A simple 3-D thermoelastic model for assessment of the long-term performance of the Hijiori deep geothermal reservoir Yani Jing a , Zhenzi Jing b, ⁎, Jonathan Willis-Richards c , Toshiyuki Hashida d a Department of Electronic Engineering, Jiangnan University, 1800 Lihu Avenue, Wuxi, Jiangsu 214122, China b School of Materials Science and Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China c Loeb Aron & Company, Georgian House, 63, Coleman Street, London, United Kingdom d Fracture and Reliability Research Institute, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan abstract article info Article history: Received 1 August 2013 Accepted 30 October 2013 Available online 8 November 2013 Keywords: Thermoelastic effect HDR FRACSIM Modeling Hijiori deep reservoir In order to assess the thermoelastic influence on the long-term performance of hot dry rock (HDR) reservoirs, a simple three-dimensional (3-D) thermoelastic model has been developed based on an assumption of a spherical- ly symmetric volume of cooled rock within reservoir. This model has been incorporated into a 3-D stochastic net- work model, FRACSIM-3D, which incorporates a fracture network designed to mimic natural fracture distributions as well as stimulation and circulation. The model has been used to evaluate the possible long- term performance of the deep HDR reservoir at Hijiori, Japan. Simulation results showed that thermoelasticity could exert a significant influence on production temperature, injection pressure and water loss. For a multi- well geothermal system, thermoelasticity seemed to have a potential to cause the development of high flow rate/rapidly cooling flow paths (thermal short circuits). © 2013 Elsevier B.V. All rights reserved. 1. Introduction Renewable energy, especially after the great east Japan earthquake on March 11, 2011, becomes more and more important in ensuring self-sufficiency of energy supply and mitigating climate change. Among renewable energy options, geothermal power generation has the advantage of providing baseload energy supply, i.e. a stable energy supply and unaffected by weather conditions, in contrast to solar and wind power generation. HDR geothermal field experiments have been carried out at several sites worldwide to explore the potential for exploiting geothermal ener- gy (Laughlin et al., 1983; Nakatsuka, 1999; McDermott et al., 2006; Hebert et al., 2010; Zaigham and Nayyar, 2010; Watanabe et al., 2011; Feng et al., 2012; Zeng et al., 2013). The concept of HDR typically in- volves: (a) stimulation, i.e. pumping highly pressured fluid through an injection well to open up predominantly pre-existing natural fractures in the ‘hot’ rock, thereby creating a man-made or “engineered” geother- mal reservoir of fractured rock; (b) circulation, i.e. circulating fluid through the stimulated fracture networks in hot rock where the fluid is heated, and recovering the heated fluid via production well(s) to move heat from the reservoir to the surface. Commercial design consid- erations typically require the operation of these systems for more than 20 years before abandonment due to drops in production rates or tem- perature. Therefore the potential effect of prolonged thermoelasticity induced by temperature drawdown of the rock mass around fluid paths needs to be considered. A priori, thermoelastic deformation is considered likely to cause both widening and narrowing of the fracture apertures in different parts of the rock mass, which will inevitably affect the permeability distribution of the fracture network and reservoir performance. To model HDR engineered systems, a three-dimensional (3-D) sto- chastic network model, FRACSIM-3D, has been presented (Jing et al., 2000), which incorporates a fracture network whose statistical param- eters are derived from borehole observations and is designed to be sim- ilar to that in the HDR reservoir under study, and addresses both the changes that occur within the rock mass during hydraulic stimulation (chiefly slip along fractures, fracture opening and resulting stress changes) and the circulation under steady-state of the heat exchange system to move heat from the reservoir to the surface. The model, how- ever, failed to incorporate the effects of thermoelasticity and water/rock chemical interaction (WRCI), thought to be very important in the evolv- ing performance of HDR reservoirs. As an extension, a water/rock chemical interaction (WRCI) module has been incorporated into FRACSIM-3D (Jing et al., 2002), and applied to estimate the effect of WRCI on the performance of the Hijiori deep reservoir. The objective of this work is to incorporate thermoelasticity into the 3-D stochastic network model, so as to complete another exten- sion of FRACSIM-3D. Thermoelastic effects on the flow path (Elsworth, 1990), on defor- mation along a single fracture (Evans et al., 1992), on thermal short cir- cuits (Duteau et al., 1994) and on the performance of HDR reservoirs (Hicks et al., 1996) have been modeled. However these existing one- or, two-dimensional (1- or 2-D) models, which used a parallel plate or Journal of Volcanology and Geothermal Research 269 (2014) 14–22 ⁎ Corresponding author. Tel./fax: +86 21 6958 0264. E-mail address: zzjing@tongji.edu.cn (Z. Jing). 0377-0273/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jvolgeores.2013.10.012 Contents lists available at ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores