| Abstract |
Reservoir deformation due to coupled thermal and hydraulic processes is important for various geological applications such as geothermal heat extraction, co2 sequestrations and nuclear waste disposal. The contraction of rock matrix and pore pressure increases during cold water injection into the reservoir, cause significant variation of reservoir porosity and permeability. In past, these evolutions were modeled considering either thermo-hydro-mechanical effects [kelkar 2014] or combined thermo-hydro-mechanical-chemical effects [taron and elsworth 2009]. When a large fracture/Fault is the main flow conduits, the modeling of fracture opening and closing (I.E. Aperture increase and decrease) Is very important to predict long-term evolution of geothermal reservoir. Some past modeling studies quantified the coupled thm effects on the single fracture [kohl et al. 1995; Rawal and ghassemI 2014; Guo et al., 2015] and fracture networks [koh et al. 2011; Fu et al, 2015]. Their result showed that cooling and overpressure resulted in reduction of effective normal stresses and increase in fracture aperture. All these studies indicated that couplings among the different physical processes occurred simultaneously, but at different time scales. Some processes such as thermo-elastic effects are present for the entire production period whereas poro-elastic effects are important in the early stage. We performed coupled thermo-hydro-mechanical (Thm) Simulations to investigate the evolution of aperture of a single fracture connecting an injection and a production wells. We used fehm code (Finite element for heat and mass transfer) For this purpose. The mass and energy balance equations in fehm is solved using control volume method and force balance equations with finite element method. It was originally developed for modeling of heat & mass transfer and deformation of porous medium. But to make fehm as capable of modeling of these processes for a facture, we considered fracture as an equivalent thin porous layer. The depth-integrated mass, momentum and energy transport in the porous layer were equated with those of the fracture [chaudhurI et al., 2013]. This approach was successfully used for thermo-hydro-chemical modeling of aperture alteration in geothermal setups [pandey et al. 2014; 2015]. For the mechanical deformation of fracture, we implemented nonlinear fracture joint model in fehm. For modeling of aperture alteration of fracture or rock joint, we considered bandis model of stress dependent fracture stiffness. Form that we derived a nonlinear stressstrain relation for the equivalent porous layer to determine the permeability alteration. |