Numerical investigation of fluid flow and heat transfer in a doublet enhanced geothermal system with CO 2 as the working fluid (CO 2 eEGS) Feng Luo, Rui-Na Xu, Pei-Xue Jiang * Beijing Key Laboratory for CO 2 Utilization and ReductionTechnology, Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China article info Article history: Received 20 April 2013 Received in revised form 15 August 2013 Accepted 17 October 2013 Available online 11 November 2013 Keywords: Enhanced geothermal system CO 2 Induced fracture Injection/production well perforation placement Numerical simulation abstract Enhanced geothermal system with CO 2 instead of water as the working fluid (CO 2 eEGS ) has attracted much interest due to the additional benefit of CO 2 geological storage during the power generation process. This paper describes numerical analyses of a doublet CO 2 eEGS system, focusing on the influence of the CO 2 injection rate, the permeability of induced fractures near the wellbores, the injection/pro- duction well perforation placement, the working fluid, and the heat transfer between the wellbores and the surrounding reservoir. The larger permeability in the induced fractures around the wellbores allows the fluid to more easily flow through the reservoir up to a critical fracture permeability, with further increases of the fracture permeability further reducing the pressure loss a little but the cost is not worth the added benefit. Induced fractures around the wellbores result in little difference among different wellbores perforation locations in the reservoir. Increased CO 2 injection rates reduce the heat transfer between the wellbores and the surrounding reservoir so that this heat transfer can be neglected at large mass flow rates. With CO 2 as the working fluid, the CO 2 temperature decreases significantly going up the production well. This paper presents some important implications for CO 2 eEGS system for further nu- merical studies as well as for practical projects. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Issues related to the unprecedented demand for energy have become more and more serious due to population growth and in- dustrial development. The environmental stresses mainly from global warming and the expanding consumption of traditional energy sources (fossil fuels) request urgent development of low- carbon-emitting energy technologies [1]. Against this back- ground, geothermal energy, especially EGS (enhanced geothermal systems) have attracted much attention worldwide due to their cleanliness and enormous potential [2e6]. A novel CO 2 eEGS concept using supercritical CO 2 instead of water as the heat transmission fluid was proposed by Brown [7] to coincide with the need to reduce carbon dioxide emissions. Besides the potential for CO 2 geological storage because of working fluid losses at great depths, investigations have shown other advantages of CO 2 eEGS including large expansivity and compressibility, favorable transport properties (larger ratio of density to viscosity under typical reser- voir conditions), low salt solubility and low chemical activity, and self-driven high flow rates due to the strong buoyancy force [7e14]. In the limited literature about experimental laboratory research on CO 2 eEGS [15e18], Ueda et al. [15] described experiments on CO 2 ewatererock/mineral interactions at elevated temperatures under hydrothermal conditions which indicated that in the outmost region of CO 2 eEGS reservoir where the fluid is a single aqueous phase with dissolved CO 2 , calcium is easily released from the rock (silicates) and may be removed as CaCO 3 and/or CaSO 4 due to chemical interactions, favoring rapid CO 2 geological storage from fluid losses. Muller et al. [16] carried out core flood experimental research on CO 2 injection impairment due to halite precipitation with the experimental results showing that the observed CO 2 permeability in the sandstone core sample was reduced by about 60% after 32 h of flushing with dry supercritical CO 2 . Zimmermann and Reinicke [17] investigated the fracture permeability changes and long-term stability of various proppant types in hydraulic fracture stimulations for developing an EGS system, suggesting that large concentrations of a HSPs (high-strength proppants) would * Corresponding author. Department of Thermal Engineering, Tsinghua Univer- sity, Beijing 100084, China. Tel.: þ86 10 62772661; fax: þ86 10 62770209. E-mail address: jiangpx@mail.tsinghua.edu.cn (P.-X. Jiang). Contents lists available at ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy 0360-5442/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.energy.2013.10.048 Energy 64 (2014) 307e322