Experimental Investigation On The Rheology-seepage Evolution Law Of Coal By Thermal-mechanica Coupling

In recent years,Coalbed methane(CBM)has become a potential alternative energy to fossil fuels for its clean property,high calorific value and large storage in the field of energy exploitation.Coal permeability is the key factor determining the effectiveness of coal-bed methane(CBM)exploitation.In the CBM production cycle,the long-term high temperature and stress make the seepage paths in coal evolve with time.The connected pores and fissures in coal are important channels for gas diffusion,seepage and migration.The evolution and deformation of these channels are not only related to the long-term temperature field and stress field around the coal,but also show significant time dependence.On one hand,the coal strength decreases,cracks extends and mineral dehydrates by the high temperature,on the other hand,pores are compressed and cracks are closed by the long-term rheology of coal,consequently,the permeability of coal and the extraction efficiency of CBM is affected.Therefore,the topic “experimental investigation on the rheology-seepage law of coal by the thermo-mechanical coupling effect” is taken as the subject of this paper and the following research are carried out:1)The uniaxial compressive multistage creep tests of lean coal are performed in YDY-100 uniaxial rock creep testing machine.In creep tests,strains are instantaneous or time-dependent at different stress loads due to the creep start stress threshold.When stress is lower than the threshold,strains is instantaneous.Once stress reaches or exceeds the threshold,strains show instantaneous strain,transient creep and steady creep,presenting time-dependent creep.The axial and radial strain is not completely coordinated in creep.At low stress levels(24.37%~34.11% of the uniaxial compressive strength),both the axial and radial strain exhibit instantaneous elastic response and the axial strain and the radial strain are quite coordinated;at the intermediate stress level where the stress reaches the axial creep start stress threshold but has not yet reached the radial creep start stress threshold(34.11%~43.86% of the uniaxial compressive strength),the axial and radial strain differences are obvious because the axial strain shows transient creep and stable creep while the radial strain is still instantaneous;at high stress levels(53.60%~76.02% of the uniaxial compressive strength),both the axial strain and the radial strain show time-dependent creep strain,but their creep strain rates are still different.2)The stress relaxation tests are conducted in micro uniaxial rock testing machine.In stress relaxation tests,stress on specimens experiences rapid relaxation and transient relaxation and finally stabilizes.Anisotropy is observable from stress relaxation tests.The failure strengths and elastic moduli of the bedding-perpendicular samples are generally greater than those of the bedding-parallel samples due to the hardening effect.At the same strain and before the final strain,the relaxation stress of the bedding-perpendicular samples is also greater than that of the bedding-parallel samples.At the final strain,for the bedding-parallel samples,the relaxation stress increases more evidently with higher initial stress relaxation rate,and the residual stress ratio decrease more remarkably.3)The instantaneous elastic modulus and the viscosity coefficient are combined to analyze the creep mechanism.During creep,the instantaneous elastic modulus increases,reflecting the stiffness change of sample due to the hardening effect,and the viscosity coefficient decreases,reflecting the damage effect of the sample during creep.At low stress,the hardening effect works and the strain is instantaneous.When the stress reaches or exceeds the critical creep start stress threshold,both the hardening effect and the damage effect work,and transient creep and steady creep are shown.During long-term creep,samples are first hardened,then weakened,and finally fail due to the large accumulation of damage.4)The micro-CT scanning is used to compare and analyze the cross-sections of samples after stress relaxation at different strain levels.During the stress relaxation of lean coal at different strain levels,the sample is compacted at the beginning and then gradually damages with the increase of strain.The damaged density increment of each scanning cross-section increases slightly at first,then gradually decreases.At the final strain,the sample failure is reached and the damaged density increments of each scanning cross-section decrease significantly.During the stress relaxation process,the microstructure adjustment is mainly manifested by the horizontal expansion along the radial direction of the sample and the relative slip along the axial direction of the sample.5)The seepage evolution experiments of coal changing with raised temperatures and the creep-permeability evolution experiments with creep consideration are both conducted.The permeability of coal decreases gradually with raised temperature at short-term static loading,and the higher the temperature,the lower the temperature sensitivity of coal permeability.When the creep is considered,the creep of coal undergoes three phases: the hardening phase before the creep start stress threshold,the creep phase with compressive volume deformation and the creep phase with expanding volume deformation.In the whole process,the permeability of coal gradually decreases and then increases with the creep strains and there is a good consistency between the permeability evolution and time-dependent volumetric strain.The increasing temperature is helpful for the occurrence of coal creep strain,resulting in the decrease of the creep start stress threshold and the ultimate failure strength.At high temperature,the creep of coal shows a greater influence on the decrease of permeability,and the maximum loss of coal permeability can reach 40.35%.6)A more accurate fractional derivative model was established on basis of the generalized Kelvin model.On basis of the hardening-damage creep mechanism of lean coal,hardening function and damage function are both introduced into the model,thus the creep constitutive equation is obtained.Under the thermo-mechanical coupling,the instantaneous elastic modulus and the rising temperature exhibit a quadratic polynomial relationship during the lean coal creep,and the viscosity coefficient decreases with the rising temperature in a linear relationship.On basis of the temperature-dependent instantaneous elastic modulus and viscosity coefficient of coal,a high temperature creep model of coal is established and introduced into the relationship between deformation and permeability.Therefore,a permeability model considering the high temperature creep effect of coal is obtained.This model can well describe the decreasing evolution law of permeability before the turning point.