Study On Nonlinear Flow Behaviors And Coupled Hydro-thermal Simulation Of Fractured Rock Masses

The fluid flow and heat transfer characteristics of rock mass is closely related to the safety and stability of oil and natural gas production and nuclear waste disposal project.A large number of joints,fractures and faults in natural rock mass are the main seepage channels,which plays an important role in the stability and permeability of rock mass.Therefore,it is of great theoretical and engineering significance to deeply understand the fluid flow and heat transfer behavior of the fracture and the fracture network.The laboratory test,theoretical analysis and numerical simulation method were used to study systematically the fluid flow and heat transfer characteristics of fractured rock mass.Firstly,the fluid flow tests in roughness-walled single fracture and intersecting fracture were conducted,respectively.The coupled hydrao-thermal?HT?model of fracture was then derived.Finally,a reservoir scale coupled HT modelling method was proposed to study geothermal reservoir exploitation.The main research points of this paper are as follows:?1?A roughness-walled single fracture visual flow test equipment was designed,and five rock fracture samples with different roughness were prefabricated.A series of seepage tests were carried out for fracture samples under different water pressure.Results show the Forchheimer equation can capture nonlinear flow behavior of fractures.The nonliear parameter is related to nonlinear flow properties and geometries of fractures and experiences an increase of 1-5 orders of magnitude,varying from 6.0e10 to 5.0e15.A semi-empirical equation of nonliear parameter,dependent on initial mean aperture,contact ratio and fractal dimension,is developed.A new empirical critical Reynolds number equation is proposed,which offers a mathematical prediction for evaluating the nonlinear flow effect.Critical Reynolds number decreases with increasing contact ratio and fractal dimension,ranging from 0 to 60.Additionally,based on the velovity of fracture element proportional to the element aperture of subfracture,a nonlinear seepage mathematical model of three-dimensional roughness-walled fracture was established and verified by seepage experiments.?2?Based on the rigorous mathematical theory,the coupled HT model between the fluid and the rock fracture considering the variation of roughness and aperture was obtained by using the fluid flow and heat transfer semi-analytical soultion of wedge-like fracture.The effect of fracture roughness and injection water pressure on heat transfer characteristics of water flow through a real single fracture obtained by three-dimensional optical scanner is analyzed.The results show that the roughness of fracture has great influence on the distribution of local heat transfer coefficient,but has little influence on the distribution of temperature.The water pressure affects the temperature and local transfer coefficient value,not affect its distribution in fracture.In addition,the relationships between the water pressure,hydraulic aperture,fractal dimension and the average heat transfer coefficient were obtained.With the increase of fractal dimension,the average heat transfer coefficient decreases.The increase of water pressure leads to the increase of average heat transfer coefficient.However,the hydraulic aperture is negatively correlated with the average heat transfer coefficient.?3?Four rock intersecting fracture samples with different intersecting angles were prefabricated,including Straight fracture,Buckling fracture,Crossing fracture and Furcating fracture.A series of seepage tests under different water pressure were carried out to study the nonlinear seepage behavior of the four intersecting fractures,and corresponding fluid flow numerical simulation was carried out.According to the fluid flow numerical and experiment results,The Forchheimer equation still provides a good relationship to quantify the nonlinear flow regime in intersecting fractures.As the buckling angle?increases,the Forchheimer coefficient?decreases for the case of Buckling fracture and the critical hydraulic gradient J_c increases.In contrary,the Forchheimer coefficient?for the case of Crossing fracture increases and critical hydraulic gradient J_c decreases nonlinearly with increasing crossing angle?.The flow nonlinearity for the case of Furcating fracture depends on the values of furcating angle?₁ and?₂.The greater the abs??₁-?₂?,the greater the value of Forchheimer coefficient?and the smaller the value of critical hydraulic gradient J_c.In addition,three types of semi-empirical relations of intersecting fracture nonlinear coefficient are obtained.This formula is related to intersecting angles??,?,??and hydraulic aperture.?4?A finite element formulation combing Reynolds equation and Forchheimer equation was proposed for three-dimensional fracture networks,and a direct iteration method was used to solve large system of nonlinear equations.The effects of fracture network connectivity and fracture surface roughness on nonlinear flow regimes in three-dimensional fracture networks are analyzed.High fracture surface roughness can easily produce preferential flow paths,i.e.,higher channeling effects,even when there is a good connectivity in the fracture networks.Due to the effect of nonlinear flow,as the hydraulic gradient increases,the transmissivity of fracture networks gradually decreases.The transmissivity increases with increasing dimensionless percolation density,but decreases with the increasing fracture surface roughness.?5?Based on the heat transfer model and the nonlinear fluid flow model,a simplified coupling HT calculation method of three-dimensioanl fracture network was establised and its reliability was verified by COMSOL code.The calculated results are consistent with the field monitoring results.a large velocity zone appears at injection and production well locations,which value is approximately 10 times bigger than that at other locations.The locations may appear turbulent flow.Near the injection well,the temperature increases sharply.When the distance from injection wall reaches about 100 m,the temperature is stable,reaching the rock temperature of 483.15 K.The injection pressure will affect the geothermal exploitation life of the reservoir.The higher the pressure lead to the longer time keeping initial temperature of the producing well and the longer the exploitation life of the reservoir.