Rapid Numerical Simulation of Heat Transfer in Fractured Geothermal Reservoirs Based on Dynamic Thermal Influence Volume

Summary The growing demand for reliable and sustainable energy with low environmental impact drives the development of geothermal energy. One of the most commonly used techniques to extract geothermal energy is the enhanced geothermal system (EGS). At present, numerical simulation is the primary tool for researching and designing an efficient EGS. In each simulation, a significant number of grids are required to model the large reservoir region, along with the complex network of natural fractures and EGS-induced artificial fractures. It significantly increases the computational burden and the time needed for simulation. Additionally, determining the optimal geothermal model for energy extraction generally requires running numerous numerical simulations. Therefore, it is essential to improve computational efficiency and reduce computational time to accelerate the design and optimization process of EGS. In this study, the authors propose a fast numerical simulation method by combining the dynamic thermal influence volume (DTIV) with the embedded discrete fracture model (EDFM) to simulate the heat extraction process of EGS. The thermal influence volume (TIV) refers to the reservoir region where the temperature disturbance can be detected (i.e., the region affected by heat extraction). In contrast to the static TIV, which represents the temperature-affected region at the end of the heat extraction process, the DTIV introduced in this work evolves over time, capturing the temporal expansion of the temperature-affected region as heat is extracted. To model this dynamic behavior, the eikonal equation is derived to describe the propagation of the heat front in porous media. Building on this foundation, the fast marching method (FMM) combined with the concept of non-neighboring connections (NNCs) is used to efficiently solve the eikonal equation and track the boundaries of the DTIV. According to the definition of DTIV, the temperature disturbance outside the DTIV is sufficiently small to be neglected. Therefore, the authors focus only on temperature disturbance within the DTIV to approximately characterize the heat extraction process of the EGS. By applying the EDFM, an efficient method for describing fluid flow and heat transfer within the complex fracture system, only within the DTIV, the proposed method significantly reduces the model size, thereby significantly improving computational efficiency compared with the traditional EDFM. The calculated results indicate that the outputs of the proposed method show a good agreement with those of the traditional EDFM method while significantly reducing the computational time.