ABSTRACT: The properties of faults and fractures under dynamic reservoir conditions are critical in determining flow and performance of geothermal doublets in fractured reservoirs. The complexity of faults obtained from geological and seismic characterization is usually not honored in flow models for fractured reservoirs. Fault zone architectures may affect flow as well as the geomechanical response of the fractured reservoir to doublet operation. In this study, an analytical model is developed that allows incorporation of fault zone architecture and multiple fault sets in the description of fractured reservoir permeability. The model is used with data from seismic fault characterization to describe bulk permeability for a potential geothermal target in fractured Dinantian carbonates in the Netherlands. The evolution of pressure and temperature during doublet operation is simulated using a 2D numerical flow simulator. The geomechanical response of the fractured carbonates to doublet operation was analyzed using a Mohr- Coulomb failure analysis. Large variations in the spatio-temporal evolution of pressure and temperature are found for different permeability constraints and doublet designs. Increased fracture permeability, reactivation of fractures, shear failure of the reservoir rock, and initiation of tensile fractures can affect flow and need to be taken into account to optimize doublet performance.
The success of geothermal projects in naturally fractured reservoirs critically depends on the properties of faults and fractures (Hickman et al., 1997; Moeck, 2014). Although the relationship between fluid flow and fault properties has been studied extensively (Frank 1965; Caine et al., 1995; Faulkner et al., 2010), proper incorporation of realistic fault populations and the dynamic response of faults and fractures to flow in fractured reservoirs are largely unresolved issues. In particular, fractured reservoir flow models usually do not account for the interplay between (1) fault zone architecture and variation of permeability in different structural fault units such as damage zones (Caine et al., 1995), (2) site-specific characteristics of fault and fracture populations determined using seismic surveys, outcrop analogues, core material or laboratory experiments (Odling et al., 1999), and (3) changes in permeability due to opening or reactivation of fractures caused by pressure or temperature variations (Safari and Ghassemi, 2016; Fang et al., 2018).