1. INTRODUCTION The presence of flaws such as joints, cracks and faults are critical factors in analyzing reservoir systems (e.g., geothermal, and shale gas). Fractures highly influence flow rate, heat extraction, and induced seismicity in the stimulation process (Ghassemi, 2012). Distortion (e.g. shear slip, dilation) occurs when fractures are exposed to stress field oscillation. For instance, shear slip happens when the normal effective stress is reduced by either pore pressure increase or the thermal stresses growth (Cornet and Jianmin, 1995; Segall, 1989). It also causes shear dilation which results in the increase of rocks’ permeability. The reservoir pressure fluctuates in the injecting and producing process. This results in stress variation that induces deformation in fractures and rock matrix. The fracture network is more susceptible to the pressure, temperature, and stress changes in comparison with the matrix. The flow behavior, therefore, is controlled by the fracture permeability. Hence, for performing a comprehensive analytical and numerical analysis of the reservoir reaction to the injection and exploitation process, the joint deformation and strength characteristics are inherent factors (Ghassemi, 2012). Crack initiation is defined when an undamaged elastic body is subjected to a stress filed, and after a finite time, a displacement discontinuity happens. In such a case the propagation velocity cannot be defined nor an incremental formulation for the problem. The energy release rate is always unbounded (Gdoutos, 2006). Fracture propagation occurs when a flaw exists in the rock crystal. In such a case the energy released during propagation is well known since Irwin, 1957. In the case of geothermal sites, the crack propagation occurs by fluid flow in either hydraulic fracturing or enhanced geothermal system (EGS) processes. Crack opening, length, and depth are the critical factors for the fluid flow rate. The fundamental difference between these two methods is the injection pressure. In EGS approach the pressure is below the minimum principal stress and only makes the joints dilate, slip and shear (Rinaldi et al., 2015). 1.1. Crack Initiation According to the several experiments on the mechanism of fracture initiation (Wong, 2009; Bombolakis, 1963; Latajai, 1974; Li, 2005; Park, 2010; Yang, 2011) tensile (wing) cracks initiated by applying loading condition (e.g. in the first stages of a uniaxial compression test), and ARMA 18– 514 A Numerical Simulation of Thermo-Mechanical Behavior of a Single Fracture in Porous Rock Azad, E. Amirkabir University of Technology, Tehran, Iran Peik, B. and Abbasi, B. University of Nevada, Reno, US Abbasi, B. Oregon State University, Portland, US Copyright 2018 ARMA, American Rock Mechanics Association This paper was prepared for presentation at the 52 nd US Rock Mechanics / Geomechanics Symposium held in Seattle, Washington, USA, 17–20 June 2018. This paper was selected for presentation at the symposium by an ARMA Technical Program Committee based on a technical and critical review of the paper by a minimum of two technical reviewers. The material, as presented, does not necessarily reflect any position of ARMA, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of ARMA is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 200 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgement of where and by whom the paper was presented. ABSTRACT: Fracture studies mainly concern with the investigation of the cracks behavior under specific conditions which cause the crack aperture or closure. Joint aperture in rock mass with presence of other factors, such as high temperature and pore pressure, is a complex case. In this paper, the extended finite element method (XFEM) and traditional finite element method (FEM) techniques are employed to study the behavior of a single crack initiation subjected to a tensile loading in pure mechanical (M), hydro-mechanical (HM) and thermo-mechanical (TM) conditions. For this purpose, water as the preferred fluid and 250 F as the desired temperature were considered. Fracture aperture is modeled for three scenarios; edge, center horizontal and center inclined crack. The numerical analysis was also verified utilizing the analytical solution. The presence of pore pressure is shown to have a considerably low impact on the crack aperture, while it is argued that the temperature highly influences the analysis. Furthermore, the pros and cons of the XFEM technique are evaluated upon conventional FEM method, and it is shown that for the stationary crack initiation and calculation of the stress intensity factor, XFEM is less favorable than the FEM.