Influence Of Shear Induced Fracture Surface Structure Evolution On Seepage And Heat Transfer Characteristics Of Rock Mass

Since the 20 th century,China has entered a new era and achieved many landmark achievements,with its comprehensive national strength having made great leaps.With rapid development,the demand for energy in China is increasing.Geothermal energy is a type of clean energy with obvious advantages: stable and reliable,not affected by climate and weather,emits no harmful substances,and is environmentally friendly.Geothermal energy can be developed and utilized through the construction of enhanced geothermal systems,and hydraulic fracturing is one of the important technical means for the construction of such systems.The main function of hydraulic fracturing technology is to increase crack opening and improve reservoir permeability.There are many natural fractures in high-temperature geothermal reservoirs,which also provide favorable conditions for hydraulic fracturing.Therefore,it is necessary to study the effect of shear deformation on the flow and heat transfer characteristics of fractured rock masses.Conducting shear flow and heat transfer experiments under hightemperature conditions is difficult,so researchers often conduct numerical simulations to explore the mechanisms involved.Currently,most numerical studies on shear-flow coupling first construct the opening field after shearing,and then study the flow law.However,there are few studies that fully consider the mechanical properties of the shear process,and rare studies that take into account shear sliding in research on flow and heat transfer.There is also currently a shortage of research on the coupling of flow and heat transfer in fractured rock masses under shear deformation conditions.Taking geothermal resource exploitation and hydraulic fracturing enhancement technology as the research background,and taking the shear-flow-heat transfer of a fractured rock as the research object,this paper has established a three-dimensional numerical simulation method that considers the elastic deformation and contact of rock masses.The simulation results explain the fluid flow and heat transfer laws of shear fractures in rock masses.The main research achievements of this paper include:(1)Direct shear test of rock is simulated to reproduce shear scratches and to study the influence of shear on the distribution of roughness crack opening and evolution of contact area.Brazilian splitting is studied to produce rough fracture surface.Point cloud data of rough fracture surface are obtained by using three-dimensional scanning system and numerical model of fractured rock mass is established.Uniaxial compressive strength,Young’s modulus,Poisson’s ratio,tensile strength,density and basic friction angle are obtained through uniaxial compressive test,Brazilian test and inclination test.Direct shear test is carried out on fractured rock mass and shear process is reduced by numerical simulation.The stress-displacement relation curve is in good agreement with laboratory test.The shear displacement contact area is superimposed to form the complete contact area during the shear process.The complete contact area is consistent with the scratches of rock samples after shear,which is rare in other studies.The numerical simulation results can well restore the laboratory test,and the opening distribution and evolution during shear process can be presented by numerical simulation,which provides a basis for subsequent research.(2)Based on the numerical simulation of direct shear test,the SHEAR-SEEPAGE coupling model of fractured rock mass is established.Based on the direct shear test numerical simulation,the shear seepage numerical model is established by extracting rough fracture surfaces with different shear displacements,and the influence of shear-induced spatial structure evolution on seepage characteristics is analyzed.With the advancement of shear displacement,the spatial structure evolution between the cracks results in an advantageous seepage path with faster flow rate and denser streamlines when the fluid flows.Shearing is the change of space structure between cracks,the change of opening distribution and contact area,and the formation of an evolving seepage basin.When the seepage catchment changes little,mechanical opening is the dominant factor of seepage characteristics such as seepage flow rate,seepage flow rate,permeability and equivalent hydraulic opening.The larger the opening,the faster the average flow rate,the larger the seepage flow,the higher the permeability and the greater the equivalent hydraulic opening.When the seepage catchment changes greatly,the influence of opening degree on seepage characteristics is small.(3)A shear-seepage-heat transfer coupling model for fractured rock mass is established considering the influence of fluid density and dynamic viscosity.The shear-seepage-heat transfer coupling model was established with rock mass as rock matrix and space between rough cracks as fluid seepage catchment area.The influence of shear on seepage heat transfer characteristics was studied by numerical simulation.When cryogenic fluid is injected into high-temperature rock by fissures,heat is absorbed from the inlet.When seepage heat reaches a stable state,the temperature between the fissures is the lowest.The cryogenic zone extends from the inlet to the outlet along with the fissure space,and the thermal cone expands outwards from the fissure.Due to the uneven distribution of three-dimensional rough fissure opening,the seepage path of fluid in the fissure is complicated.The temperature on the surface of rock mass is related to the fissure shape.In the area with larger opening,the fluid velocity is faster and the flow line is denser.The temperature of the main seepage path formed is lower,the cold front extends forward along the main seepage path,and the fluid absorbs more heat.The smaller the equivalent hydraulic opening,the larger the average convection heat transfer coefficient and the more heat transfer from the high temperature rock mass are under the condition of unchanged fracture spatial structure.The space structure between the cracks is changing constantly during the shear process.The relationship between equivalent hydraulic opening and convection heat transfer coefficient will not strictly follow this rule,but it still shows that the smaller the equivalent hydraulic opening is,the larger the average convection heat transfer coefficient is.