Numerical Method For The Coupled THM Processes In Deep Geothermal Reservoirs At City Scale And Application

Geothermal energy is a clean and environmentally friendly renewable energy,and geothermal reinjection is of great significance for the sustainable utilization of geothermal energy.City geothermal reservoirs are often buried deep and in a complex environment of high temperature and pressure.The dynamic evolution of temperature,seepage and stress fields significantly affects the operation of the geothermal system.Therefore,an understanding of the coupled multiphysics mechanism in deep geothermal reservoirs at city scale is critical to predict thermal breakthrough of production wells,optimize geothermal field management scheme,monitor surface subsidence and prolong the operation life of geothermal system.In this thesis,the city scale deep geothermal reservoirs is focused on,and the multi-scale studies from geothermal well pair to geothermal mutiple well system are carried out through field test,theoretical analysis and numerical simulation.A theoretical model of equivalent flow channel of heterogeneous porous geothermal reservoirs is proposed,and numerical method for city scale geothermal multiple well system under complex geological conditions is established.The coupled thermal-hydro-mechanical processes in deep geothermal reservoirs during the long term well operation is discussed at city scale.The major advances and conclusions can be drawn as follows:(1)In order to study the seepage and heat transfer characteristics of heterogeneous porous geothermal reservoirs,heterogeneous porous geothermal reservoirs models considering the spatial distributions of permeability and rock heat capacity are generated.In total,200 realizations with different permeability fields are considered.A fully coupled modeling of reservoir deformation,fluid flow,and heat transfer is performed on each realization using the finite element method.The results showed that heterogeneity induced channeling appears in the geothermal reservoirs with increasing heterogeneity of permeability,through which the injected production water can quickly reach the production well.This causes the shorter thermal breakthrough time at production wells,and the lower reservoir surface settlement.In geothermal reservoirs models considering the correlation between heat capacity and permeability,the thermal breakthrough curves and the reservoir surface settlement are the same as the reservoir models of the constant heat capacity.With increasing correlation length,the possibility of flow channels appearing in well pair system increases,causing a short average thermal breakthrough time and a lower surface settlement around the injection well.The existence of preferential flow channels between injection and production wells indicates that an equivalent flow channel model can be approximately simulate the coupled multiphysics mechanism of geothermal doublets in heterogeneous reservoirs.(2)By extension of the equivalent flow channel model of fractured media to heterogeneous porous geothermal reservoirs,a parameter inverse method for equivalent flow channel of heterogeneous geothermal reservoir based on tracer test that combines an analytical solution of the tracer transport equation with a differential evolution method for global optimization is established.Combined with an analytical expression for heat advection and conduction,a prediction method of thermal breakthrough time for geothermal well pair system in heterogeneous reservoirs is presented.An application case uses the modeling framework in combination with data from tracer tests to calculate the parameters of equivalent flow channels that link two distinct geothermal well pairs and predict the thermal breakthrough time of production well at Dezhou geothermal field,Shandong,China.The results showed that channel parameter values are similar for the two well pairs.A sensitivity analysis is carried out to understand the roles of the flow channel dimensions and the flow velocity of equivalent flow channels in thermal breakthrough curves at production wells.(3)Deep geothermal field in city scale usually contain dozens or even hundreds of geothermal wells.Due to the scale disparity between wells and geothermal reservoirs(~dm vs.~km),the numerical model of geothermal multiple well system is facing great challenges in mesh generation and calculation speed.To improve the computational efficiency without losing accuracy,a robust numerical method for modeling multiple wells in city scale geothermal field based on simplified one-dimensional well model is proposed.This model considering heat convection and conduction occurs along axial direction and heat transfer between geothermal fluid and rocks is considered by an equivalent heat transfer coefficient.Compared with the traditional finite element method,a benchmark example is used to examine the reasonability and efficiency of the proposed model of geothermal wells.The computation time is significantly reduced by avoiding mesh refinement near geothermal wells.Finally,we further apply this simplified model to practical geothermal engineering.Two case study of multiple well system in Decheng district of Dezhou and southeast district of Beijing are presented.The evolution characteristics of temperature,seepage and stress fields of geothernmal reservoir under periodic production in cold season are simulated,and thermal breakthrough time of production well is also predict.The optimal design of production and renjection scheme is put forward.Moreover,the interactions between different geothermal reservoirs,and the role of faults in reservoir performance are also discussed.Finally,the efficient simulation of city scale multiple geothermal well system under complex geological conditions is realized.This dissertation includes 103 figures,13 tables and 180 references.