EGS Analytical Model And Parameter Inversion Based On Discrete Fracture Network

In recent years,the voice of vigorously developing new renewable energy resources with large reserves and friendly environment has been unusually high.Hot dry rock geothermal energy is different from traditional hydrothermal geothermal energy,which is widely distributed and has more abundant reserves.It is considered as the most potential new energy in the 21 st century.Enhanced geothermal system(EGS)is an artificial geothermal system which extracts heat from hot dry rock.At present,there is still a gap in the field mining and tracer test of EGS in China,and further theoretical simulation research is urgently needed.In this study,the enhanced geothermal system is taken as the research object.Firstly,based on the discrete fracture network,the analytical model of EGS heat flow coupling is established,and the analytical solution of the temperature of the thermal reservoir fracture network is deduced by Laplace transform method.Different injection-production well patterns are constructed and the sensitivity of parameters is analyzed.The traditional analytical solution of single fracture is extended to the analytical solution of discrete fracture network with multiple fractures communicating with each other,and the practicability of the analytical model is greatly improved.Secondly,based on the fractal theory,the tree-like fractal branching network is used to characterize the discrete fracture network in the artificial thermal reservoir.The EGS analytical model is improved and solved,and the influence of the structural parameters of the branching fracture network on the thermal recovery is analyzed.Then,the improved analytical model of EGS is applied to the tracer test.The extraction concentration solutions of EGS tracer test under different injection modes are derived and the parameters are analyzed.Finally,according to the heterogeneous characteristics of EGS artificial reservoir,a multi-layer model for inversion of fracture network parameters is established.Combining with the measured concentration data of Soultz EGS tracer test in France,the inversion model is solved by PSO optimization algorithm.Based on the inverted fracture network parameters,the thermal recovery simulation and thermal breakthrough prediction of thermal recovery wells are carried out.The results show that the characterization of discrete fracture network is very important to the correctness of thermal simulation,and different injection-production wells have influence on the temperature change law and thermal breakthrough time at the outlet of fracture network.Under the same injection-production model,the larger the well spacing or the slower the injection rate,the later the thermal breakthrough occurs at the outlet of fracture network and the slower the temperature drop at the outlet.The parameters of injection-production well type and well spacing should be reasonably optimized in order to maximize heat recovery.When simulating with EGS analytical model based on tree-like fractal branching network,the complexity of fracture network is mainly affected by fracture branching levels.Fracture branching levels,fracture length ratio and fracture opening ratio are positively correlated with the temperature at the outlet of fracture network and the increment of cumulative heat produced by the system.Increasing the injection rate will increase the heat recovery of EGS in a short time,but at the same time,thermal breakthroughs will occur earlier.The lower the injection temperature,the more accumulated heat produced by the system.In EGS tracer test,when tracer is injected with a fixed mass slug,the injection speed only affects the arrival time of concentration peak,while the longitudinal dispersion only affects the value of concentration peak;the larger the fracture opening,the smaller the peak value of concentration curve and the later the arrival time of concentration peak;the bigger the fracture branching levels,the smaller the peak value of concentration curve and the earlier the arrival time of concentration peak.The results of this study are intended to provide some theoretical guidance for the construction of EGS demonstration site and the development of tracer site test in China.