Experiment And Simulation Study Of Preferential Flow And Tracer In Bifurcate-cross Fracture

With the development of society and the increase of groundwater engineering,the safety of nuclear waste geological disposal,oil exploitation and carbon dioxide geological storage has been paid more and more attention worldwide.The study of fracture water transport in bedrock has become an important topic in hydrogeology.As an important part of natural network fracture,the mechanism of water transport in cross fracture is not fully clear due to strong heterogeneity and anisotropy.Among them,quantification of seepage and solute transport behavior and its controlling factors are still challenging problems.In this paper,a physical model of bifurcate-cross fracture laboratory is constructed,and water flow and temperature tracing tests in different bifurcate fracture(one entry and two exit,one entry and three exit)and crosse fracture(two entry and two exit)are carried out respectively.The seepage movement state of different inlet and outlet modes in bifurcate-crosse fracture is analyzed quantitatively,The influence mechanism research and numerical simulation of dominant flow and temperature tracer migration under different inlet and outlet modes,inlet and outlet angles,roughness,flow rates and hot water injection rates are conducted systematically.The main conclusions are as follows:(1)There is a non-linear relationship between velocity V and hydraulic gradient J in bifurcate-crosse fracture.The flow is non-linear,and the fitting accuracy of Forchheimer formula is higher.The coefficients of the quadratic term of Forchheimer's formula show a decreasing law with the increase of the convexity or the decrease of the inlet and outlet angles,and the flow tends to Darcy flow.(2)There is a good linear fitting relationship between the dominant flow ?q and the total flow Q in the fracture of the different types of inlet and outlet water,and the roughness of the fracture and the angle of the inlet and outlet water have obvious influence on the dominant flow effect.In cross fracture with two entry and two exit and bifurcate fracture with one entry and three exit,the bigger the roughness difference between the outlet channel and the inlet and outlet angle,the more obvious the dominant flow effect.In the bifurcate fracture with one inlet and two outlets,the roughness of straight fracture is bigger,and deflection fracture is the dominant channel.The dominant flow effect is positively correlated with the difference of roughness,and negatively correlated with the angle of inlet and outlet water.There is a critical roughness difference between 0-1mm roughness and the critical angle of inlet and outlet water from 30 to 45 degrees,which makes the discharge of two single fracture channels of outlet water consistent.(3)In the temperature tracer migration test,the asymmetry of temperature penetration curve is non-normal distribution,and there is obvious tailing phenomenon,the higher the peak value of the temperature curve,the larger the flow rate in the channel.The peak value of temperature curve does not decrease with the increase of migration distance.The peak value mostly appears at probe point 2 or probe point 3,which indicates that the temperature mixing mode at the intersection of fracture is between the complete mixing mode and the linear mode.(4)The peak value of temperature penetration curve changes with the different hot water injection.With the increase of hot water injection,the time of arrival of temperature peak value gradually delays,and the peak value gradually increases.Peak value increases with the increase of flow rate and decreases with the increase of roughness and inlet and outlet angle.Peak arrival time is inversely proportional to flow rate and is proportional to roughness and inlet and outlet angle.It shows that there is interaction between seepage field and temperature field.(5)Using ADE model and TPL model to simulate the temperature penetration curve,and comparing and analyzing the fitting effect.For the trailing phenomenon of the temperature curve,the fitting effect of TPL model is obviously better than that of ADE model,and the simulation accuracy is higher,which is more consistent with the measured curve.