| Abstract |
Increasing permeability of the reservoir and engaging a larger portion of hot dry rock volume is critical for sustainable heat production in engineered geothermal projects. The low permeability of the igneous formations suitable for geothermal projects often necessitates application of different stimulation techniques to enhance circulation of water. Shear stimulation or hydro-shearing is the method of injecting a fluid into the reservoir with the aim of increasing the fluid pressure in the naturally fractured rock and inducing shear failure or slip events. This mechanism can enhance the system’s permeability through permanent dilatational opening of the sheared fractures. This paper is focused on evaluating the potential of shear stimulation in enhancing productivity of low permeable rocks using a computational modelling approach. It is believed that the characteristics of the Discrete Fracture Network (DFN) are key to the rate and sustainability of the hear production, and thus, the network of fractures need to be characterized in field and explicitly represented in the numerical model. This paper aims to evaluate the interaction between fluid injection, DFN, and the potential growth of a hydraulic fracture. Numerical analyses were completed using distinct element codes UDEC (Itasca, 2011) in a two-dimensional framework. UDEC represents rock masses as an assembly of interacting blocks separated by fractures. It allows for simulation of fracture propagation along the predefined planes only (i.e., the trajectory of the hydraulic fracture is not part of the solution of the problem). Thus, the hydraulic fracture is assumed to be planar, aligned with the direction of the major principal stress. The two “incipient surfaces” of the plane of the hydraulic fracture initially are bonded with a strength that corresponds to specified fracture toughness. Propagation of the hydraulic fracture corresponded to breaking of these bonds. The pre-existing fractures were represented explicitly. They are discontinuities which deform elastically, but also can open and slip (Coulomb slip law) as a function of pressure and total stress changes. The model was fully coupled hydromechanically. The paper discusses sensitivity of the interaction between the hydraulic fracture and pre-existing fractures and the rock mass stimulation to a number of parameters including the characteristics of the fracture network, in-situ conditions (e.g., orientation and magnitude of the principal stresses, mechanical properties of intact rock and pre-existing fractures) and operational parameters (e.g., injection rate and viscosity of injected fluid). |