Investigating the Role of Temperature on Thermal Stress and Fracture Propagation in Geothermal Systems

Available geothermal energy extractable by conventional techniques is in dry and comparatively impermeable rocks. Enhanced Geothermal System (EGS) technologies enhance geothermal resources in the hot dry rock (HDR) through fracture operations, usually through hydro-shearing. Large scale deployment of geothermal power production requires the demonstration of successful EGS projects extracting heat from reservoirs constituting a variety of geological conditions. In this part, numerical models are very important to show how geothermal power plant operations can be less risky and safer. Owing the fact that, some major challenges in these operations are interaction between shear and tensile fractures with natural faults. These interactions can be seen in two different cases, either these faults are badly oriented or these faults are fill in pore fluids or gases which are mainly high pressure. Fluids and gases are important on account for because of the fact that these pore fluids can over whelmed the injection pressure and cause well blow out. Furthermore, to prevent these operational hazards, we use field data and analysis in combination with experimental tests and numerical/analytical models with finite element method software such as COMSOL Multiphysics. Further work will be required for improving enhanced geothermal production by optimizing hydro-shearing practices. friction coefficient and far field stresses. But these parameters were not considered in these set of presenting models. HF in medias with initial faults has been studied by different methods. Akulich and Zvyagain have studied interaction between HF crack and natural fractures using a BEM procedure, where viscosity effects have been taken into account. Simulation of hydraulic fracture propagation near a natural fracture using virtual multidimensional internal bonds is represented by Zhang and Ghassemi. Zhang et al. has studied layered medium where the effects of different sequences of soft and stiff layers has been investigated. Gudmunsson and Brenner have performed similar study with more than two layers taken into account. In this paper the finite element method will be used to model HF in sedimentary rock samples and its interaction with natural faults. The crack tip behavior will be assumed to be linear, where asymptotic enrichment functions will be employed. The contact between surfaces of the natural fault is modeled. The HF is assumed to be non-viscous, producing a uniform pressure over the interface where no leak-off is considered for the current along the crack into the domain. Stress intensity factor based criteria are employed for the fracture initiation and propagation direction detection. Finally, in order to show the accuracy and robustness of the proposed algorithms several numerical examples are presented. HF is a handful process employed extensively in different engineering applications. One of the most appealing uses is in oil industry where HF is applied in order to break hard layers of rocks to reach pressurized oil and gas reservoirs. In such cases, massive amount of fluids – especially water – is injected into reservoirs resulting in breakage of hard layers. This method is very common and significantly improves the total revenue of the field. The simulations were performed by Comsol multiphysics. 2. GOVERNING EQUATIONS Consider a body  as shown in Fig1. With boundary  that consists of external parts of t  and u  referred to loaded and restrained zones together with internal discontinuities of d  which could be both pressurized or traction free. The Lagrangian framework is employed in order to implement the formulation, thus the reference configuration is used. And the continuity equation results in 0 J    , where J is the determinant of deformation gradient. Assuming quasi-static condition, the equilibrium equation could be written as follows, (1) . 0     σ b Where b is the body force per unit mass vector which is generally gravity,  is the average density of the medium and σ is the Cauchy stress tensor. u