Effects of fracture distribution on heat extraction through CO₂-EGS

Authors

ZHOU Qing, College of Civil Engineering, Guizhou University, Guiyang 550025, ChinaFollow
LIAO Jianxing, College of Civil Engineering, Guizhou University, Guiyang 550025, ChinaFollow
XU Bin, College of Civil Engineering, Guizhou University, Guiyang 550025, China
YAO Liang, College of Civil Engineering, Guizhou University, Guiyang 550025, China
ZHOU Haoyu, College of Civil Engineering, Guizhou University, Guiyang 550025, China
ZHAO Quan, College of Civil Engineering, Guizhou University, Guiyang 550025, China

Abstract

Geothermal energy, a promising source of renewable energy, has attracted considerable attention. In deep geothermal reservoirs, complex fracture networks formed by artificial stimulation provide predominant channels for heat extraction. Therefore, the spatial distribution of fractures directly affects the heat extraction efficiency. This study aims to explore the effects of different spatial distributions of fractures on the heat recovery performance. Based on the fracture network following a power-law distribution, this study systematically investigated the effects of the fracture network with different values of fracture length index (a) and density (β) on the heat recovery performance of a new enhanced geothermal system with CO₂ as injection fluid (CO₂-EGS) using the THM coupling model established under the TOUGH2MP-FLAC^3D framework. Furthermore, this study presented a detailed evaluation of the thermal performance of CO₂-EGS using five evaluation indicators: heat breakthrough time, CO₂-EGS life, heat production rate, total heat production, and heat production efficiency, obtaining the following results. Under a constant injection rate, an increase in a corresponded to a smaller proportion of long fractures, a smaller number of penetrating fractures between the injection and production wells, a larger fracture width, and higher decreasing rates of the production temperature and heat production rate. These led to earlier heat breakthroughs, thereby shortening the CO₂-EGS life and reducing the total heat production. In the case of a constant a, a greater fracture density β was associated with a greater number of fractures and lower decreasing rates of the production temperature and heat production rate. These prolonged the thermal breakthrough time and CO₂-EGS life and improved heat production. Specifically, the heat breakthrough time, EGS life, and total heat production could increase by up to 15.65 a, about 10 years, and about 22.77%, respectively. In contrast, increasing a decreased the thermal breakout time and total heat production by 13.1 a and 20.8%, respectively. Therefore, increasing the proportion of long fractures and fracture density is instrumental in improving fracture connectivity, promoting convective heat transfer of fluids, improving the effects of fractures in heat recovery, and enhancing heat production. The results of this study can serve as a theoretical guide for the hydraulic fracturing of hot dry rocks to generate fractures and enhance their permeability.

Keywords

enhanced geothermal system (EGS), fracture distribution, thermal performance, evaluation indicator, numerical simulation

DOI

10.12363/issn.1001-1986.23.12.0827

Recommended Citation

ZHOU Qing, LIAO Jianxing, XU Bin, et al. (2024) "Effects of fracture distribution on heat extraction through CO₂-EGS," Coal Geology & Exploration: Vol. 52: Iss. 1, Article 12.
DOI: 10.12363/issn.1001-1986.23.12.0827
Available at: https://cge.researchcommons.org/journal/vol52/iss1/12

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