Coupled Evolution Mechanism Of Seepage And Heat Transfer In Fractured Rock Mass And Evaluation Of Its Connectivity

With the development of national economy,the coupling of seepage and heat transfer in rock mass engineering has been paid more and more attention by researchers.Whether it is the exploitation of geothermal energy or the construction and excavation of nuclear waste storage,it will involve the seepage and heat transfer of underground rock mass.In addition,the connectivity of rock mass and the distribution of fractures will directly affect the exploitation efficiency of heat energy and the safety of nuclear waste burial.Therefore,it is of great practical significance to study the evolution mechanism of seepage and heat transfer in fractured rock mass and its connectivity.In this paper,the evolution mechanism of seepage and heat transfer in fractured rock mass and the evaluation of its connectivity are systematically studied by means of laboratory tests,theoretical derivation and numerical simulation.Firstly,the related characteristics of fractured rock mass are studied based on the laboratory size.Secondly,the evolution mechanism of seepage and heat transfer in fractured rock mass is analyzed based on discrete fracture network.Finally,the connectivity evaluation method is proposed and improved by calculating a practical example.The main research work and achievements are as follows:(1)The effect of roughness on fracture seepage was studied through nonlinear seepage tests in rough fractured media.The experimental results show that the hydraulic gradient is proportional to the fluid flow rate under the same confining pressure.With the increase of hydraulic gradient,the growth rate of seepage flow decreases continuously.This is caused by the additional pressure loss caused by inertia effect,and the seepage appears nonlinear phenomenon.The larger the fracture roughness coefficient JRC is,the lower the flow is under the same hydraulic gradient condition,and the easier it is to form nonlinear seepage.The nonlinear influence factor E increases with the increase of hydraulic gradient and confining pressure.Compared with the condition of low confining pressure,the nonlinear seepage is more sensitive to the roughness under the condition of high confining pressure.(2)The influence of roughness on shear behavior was studied by direct shear test and compression test after high temperature action,and the failure mode of fractured rock mass after high temperature action was analyzed.The experimental results show that the shear stress-shear displacement curve is dilatancy under small normal load.Under the condition of high normal load,it is generally shear type,and the stage of wear-occlusion-wear is repeated in the shear process.The stress-strain curve of rock mass presents concave shape after high temperature action.With the increase of temperature,the concave stage of the stress-strain curve becomes longer.With the increase of temperature,the linear phase of the stress-strain curve becomes shorter and the non-linearity becomes more obvious.With the increase of temperature,the peak compressive strength of rock samples decreases.(3)A heat flux coupling model considering the correlation of fracture parameters was established to simulate the complex two-dimensional fracture network.Firstly,based on laboratory data,the relationship between fracture length and aperture was established to improve the reliability of model evaluation.Secondly,the roughness coefficient distribution function is introduced to evaluate the influence of roughness on the coupling of seepage and heat transfer in fractured rock mass.Finally,based on the relationship between confining pressure and fracture aperture,it is assumed that fracture aperture changes with confining pressure in the simulation process,and the influence of confining pressure on the heat transfer of seepage is analyzed.The results show that the correlation between fracture length and aperture has a significant effect on heat flux coupling.After considering the correlation of parameters,the flow rate and heat transfer rate of the model increase,the flow rate at the exit boundary reaches the maximum,and the average temperature at the exit decreases rapidly.The fracture surface roughness reduces the outlet flow.At the same time,during the heat transfer process,the average temperature at the outlet decreases relatively slowly.The outlet flow of the model with variable fracture aperture decreases with time,which is consistent with the seepage law under creep condition.(4)A three-dimensional coupled heat flow model of fractured rock mass is established,and the nonlinear seepage characteristics of fractured rock mass during heat transfer are analyzed by considering both single fracture and complex fracture.By controlling the geometric parameters of the fracture,the influence of the direction of the size and density of the fracture and the percolation state on the heat transfer of the seepage was analyzed.In addition,the influence of hydraulic pressure gradient,fracture aperture and roughness coefficient on the heat transfer process of seepage is further studied.The results show that the cooling rate near the fracture surface is faster during the heat flux coupling process,which is consistent with the experimental observation.The larger the hydraulic pressure gradient is,the faster the outlet temperature drops.However,when the hydraulic pressure gradient is high to a certain extent,the sensitivity of the temperature at the fracture outlet to the hydraulic pressure gradient decreases,and the difference between the temperature change curves at the fracture outlet decreases.With the increase of the water pressure gradient,the nonlinear effect becomes stronger,but the growth rate of the nonlinear flow decreases,and the average temperature at the exit of the model increases.The hydraulic slope is sensitive to the change of fracture aperture,and the change of local fracture aperture will cause the change of the descent path of hydraulic pressure gradient.The outlet temperature is not sensitive to the fracture surface roughness coefficient JRC.In percolation model,penetration fractures provide seepage channels for fluid,and the flow and velocity of percolation model are usually larger than that of non-percolation model.The fluid with lower temperature rapidly exchanges heat with the surrounding high-temperature rock mass when passing through the fissure channel,and the average temperature at the outlet drops faster than that in the model without percolation.(5)An improved evaluation method for two-dimensional connectivity is proposed.In order to make the distribution of fracture network more realistic,the fracture aggregation phenomenon in real situation is simulated by using non-homogeneous Poisson stochastic process and simulated annealing method to generate the location of fracture.Based on the correlation function of fracture aperture and track length,a weighted factor was introduced to establish the connectivity function considering the correlation between fracture length and aperture,which improved the reliability of connectivity analysis.Related area is obtained by field aerial outcrop data of joints,will consider relevance and not considering correlation connectivity comparing the calculation results under two kinds of situations,found considering fracture length and diameter of correlation improved the connectivity evaluation and internal fracture network flow distribution uniformity,extending the application of the connectivity evaluation value.The potential zones of hydraulic fracturing process can be well predicted in geothermal exploitation.(6)A connectivity evaluation method for three-dimensional fracture network was established.The polygonal fractures are used and the algorithm is optimized to improve the calculation efficiency.Based on the location of the relationship between fracture,this paper puts forward that under the condition of three dimensional fracture connected field assessment,connectivity with the traditional evaluation methods based on fracture density,size,intersection point is different,the method can be adopted by the good preservation and the use of intersection between the fracture information,more realistic simulation of fracture seepage field,potential flow path can predict research area.Engineering examples show that connectivity assessment is a very effective method,especially in evaluating reservoir characteristics(such as flow characteristics)based on fracture systems.The results show that the three-dimensional connectivity evaluation method is feasible and more realistic and comprehensive than the two-dimensional connectivity evaluation method.This method greatly improves the reliability of connectivity assessment,and plays an important role in the location selection of the sources of nuclear waste disposal,the prediction of the potential flow path and the reduction of nuclear pollution.In addition,the 3D connectivity field can be used to determine the optimal drilling location of a reservoir,such as creating new Wells in areas with the greatest connectivity to maximize reservoir productivity.This method also has a certain theoretical reference significance to the seepage problem of fractured rock mass commonly encountered in hydraulic engineering.