**2. Requirements for a Successful Geothermal Well (EGS)**

Simple calculations suggest that proximal and interconnected fracture systems, natural or otherwise, are required for economic viability in all but the hottest geothermal scenarios. Currently, geothermally derived power is associated with "natural" hydrothermal sys‐ tems. These are reasonably permeable and have equilibrated fluid circulation systems, with heat delivered by deep convection. They are characteristically naturally fractured and/or faulted, at least to some extent. Stimulation of fractured wells to enhance fracture conductivity is an opportunity for engineering massively stimulated systems – engi‐ neered geothermal prospects – using hydraulic fracturing. Pritchett, 2012 [1], cautions that the practicality of these scenarios depends on fractures that are conductive enough to support the required high geothermal flow rates. "Performance will be significantly impaired if the average fracture separation is greater than 50 meters or so. … The crea‐ tion of such extensive and pervasive artificial fracture networks at costs that will prove acceptable … is the fundamental challenge for EGS. … New stimulation paradigms may be required." These fractures need to be close enough together and appropriately orient‐ ed to encourage heat sweep and thermal energy recovery.

Following Pritchett's logic, for a very specialized generic reservoir analysis, assume 100 kg/s are required for an economic system. To adequately delay thermal breakthrough (on the order of 30 years), required fracture spacing varies from 20 to 70 m.The time to breakthrough decreases as the fracture spacing becomes larger. Wu et al., 2012 [59] demonstrate fracture spacing issues numerically.

What non-specialists don't always realize is the throughput that is required to ensure an economic geothermal prospect. If there is a single producer, the criterion for economic throughput is colloquially expressed as 100 kg/s by some; 2000 gal/minute by others. In any case, at 200°C this is between 62,000 and 69,000 BWPD at the sand face. To accommodate such high rates with nominal friction, large diameter casing is conventionally used with large diameter barefoot sections. This philosophy may be acceptable in conventional systems. If EGS is planned, large contact between one or more fractures and the wellbore is essential. Without effective diversion, multiple fracturing in an open hole is extremely difficult. Extended fracture contact with the well and/or multiple fracture intersections seem to be essential for EGS. This brings up the contentious topic of whether cased and perforated completions would be acceptable in geothermal environments.
