A series of simple discrete fracture network (DFN) models explore the factors that control the performance of Enhanced Geothermal Systems (EGS) by studying the behavior of the long-term circulation experiments conducted at Rosemanowes in the UK during the late 1970’s through the 1980’s. The Rosemanowes site was chosen for the richness of its database and the existence of a thermal decline curve. The Rosemanowes program advanced microseismic technology and understanding of natural fracture stimulation but ultimately did not reach its goal of commercial power production due to low flow rates and early thermal decline. The injection and production wells tapped mainly the shallow portion of a stimulated rock volume that migrated to depths greater than 4 km. The models in this study explore whether or not larger well spacings and different rates could have produced slower thermal decline. Matching the existing thermal decline curve required enhancing the fracture intensity and adding cross-fractures to a parallel fracture model to create a system that drew its heat preferentially from rock blocks within the stimulated zone over heat flow from outside the zone. Once calibrated the model explored changing the well spacing in 200 m increments from 200 to 800 meters. These changes delayed the thermal breakthrough from less than a year to six years, but were short of the 20-30 year target. Decreasing flow rates from the 14 l/s of the experiment to 9 and 5 l/s further delayed the thermal breakthrough with 10% decline occurring at 20 years for the lowest rate case. While this case maintains the temperature of the production, the single stimulated volume falls short of rates that may be needed for production. Meeting the joint requirement of low thermal decline and total production rate may require multiple independent stimulated volumes as suggested by MacDonald and others (1992) in their review of the Rosemanowes program. While this study is not adequate to develop design guidelines, it shows that useful information can be gathered from past EGS experiments for designing future development of this important resource.

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