A number of challenges are associated with the engineered geothermal system (EGS) reservoir development, including the ability to reliably predict hydraulic fracturing, the deformation of natural fractures, and estimating permeability evolution of the fracture network with time. In order to research the behavior of EGS reservoir, we have developed a three-dimensional geomechanical model which couple rock deformation and fracturing with network fluid flow model. In discrete element method (DEM), solid rock is represented by a network of discrete elements (often referred as particles) connected by various types of mechanical bonds such as springs, elastic beams or bonds that have more complex properties(such as stress-dependent elastic constants). Fracturing is represented explicitly as broken bonds (microcracks), which form and coalesce into macroscopic fractures when external and internal load is applied. The natural fractures are represented by a series of connected line segments. Mechanical bonds that intersect with such line segments are removed from the DEM model. A network flow model using a conjugate lattice to the DEM network is also developed and coupled with the DEM. The fluid pressure gradient exerts forces on individual elements of the DEM network, which therefore deforms the mechanical bonds and breaks them if the deformation reaches a prescribed threshold value. Such deformation/fracturing in turn changes the permeability of the flow network, which again changes the evolution of fluid pressure, intimately coupling the two processes. The intimate coupling between fracturing/deformation of fracture networks and fluid flow makes the meso-scale DEM-network flow simulations necessary in order to accurately evaluate the permeability evolution, as this method has substantial advantages over conventional continuum mechanical models of elastic rock deformation. The challenges that must be overcome to simulate EGS reservoir stimulation, preliminary results, progress to date and near future research directions and opportunities will be discussed.

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