Pore Evolution After Fracturing With Cyclic Liquid Nitrogen And The Mechanism Of Permeability Enhancing

In China,the coal seam with high gas content is characterized by the complicated geological structures,the existence of micro-pores,high adsorption of CBM and low permeability.As a result,the CBM is hard to be extracted.The permeability of coal seam is the main bottleneck restricting gas extraction.With the development of coal seam fracturing technology,many hydration crack anti-reflection measures applied and obtained the widespread attention,its characteristic is using nonaqueous material as coal seam crack anti-reflection medium.Especially in the western part of our country where water resources are scarce is more urgent.Based on the freeze-thaw erosion and the expansibility of water while freezing to ice,this research put forward a method for extracting CBM through increasing the permeability of the coal seams using the freeze-thaw cycle of liquid nitrogen?LN₂?.The joint influences of the frost heave and the expansive force induced by the gasification of LN₂ and the damage caused by LN₂ at low temperatures in the coal,promote the extension and connection of macroscopic and microscopic fractures.In this way,the fracture network is developed,so that the permeability of the coal seam is increased.This dissertation is based on the study of the pore structure evolution of the LN₂cycle and its cracking mechanism.Using laboratory experiments,numerical simulation and theoretical analysis,set up a LN₂ crack test system.Based on the relaxation spectrum analysis technique and scanning electron microscopy technique,the fine quantitative characterization of pore distribution of coal in the process of LN₂ cracking was realized.The relationship between the parameters of LN₂ injection and the characteristic parameters of coal fracture was obtained.The expansion and connectivity of macroscopic and microfissure space and the mechanism of LN₂ cracking in the process of LN₂ cracking are revealed.This dissertation proves the feasibility of improving the pore structure of the coal seam by LN₂.The crack mechanism of LN₂freezing coal was studied and the application thought of the coal bed methane was put forward.The main conclusions are as follows:1)Along with the freeze-thaw action,various sizes of pore were developed and more new pore and fissure sizes were generated in the coal.When the coal was frozen to some extent,lager size pores connected mutually to form the fractures with greater size.The results demonstrated that the rate of increment of both the effective porosity and total porosity of the coal are positively correlated with the LN₂ freezing time and number of cycles but the residual porosity showed a negative correlation.Results obtained using a scanning electron microscope?SEM?indicated that with the increase of freeze-thaw cycles using LN₂,an interconnected fracture network was gradually formed along the directions of cleats in coal masses.As a result,macroscopic cracks were generated on the surface of coal.There is found to be a pattern of circularly alternating stress?freezing shrinkage–frost heave–freezing shrinkage?in coal subjected to freeze–thawing using LN₂.Therefore,LN₂ freeze–thawing cycles can realize cyclic loading of this alternating stress so as to promote the development of fractures in the coal.The results suggest that appropriate control of the number of freeze–thaw cycles can result in effective fracturing of coal.2)The coal samples were subjected to different LN₂ freezing times and to freeze-thaw cycles and coals of different rank and with different moisture contents were tested.Changes in these four freeze-thaw variables changed the petrophysical properties of the frozen-thawed coal samples;the pore structure,porosity,and permeability of the coals were modified.Of these variables,the number of freeze-thaw cycles had the most substantial effect on modifying the coal's petrophysical properties.The degree of modification on the coals of different rank was affected by the coal's initial porosity.In general,lignites were modified the most,anthracite coal was modified less,and bituminous coal was modified the least.The study analyzed three of the classic NMR transforms for determining permeability and found that the Schlumberger-Doll Research?SDR?model matched the measured gas permeabilities most consistently.Based on this SDR permeability model,equations suitable for predicting the permeability of frozen-thawed low-rank coals were derived.3)The nuclear magnetic resonance?NMR?and fractal dimension theory were used to determine the properties of the coal's pore system after samples of low rank coal had been frozen and then thawed.A number of different freeze-thaw procedures using LN₂were employed.After the expression of the fractal dimensions based on the NMR T₂transverse relaxation times and porosity had been deduced,correlation analysis was conducted on the characteristics of the coal's pore system.The fractal dimensions of pores in frozen-thawed coal samples were divided into five types according to pore size and the state of the fluid in the coal pores.The results showed that the fractal dimension D_A of adsorption pores was less than two,indicating that these pores did not exhibit fractal characteristics.The fractal dimensions D_ir and D_T representing closed pores and total pores presented low fitting precision,so the closed pores showed insignificant fractal characteristics.However,the fractal dimensions D_F and D_S representing open pores and seepage pores had high fitting precision,suggesting that open and gas seepage pores exhibited a favorable fractal characteristic.Correlation analysis revealed that D_F and Ds were negatively correlated with LN₂ freezing time and the number of freeze-thaw cycles.After being frozen and thawed,coal porosity and permeability showed a strong negative correlation with fractal dimension and this relationship allowed predictive models for permeability and fractal dimensions?D_F and D_S?to be constructed.The models showed that the smaller the fractal dimension,the more uniformly the pores were distributed and the higher their degree of connection.These properties favor the production of CBM.4)This study investigated the mechanical characteristics,acoustic emission characteristics and development laws of fractures at four stages?compaction,elasticity,yield and failure?in the failure process of frozen-thawed coal samples under uniaxial compression.The compressive strength of the elastic modulus of the coal was reduced after the LN₂ was frozen,and the poisson ratio increased.By establishing a relational model for an elastic modulus based damage variable D and the LN₂ freezing conditions,it was found that variable D increased to and stabilized at 0.12 with the single freezing experiments.However,the damage to the coal caused by cyclic freezing and thawing was continuous and damage accelerated after 20 freeze-thaw cycles.To evaluate the different freeze-thaw variables for modifying the coal's mechanical properties,the effects of freezing time,number of freeze-thaw cycles,and the coal's moisture content were studied using uniaxial compression.Freezing the samples with LN₂ for increasing amounts of time degraded the coal's strength within a certain range,but freeze-thaw cycling caused much more damage to the coal.The third variable studied,freeze-thaw damage resulting from high moisture content,was restricted by the coal's moisture saturation limit.5)In order to study heat transfer and fracture propagation in in-situ geological formations fractured by LN₂ injections,this research studied heat transfer and fracturing in samples injected with LN₂ under true triaxial stress conditions.The results show that a single LN₂ injection mainly transfers heat through the solid media and only damages areas adjacent to the injection tubes.However,cyclic injections formed an effective fracture network and the heat was transferred along the fractures.Moreover,plastic deformation occurred in the whole volume of the sample and the main fractures coalesced until the sample failed.The results show that compared with a single injection,cyclic LN₂ injections resulted in more fractures appearing in the samples,and temperature decreased and rose more quickly during the freezing and thawing processes,respectively.The research revealed that high-pressure nitrogen gas transferring liquid water to the tips of new fractures is essential for cyclic LN₂ injection to form effective frost-heaving forces and fracturing.Therefore,the fracturing efficiency of the cyclic LN₂ injection is far higher than that of single LN₂ injections.