Study On The Evolution Law And Mechanism Of Gas Migration In Unsaturated GMZ Bentonite Considering Temperature Effects

Bentonite is preferably used as a buffer/backfill material during the long-term operation of the high level waste（HLW）repository.Due to the seepage of groundwater,the generation and accumulation of gases due to complex physico-chemical reactions,as well as the convergence of the surrounding rock and other effects,bentonite is exposed to complex hydraulic-stress coupling problems.In addition,the heat generated by the decay of radioactive waste has an important effect on the hydraulic properties of the bentonite.Temperature effects lead to important changes in the hydraulic-stress coupling process of bentonite,which in turn affects the gas migration properties.As the preferred buffer/backfill material for the construction of HLW repositories in China,the study of temperature effects on the hydraulic and gas permeability properties of GMZ bentonite is of great practical importance.Based on the multiple pore structure of bentonite,from the perspective of macro-micro double porosity,the current study combines theoretical derivation,experiments,digital images,numerical simulations,and other means to systematically study the gas migration properties and mechanism of GMZ bentonite.The following research results were obtained.（1）The hydraulic properties test of GMZ bentonite considering temperature effects was conducted,and it was found that the temperature effects led to the phenomenon of weakened water-holding capacity.Considering the multi-scale pore structure inside the bentonite,the pores inside the bentonite are divided into inter-aggregate pores（macrostructural large pores）and intra-aggregate pores（microstructural small pores）,and then based on the macro-micro double porosity structure,a macro-micro double porosity water retention model considering the temperature effects was established,and the hydration characteristics were elaborated from the perspective of double porosity.Considering temperature effects on the hydraulic properties of bentonite,the effects of temperature-induced suction changes should be considered on the one hand,and the effects of microstructural deformation characteristics should be considered on the other hand.（2）The pore structure observation test of GMZ bentonite considering temperature effects was performed.Base on fixed-point scanning,the pore structure change of GMZ bentonite with different relative humidity were investigated at different hydration temperatures.It was found that the interaggragate porosity increased with the temperature in the hydration process of low humidity samples,while for high humidity samples,the inter-aggragate porosity occurred to increase first,then decrease,and then increase again with the increase of temperature.In the hydration process at different temperatures,the change of pore structure was influenced by the initial water content.When the temperature increases,the bentonite undergoes two water loss processes:inter-aggregate pore water loss and intra-aggregate pore water loss,while this process also confronts the effect of thermal expansion.Under the competing effects of water-loss shrinkage and thermal expansion,the porosity and pore size distribution of GMZ bentonite are complexly changed.（3）The gas permeability tests of GMZ bentonite during real-time heating were conducted.For different humidity samples,tests on the variation of gas permeability with temperature were carried out at different confining pressures.Overall,it was found that the gas permeability drecreased with increasing temperature.More particularly,for samples with high relative humidity,a sharp increase in gas permeability was observed at 80℃as a result of gas pushing out the moisture,while the increase was influenced by the confining pressure.For samples with an initial equilibrium humidity of 97.3%,the permeability decreases to the order of 10^-19m² when the confining pressure reaches7 MPa and the temperature increases to 40°C.When the confining pressure reaches 10MPa,although a sharp increase in gas permeability occurs when the temperature reaches 80°C,the increased permeability is still on the order of 10^-19m²,which basically meets the requirements of gas tightness.（4）Based on the macro-micro double pore structure,a coupled THM model of unsaturated GMZ bentonite was developed.The model is able to simulate the evolution characteristics of macrostructural large pores and microstructural small pores during hydration at different temperatures,and effectively reflecting the hydraulic properties of GMZ bentonite during hydration.Meanwhile,considering that the gas permeability is sensitive to the influence of confining pressure and temperature,the intrinsic gas permeability model considering the influence of stress-temperature-gas pressure was established,and then combined with the characteristics of gas flow mainly in the pores of the macrostructure,the change in effective gas permeability characteristics were obtained,which can simulate the changes in the gas migration characteristics of unsaturated bentonite when affected by confining pressure and temperature.This dissertation contains 60 figures,10 tables,and 257 references...