Study On Cracking Effect And Permeability Enhancement Mechanism Induced By Liquid Nitrogen In Low Permeability Coal

Liquid nitrogen(LN₂),a fracturing medium with excellent properties,has attracted more attention due to its multiple cracking effects on coal.Its potential application in coal bed methane exploitation has become a research hotspot.In this paper,laboratory experiments,theoretical analysis and numerical simulation were used to explore the key scientific issues of LN₂ cracking low permeability coal reservoirs.The macro-micro mechanical mechanism of coal subjected to LN₂ cooling was systematically investigated.Then,the influence of LN₂ freeze-thaw cycles on deformation,failure and permeability of coal was analyzed.On the basis of revealing the influence of multiple cracking effect of LN₂ on results of coal fracturing,the fracture-crack propagation criterion was established.Finally,the laws of the pore pressure distribution and fracture pressure evolution of coal under the freeze-thaw cycles were explored.The main conclusions are as follows:(1)The influence of LN₂ cryogenic cooling on the physico-mechanical properties and macro-micro structure of coal was analyzed in detail.Uniaxial compression,Brazilian splitting,three-point bending,and scanning electron microscopy tests were conducted to study the changes of compressive/tensile strength,fracture toughness,failure characteristics and macro-micro structure of coal subjected to low temperature LN₂ treatment.The results showed that LN₂ treatment can promote the intergranular and transgranular microcracks propagation,and simultaneously induce the destruction of pore structure and matrix inside the coal,resulting in the deterioration of physical and mechanical properties of freeze-thaw coal.It should be noted that the low temperature cooling environment induced by LN₂ had a certain strengthening effect on the mechanical properties of coal.Moreover,the two treatment methods of LN₂,including freeze-thaw and low temperature freezing,can improve the degree of coal fragmentation,especially for the freeze-thaw treatment.In summary,freeze-thaw treatment can crack low-permeability coal more efficiently.(2)Based on the fractal theory,the LN₂ low temperature cracking effect on coal was evaluated quantitatively,and its influencing mechanism on the formation of crack network was analyzed.The porosity distribution,acoustic emission parameters,fragmentation distribution and fracture surface morphology of coal treated by LN₂freeze-thaw were obtained by laboratory experiments,and their fractal characteristics were investigated.The results showed that after the LN₂ freeze-thaw treatment,the pore structure distribution of coal was more homogeneous,the its connectivity was better,and the disorder of crack propagation in space was enhanced,which led to the increase of the fragmentation uniformity and number,and the increase of fracture surface roughness.The fractal study showed that the crack network complexity and volume fracture degree of the low permeability coal increased under the cracking effect of LN₂freeze-thaw.(3)The permeability enhancement mechanism and the deformation failure law of coal induced by LN₂ freeze-thaw cycles were explored.The experimental methods of uniaxial/triaxial compression test and permeability test were used to study the influence law on deformation failure and permeability of coal treated by LN₂ freeze-thaw cycles.The results showed that with the increase of LN₂ freeze-thaw cycles,the micro-cracks continued to grow,and the failure mode of the micro-structure changed from the primary crack propagation to the fracture of the matrix skeleton.These led to the enhanced connectivity of the pore structure and the weakening of the deformation resistance of the matrix framework,then the coal permeability and the volume fracturing degree at the loading increased These led to the enhancement of the pore structure connectivity and the decrease of matrix framework deformation resistance,then,the coal permeability and the volume fracturing degree at the loading increased.Finally,the mechanical properties,deformation failure,and permeability of coal had changed in the direction that was favorable for coal fracturing.The results showed that LN₂ freeze-thaw cycles treatment can be used as an effective mean to greatly improve the permeability and the volume fracturing degree of coal.(4)The LN₂ fracturing effects on coal with different initial states were studied,the effects of thermal shock,low-temperature cracking and freeze-thaw damage induced by LN₂ on coal fracturing were revealed.The LN₂ fracturing simulation experiment for coal was designed and conducted.The fracturing characteristics of the coal with different initial states were obtained,and combined with the numerical simulation results of temperature field and stress field around the hole wall,the multiple LN₂cracking effects on coal fracturing were explored.The results showed that the coal with higher initial temperature had the lager damage zone around the hole wall,the little fracturing pressure as well as the higher volume fracturing degree.Under the influence of low-temperature cracking effect,the crack opening degree of the freezing coal increased,and the difficulty of fracturing fluid entering into coal were depressed.This led to the transfer of the fluid pressure to the opening crack,then the crack promoted.Compared with the untreated or freezing coal,the freeze-thaw coal had the largest damage zone around the hole wall,the least fracturing pressure and the highest volume fracturing degree.Associated with the engineering practice,the method‘LN₂ freeze-thaw cycles treatment+the nitrogen fracturing with high pressure'was proposed so that the multiple fracturing effects of LN2 can be fully used(5)The crack initiation criterion of coal fracture under the influence of LN₂ was established based on the experimental results by introducing the degradation coefficient of fracture toughness,and the influence of LN₂ freeze-thaw cycles on pore pressure distribution and fracture pressure was studied by using fluid-solid coupling numerical simulation method.The results showed that both the fracture pressure and the pore pressure around the borehole at the time of coal fracture decrease gradually with the increase of freeze-thaw cycles caused by the increase of pore pressure distributed around the borehole and the decrease of coal fracture toughness.Therefore,the LN₂freeze-thaw cycles assisted coal fracturing is expected to be an effective method for reservoir stimulation.There are 88 figures,12 tables and 153 references.