The effects of fractures on porous flow and heat transfer in a reservoir of a U-shaped closed geothermal system

Abstract

In geothermal reservoirs, fractures, serving as preferential channels for heat transfer fluid circulation, can cause complicated migration patterns and heat transfer characteristics of the fluids. However, the crucial issue of whether and how fractures within a hydrothermal reservoir of a closed geothermal system (CGS) affect reservoir heat transfer fluid migration and the system’s heat transfer is still poorly understood. In this study, we construct a thermo-hydraulic coupling finite-element model to investigate the effects of fractures on fluid flow and heat transfer enhanced by natural convection in a U-shaped closed geothermal system (UCGS) with a horizontal well. We systematically analyze the responses of fluid migration and heat extraction in the system to key factors, such as fracture aperture, fracture azimuth, fracture distribution, and reservoir permeability. The results show that, in a high permeability reservoir (such as 2 × 10−13 m2), the fracture with an aperture of a millimeter scale can greatly strengthen and control the natural convection induced by heat transfer around the horizontal well in a UCGS, achieving a heat recovery efficiency of six times higher than conduction alone within the reservoir. The fracture azimuth can control the heat production region by guiding the evolution of natural convection. Intersecting network-distributed fractures can effectively improve heat recovery area and performance (more than 15%) compared to a single fracture. Some specific fracture segments within a fracture network may exhibit synergy or inhibition in different heat extraction stages, while a fracture connecting in the deep reservoir and orienting to the horizontal well may lead to heat breakthrough. In addition, a low-permeability interlayer below the horizontal well can restrict the heat recovery (nearly 20% drop in our models), but a fracture penetrating the layer can compensate for this hindrance. The findings of this study can contribute to understanding the thermodynamic mechanisms of fluid flow and heat transfer in a fractured reservoir, and then utilizing them to maximize heat recovery rather than considering conduction only, thus providing scientific insights into engineering practices of the UCGS.