Mechanism And Application Research Of Liquid CO₂ Phase Transition Fracturing Coal Seam Based On BLEVE To Enhance Permeability

The liquid CO₂ phase transition blasting of is an enhanced gas drainage measure with the characteristics of unknown fire and no water pollution.It has attracted extensive attention in the process of increasing the permeability of a single coal seam.The liquid CO₂ phase transition blasting can not only improve the permeability of a single coal seam and improve the gas utilization rate,but also achieve the purpose of reducing CO₂ emission.However,at present,the research on liquid CO₂ phase transition blasting mainly focused on the impact of blasting pressure on the application.The BLEVE effect in the liquid CO₂ phase transition blasting is rarely considered.And there are few studies on the impact of the setting of blasting parameters on the coal fracture mode.In this paper,the theoretical knowledge of thermodynamics,hydrodynamics and seepage mechanics was mainly used.In addition,the BLEVE release characteristics,fracture distribution characteristics and surface strain field evolution law of liquid CO₂ phase transition blasting were analyzed by using the independently developed BLEVE release and coal fracture measuring device and 2D high speed DIC surface strain field measuring system.The mechanism of damage to coal body and the permeability enhancement of coal seam by liquid CO₂ phase transition blasting were revealed.The influence of blasting parameters on coal fracture mode was analyzed by orthogonal experiment method.And the idea of changing the mode of improving coal seam permeability by adjusting the parameters of liquid CO₂phase transition blasting was obtained.The suggestions for blasting parameters setting to enhance coal permeability was explored.Besides,the effect of enhanced gas drainage with different blasting parameters was verified by field application.The main conclusions of this paper are as follows:（1）Based on the thermodynamic theory and the engineering background of liquid CO2 phase transition blasting,it is found that when the pressure condition is fixed above the critical pressure,even if the temperature rises to 120℃,the CO₂ density remains above 800 kg/m³,which is close to the density of liquid CO₂.In the pressure range of 20 MPa-110 MPa,the viscosity decreases by about 60%-40%with the increase of temperature.The analysis of the process characteristics of liquid CO₂ BLEVE indicated that the occurrence mechanism is that the gas-liquid two-phase equilibrium state is destroyed due to the instantaneous release of gas in the system,and the system enters metastable or unsteady state.After meeting the specific conditions,the system can completely lose stability and produce rapid phase transition of liquid CO₂.The BLEVE impact characteristic analysis shown that the storage pipe filled with liquid CO₂ will push the surrounding atmosphere like a piston at the initial stage of opening,and the compression wave will catch up with the stack to form a shock wave.When the storage pipe is fully opened,the pressure will decay to a lower value,and the liquid CO₂ can be in a metastable state of superheat.（2）The measurement results of release characteristics of liquid CO₂ phase transition blasting indicated that the pressure-time curve can be divided into three stages:heating stage,blasting stage and quasi-static release stage.And a rebound pressure can be generated in the quasi-static release stage.When the filling pressure is increased or the release diameter is reduced,the rebound pressure is 1.27 and 1.54 times of the blasting pressure respectively.When the blasting pressure is increased,the pressure drop rate is increased by 1.68 times,and the rebound rate is only increased by 0.5 times.When the release diameter is reduced,the pressure drop rate is less than the rebound rate.It indicats that increasing the blasting pressure and reducing the release diameter at the same time may inhibit the BLEVE effect of carbon dioxide.Under the same blasting pressure and release diameter,the temperature before blasting rupture and the temperature during pressure rebound are increasing with the filling pressure increase.After reducing the release diameter or increasing the blasting pressure,the release temperature increases significantly.There is no obvious difference between release pressure and temperature at lower blasting pressure.It shows that the viscosity of CO₂under low burst pressure will not have a great influence on wedging in the fracture.With the increase of bursting pressure and temperature,the viscosity is about 30%higher,which is not conducive to the propagation of cracks.（3）The results of static and dynamic load test and coal crack response shown that with the increase of confining pressure,the maximum static peak strength of coal sample is 82.99 MPa,which is 13.92 times that of uniaxial load,and the maximum peak dynamic compressive strength of coal sample is 136.49 MPa.When the confining pressure is 10 MPa and the dynamic impact velocity is 9.43 m/s,11.63 m/s and 13.78m/s respectively,the peak compressive strength of coal samples is 83.61 MPa,97.30MPa and 116.80 MPa respectively.The higher confining pressure can hinder the crushing and pulverization of coal body and change the formation form of cracks,and higher dynamic loading speed can increase the density of coal cracks and promote coal crushing.Under the confining pressure of 5 MPa,10 MPa and 15 MPa,the increase of permeability of corresponding coal samples increased from 125.8 times,81.2 times and116.5 times respectively.（4）The fracture characteristics of coal body by liquid CO₂ phase transition blasting shown that when the blasting pressure is 5 MPa,the borehole expansion rate without confining pressure is 2.3 times higher than that with confining pressure.With the increase of blasting pressure,the expansion rate of borehole the initial number of fractures can increase significantly.With the increase of volume,the borehole expansion rate decreased slightly.And there is no obvious change in the number of initial fractures.The application of confining pressure will significantly reduce the number of cracks.Besides,the variance analysis of orthogonal experiments indicated that the blasting pressure is the most important factor to determine the crushing area,and the contribution rate is 55.51%.Accordingly,the filling volume has the greatest influence on the fracture area,and the contribution rate is 80.12%.It shows that the blasting pressure is the most important factor affecting the smash of coal,and the filling volume is the most important factor affecting the fracture network of coal.Based on the fracture characteristics,it is summarized that the enhancement permeability mechanism of liquid carbon dioxide phase transition blasting can be divided into the following two aspects:pressure relief antireflection effect and fracture structure change effect.（5）The investigation on the permeability enhancement effect of liquid CO₂ phase transition blasting shown that the coal seam permeability after blasting with the two blasting parameters is increased by 41.2 times（0.887 m D）and 23.4 times（0.513 m D）respectively compared with ordinary drilling.The gas emission per hundred meters from blasting boreholes is 3.87 m³/min.hm,which is increased by 8.21 times.In the coal roadway with ordinary drilling section,the total time for driving 300 m is 107.2 d,and the average driving speed is 2.8 m/d.The coal roadway excavation in the drilling control section of liquid CO₂ phase transition blasting only takes 58.8 d,the average excavation speed is 5.1 m/d.The excavation time is shortened by nearly half（45.15%）,and the maximum values of the indexed including S,q andΔh₂ were 4.8 kg/m,3.7 L and 140 Pa respectively,which effectively reduced the outburst risk.The dissertation has 108 figures,28 tables and 193 references.