(392ac) Identifying Key Factors for Maximizing Energy Extraction from Enhanced Geothermal System Reservoirs

Enhanced Geothermal Systems (EGS) provide a sustainable means to tap into Earth’s deep heat reserves, making reservoir optimization essential for their success [1]. This study explores a triplet configuration with one injection well and two production wells—a 1-2 triplet—designed to enhance energy recovery from an EGS reservoir. By conducting sensitivity analysis, we examine critical planning and operational parameters that influence performance, including the spacing between the injection well and production wells, the number of hydraulic fracture clusters along each well, the spacing between fractures, the injection flow rate, and the temperature of the injected fluid. Flow simulations guide the analysis, offering insights into how these factors interact within the reservoir [2]. Unlike doublet systems, the 1-2 triplet aims to increase heat extraction by leveraging dual production wells to capture fluid circulated through a shared fracture network. The goal is to pinpoint the most impactful elements that drive efficiency and sustainability, ensuring the reservoir delivers maximum energy output over time. The objective of this work is to identify the key factors that unlock the full potential of an EGS reservoir, providing a practical framework for optimization.

The 1-2 triplet relies on a single injection well to push fluid through a fractured reservoir, with two production wells positioned to collect the heated output, potentially amplifying recovery compared to simpler designs. Well spacing is a foundational planning decision: if the wells are too close, the reservoir cools rapidly, reducing long-term yield; if too far apart, heat remains untapped, lowering efficiency [3]. Sensitivity analysis reveals an optimal spacing that balances these trade-offs, sustaining production without wasting thermal resources. The number of fracture clusters—stimulated zones along each well—boosts fluid flow by increasing permeability, but too many clusters lead to overlap and unnecessary costs, suggesting a practical limit for effectiveness. Fracture spacing also shapes the system: tight spacing strengthens connectivity between wells, while wider spacing preserves heat exchange with the surrounding rock, with an intermediate range emerging as the best compromise.

Operationally, the injection flow rate determines how much heat the system can extract. A low rate allows the fluid to absorb more heat but limits the total energy produced, while a high rate speeds up output at the expense of faster cooling or potential instability[4]. Sensitivity analysis identifies a flow rate that maximizes energy yield while keeping the reservoir viable. The temperature of the injected fluid influences heat uptake: cooler fluid increases the temperature difference with the rock, enhancing extraction, but if too cold, it may complicate operations by stressing the system. A moderate temperature stands out as the most reliable choice for steady performance. These operational adjustments are critical to fine-tuning the triplet’s output and ensuring it operates at peak efficiency.

Simulations model the reservoir’s fluid flow and heat transfer, providing a foundation for sensitivity analysis. Statistical techniques assess how each parameter affects energy production over a 10-year period, showing that well spacing and flow rate have the greatest influence, followed by fracture cluster count and spacing, while injection temperature matters more early on and less as the reservoir stabilizes. A simplified predictive model, derived from these simulations, maps out how these factors work together, offering a tool to find the best configurations. This approach keeps the analysis focused on essential behaviors, avoiding unnecessary complexity while still capturing what drives performance.

Getting the most out of an EGS reservoir requires aligning planning and operational choices. A triplet with ideal spacing can falter if the flow rate or temperature is off, but flexible adjustments—like varying the injection rate—can recover efficiency. This study shows that by focusing on key factors—well spacing, fracture setup, flow rate, and temperature—the reservoir’s energy potential can be fully realized. Sensitivity analysis highlights the trade-offs, such as balancing fracture density against cost or flow rate against thermal decline, guiding decisions that boost output and reduce risks. The result is a clear strategy for optimizing EGS reservoirs, making them a dependable source of renewable energy.

References:

[1] P. Olasolo, M. C. Juárez, M. P. Morales, S. D´Amico, and I. A. Liarte, ‘Enhanced geothermal systems (EGS): A review’, Renewable and Sustainable Energy Reviews, vol. 56, pp. 133–144, Apr. 2016, doi: 10.1016/j.rser.2015.11.031.

[2] P. Asai, P. Panja, R. Velasco, J. McLennan, and J. Moore, ‘Fluid flow distribution in fractures for a doublet system in Enhanced Geothermal Systems (EGS)’, Geothermics, vol. 75, pp. 171–179, Sep. 2018, doi: 10.1016/j.geothermics.2018.05.005.

[3] S.-M. Lu, ‘A global review of enhanced geothermal system (EGS)’, Renewable and Sustainable Energy Reviews, vol. 81, pp. 2902–2921, Jan. 2018, doi: 10.1016/j.rser.2017.06.097.

[4] A. C. Gringarten, P. A. Witherspoon, and Y. Ohnishi, ‘Theory of heat extraction from fractured hot dry rock’, J. Geophys. Res., vol. 80, no. 8, pp. 1120–1124, Mar. 1975, doi: 10.1029/JB080i008p01120.