explosion overpressure are affected by the length at different crack equivalent scales.When the crack equivalent diameter is 1.64mm,the increase of the length of the crack zone has an exciting effect on the methane explosion propagation,while when the flame propagates in the pore network with a smaller crack size,the length has little effect on the flame explosion pressure peak,indicating that there is an optimal crack equivalent diameter,which makes the methane explosion intensity in the limited space the lowest.(3)The experimental study on the influence of different crack scale combinations on the propagation process of methane detonation shows that,before the flame enters the crack zone in the middle of the explosion tube,the front propagates forward in the form of ellipsoidal surface,and the shape is more regular than that of a single pore channel.With the decrease of crack scale,the flame brightness gradually darkens and the laminar combustion degree decreases.And the time to contact the boundary of pore space increases with the decrease of the size of the composite crack.After the flame wave passes through the composite crack zone,the flame tip tends to sag inward under the condition of 2.74-2.19mm fracture combination,and a tulip-shaped flame appears with bright color.In the crack combination of 2.19-1.64mm and 1.64-1.10mm,there is no depression after the flame front passes through the crack region,and the degree of turbulence effect is reduced.When the flame front tip reaches the pore space,the corresponding velocity peaks are 2.64m/s,1.51m/s and 0.49m/s,respectively.Compared with the single pore network,when the flame passes through the pore space of the fractured zone,with the increase of the crack size and the number of layers,The peak velocity reached by the flame front tip in the pore channel will decrease and the time will lag relatively.The variation trend of the peak overpressure in the blast tube with different fracture sizes is basically the same.Compared with the empty pipe condition,the peak overpressure decreases by 46.17%,59.61%and 44.56%.Compared with the single pore structure,the peak overpressure weakens,but the attenuation amplitude is different,and the reduction degree fluctuation is smaller than that of the single pore structure.The maximum explosion pressure first decreases and then increases with the increase of fracture size.(4)The change characteristics of flame explosion pressure and temperature at different crack scales were studied by numerical simulation.It can be seen that cracks with different aperture can intensify flame turbulence.After passing through the crack zone,the coupling of pressure wave and flame wave forms a vortex on the lower wall surface of the pipeline,and the methane explosion temperature fluctuates when reflected back to the pore channel.The decreasing trend of the explosion pressure is consistent with the experimental results,which proves that reducing the crack size can weaken the intensity of the explosion reaction.This paper has 60 figures,12 tables and 65 references. |
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