Developing hot dry rock geothermal energy via enhanced geothermal systems (EGS) is crucial for sustainable energy; however, optimizing extraction in heterogeneous reservoirs remains challenging. Existing studies predominantly focus on planar flow extraction patterns in homogeneous reservoirs, lacking a comprehensive understanding of the fluid dynamics mechanisms and heat transfer behaviors under the coupling effect of heterogeneous strata and fractures. In this study, a three-dimensional (3D) coupled thermo-hydraulic finite element numerical model was established to systematically quantify the impacts of flow patterns, well spacing parameters, stratigraphic heterogeneity, and the geometric characteristics of large irregular fractures on the heat extraction performance of an EGS over a 60-year lifespan. The accuracy of the model was verified using the Lauwerier analytical solution (maximum temperature error 2 °C). The results demonstrate that in homogeneous reservoirs, the spherical flow pattern exhibits optimal long-term thermal stability. Its 60-year cumulative heat extraction reaches 8.79 × 1016 J, which is marginally lower (by 0.57%) than that of the planar flow pattern, but its late-stage temperature decline rate is reduced by 32.1%. Well spacing optimization indicates that a vertical well configuration with a 400 m spacing yields the best heat extraction performance, increasing the cumulative heat production by 27.9% compared to the 200 m spacing. In heterogeneous reservoirs, a stratigraphic sequence where permeability increases with depth (with permeabilities of 5 × 10−17, 1 × 10−16, and 3 × 10−16 m2 for layers 2 to 4, respectively) effectively suppresses interlayer thermal breakthrough, achieving a cumulative heat extraction of 9.23 × 1016 J. When large irregular fractures are present in the reservoir, a fracture with a 90° dip angle can uniformly divert the fluid and block the preferential flow paths between the injection and production wells, thereby elevating the cumulative heat extraction to 9.20 × 1016 J. This paper reveals the core heat transfer mechanisms in heterogeneous reservoirs—specifically, the preferential thermal breakthrough at lithological interfaces and the drag effect of the cold front in high-permeability layers on the overlying strata. Furthermore, it proposes design principles that prioritize deep high-permeability zones for enhanced heat exchange and shallow low-permeability zones for controlling thermal breakthrough. This study provides a quantitative theoretical basis and engineering guidance for the efficient development of EGS under complex geological conditions.
Cai et al. (Mon,) studied this question.