Development of a geological model for an Enhanced Geothermal System (EGS) reservoir requires a characterization of the fracture network. Previous studies have identified fractures from an acoustic image log run in the crystalline basement section of the Basel-1 well and described fracture sets and fracture zone characteristics. Using the Cantor's Dust method, we investigated the fractal behavior of four major natural fracture sets intersected by the Basel-1 well. These sets are differently distributed between 2600 m and 5000 m (i.e., the logged depth) along the well below the rotary table of the rig. The obtained fractal dimensions (D) vary between 0.35 and 0.60 for the four fracture sets. Similar D values ranging for sets 1–3 in the interval between 2600 m and 3000 m emphasize the role of the geological setting in the distribution of fractures. Moreover, the spacing distribution of sets 2 and 3 show a clear power-law distribution.
An Enhanced Geothermal System (EGS) seeks at extracting heat from deep underground by circulating a fluid between injection and production wells. In most EGS projects, fluid flow occurs essentially along fractures as the matrix permeability is small (Genter et al. 2010). In general, the low permeability of such reservoirs prevents production at economical rates. The enhancement of fracture permeability to facilitate fluid circulation is required. One approach to enhance and create the permeability is to perform hydraulic stimulations. In order to design and assess EGS reservoir stimulation strategies, development of a geological model with representation of lithological domains and characterization of the fracture network is required. This characterization includes a correct statistical distribution of fractures and fracture set properties. Such a geological model is crucial for geomechanical simulations during the life-time of an EGS reservoir, too.
The existence of fractures within the target EGS rock mass plays a controlling role on the mechanical behavior during reservoir stimulation. Valley and Evans (2007) have identified more or less continuous, local stress variations from analyzing wellbore failures along deep wells at the Soultz-sous-Forêts EGS in France, and attributed these stress changes to the existence of natural fractures cut by the deep boreholes. Similar observations were made at the Basel EGS project in Switzerland (Valley and Evans 2009).
