Proceedings World Geothermal Congress 2015 Melbourne, Australia, 19-25 April 2015 1 A Thermal-Hydraulic-Mechanical Fully Coupled Model for Heat Extraction in Enhanced Geothermal Systems Wenjiong Cao, Wenbo Huang, Fangming Jiang* Laboratory of Advanced Energy Systems, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences (CAS), Guangzhou 510640, China jiangfm@ms.giec.ac.cn Keywords: THM coupling; local thermal non-equilibrium; thermal-pore-elastic; EGS ABSTRACT During the heat extraction in enhanced geothermal systems (EGS), the porosity and permeability of the heat reservoir can be greatly affected by the multi-physical coupling of Thermal (T), Hydraulic (H), and Mechanical (M) processes. In the present work we develop a three-dimensional transient model fully coupling the subsurface THM behaviors during EGS heat extraction process. The local thermal non-equilibrium is assumed when describing the heat exchange between the rock matrix and heat transmission fluid, and the variable properties of the fluid are combined to realize the bi-directional coupling of thermal and hydraulic behaviors. The thermal-pore-elastic model is adopted to calculate the effective stress of the rock matrix and further to derive the local porosity and permeability. Case study with respect to an imaginary EGS demonstrates the validity and capability of the developed model. Results indicate that the model can be used for heat extraction simulation of more practical EGSs. 1. INTRODUCTION Simulating the heat extraction process and predicting how much heat can be extracted from an enhanced geothermal system (EGS) reservoir are relatively involved tasks and thus remain still as major challenges for the geothermal industry (MIT Report, 2006). To address such challenges it is desirable to develop reliable numerical models dealing with the inter-coupled Thermal– Hydraulic– Mechanical (THM) processes in EGS subsurface reservoir. Koh et al. (2011) mentioned that circulating cold fluid into the reservoir can lead to large amounts of stress energy release inside the rock matrix. How does EGS performance including its production temperature, mass flow rate of circulation fluid, lifetime, and heat extraction ratio etc. relate the THM coupling behaviors? This can be unraveled or better understood via numerical modeling. During the past decades, considerable efforts have been expended in the THM model development and numerical studies relevant to EGS heat extraction. Rutqvist (2002, 2011)and Jeanne et al. (2014) reviewed the developed models of hydromechanical coupling in an aquifer and summarized a number of well-known empirical approaches for estimations of normal stress across fractured medium. Ghassemi and Zhou (2011), Ghassemi et al. (2007), and Rawal and Ghassemi (2014) numerically investigated the poroelastic and thermoelastic responses of a reservoir or fractures in a reservoir upon the injection of cold water. McDermott et al. (2005, 2006) studied the influence of THM coupling on the heat extraction from reservoir in crystalline rocks with an experimentally validated geomechanical model developed by themselves. The results indicated that changes of fracture aperture and closure were caused by changes of normal stress acting on fractures, which was the dominant mechanism for thermal stress inducing changes of reservoir permeability. The physical properties of the injected fluid, such as density, viscosity and heat capacity are speculated to play a significant role in the fluid-rock interactions. Nevertheless, to the best of our knowledge, there are no published THM numerical models that have fully coupled the effects of variable fluid properties. In the present work, we present a three-dimensional transient model fully coupling the subsurface THM behaviors during EGS heat extraction process. The local thermal non-equilibrium assumption is adopted to describe the heat transport in the reservoir, and variable fluid properties are considered to realize the bi-directional coupling of thermal and hydraulic behaviors. A thermal-pore- elastic model is employed to calculate the effective stress of the rock matrix and further to derive the local porosity and permeability. A case study with respect to an imaginary EGS will be performed to demonstrate the validity and capability of the developed model. 2. THM COUPLING MODEL FOR EGS HEAT EXTRACTION 2.1 Model equations We focus on the heat extraction from EGS subsurface reservoir and consider an imaginary EGS consisting of an injection well, a production well or multiple production wells, an artificial heat reservoir, and rocks enclosing the reservoir as described in our previous publications(Jiang et al., 2013). The reservoir is taken as an equivalent porous medium with uniform porosity  and permeability K. Model equations describing the heat transport and fluid flow are listed below. Fluid continuity equation: f f ( ) ( ) 0 u t       (1) Fluid momentum equation: