Coal Geology & Exploration


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-FLAC3D 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.


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




[1] SUN Zhixue,ZHANG Xu,XU Yi,et al. Numerical simulation of the heat extraction in EGS with thermal–hydraulic–mechanical coupling method based on discrete fractures model[J]. Energy,2017,120:20−33.

[2] 令兰宁,姚尔人,孙昊,等. 中深层地热能同轴套管换热器储能发电系统热力学性能分析[J]. 西安交通大学学报,2024,58(1):126−137.

LING Lanning,YAO Erren,SUN Hao,et al. Thermodynamic performance analysis of medium and deep geothermal energy coaxial tube heat exchanger energy storage and power generation system[J]. Journal of Xi’an Jiaotong University,2024,58(1):126−137.

[3] 张建英. 增强型地热系统(EGS)资源开发利用研究[J]. 中国能源,2011,33(1):29−32.

ZHANG Jianying. Research on developing enhanced geothermal systems resource[J]. Energy of China,2011,33(1):29−32.

[4] 韩东旭,张炜韬,焦开拓,等. 基于嵌入式离散裂缝模型的增强型地热系统热–流–力–化耦合分析[J]. 天然气工业,2023,43(7):126−138.

HAN Dongxu,ZHANG Weitao,JIAO Kaituo,et al. Analysis of thermal–hydraulic–mechanical–chemical coupling for EGS based on embedded discrete fracture model[J]. Natural Gas Industry,2023,43(7):126−138.

[5] BROWN D W. A hot dry rock geothermal energy concept utilizing supercritical CO2 instead of water[C]//Proceedings of 25th Workshop on Geothermal Reservoir Engineering. California:Stanford University Press,2000:233–238.

[6] PRUESS K. On production behavior of enhanced geothermal systems with CO2 as working fluid[J]. Energy Conversion and Management,2008,49(6):1446−1454.

[7] ZHONG Chenghao,XU Tianfu,GHERARDI F,et al. Comparison of CO2 and water as working fluids for an enhanced geothermal system in the Gonghe Basin,northwest China[J]. Gondwana Research,2023,122:199−214.

[8] 赵悦安. 超临界地热系统水和CO2取热性能分析:以意大利Larderello地热田为例[D]. 长春:吉林大学,2023.

ZHAO Yue’an. Analysis of heat extraction performance of water and CO2 in supercritical geothermal system:A case study of Larderello geothermal field in Italy[D]. Changchun:Jilin University,2023.

[9] 任韶然,崔国栋,李德祥,等. 注超临界CO2开采高温废弃气藏地热机制与采热能力分析[J]. 中国石油大学学报(自然科学版),2016,40(2):91−98.

REN Shaoran,CUI Guodong,LI Dexiang,et al. Development of geothermal energy from depleted high temperature gas reservoir via supercritical CO2 injection[J]. Journal of China University of Petroleum (Edition of Natural Science),2016,40(2):91−98.

[10] LEI Qinghua,LATHAM J P,TSANG C F. The use of discrete fracture networks for modelling coupled geomechanical and hydrological behaviour of fractured rocks[J]. Computers and Geotechnics,2017,85:151−176.

[11] 熊峰. 裂隙岩体非线性渗流特性及水热耦合模拟研究[D]. 武汉:武汉大学,2020.

XIONG Feng. Study on nonlinear flow behaviors and coupled hydro–thermal simulation of fractured rock masses[D]. Wuhan:Wuhan University,2020.

[12] MA Yueqiang,ZHANG Yanjun,YU Ziwang,et al. Heat transfer by water flowing through rough fractures and distribution of local heat transfer coefficient along the flow direction[J]. International Journal of Heat & Mass Transfer,2018,119:139−147.

[13] DAVY P,SORNETTE A,SORNETTE D. Some consequences of a proposed fractal nature of continental faulting[J]. Nature,1990,348:56−58.

[14] DARCEL C,BOUR O,DAVY P. Stereological analysis of fractal fracture networks[J]. Journal of Geophysical Research:Solid Earth,2003,108(B9):1−13.

[15] 孙致学,姜传胤,张凯,等. 基于离散裂缝模型的CO2增强型地热系统THM耦合数值模拟[J]. 中国石油大学学报(自然科学版),2020,44(6):79−87.

SUN Zhixue,JIANG Chuanyin,ZHANG Kai,et al. Numerical simulation for heat extraction of CO2–EGS with thermal–hydraulic–mechanical coupling method based on discrete fracture models[J]. Journal of China University of Petroleum (Edition of Natural Science),2020,44(6):79−87.

[16] ZHOU D,TATOMIR A,NIEMI A,et al. Study on the influence of randomly distributed fracture aperture in a fracture network on heat production from an enhanced geothermal system (EGS)[J]. Energy,2022,250:123781.

[17] DE DREUZY J R,DAVY P,BOUR O. Hydraulic properties of two–dimensional random fracture networks following a power law length distribution:2. Permeability of networks based on lognormal distribution of apertures[J]. Water Resources Research,2001,37(8):2079−2095.

[18] GUDALA M,GOVINDARAJAN S K,YAN Bicheng,et al. Numerical investigations of the PUGA geothermal reservoir with multistage hydraulic fractures and well patterns using fully coupled thermo–hydro–geomechanical modeling[J]. Energy,2022,253:124173.

[19] 单丹丹,李玮,王艳,等. 井筒布置及裂隙分布特征对增强型地热系统采热性能的影响[J]. 地球物理学进展,2023,38(2):600−611.

SHAN Dandan,LI Wei,WANG Yan,et al. Effect of wellbore layout and fracture distribution characteristics on heat recovery performance of enhanced geothermal system[J]. Progress in Geophysics,2023,38(2):600−611.

[20] 王丹丹,党志伟,石哲伟,等. 裂隙分布及井间距对增强型地热系统采热性能的影响[J/OL]. 地球物理学进展,2023:1–18[2023-11-09]. https://link.cnki.net/urlid/11.2982.P.20231109.1128.002.

WANG Dandan,DANG Zhiwei,SHI Zhewei,et al. Effect of fracture distribution and well spacing on heat recovery performance of enhanced geothermal system[J/OL]. Progress in Geophysics (in Chinese),2023:1–18[2023-11-09]. https://link.cnki.net/urlid/11.2982.P.20231109.1128.002.

[21] ZHANG Keni,YAMAMOTO H,PRUESS K. TMVOC–MP:A parallel numerical simulator for three–phase non–isothermal flows of multicomponent hydrocarbon mixtures in porous/fractured media[R]. Berkeley:Lawrence Berkeley National Laboratory,2008.

[22] LIAO Jianxing,XU Bin,MEHMOOD F,et al. Numerical study of the long–term performance of EGS based on discrete fracture network with consideration of fracture deformation[J]. Renewable Energy,2023,216:119045.

[23] LIAO Jianxing,HU Ke,MEHMOOD F,et al. Embedded discrete fracture network method for numerical estimation of long–term performance of CO2–EGS under THM coupled framework[J]. Energy,2023,285:128734.

[24] GUO Tiankui,TANG Songjun,SUN Jiang,et al. A coupled thermal–hydraulic–mechanical modeling and evaluation of geothermal extraction in the enhanced geothermal system based on analytic hierarchy process and fuzzy comprehensive evaluation[J]. Applied Energy,2020,258:113981.

[25] ZHONG Chenghao,XU Tianfu,YUAN Yilong,et al. The feasibility of clean power generation from a novel dual–vertical–well enhanced geothermal system (EGS):A case study in the Gonghe Basin,China[J]. Journal of Cleaner Production,2022,344:131109.

[26] SUN Zhixue,JIANG Chuanyin,WANG Xiaoguang,et al. Joint influence of in–situ stress and fracture network geometry on heat transfer in fractured geothermal reservoirs[J]. International Journal of Heat and Mass Transfer,2020,149:119216.

[27] ZHANG Xu,HUANG Zhaoqin,LEI Qinghua,et al. Impact of fracture shear dilation on long–term heat extraction in enhanced geothermal systems:Insights from a fully–coupled thermo–hydro–mechanical simulation[J]. Geothermics,2021,96:102216.

[28] YU Guojun,LI Huyu,LIU Cong,et al. Thermal and hydraulic characteristics of a new proposed flyover–crossing fracture configuration for the enhanced geothermal system[J]. Renewable Energy,2023,211:859−873.



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