Study On Nonlinear Seepage Mechanisms In Pore-Fracture Combination Rock After High Temperature

Cracks usually develop in natural rock mass due to crustal movement and in-situ stress.There will be a lot of pores in the rock after high temperature.As for fractured rock mass,the permeability characteristics of porous rock mass after high temperature has always been a hot issue concerned by scholars.In the study of nuclear waste disposal and underground energy development,the pore-fracture combination of rocks after high temperature is closer to the actual geological model.In this paper,the rock pore-fracture combination after high temperature is taken as the study object.By means of laboratory test,theoretical analysis and numerical simulation,the seepage characteristics of the split fracture of rock,the porous rock after high temperature and rock pore-fracture combination after high temperature are studied.The main works are as follows:(1)Measurement of physical and mechanical properties of rocks with different lithology under saturated condition.The physical and mechanical properties of rock in saturated state are of great significance to the study of rock permeability.The standard samples of saturated coarse-grained granite,medium-grained granite,marble,red sandstone and limestone were tested by MTS815.02 test system.The uniaxial compressive strength,tensile strength,cohesion and acoustic emission parameters of different lithology were analyzed.The difference of mechanical parameters such as internal friction angle,Poisson’s ratio and elastic modulus under saturation condition.Based on these,the influence of lithology on permeability characteristics can be further discussed.(2)Non-linear seepage mechanism of three-dimensional rough single fracture with different lithology.In order to obtain a fairly wide range of roughness coefficients,coarse granites,medium granites,marble,limestone and red sandstone are used in order to obtain a fairly wide range of roughness of the same rock since the roughness of the same rock is basically the same.After machining into a standard rock sample,the center split into a rock sample containing a single three-dimensional rough crack.Non-contact JR three-dimensional scanning system was used to scan the three-dimensional features of the surface of the split rock samples,and the JRC curves of different lithology were obtained.On the basis of these,the permeability characteristics of fracture surface after different lithology splitting were studied.(3)Study on permeability characteristics of damaged granite by high temperature.There will be a large number of irregular pores and cracks in the rock after different high temperature exposure,these defects will have an innegligible effect on the seepage characteristics of the rock.The pore characteristics of the rock after high temperature have been systematically studied by the mercury compression test.Then,the rock permeability test system of China University of Mining and Technology was used to test the permeability of granite samples under different confining pressure after different high temperature,and the microstructure analysis such as SEM was carried out.The relationship between hydraulic gradient and volume velocity of damaged granite after high temperature is obtained,and the parameters such as equivalent permeability coefficient and critical hydraulic gradient of rock are further calculated.The seepage characteristics of granite samples after high temperature damage are discussed.(4)Study on nonlinear seepage characteristics of pore-fracture combination.According to the above-mentioned research contents,we can know that the seepage characteristics of rock in porous medium after high temperature and rock with single fracture are linear and non-linear,respectively.In this paper,the seepage characteristics under the combination of pore-fracture rock after high temperature were studied,and the contribution characteristics of pores and fractures to the whole seepage process were discussed.And the related theories such as Forchheimer formula are further analyzed in the light of this experiment.(5)Numerical simulation of seepage flow in pore-fracture media.Combined with the parameters of the model and the boundary conditions obtained in the test part,the numerical simulation of seepage flow with single rough fracture and pore-fracture combination is carried out by using the COMSOL Multiphysics finite element numerical simulation software.And the numerical simulation of seepage flow is carried out by using the finite element simulation software COMSOL Multiphysics.Furthermore,the seepage and flow characteristics of fluid in porous and fractured media were found,and the spatio-temporal evolution of pore water pressure in flowing medium was analyzed,which is corroborated by the experimental results.