| Title | Identification of Fluid Circulations at the Borehole Scale from Temperature Logs: Insights from Deep Geothermal Wells in the Upper Rhine Graben |
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| Authors | Jeanne VIDAL, Régis HEHN, Carole GLAAS, Albert GENTER |
| Year | 2020 |
| Conference | World Geothermal Congress |
| Keywords | permeable fracture zones, temperature profiles, production logs, fluid circulation, geothermal wells, Rittershoffen, Soultz-sous-Forêts, Upper Rhine Graben (URG) |
| Abstract | In the Upper Rhine Graben (URG), geothermal projects since 40 years targeted hot geothermal anomalies with temperatures above 140°C at 2-km, observed at the regional scale. They are mainly concentrated on the western side of the URG, around N-S striking local faults that channelize hot geothermal resource corresponding to native brine. The so-called Soultz and Rittershoffen anomalies located in France were widely investigated from the regional to the more local scale. At the borehole scale, the vertical circulations of the brine in the fracture and fault zones surrounded by a damage zone are also evidenced in temperature (T) logs. T logs are affected by discrete T anomalies in the convective regime section that spatially match with several meters-thick fracture zones (FZ) observed on core or cutting samples, geophysical logs and image logs. At Soultz, these T anomalies were all negative at thermal equilibrium and interpreted as the remnant cooling of porous damage zones after mud invasions during drilling operations and water injection during stimulation operations. In this study, the dataset of T logs in production and at thermal equilibrium in the wells of Soultz and Rittershoffen evidences a new interpretation of these T anomalies. At equilibrium, a negative anomaly indicates that geothermal brine circulating through the FZ is colder than the surrounding rock formation. During production, this T anomaly can be positive that indicates inflow of water hotter than the mix of water circulating below. Thus, the polarity of the T anomaly can change over the time depending of the state of thermal equilibrium. Negative anomalies are more often observed because fractures are direct paths for cold mud or water during drilling and hydraulic operations and are thus cooled faster than the rest of the rock formation. Because temperature flux and geothermal fluids are intimately linked, the T logs are a useful, reliable and very sensitive tool to localize and characterize the inflow of geothermal water through FZs at the borehole scale. The recent T logs acquired in injection in the newly drilled well GIL-1 at Illkirch geothermal site (Strasbourg, France) allow to localize the permeable zones in the granitic reservoir. T logs of the URG evidenced that the most permeable reservoirs were intersected at the first hundred meters of the granitic basement; 700m at Soultz, 200-400m at Rittershoffen, more than 400m at Illkirch. |