Study On Evolution Law Of Gas Permeability Of GMZ Bentonite Under Hydromechanical Coupling

The use of nuclear energy is of great significance to alleviate the energy pressure and reduce environmental pollution in China.However,it will produce radioactive waste,the rational and safe disposal of which is a key issure to be solved urgently.At present,deep geological disposal is widely accepted,China has basically decided to use GMZ bentonite in Inner Mongolia as a barrier of disposal of reservoir engineering.With the passage of time,gases are generated in the repository,accompanied by the effect of groundwater and surrounding rock,the gas tightness under hydromechanical coupling is one of the key scientific issues in the later stage of deep geological disposal of high-level radioactive.In this paper,the hydromechanical coupling characteristics of high compacted GMZ bentonite were studied by water retention test,then the samples were used to test the gas permeability.Further more,the gas-liquid-solid three-phase coupling equation was established through theoretical analysis,and the numerical simulation was compared with the experimental results,which provides references for investigating the migration law of gas in high compacted bentonite and evaluating the gas tightness of high level radioactive waste disposal system.The specific research content are as follows:?1?The water retention test of high-compacted GMZ bentonite samples was carried out.The soil-water characteristic curve of bentonite was measured.It can be found out that the water content of the samples under free-expansion conditions is significantly affected by the change of suction,the relative mass and volume under different humidity would have a significant change,the changes can be reach to 7.16%and 16.51%respectively in the 98%relative humidity.?2?Gas permeability test of high-compacted GMZ bentonite was conducted.The gas permeability of the sample after the water retention test was measured under different confining pressures.It is found that the permeability ranges from 10^-19 to 10^-14m²,which shows a great changes.The permeability of the specimen was obviously influenced by the confining pressure and the ambient humidity.The permeability of the sample under a relative humidity of 98%under pressure from 1 MPa to 10 MPa is even reduced by three orders of magnitude.When the sample is first compressed at 1 MPa,with the increase of relative humidity,the permeability of the sample increased first and then decreases.After cyclic loading,the permeability decreases with the relative humidity increasing,which can be attribute to the changes of saturation and porosity with the environmental humidity.The permeability of the specimen is not only influenced by the confining pressure and the ambient humidity,but also influenced by the pressure of gas injection,the gas permeability increases with the gas injection pressure.This phenomenon is related to slippage effect,the slip factor decreases with the intrinsic permeability increasing.?3?The hydromechanical coupling control equation of high-compacted GMZ bentonite was established.The deformation caused by suction and the effective porosity changes by the special pore structure of bentonite was considered.The adsorption and hydration process of high-compacted bentonite is simulated by Comsol Multiphysics software,and the changes of saturation and porosity is studied,and its feasibility and accuracy is proved by compared with test results.Further more,the effect of saturation and porosity on the permeability is studied,the permeability decreases with the increase of saturation and increases with the increase of porosity.Since the ambient humidity influences the saturation and porosity at the same time,the permeability of the sample increased first and then decreases with the ambient humidity increasing.