Review of the Hydraulic Development in the Multi-Reservoir / Multi-Well EGS Project of Soultz-sous-Forêts

Abstract

Over 20 years of research at the Soultz EGS project have led to the development of three vertically positioned reservoirs. They were explored in approximately 2, 3.6, 5 km depth by the 5 geothermal wells EPS1, GPK1, GPK2, GPK3, and GPK5. Natural injectivity of the three reservoir levels decreases with increasing depth from II = 9∙10-10 m3 Pa-1 s-1 in the upper reservoir at GPK1 to II = 1-2∙10-10 m3 Pa-1 s-1 in the deep reservoirs at GPK2 and GPK4. In the framework of investigating possible stimulation success and temperature development under production condition, we have reviewed the complete hydraulic datasets of from 1988 to 2013. Stimulation experiments involve exclusively hydro-mechanic stimulation in the upper (GPK1) and intermediate (GPK1 and GPK2) reservoirs. Acidification using HCl and further chemical treatment has been applied to the three wells GPK2 to GPK4 reaching the deep reservoir. Our review shows that the most complete hydraulic history exists for GPK2. In the intermediate reservoir, extraordinary hydraulic similarity between GPK1 and GPK2 is observed. Starting from a similar injectivity, a ratio between PI to II of 2:1 is obtained in first high volume stimulation. When productivity in GPK2 reached its maximum, a related improvement of GPK1 indicates good connection. The same ratio is reproduced when inverting the wells. In the deep reservoir we conclude that most likely since 2003 or before, Soultz is operated in a multi-reservoir mode as revealed by hydraulic data in GPK2 due to the flow contributions from a fault zone in 3860m. The rather low production temperature since 2003 gives a further hint to this scenario. It can be considered as indicator of the complex flow field excluding a simple linear pressure or flow influence to the production temperature.