Yuanxiu Sun

To efficiently develop deep geothermal resources, a novel multi-branch horizontal well closed-loop geothermal system (MBHW-CLGS) was proposed. The proposed design effectively expands the formation heat exchange area by optimizing the wellbore structure. The working fluid velocity within the branches was reduced to prolong the heat exchange time, thereby improving the heat extraction and economic performance of the system. A three-dimensional unsteady-state heat transfer model was established to verify the advantages of MBHW-CLGS. In this model, outlet temperature, heat extraction power, power generation, and levelized cost of energy (LCOE) were selected as evaluation criteria. The results indicated that the power generation of the proposed system was 3.803 and 1.411 times that of the traditional U-shaped and horizontal well CLGS, respectively, while the LCOE decreased by 67.94% and 16.32%, respectively. Then, the effects of the number of horizontal branches, branch well spacing, branch well diameter, volumetric flow rate, injection temperature, and geothermal gradient on the heat extraction and economic performance of the system were investigated. The influence of working fluid transport, flow distribution, and internal flow velocity on heat exchange within the multi-branch system was clarified. In summary, the multi-branch horizontal well configuration displayed significant advantages in closed-loop geothermal development, providing a theoretical reference for the structural design and parameter optimization of deep hot dry rock geothermal projects.

To efficiently develop deep geothermal resources, a novel multi-branch horizontal well closed-loop geothermal system (MBHW-CLGS) was proposed. The proposed design effectively expands the formation heat exchange area by optimizing the wellbore structure. The working fluid velocity within the branches was reduced to prolong the heat exchange time, thereby improving the heat extraction and economic performance of the system. A three-dimensional unsteady-state heat transfer model was established to verify the advantages of MBHW-CLGS. In this model, outlet temperature, heat extraction power, power generation, and levelized cost of energy (LCOE) were selected as evaluation criteria. The results indicated that the power generation of the proposed system was 3.803 and 1.411 times that of the traditional U-shaped and horizontal well CLGS, respectively, while the LCOE decreased by 67.94% and 16.32%, respectively. Then, the effects of the number of horizontal branches, branch well spacing, branch well diameter, volumetric flow rate, injection temperature, and geothermal gradient on the heat extraction and economic performance of the system were investigated. The influence of working fluid transport, flow distribution, and internal flow velocity on heat exchange within the multi-branch system was clarified. In summary, the multi-branch horizontal well configuration displayed significant advantages in closed-loop geothermal development, providing a theoretical reference for the structural design and parameter optimization of deep hot dry rock geothermal projects.

As the environmental and economic problems caused by traditional fossil energy have become increasingly severe, geothermal energy of depleted oil and gas reservoirs has attracted wide attention due to its natural advantages and development potential. Previous studies mainly focused on the reservoir and fracture systems, while the wellbore structure was often simplified. The blocking effect of the casing on CO2 heat and mass transfer was not fully considered. In this study, a discrete fracture network was generated using a random function to simulate natural fractures, and hydraulic fractures connecting the wellbore and the reservoir were established. Based on these features, a thermal–hydraulic–mechanical (THM) numerical model was constructed in which CO2 mainly flowed and exchanged heat through the perforated intervals and the end of the wellbore. The effects of well placement patterns, the number of natural fractures and hydraulic fractures on heat-extraction performance were systematically analyzed. We also proposed a non-constant injection temperature strategy. The injection temperature was changed over time to control the temperature difference between CO2 and the reservoir. This method improved the heat-extraction efficiency. Finally, each key parameter was changed in equal proportions to study its relative effect on heat-extraction efficiency. Production temperature and heat-extraction power were used as evaluation indicators, and the relative influence of each parameter was also quantified in this study. The results provide an important reference for optimizing wellbore design and parameter control in geothermal development of depleted oil and gas reservoirs.

Under the dual strategic impetus of the ”dual carbon” goals and energy structure transformation, the carbon dioxide enhanced geothermal system has emerged as a pivotal technology for the efficient exploitation of hot dry rock resources. However, its engineering application is constrained by the complexity and uncertainty of thermo-hydro-mechanical multi-field coupling during heat extraction. To address this challenge, this study develops a coupled thermo-hydro-mechanical numerical model for rough fracture networks, which comprehensively accounts for the dynamic evolution of carbon dioxide thermal properties with temperature and pressure, as well as the time-dependent permeability of the reservoir-fracture system under multi-physics coupling. Based on this model, systematic parametric simulations are conducted to investigate mixed injection modes, fracture roughness heterogeneity, variable fracture width, and intermittent injection-production under thermal compensation. There is an optimal ratio range for the mixed working medium, and the bottom hole temperature of the production well can reach the highest when the mass fraction ratio of carbon dioxide to nitrous oxide is 1:1. Fracture roughness significantly hinders carbon dioxide convective heat transfer, and this effect is positively correlated with the fracture fractal dimension. Ignoring fracture opening evolution leads to notable deviations between simulated and actual behavior. The heat extraction performance under intermittent injection-production is significantly better than under continuous operation, with different intermittent periods exhibiting distinct regulatory characteristics. Injection pressure and temperature exert opposite effects on thermal recovery power. This study provides a theoretical basis and technical support for parameter optimization and engineering design of enhanced geothermal system thermal extraction systems.

Under the dual strategic impetus of the “dual carbon” goals and energy structure transformation, the carbon dioxide enhanced geothermal system has emerged as a pivotal technology for the efficient exploitation of hot dry rock resources. However, its engineering application is constrained by the complexity and uncertainty of thermo-hydro-mechanical multi-field coupling during heat extraction. To address this challenge, this study develops a coupled thermo-hydro-mechanical numerical model for rough fracture networks, which comprehensively accounts for the dynamic evolution of carbon dioxide thermal properties with temperature and pressure, as well as the time-dependent permeability of the reservoir-fracture system under multi-physics coupling. Based on this model, systematic parametric simulations are conducted to investigate mixed injection modes, fracture roughness heterogeneity, variable fracture width, and intermittent injection-production under thermal compensation. There is an optimal ratio range for the mixed working medium, and the bottom hole temperature of the production well can reach the highest when the mass fraction ratio of carbon dioxide to nitrous oxide is 1:1. Fracture roughness significantly hinders carbon dioxide convective heat transfer, and this effect is positively correlated with the fracture fractal dimension. Ignoring fracture opening evolution leads to notable deviations between simulated and actual behavior. The heat extraction performance under intermittent injection-production is significantly better than under continuous operation, with different intermittent periods exhibiting distinct regulatory characteristics. Injection pressure and temperature exert opposite effects on thermal recovery power. This study provides a theoretical basis and technical support for parameter optimization and engineering design of enhanced geothermal system thermal extraction systems.

Under the dual strategic impetus of the “dual carbon” goals and energy structure transformation, the carbon dioxide enhanced geothermal system has emerged as a pivotal technology for the efficient exploitation of hot dry rock resources. However, its engineering application is constrained by the complexity and uncertainty of thermo-hydro-mechanical multi-field coupling during heat extraction. To address this challenge, this study develops a coupled thermo-hydro-mechanical numerical model for rough fracture networks, which comprehensively accounts for the dynamic evolution of carbon dioxide thermal properties with temperature and pressure, as well as the time-dependent permeability of the reservoir-fracture system under multi-physics coupling. Based on this model, systematic parametric simulations are conducted to investigate mixed injection modes, fracture roughness heterogeneity, variable fracture width, and intermittent injection-production under thermal compensation. There is an optimal ratio range for the mixed working medium, and the bottom hole temperature of the production well can reach the highest when the mass fraction ratio of carbon dioxide to nitrous oxide is 1:1. Fracture roughness significantly hinders carbon dioxide convective heat transfer, and this effect is positively correlated with the fracture fractal dimension. Ignoring fracture opening evolution leads to notable deviations between simulated and actual behavior. The heat extraction performance under intermittent injection-production is significantly better than under continuous operation, with different intermittent periods exhibiting distinct regulatory characteristics. Injection pressure and temperature exert opposite effects on thermal recovery power. This study provides a theoretical basis and technical support for parameter optimization and engineering design of enhanced geothermal system thermal extraction systems.