Numerical Simulation Study Of Enhanced Geothermal Systems (EGS) Considering Fluid Losses

The present society has an increasing demand for energy,however,the existent major energy is still fossil fuel,which not only faces with exhaustion but also brings serious environmental problems.Therefore,it is necessary to vigorously develop renewable and environmentally friendly energy.Geothermal energy is very abundant and is an important kind of clean and renewable energy,and EGS(enhanced geothermal systems)can offer continuous base-load power,and have an enormous potential to extract geothermal energy and have attracted more and more attention.Some countries have been active to develop EGS projects,and many researchers have done lots of work on EGS.So far,the working fluids of the EGS mainly include water and CO2,and because of fluid leakage to the formation,there exists fluid losses during the process of EGS heat extraction.However,there have been only a few researches considering water losses in the EGS using water as the working fluid(H2O-EGS),and only a few researchers have approximately evaluated CO2 losses(i.e.CO2 sequestration)in the EGS using CO2 as the working fluid(CO2-EGS);besides,the influences of some important factors on heat extraction from H2O-EGS and CO2-EGS have not been analyzed,and the influences of some factors on CO2 sequestration in CO2-EGS have not been investigated before.Therefore,this paper has numerically simulated heat extraction from EGS,and mainly makes a detailed investigation on the influencing factors of H2O-EGS and CO2-EGS.The thermophysical properties of the formation are important parameters of EGS,but are not easy to measure,therefore,firstly this paper proposes a new method to analyze spatial distributions of the thermophysical properties of the EGS formation.In this method,cold water is injected into the outer wellbore of double-wellbore geothermal well and is extracted from the inner wellbore,and meanwhile the temperatures of water in the outer wellbore are measured at different time,which is used to predict the thermal conductivity and volumetric heat capacity of the EGS formation based on a relationship between the water temperatures and the thermophysical properties by using Monte Carlo inversion method.To assess the feasibility of the proposed method,a virtual geothermal well is analyzed,the result of which shows that the mean relative error of thermal conductivity is 1%and the mean relative error of volumetric heat capacity is 5%.The proposed method could estimate the thermophysical properties of the EGS formation under in situ conditions,which is very convenient and cost-effective.To precisely analyze the heat extraction process in double-well EGS with fluid losses,a three-dimensional thermal-hydrologic(TH)single-porosity numerical model is established and verified.The TH numerical model includes two parts:the wellbore heat and mass transfer model and the porous media heat and mass transfer model.Compared to traditional models considering fluid losses,the TH numerical model considers the heat transfer between the wellbores and the formation,three-dimensional fluid losses from the reservoir to the formation,the reservoir permeability anisotropy and heat extraction rate.The TH numerical model is applied to Fenton Hill EGS,and the simulated injection and production flow rates were compared with experimental data,with the mean relative errors 5%and 9%,respectively,showing the feasibility of the TH numerical model to simulate EGS heat extraction.Based on the wellbore heat and mass transfer model and a two-dimensional wellbore numerical model,the heat transfer between the wellbores and the formation is analyzed,and the results are as follows:the efficient heat transfer radius of the formation is about 40 m;as the well spacing in a general EGS is more than 80 m,therefore,the two or more wells will not be influenced by each other;the heat transfer in the wellbores has an important effect on EGS performance,and thus it is necessary to consider the heat transfer in the wellbores.Based on the verified TH numerical model,eleven influencing factors of H2O-EGS heat extraction are analyzed.The findings are concluded as follows:as the formation permeability increases,water loss rate increases and production flow rate decreases,leading to decreased steady-state heat extraction rate and almost unchanged thermal breakthrough time,therefore,if the existing water losses from the reservoir to the formation are ignored,heat extraction rate will be overestimated during all the operation time;water loss ratio decreases with increasing ratio between average reservoir permeability and the formation permeability,and water losses should be considered when the permeability ratio is less than about 2000;the reservoir permeability is assumed to be homogeneous in many researches,which would result in overestimated heat extraction;vertical reservoir permeability component has little effect on heat extraction,while the two horizontal reservoir permeability components have an important effect on heat extraction,and heat extraction is the greatest when the ratio between the two horizontal reservoir permeability components is about 1:1;the initial temperature of the porous media has an important influence on heat extraction,and it is suggested to consider the changes of the fluid physical properties with the cooling of the reservoir;when the injection or production well approaches the edge of the reservoir,production flow rate decreases and thermal breakthrough time is independent on the distance between the injection well and reservoir center,which would restrain heat extraction,so the injection and production wells should be located about 20-25 m from the edge of the reservoir;with fixed well locations,enlarged reservoir volume has little effect on heat extraction,and therefore,to improve EGS performance,it is suggested to enlarge reservoir volume and to optimize well layout as well;open-hole length is optimized to be equal to vertical reservoir dimension;with the increase of production pressure,water loss rate increases slowly and production flow rate decreases,which would restrain heat extraction;for each influencing factor,it can be concluded that thermal breakthrough time is mainly dependent on production flow rate and water loss rate,showing that water losses have an important effect on heat extraction.The three-dimensional TH numerical model is further developed by considering the changes of flow rates with time,which can fulfill dynamic and simultaneous simulation of heat extraction and CO2 sequestration in CO2-EGS,and then CO2-EGS and H2O-EGS are simulated and compared.With fixed difference between the injection and production pressures,CO2-EGS and H2O-EGS are simulated for different injection pressures,and the results show that the average reservoir permeability and average reservoir fluid pressure of CO2-EGS with higher injection pressure are similar with those of H2O-EGS with lower injection pressure,besides,CO2-EGS needs high injection pressure while H2O-EGS does not need high injection pressure,therefore,it is more reasonable to compare CO2-EGS with higher injection pressure and H2O-EGS with lower injection pressure.The comparisons between the two EGS under different parameters show that compared with H2O-EGS,CO2-EGS has greater heat extraction,and the advantage of CO2-based EGS is larger for lower average reservoir permeability,lower initial reservoir temperature,smaller wellbore depth and higher surrounding formation permeability.Compared with H2O-EGS,CO2-EGS has much higher fluid loss rate(especially at high average reservoir permeability,high initial reservoir temperature and small wellbore depth),and has much lower fluid loss ratio(especially at low average reservoir permeability and low initial reservoir temperature),indicating that the present models are not accurate to calculate CO2 sequestration based on flow tests at H2O-EGS geothermal fields.For CO2-EGS,as surrounding formation permeability increases,both fluid loss rate and ratio increase,and heat extraction rate decreases slowly,suggesting that surrounding formation permeability should be considered to calculate CO2 sequestration and heat extraction,and it is necessary to consider CO2 losses from the reservoir to the surrounding formation.The above results show that there exists heat extraction and CO2 sequestration in CO2-EGS,therefore,lastly the influences of injection parameters on heat extraction and CO2 sequestration are then investigated based on the developed three-dimensional TH numerical model.As injection pressure increases,both production flow rate and steady-state heat extraction rate increase,and both fluid loss rate and fluid loss ratio increase;therefore,increasing injection pressure will enhance heat extraction and CO2 sequestration.As production pressure increases,both production flow rate and steady-state heat extraction rate decrease,and both fluid loss rate and fluid loss ratio increase;therefore,increasing production pressure will constrain heat extraction but enhance CO2 sequestration.As injection temperature increases,both production flow rate and steady-state heat extraction rate decrease,and both fluid loss rate and fluid loss ratio decrease;therefore,decreasing injection temperature will enhance heat extraction and CO2 sequestration.The results can offer some advice for injection parameters optimization considering the overall performance of CO2-EGS.