Numerical simulation of heat extraction performance in enhanced geothermal system with multilateral wells

Supercritical CO 2 (SCCO 2 ) is considered as a promising alternative heat extraction fluid in the enhanced geothermal system (EGS) which can reduce atmospheric CO 2 emissions and water waste. However, the heat extraction performance of SCCO 2 is greatly affected by its thermophysical properties, which are high sensitivity to temperature/pressure. In this study, by considering the impacts of temperature and pressure on thermophysical properties of SCCO 2 , a thermo-hydro-mechanical coupling numerical model was established to investigate the effects of the initial reservoir temperature as well as the SCCO 2 injection pressure and temperature on heat extraction. The variations of reservoir permeability and mass productivity as well as the net heat extraction rate (HER) were simulated by implementing the model into COMSOL Multiphysics, which showed a good consistency compared with the hightemperature/pressure triaxial seepage experiments. Simulation results indicate that there are three distinct stages in mass productivity including the slow decline stage, the rapid increase stage, and the stabilization stage. Increasing the initial reservoir temperature or the injection temperature will reduce the net HER to varying degrees.Appropriate increase of the injection pressure and injection temperature can extend Stage 2 duration which can subsequently enhance the mass productivity and the net HER. Meanwhile, the higher injection pressure or the lower injection temperature will inhibit heat compensation within rock masses and accelerate the reservoir heat depletion. This study provides guidance for optimizing the injection parameters of CO 2 -EGS and aims to enhance the net HER while ensuring a reasonable reservoir production life. K E Y W O R D S enhanced geothermal system, heat extraction experiment, high temperature/pressure, numerical simulation, supercritical CO 2 How to cite this article: Li P, Wu Y, Liu J-F, et al. Effects of injection parameters on heat extraction performance of supercritical CO 2 in enhanced geothermal systems. Energy Sci Eng. 2019;7:3076-3094. https ://doi.

Section: Simulation Results and Discussionmentioning

confidence: 99%

Effects of injection parameters on heat extraction performance of supercritical CO₂ in enhanced geothermal systems

Liu

et al. 2019

“…Most of the parameters required in this simulation were collected from the literatures 8,9,20,25,33,[38][39][40][41]56 and listed in Table 4. The temperature-dependent fluid thermodynamic properties are described by some empirical formulas and are not counted in Table 4.…”

Section: Basic Input Parametersmentioning

confidence: 99%

“…[17][18][19] At present, numerical simulation is still the most efficient method to analyze the THM coupling problem. Based on the thermal-hydraulic (TH) coupling theory, the effects of different fracture morphology, 20,21 circulating fluids, 21,22 and conceptual models or well layouts [23][24][25] on heat extraction performance were investigated, respectively. In their models, it was assumed that the local thermal equilibrium (LTE) between the rock matrix and the pore fluid can be achieved instantaneously.…”

Section: Introductionmentioning

confidence: 99%

Investigation on heat extraction characteristics in randomly fractured geothermal reservoirs considering thermo‐poroelastic effects

Han

Cheng

Gao

et al. 2019

A fully coupled thermal‐hydraulic‐mechanical (THM) model was developed to investigate the underlying response mechanisms during heat extraction in fractured geothermal reservoirs. The random fracture network in the stimulated zone was reproduced based on the fractal theory. The coupled model accounts for the dominant physical phenomena including (a) fluid flow, heat transport, and solid deformation in porous media and fractures; (b) local thermal nonequilibrium (LTNE) between rock matrix and flowing fluid; and (c) temperature‐dependent fluid thermodynamic properties and stress‐dependent pore and fracture permeability. The proposed model was validated by several analytic solutions. Sequentially, the evolution of pore pressure, equivalent temperature, effective stress, and reservoir permeability was analyzed. The sensitivity of the heat extraction performance to fracture network morphology was discussed. Results show that interconnected large‐scale fractures dominate mass and heat transport. Widely distributed small‐scale fractures contribute to the cooling of the heat rock mass along many flow paths in parallel. The change in effective stress associated with fully coupled thermo‐poroelastic effects may induce fracture shear dilation and pore expansion, resulting in an enhancement of the overall reservoir permeability. There is a significant temperature difference between the solid phase and the fluid phase in fractures, but not in porous media. It is more reasonable to use the LTNE theory to analyze the heat extraction of fractured geothermal reservoirs. The fracture network morphology has a profound effect on heat extraction efficiency and injectivity. Undeveloped fracture networks will result in a higher injection pressure, earlier thermal breakthrough, and shorter lifetime, but higher heat extraction ratio. Properly increasing the fracture density can delay the thermal breakthrough time, prolong the service life, and improve the injectivity. Fully connected fracture networks may result in thermal short‐circuiting and earlier thermal breakthrough. Generating a complex and scattered fracture network but without preferential channels is conducive to extracting more heat from geothermal reservoirs.

“…The transient three‐dimensional numerical model considers local thermal nonequilibrium between the rock matrix and water flowing in the fracture and fault. The following assumptions are adopted in the present study: The water flows comply with the Darcy's Law and keep a steady state in the fractures and rock matrix. The water remains liquid and does not vaporize under the conditions of 40‐80MPa pressure and 323‐513 K temperature The model neglects the variation of fracture aperture and chemical effect. The continuous porous media properties of rock matrix are isotropic.…”

Section: Description Of the Numerical Modelmentioning

confidence: 99%

Production capacity and mining plan optimization of fault/fracture‐controlled EGS model in Gonghe Basin

Zhang

Xie

Zhang

2019

Taking full advantage of geological conditions is beneficial to reduce the development cost and enhance the exploit of geothermal energy in the Gonghe Basin. In this paper, a thermal-hydraulic coupled model is put forward to study the heat extraction for Gonghe Basin based on local thermal nonequilibrium theory considering the natural fault. An excellent heat recovery system and reasonable combination of parameters are the premise of sustainable geothermal development. Two doublet systems are introduced to investigate the efficiency of the fault/fracture-controlled EGS model.A comparative analysis is performed from the aspect of production temperature, heat extraction ratio, flow impedance, injection pressure, thermal power, and electric power. Key parameters (injection rate, injection temperature, production pressure, number of fractures, vertical interval between the two storages, and the storage permeability) are discussed. The results show that the fault-fracture-controlled doublet model is more suitable for EGS to heat transfer than the common doublet model. The doublet model at a depth of 3300-3800 m has a temperature gradient of 0.04°C/m.For fault-fracture-controlled EGS model, the optimal values are 50 kg/s, 15 MPa, 60°C, 310 m, 4 × 10 −14 m 2 , and two for injection rate, production pressure and injection temperature, vertical interval, storage permeability, and fracture number, respectively. The best well pattern layout method for the Gonghe Basin is one injection well and two production wells. The production temperature varies from 196.8°C to 192.7°C when the heat extraction ratio is 0.13 at the end of the operation. Thus, the fault-fracture-controlled EGS model can provide a mining method and advance the development of geothermal energy for the Gonghe Basin in the future. K E Y W O R D Senhanced geothermal system, fault/fracture-controlled model, local thermal nonequilibrium, parameter optimization | 2967 ZHANG et Al.