Study On Numerical Modelling Based On Thermo-hydro-mechanical Coupling And Heat Transfer In Enhanced Geothermal Systems

As a promising advanced technology,Enhanced Geothermal System(EGS)utilizing deep geothermal resources has gained increasing attention.A thorough understanding of heat and mass transfer processes during production from hot dry rock is crucial to predict the reservoir response,evaluate and further optimize production and stimulation schemes.Field-scale reservoir simulation was carried out to study the mass flow and heat transfer of fluid in EGS.The studied domain extends from the injection well,through the reservoir and eventually to the production well.The model was established based on the geological survey of a targeted EGS area.From a physical perspective,this study incorporates three aspects,mass flow: wellbore flow from the surface to the reservoir,seepage through the fractured area;heat transfer: convection heat transfer of fluid in the wellbores and heat transfer between fluid and rock matrix in the fractured area;displacement: the stress field is not constant due to fluid injection and production,and simultaneously the cooling effect of rock matrix,and thus displacement occurs.The thermo-hydro-mechanical process is fully coupled and exists throughout the whole production.This paper is organized according to the topics regarding the numerical model,each part is involved with the multi-field coupling.The covered contents included: THM coupling of the reservoir,local thermal non-equilibrium simulation,wellbore simulation and reservoir-wellbore coupling,and effects of natural convection on mass and heat transfer in the reservoir.For THM coupling,the governing equation of rock thermo-elasticity was solved in addition to the mass and energy balance equations.Persistent injection of cold water in EGS incurs more significant influence of THM coupling on the reservoir response compared to that in conventional hydrothermal systems.Pore pressure and thermal stress enhance injectivity around the injection well,while they exert opposite effects around the production well.However,due to the cooling effect in the whole reservoir,thermal stress takes the dominant role during the production.Thus,both heat and mass transfer are enhanced due to the rock matrix contraction.Based on the THM study of the reservoir,different thermophysical properties,mechanical parameters,boundary conditions and production schemes were employed for a parametric study regarding the production capability.To take account of the temperature difference between the fluid and the rock matrix,local thermal non-euilibrium simulation was implemented.Firstly,an experiment using a single fracture was carried out for correlations of Poisuille number and Nusselt number.Instead of being a constant in macro-scale pressure-driven flow,Po of water flowing through the fracture decreases with Re,Nu decreases with the dimeniosnless distance.To consider the influence of roughness,roughness-viscosity model was employed to modify the numerical model,the derived results agree well with the experimental results.Based on the correlations of Po and Nu,two energy balance equations were employed and local thermal non-equilibrium was considered.The numerical results demonstrate that under LTNE,the cooling effect of the reservoir is delayed.Generally,the depth of hot dry rock is several kilometers from the earth surface.Reservoir and wellbore were indirectly coupled as the governing equations for these two parts are different.A series of run at different mass flow rates and different temperatures was carried out,and subsequently,these results were supplied to the reservoir simulation for the wellbottom pressure through interpolation.Comparison of the results with and without coupling indicates that,the mass flow rate from production well is smaller than that obtained without coupling,especially at the later stage of production.Wellbore simulation was carried out individually to study pressure and temperature distribution along the wellbores and heat influence radius around them.According to the reservoir simulation,buoyancy-driven flow plays a significant role regarding mass flow and heat transfer in the fractured area.A single fracture model targeting the natural convection influence was established to study the buoyancy-driven flow qualitively.To concentrate the computational efforts on convection in the fracture,boundary element method was used,and green's function was employed to calculate the heat flux between the rock matrix and the water flowing in the fracture,the governing equations of fluid in the fracture was solved using the finite element method.Numerical results demonstrate that flow path of fluid in the fracture is extended under the influence of buoyancy force,and thus heat transfer area is expanded.With equivalent dimensionless time of production,the output temperature is evaluated significantly with buoyancy-driven flow compared to that without buoyancy,and horizontal wells are superior to vertical wells in terms of heat extraction rate from the fractured area.