| As a new type of renewable energy,geothermal resources have received extensive attention from all over the world and researchers.According to its storage form,geothermal resources can be divided into two types:hydrothermal type and dry-hot rock type.At present,the development of hot dry rock-type geothermal resources is mainly through the Enhanced Geothermal System(Enhanced Geothermal System,EGS),and the key step in the construction of EGS is the reservoir reconstruction project.At this stage,the reservoir reconstruction is mainly realized by hydraulic stimulation.At present,most of the EGS development sites in the world are facing difficulties and challenges caused by the difficulty in constructing geothermal reservoirs that meet the heat transfer requirements in reservoir hydraulic fracturing projects.It is now accepted believed that the mechanism of permeability enhancement of hydraulically stimulation in HDR reservoirs is mainly fracture disruption due to fluid injection,followed by the generation of an effectively connected fracture network causing permeability enhancement.In practical engineering studies,the fracture models of hydraulic and mechanical processes are often substantially simplified due to the limitations of problem size and computational power.The simplified fracture model generally only considers the permeability enhancement due to natural fracture shearing,and does not provide a good description of the fracture initiation,propagation and interconnection processes,so there are some deviations in the simulation accuracy and precision.Based on this situation,it is necessary to clarify the fracture behavior and fracture mechanics process in the hydraulic stimulation process propagation and connection process,and establish a set of methods that can be applied and field-scale research.Calculate the theory and method,and develop a set of numerical simulation programs suitable for site research to realize the description of the coupling process of thermal-fluid-mechanical-fracture damage coupling process in the hydraulic stimulation of hot dry rock,and the quantitative characterization of the mutual feedback.Provide theoretical support and tool support for the research and design of hydraulic fracturing process of hot dry rock on site.This paper firstly clarifies the purpose and content of the research by summarizing the existing research.Based on the theory of meso-fracture mechanics,the criterion of fracture initiation and propagation and the quantitative calculation method of fracture propagation behaviour under different fluid pressures are reconstructed.The results are integrated into the calculation of stress field(elastoplastic constitutive model)and seepage field(dynamic permeability model),and the thermal-fluid-mechanical-fracture damage coupling mathematical model is constructed.On this basis,the fracture-damage calculation module and the nonlinear mechanics calculation module are developed,and the sequential coupling method is used to lap each module,and the thermal-fluid-mechanical-fracture damage coupling numerical simulation program TOUGH2-EPFD3D.It can be used for site scale simulation is obtained.Subsequently,the meso-fracture of rock fissures and the macro-deformation of fractured rocks are simulated and verified,which preliminarily confirms the accuracy and reliability of the construction program of this study.After the development and preliminary verification of the program,this study takes the vertical well 58-32 in Milford forge site,Utah,as the research object.According to the actual structure and site parameters of well 58-32,a coupling model of seepage heat transfer stress fracture damage process is established.According to the injection test data of the site in 2017,the model is used to fit all the 8 injection test data of the test fracturing,correct the physical and mechanical parameters of the model,and verify the feasibility of the coupling theory and model.By comparing the simulation results with the numerical simulation results based on the classical hydraulic shear model(T2Biot),the importance of the fracture behaviour in the hydraulic fracturing process and the deficiency of the underwater shear model are explained.In addition,the evolution process of the reservoir fractures and the reservoir behaviour during the trial fracturing process are deeply analysed.It is pointed out that there are differences in the fracture behaviour of the reservoir fractures when hydraulic fracturing is carried out in the reservoirs with multiple groups of fractures.Besides,the competitive fracture mechanism of the fractures are put forward.Finally,the effects of temperature field and additional strain,different injection fluid temperature,horizontal in-situ stress difference and fracture strength(fracture toughness)on fracturing results are discussed in detail.Considering that the research on staged fracturing of horizontal wells in HDR reservoirs has attracted more and more attention,this study takes well 16A(78)-32 in Milford site as the research object,establishes a staged fracturing model of horizontal wells by using the field measured data,and predicts and evaluates the results of hydraulic fracturing.The results show that after 45 hours of large displacement injection,176×10~6m~3 of reservoir are stimulated and the permeability of this reservoir part increases by 2～3 orders of magnitude.Besides,the research show that increasing the injection displacement can effectively improve the fracturing reservoir volume and the staged injection sequence has little effect on the fracturing area.In addition,the competitive fracture mechanism of fractures plays a decisive role in optimizing the staged fracturing effect of horizontal wells.In this study,a fracture damage model describing the hydraulic stimulation process of HDR is established,and a numerical simulation program TOUGH2-EPFD3D is developed and applied to EGS site research.The results and conclusions obtained can provide a reference for the calculation of the fracture damage process of natural fractures in the process of hydraulic fracturing of dry hot rock,and provide a theoretical and methodological basis for the future theoretical research of hydraulic fracturing,the design and optimization of construction scheme and the construction of EGS project. |
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