Study On Heat Extraction And Thermal Mass Migration In Heterogeneous Geothermal Reservoirs

Deep geothermal reservoir,especially hot dry rock(HDR),is regarded as a key potential energy source due to its wide distribution and huge reserve.The extraction and utilization of geothermal energy is of great significance to the optimization of China’s energy consumption structure.So far,there is no successful commercial operation in the enhanced geothermal system(EGS).In order to investigate the failure mechanisms for the previous EGS tests,this dissertation carried out theoretical and numerical simulation studies on the heat transfer and heat extraction evaluation from fractured geothermal reservoirs.Through this study,the key factors to the success of geoethermal energy extraction are identified.These can provide some engineering guidances and decision-making supports for the development of geothermal energy.The main contents and results in this dissertation are summarized as follows:(1)A multiphysical coupling simulation method is proposed for seepage and heat transfer in a multi-scale self-affine single fracture and the effect of rock fracture roughness constructed by Weierstrass-Mandelbrot(W-M)function on heat transfer is studied.The single-scale contour is extracted from a multi-scale fracture based on scaleplate’s walk,which can avoid the difficulty in establishing geometric model with local microstructures.Further,the residuals between the scaleplate and fractal fracture are calculated and introduced into local roughness factors which are included in the governing equations of the multi-phyiscal coupling simulation model.Finally,the effects of different coupling and paramaters on heat transfer are analyzed through numerical simulations.The approach in this study can solve the size contradiction between FEM mesh and local fractal microstructure in multi-scale fractures.(2)A thermal(T)-hydraulic(H)-mechanical(M)coupling model is further estabilished for simulating the seepage heat transfer performance of working medium in fractured reservoir with discrete fracture networks.The discrete fracture networks are generated by MATLAB codes.Both fracture aperture variation with effective stress and fracture roughness are introduced into the coupling model.The coupling effects of TH and HM are verified by analytical solutions and literature data.The influences of fracture networks on the distributions of temperature and pressure are explored through numerical simulations.The evolution mechanisms of fracture aperture with heat extraction are revealed.(3)The heat extraction performances are compared through numerical simulations on three preset fracturing domains.The effects of the permeability enhancement heterogeneity in the fracturing domains on heat production are qualitatively analyzed.A large number of previous simulation studies have simplified the stimulated reservoir volume(SRV)into a homogeneous cuboid domain.This is obviously contrary to the in-situ microseismic observations that the permeability enhancement effect gradually weakens from the fractured well to the surrounding.With the consideration of permeability enhancement by multi-stage hydraulic fracturing,this study proposed three SRVs with the same volume but different features.Then,a unified TH coupling model is applied to the three SRVs for geothermal extraction simulation.The results show that the improper assumption of SRV will overestimate the output performance of an EGS.The heterogeneity of SRV should be carefully considered.(4)A uncertainty analysis for EGS performance is conduced to consider the whole certain trend and local uncertain distribution of permeability in the fractured domain.In the uncertainty analysis,the permeability enhancement is gradually weakened from the fractured well to the surrounding reservoir.At the same time,the overall general trend contains a random distribution of permeability due to nature and enhancement.This paper proposes an approach for stochastic simulations and analyzes the uncertainty of system productivity under the above permeability distribution in an artificial reservoir.Firstly,the permeability distribution is constructed by random field simulation and includes global certainty trend and local uncertainty.Then,the generated permeability distribution is applied to a THM coupling model for numerical simulations.Finally,the system output is statistically evaluated by our proposed indices.The index distribution is obtained from the simulation results.The results show that under the current conditions of antireflection technology,the establishment of EGS in low permeability hot dry rock reservoirs may be in a serious productivity risk.(5)A multi-objective optimization approach is proposed by response surface method through the alternative model for a fault geothermal system.Being different from conventional optimization scheme used in previous studies,this methodology can realize the global search optimization of system parameters.This study established a numerical model for fault geothermal system and defined three objective functions for system performance evaluation.Then,I-optimal design is ued to give a parameter list and the corresponding objectives’ value is obtained through numerical simulation.Thirdly,the response surface models of system objectives are derived through the least square fitting.Finally,the global search optimization of operation parameters is realized through applying the multi-objective genetic algorithm and the response surface models are derived.The optimization obtains the Pareto front of three objectives,and the specific parameter scheme of system operation is output.This optimization scheme based on an alternative model can largely reduce numerical simulation cost,thus being specially useful in the parameter optimization of a complex system.There are 84 figures,26 tables and 260 references in this dissertation.