Numerical Simulation Of Seepage And Heat Transfer In Rough Fractured Rock Mass

With the rapid and sustainable development of economy,the reserves of traditional fossil energy can not meet the energy demand of human development.Geothermal energy has broad application prospects and market because of its rich reserves,wide distribution and renewable characteristics.In the process of geothermal energy development,there are many problems and technical difficulties that need to be overcome inevitably,among which the seepage and heat transfer of dry hot rock is the key to affect the exploitation of geothermal energy.In this paper,based on the general fracture seepage coupling problem,considering the roughness,the rough fracture is taken as the basic geometric characteristics of the model,and the numerical simulation is carried out by the finite element software COMSOL multiphysics.On this basis,the influencing factors and characteristics of seepage and heat transfer in rough single fracture and rough cross fracture are calculated and analyzed,and the laws are summarized:In this paper,roughness is considered as the basic geometric characteristics of rock fracture seepage and heat transfer coupling model.Five of Barton's standard roughness profile curves(JRC)are selected as the Roughness Characterization of rock fracture.On this basis,a numerical model is established to simulate the rock fracture seepage and heat transfer coupling.Firstly,the influence of several parameters on the rough single fracture model is studied,in which the fluid velocity and fracture opening have a greater influence on the seepage heat transfer,while the roughness and shear displacement have a relatively small influence.Larger flow velocity and fracture opening increase the flow into the rock,and more fluid exchanges heat with the heat reservoir in a short time,and then is exploited.In this trend,the thermal breakthrough time of fluid outlet temperature is earlier.The temperature of the inner surface of the fracture is greatly affected by the undulation of the fracture.With the decrease of fracture opening,the temperature of fracture inner surface increases gradually.The seepage and heat transfer in the fracture under shear displacement is more complex.After dislocation,the pore size distribution is uneven,and the local pore size is very small,where the heat transfer intensity is high.When the fluid passes through the place with the smallest aperture,it is easy to form vortices,resulting in the decrease of the seepage channel,where the heat flow forms thermal vortices,affecting the thermal convection between the fluids.Based on the study of single rough fracture,the number of cracks is increased and connected with each other,and a rough cross fracture model is constructed to study the seepage and heat transfer characteristics when multiple cracks cross.Compared with single fracture,the temperature field and seepage field of cross fracture have changed greatly.The increased seepage path increases the area of fluid and rock exchange,which makes the heat recovery more fully.Under the condition of multiple injection ports,the flow into the model increases,and the heat produced by the fluid also increases.Then,the influence of the geometric characteristics of the secondary fracture on the seepage and heat transfer of the model is considered.By changing the opening of the secondary fracture,it is found that with the decrease of the opening of the secondary fracture,the advantage of the main fracture as a fluid path increases,and it is more difficult for the fluid to enter the secondary fracture.The hydraulic gradient is lost at the intersection,which makes the flow velocity into the secondary fracture decrease significantly,and then affects the heat transfer If the interval is longer,the fluid at the exit of the main fracture will break through prematurely.When the fractures are perpendicular to each other in the model,the heat exchange between the fracture fluid and the rock heat storage is sufficient,the temperature of the outlet fluid remains relatively stable,the temperature decline slows down,and the heat storage of the rock is absorbed more fully,which is conducive to the heat recovery efficiency and prolonging the heat recovery period of the system.