Simulation of fluid flow through fractured rock, a smeared approach

Abstract

Geothermal energy reservoirs can be used as sources of renewable and sustainable energy. The concept of geothermal technology relies on the harvesting of heat from the deep rock by injecting fluid that circulates through the porous, fractured reservoir rock between the injection and production boreholes, and thereby transports the heat to the ground surface. The presence of fractures can strongly increase the permeability of the rock and thus enhance the flow circulation in the system. Properly being able to model the fluid flow through the porous, fractured rock is therefore of importance in the optimisation of reservoir design. For that reason, this study proposes a new material model, which relates fracture initiation and propagation to a change in the permeability of the rock, assuming a smeared approach. The fracture-dependent permeability model is developed using the Diana FEA software, and can be used in combination with other fracture models and the mixture analysis for the simulation of hydro-mechanical behaviour of rock. The study investigates two different methods for the formulation of a fracture-dependent permeability, whereby both methods adhere to the cubic law, which defines the permeability as a quadratic function of its aperture. Evaluation of the model shows that it is capable of simulating extensional (mode I) fractures in the vicinity of geothermal boreholes. However, the analyses also indicate a mesh-dependency as well as a sensitivity to its input parameters. Furthermore, the fracture-dependent permeability model can only simulate hydro-mechanical behaviour in situations other than (enhanced) geothermal energy systems to a limited extent. Further model development is therefore recommended.