Research And Application Of Heat And Mass Transfer Coupling Method For Complex Fracture Medium In Enhanced Geothermal System

Hot Dry Rock(HDR)geothermal resources have attracted increasingly attention worldwide given its characteristics of widespread distribution,abundant reserves,and significant development potential.Enhanced Geothermal System(EGS)is considered as one promising way to extract energy from HDR by reservoir reconstruction technologies such as hydraulic fracturing.Heat and mass transfer processes in the EGS determines the energy extraction efficiency,operational lifespan,and economic benefits of geothermal projects.Therefore,evaluating the heat and mass transfer processes in complex fractured medium accurately and reasonably in of crucial importance for both engineering and theoretical research.In addressing the numerical simulation challenges of heat and mass transport within complex fractured media in EGS,this work introduced a novel modeling approach.This approach combines implicit discrete fracture models with embedded discrete fracture models to achieve high-precision characterization of multi-scale complex fracture systems.Compared to previous fracture modeling methods,this approach overcomes the challenges of multi-scale fracture modeling,providing new insights for modeling complex fractured media formed either naturally or through artificial induction.Additionally,a mathematical model for heat and mass transport within multi-scale complex fractured reservoirs in EGS has been developed.The threedimensional fracture media hydrothermal coupling transport numerical simulation program,T2 EDFM,was created to enable efficient computation of hydro-thermal coupling transport processes in complex fractured media.Validation against laboratory experiments,analytical solutions of simple problems,and numerical solutions demonstrates the accuracy of the new program in quantifying hydrothermal transport in fractures,offering a new numerical simulation tool for HDR development research.Taking the Utah FORGE as a case study,we developed an injection-production test model for the EGS based on the complex hydraulic fracture network structure interpreted from micro-seismic events induced by hydraulic fracturing.Utilizing the hydro-thermal coupling numerical simulation code developed in this study,we analyzed the heat and mass transfer process in the geothermal reservoir under the influence of hydraulic fractures.This analysis predicted possible preferential flow paths and hydrothermal production characteristics under different horizontal well configurations and injection-production schemes,providing theoretical guidance for the ongoing horizontal production well drilling and hydraulic circulation testing in the project.Quantitative analysis of the hydro-thermal coupling model indicates that,under the current interpreted hydraulic fracture network conditions,thermal breakthrough might occur at 1 day,3 days,and 23 days for injection-production well spacings of 75 m,100 m,and 150 m,respectively.Based on the fracture interpretation data and hydro-thermal analysis results,we recommend a vertical distance of 150～200 m between the injection and production wells.Additionally,the spatial connectivity of hydraulic fractures in the EGS reservoir dominates the non-uniform fluid seepage and inter-well thermal breakthrough processes during EGS development,significantly impacting the heat extraction performance of the EGS.Focusing on the demonstration site for HDR development in the Gonghe Basin of Qinghai,China,this study utilized the proposed complex fracture modeling method for EGS to construct a reservoir model that comprehensively considers the distribution characteristics of natural and hydraulic fractures at the site.The model’s accuracy was validated by fitting the field tracer test data.Based on this model,the developed code was used to investigate the characteristics of heat and mass transfer and the spatiotemporal evolution of the temperature and pressure fields in the thermal reservoir under the combined influence of natural and hydraulic fractures.This work clarified the impact mechanisms of the combined effects of natural and hydraulic fractures on the long-term hydrothermal transport process in complex fractured thermal reservoirs.Quantitative analysis revealed that,under given conditions,during 30 years of continuous geothermal extraction,the average injection rate is approximately 1.2 times the average production flow rate,with a noticeable loss of circulating water.The widespread distribution of natural fractures in the reservoir provides a sink for the injected water,causing an imbalance in injection and production flow rates.Previous simulations based on equivalent continuous medium models or those considering the matrix as impermeable rock layers overlooked the water loss process,leading to overestimation of heat production efficiency.Therefore,when modeling HDR development at a research scale of meters to hundreds of meters,the distribution characteristics of both natural and hydraulic fractures should be considered to avoid overestimating the geothermal development potential of fractured thermal reservoirs.The complex fracture modeling method for EGS reservoir proposed in this work overcomes the multiscale fracture modeling challenge,balancing computational efficiency and fracture complexity in modeling.It provides new insights for modeling complex fractured medium formed naturally or artificially.Additionally,the EGS fractured reservoir TH coupling code developed independently in this paper can achieve high-precision simulation of complex fractures with any tendency and dip angle in2D/3D space,providing an improved numerical simulator for analyzing the hydrothermal migration characteristics in high-temperature complex fractured.