Evolution Of Pore-Fracture Structure And Mechanism Of Fracturing And Enhanced-Permeability Of Loaded Coal Under Microwave Irradiation

It is a critical technical problem to solve the problem of difficult gas extraction in low-permeability coal seam for gas disaster prevention and control field.Microwave fracturing technology is an exploratory new approach to solve this challenge.Although the feasibility and effectiveness of this technology have been explored from the perspective of“promoting desorption and increasing permeability”,it only involves the effect of microwave irradiation on the pore-fracture structure of coal under unloaded conditions.During microwave fracturing,coal will be subjected to a combination of in-situ and(internal)thermal stress generated by microwave irradiation on the coal body.At this time,how much microstructure damage can microwave irradiation cause to coal?And the microstructure damage beneficial to the increase of coal permeability can be formed in large quantities under what microwave irradiation parameters?This is the key to determining the effectiveness of microwave fracturing and increasing the permeability.Therefore,to address the mechanism of fracturing and enhanced permeability of loaded coal under microwave irradiation,this thesis independently developed an experimental device for microwave fracturing of loaded coal.The pore structure evolution,fracture structure development,macroscopic thermal rupture,permeability change and temperature rise characteristics of coal samples under the combined action of applied load and microwave irradiation were studied.The evolution of pore-fracture structure and mechanism of fracturing and enhanced permeability of loaded coal under microwave irradiation were explored.The engineering application concept of microwave fracturing and permeability enhancement technology was also established.This thesis has achieved the following innovative research results:(1)The evolution law of the pore structure of loaded coal under different microwave irradiation parameters is clarified,and the relationship between microwave irradiation parameters and the number,scale and connectivity of the pore structure of loaded coal is obtained.The results show that under the combined action of applied load and microwave irradiation,the number of micropores within the coal sample continuously decreases and evolves to larger-scale pores,increasing the number of mesopores,macropores and microfractures.This effectively improves the connectivity between the various pore structures within the coal.Moreover,the appropriately increased microwave power or prolongated microwave irradiation time can promote the evolution of coal pore structure to a greater extent.The high power and short time microwave loading mode under the same microwave energy condition is more effective in increasing the number,scale and connectivity of pores.(2)The fracture structure evolution characteristics of the loaded coal under different microwave irradiation parameters are revealed,and the variation laws between microwave irradiation parameters and the fracture rate,fracture volume,fracture connectivity and damage factor of the loaded coal are determined.The results show that the loaded coal body under microwave irradiation produces not only new fractures,but also the proportion of pore throats with an equivalent radius of 0.4-1.2 mm and pore throats>4 mm in length in the primary fractures are greatly increased.This promotes the interconnection of the isolated initial fractures in coal,resulting in a maximum increase of 425.6%in the fracture rate of the coal body and an increase in the number of fissures with fracture volume>10mm~3.The above fracture evolution inside the coal body manifests itself as a damaging process in which the P-wave velocity of the coal body decreases and a large number of surface fractures develop.The damage factor of coal increases with microwave power and irradiation time.Under the same microwave energy condition,the high power and short time microwave loading mode have better damage and fracturing effect on coal.(3)The stress-strain process,mechanical characteristic parameter change law and thermal rupture morphology of loaded coal under different microwave parameters are clarified,and the mechanical response and thermal fracture characteristics of loaded coal under microwave irradiation are revealed.The results show that the modulus of elasticity of loaded coal under microwave irradiation decreases less,while the uniaxial compressive strength and peak strain all decrease with the increase of microwave power.This shows that the firmness of coal decreases and the ductility increases.With the increase of microwave power,the thermal rupture morphology of coal samples presents as a monoclinic shear failure,local X-type failure and overall composite Y-type failure.(4)The heating characteristics and permeability evolution law of loaded coal under microwave irradiation are obtained,and the range of microwave irradiation parameters beneficial to coal permeability enhancement is determined.The results show that the average temperature,maximum temperature and standard deviation of temperature on the surface of coal samples increased with the increase of microwave power and irradiation time.The resulting thermal stress exacerbates the development of loaded coal’s pore and fracture structure,which increases coal’s permeability by 364.7 times Under the action of 150 k J microwave energy,the microwave loading mode of 5 k W and 30 s increases the optimum effect of coal body permeability.(5)The mechanism of fracturing and permeability enhancement of loaded coal under microwave irradiation is as follows:On the one hand,the differential thermal effect of the microwave on the internal minerals of the coal body(the maximum standard deviation of the surface temperature of the coal sample is 57.8℃),which causes the obvious local temperature difference inside the loaded coal body and produces significant thermal stress(the thermal stress of the coal body reaches 6.96MPa under the microwave parameters of 3 k W and 50 s).Under the combined action of applied load-thermal stress,the microstructure(pore-fracture structure)of the coal body is damaged,which not only causes the compression closure,tensile failure and tip extension expansion of the primary pores and fractures in the coal body,but also causes the generation of new fractures in the coal body that are approximately perpendicular to the primary fracture wall and new non-directional micro-fissures between the interfaces of the minerals,forming the thermal fracturing effect of microwave on the loaded coal and increasing the permeability of coal body.Compared with the no-load condition,the unidirectional loading intensifies the microstructure development of coal,which makes the coal body increase the permeability better.On the other hand,the migration process of water evaporation,gas escape and particle volatilization produced by the pyrolysis of unstable minerals in coal under microwave irradiation has a reaming/hole-opening effect on the pore and fracture structure of coal.(6)The technical system of engineering application of microwave fracturing and permeability enhancement is constructed,including five aspects of coal quality information collection,ground fracturing experiment,drilling parameter design,microwave parameter selection and coal-fracturing effect evaluation.The prediction results show that the gas extraction rate increased by 31.9%compared to conventional extraction after 100 d of microwave-enhanced extraction.At the frequency of 0.915GHz,increasing the microwave power has the greatest improvement on the effect of coal seam gas extraction.There are 148 figures,21 tables and 217 references in this thesis.