Damage Evolution Law And Permeability Enhancement Mechanism Of Coal Induced By Liquid Nitrogen

In China,coal-bed methane（CBM）resources are abundant and promising for exploitation.However,the reservoir has a complicated geological structure,with low permeability,high ground stress and strong heterogeneity.Liquid nitrogen（LN₂）,a new type of waterless fracturing medium,can effectively fracture coal,and has the advantages of low reservoir damage,low environmental pollution and excellent permeability enhancement effect.In this study,considering the metamorphism degree of coal,current structures and in-situ stress state during the process of coal formation,the damage evolution law and permeability enhancement evaluation of coal subjected to LN₂ fracturing were thoroughly investigated from five aspects,including mechanical property degradation characteristics,macro/micro fracture evolution law,LN₂permeation boundary motion,damage evolution law and gas production effect evaluation.The optimization scheme of LN₂ combined fracturing reservoir to promote the efficient development of CBM was proposed.The main conclusions are as follows:（1）Degradation experiment of coal subjected to LN₂ freeze-thaw,the low temperature cracking effect of LN₂ on coal was quantitatively evaluated and the cracking cause was analyzed from the molecular point of view.Researches showed that low temperature LN₂ can promote the cracking of weak bridge bonds in coal molecules,accelerate the initiation and propagation of thermal cracks,improve the connectivity between pores/cracks,and reduce the effective bearing capacity of coal.During the loading process,the randomly distributed thermal cracks progressively affect the propagation path of macrocracks,increase the complexity of the fracture surface after failure,and improve the degree of coal fragmentation.LN₂ can be used as an excellent fracturing medium for enhancing permeability in coal seams with low permeability.（2）Exploratory experiments on the degradation effect of LN₂ cooling on coal under different storage and geological conditions were conducted.The differences in mechanical properties degradation and failure characteristics of coal under various factors were revealed.It was found that LN₂ fracturing was more suitable for use in shallow and low rank coal seams.After LN₂ cooling,a transition from mixed-mode failure to tensile failure occurred at the 60°bedding angle.This transition caused the maximum reduction in tensile strength and an inflection point occurred on the fracture surface complexity curve.The decrease of tensile strength and the increase of fracture surface roughness decreased with the increase of coalification degree.The complexity of crack propagation path and the degree of fragmentation gradually weaken with the increase of coalification degree.With the increase of confining pressure,the fracture surface roughness increased in sequence,and the decrease of compressive strength decreased first and then tended to be stable.（3）A thermal-hydro-mechanical-moving boundary coupling model during LN₂fracturing was established by simultaneously considering the effects of water-ice phase change and fluid permeation boundary motion.The distribution law and evolution characteristics of pore pressure,temperature and stress in coal were accurately calculated during LN₂ fracturing.The effect of LN₂ permeation boundary motion on fracturing results was revealed.The difference of LN₂ penetration depth under the effect of coal rank and bedding plane was further analyzed.The studies indicated that the penetration depth of LN₂ was positively correlated with the permeability of coal seam,which directly affects the stress distribution in coal,and then affects the damage degree of coal.（4）A new thermal-hydro-mechanical-damage coupling model suitable for LN₂fracturing coal was constructed.The low-temperature fracturing effect of LN₂ on coal was quantitatively characterized by the initiation pressure,fracture pressure and fracturing crack propagation law.The damage degree of coal was quantitatively evaluated by counting the damage number during fracturing.The differences in damage response of various factors were further revealed.The studies indicated that bedding angle and lateral pressure coefficient can change the direction of crack propagation.The coal rank had a significant effect on the uniform degree of damage distribution in coal after fracturing.The buried depth of coal seam was positively correlated with fracture pressure.（5）Simulation of coal permeability enhancement under various fracturing methods such as water,nitrogen（N₂）and LN₂ was carried out.The evolution law of damage and permeability of coal was analyzed during fracturing.The effect of coal permeability enhancement after fracturing was quantitatively evaluated by attenuation coefficient,stability coefficient and strengthening coefficient of gas production rate.The studies indicated that LN₂ fracturing had the best effect in improving permeability,followed by N₂ fracturing,and finally hydraulic fracturing.Based on the consideration of engineering economic benefits,the combined fracturing scheme of“LN₂-N₂”was proposed.Considering the influence of the LN₂ injection time ratio on the effect of coal permeability enhancement,the technical parameters of the combined scheme were optimized,to provide theoretical guidance for the efficient development of CBM in low permeability coal seam.The dissertation has 91 figures,25 tables and 189 references.