Study On Estimation Of Permeability Tensor For Deep Fractured Rock Mass With Multi-physics Coupling

Enhanced Geothermal System refers to the system of establishing artificial thermal reservoir in deep high temperature rock mass to obtain geothermal energy,which is generally composed of injection wells,production wells and fracture networks.As the main channel of seepage and heat transfer,fracture network permeability is the key factor affecting the operation efficiency and heat extraction performance of the enhanced geothermal system.Because the fracture network of thermal reservoir is in the complex environment of high ground stress,high osmotic pressure,high temperature,the field and laboratory test of the permeability coefficient of fractured rock mass are both difficult.In addition,the scale effect and anisotropy of the permeability coefficient of fractured network also make it difficult to determine the permeability tensor of fractured rock mass effectively.Therefore,it is important to compare and analyze the effects of different coupling conditions(water-thermal coupling,thermal-hydraulicforce coupling)on the permeability characteristics of fractured rock masses.Based on the geological survey data,the probability density function of the geometric characteristics of fractures(trace length,dip angle and center point)in rock mass was determined.Using Monte Carlo method with MATLAB program,a series of two-dimensional random fracture network models of different sizes was generated.The numerical calculation of seepage,water-thermal coupling and thermal-hydro-mechanical coupling was carried out by using COMSOL to analyze the permeability characteristics under the condition of fracture network coupling,that is,the variation of the equivalent permeability coefficient with the model size under the coupling condition.According to the visual judgment method and ellipse fitting method,after the equivalent permeability coefficients in different sizes and different directions were fitted and error analyzed,the permeability ellipse under the corresponding conditions of fractured rock mass and the size of the REV,principal direction,principal permeability coefficient and its permeability tensor in the study area were determined.By using the numerical simulation method to determine the permeability tensor of the fracture network under the above coupling conditions,the fracture network in the artificial thermal reservoir of the European Soultz geothermal system is numerically simulated,and the permeability tensor under the corresponding conditions is determined,which can be used for the calculation and analysis of the corresponding seepage field.It has been found that the equivalent permeability coefficient for deep fractured rock mass with Multi-physics coupling also presents the characteristics of scale effect and anisotropy.In the case of water-thermal coupling,the equivalent permeability coefficient of the fracture network specimen will decrease with the increase of the specimen size and gradually tends to be stable.According to the ellipse fitting method,the representative elementary volume and permeability tensor of the fracture rock mass were determined.Compared with the single seepage,the size of the REV is increased and the main permeability direction is deflected,and the equivalent permeability coefficient is increased.In the case of thermal-hydro-mechanical coupling,the equivalent permeability coefficient of fracture network specimens tends to be stable with the increase of size,and the REV changed compared with the single seepage and hydro-thermal coupling.Due to the effect of temperature on fluid density,dynamic viscosity and fracture width,the main permeability direction of the fracture network is deflected and the equivalent permeability coefficient decreases compared with the hydro-thermal coupling because of the thermal expansion and contraction of the matrix rock mass.