| Most coal seam belongs to low permeability and high gas single coal seam in China,pressure-relief gas pre-drainage effectively is one of the most effective methods to reduce the mine gas disaster.For enhancing permeability of single coal seam,gas disaster prevention and control,the mechanism and application of high-pressure water jet assisted hydraulic fracturing were studied in this paper.The numerical simulation of FPPA and COMSOL Multiphysics were used to carry out the simulation test of the stress distribution and evolution law of the coal body,damage cracking and crack propagation process.A permeability model of equivalent stress was used,it studied seepage field spatial and temporal evolution of single-borehole high-pressure water jet assisted hydraulic fracturing,influence of hole spacing on spatial and temporal evolution of high-pressure water jet assisted hydraulic fracturing.In the finally,it studied technology and equipment of high-pressure water jet assisted hydraulic fracturing coupled seepage enhancement,and then,field test and application of this technology was implemented.The rock breaking process divided into the initial preparation,broken initial,main,and end stage in this paper.Because of the water jet self-modulus,its shape is Spindle-like cavitation,normally in the coal mine its shape 0.3 m diameter of section,2-3 m long High-pressure water jet assisted hydraulic fracturing was hydraulic fracturing under water jet hole,its cavity is 100 times larger than normal hydraulic fracturing.High-pressure water jet assisted hydraulic fracturing fracture developed well,its major fracture throughout elastic-plastic area of water jet.It formed expanded"Three-Zone"of coal rock bursting.On this basis,RFPA and COMSOL Multiphysics software were used to analysis of high-pressure water jet and hydraulic fracturing,which revealed high-pressure water jet assisted hydraulic fracturing pressure relief and permeability improvement.Numerical simulation about single-borehole high-pressure water jet assisted hydraulic fracturing showed that the pressure was 24 MPa,it appeared a large of micro-fractures and major fractures are produced,especial more cracks appeared on both sides of the cavity.Its shape was ellipse with rectangular in the middle,extended 18-20 m.24 m double-borehole high-pressure water jet assisted hydraulic fracturing fracture pressure was 22 MPa,coupled and developed a large of fractures in middle area d between two holes,no non-rupture zone.Numerical simulation using COMSOL Multiphysics software about single-borehole high-pressure water jet assisted hydraulic fracturing under permeability model of equivalent stress showed that the coal seam permeability has increased more than 160 times,after gas extraction for100d,gas pressure decreased to 0.74 MPa,its damage rupture range was 18.5 m.When double-borehole high-pressure water jet assisted hydraulic fracturing with the distance between the two holes 18 m,24 m,32 m,the permeability improvement increased more than 180 times,140 times,130 times.After gas extraction for 100d,gas pressure decreased to as follows 0.27 MPa,0.61 MPa,0.7 MPa.Therefor,34 m double-borehole high-pressure water jet assisted hydraulic fracturing was relatively reasonable,its damage rupture range was 1.7 times the single-borehole.Technology and equipment research of high-pressure water jet assisted hydraulic fracturing coupled seepage enhancement for gas extraction were developed in this paper.Using systematic thinking,12 steps the construction flow and method and 3hierarchies regional division was proposed.High-pressure water jet assisted hydraulic fracturing was tested,it showed that its effect was very ideal.The test results showed that:its damage rupture range was 20-25 m,its fracture pressure was 26-29.7 MPa.After gas extraction for 100 d,its one-time gas drainage rate was 61%,hectometre borehole gas flow rate enhanced from 0.098 m~3/min.hm to 0.342 m~3/min.hm,increased more than 250 times.Its expand the coal seam was 3.65‰.It reduced the sensitivity of outburst prevention and prediction index to 5%,driving speed enhanced from 50-75 m/month to 110 m/month;permeability coefficient of coal seam increased by 40-200 times.The results of engineering test were basically consistent with the results of simulation analysis.There are 98 figures,19 tables,and 271 references in this paper. |
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