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Background: The thermal regime of the Upper Rhine Graben (URG) is characterized by a series of anomalies near Soultz-sous-Forêts (France), Rittershoffen (France), and Landau (Germany). These temperature anomalies are associated with groundwater circulation in fractures and faults distributed in the Cenozoic and Mesozoic sedimentary cover associated with and connected to fractures originating deep within the Paleozoic basement. The present study helps to understand the convective cell structure in order to optimize geothermal borehole trajectories. Methods: The work concentrated on a detailed interpretation of the geophysical and geological logs from Soultz geothermal wells mainly from the topographic surface to the Triassic formations, at between 800-and 1,400-m depth above the deep granitic basement.

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Fluid circulation in zones of fractures are a key challenge to exploit deep geothermal heat from natural reservoir. At Rittershoffen (Upper Rhine Graben, France), two geothermal boreholes, GRT‐1 and GRT‐2, were drilled in 2012 and 2014, respectively. They targeted the local Rittershoffen normal fault, which strikes N‐S and dips westward. In this study, major natural fractures were observed in the open holes of both wells from acoustic image logs correlated with other standard geophysical logs (gamma ray, neutron porosity, and caliper). Their permeability was evaluated at the borehole scale from temperature logs, mud losses, and gas surveys. One originally permeable (OP) fracture zone was observed in the granite of GRT‐1. In GRT‐2, four OP fracture zones were observed in the granite and two in sandstones. In GRT‐2, fracture zones are composed by several fluid pathways that could explain the higher natural permeability than in GRT‐1. All OP fractures are associated with positive temperature anomaly, interpreted as circulation of hot geothermal water through the permeable fracture, or negative one, interpreted as the cooling of a porous, altered and fractured zone around the permeable fracture after drilling operations. Permeability of natural fracture oriented N170° seems to be intimately linked to the secondary mineral deposits resulting from paleocirculations. The geometrical fracture model along the wellbore suggests that the inclined trajectory of GRT‐2 increases the connection between the borehole and the nearly vertical fracture network associated to the local fault. A good characterization of zones of fractures in a targeted natural reservoir allows an optimal exploitation of geothermal resource.

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Two deep wells were drilled at Rittershoffen (Alsace, France) to produce high-temperature fluids to supply heat to a biorefinery. The GRT-2 production well was drilled to a depth of 3196 m MD and was deviated to target a permeable local fault in the granitic basement buried beneath a thick sedimentary cover. The objective of this study is to better understand the permeability of fractured reservoirs within crystalline rocks, focusing on the production well GRT-2. Based on a petrographic and mineralogical analysis of cutting samples, several granitic facies associated with hydrothermal alteration were identified on the basis of the amounts of illite, chlorite, anhydrite, secondary geodic quartz, and oxides. These observations were correlated with various geological and geophysical datasets (gamma ray, porosity, density, electrical resistivity, caliper, borehole image logs, temperature, rate of penetration, and mud losses) to localize and identify permeable fracture zones. In sections where acoustic image logs were not available, such as in the deepest part of the well, the geometries of the fracture zones were interpreted from an oriented caliper log. The caliper log interpretation detected one-third of the fractures detected by acoustic image logs. However, two major fracture sets striking N-S and dipping eastward or westward were observed. Furthermore, a synthetic resistivity log that fits the measured resistivity log relatively well was built using the Archie and Waxman and Smits models. This approach is a proxy for estimating the porosity and the mineralogical changes based on the cation exchange capacity, which is controlled by the chlorite/illite ratio, derived from electrical logs in granitic formations. The correlation of all these results allowed the identification of a resistivity signature of a permeable fracture zone that spatially fits with the temperature signature. The major contribution of this study is the identification of a hierarchy of permeable fractures based on petrophysical signatures. The geophysical signature of fracture zones with low residual permeability exhibits a broad depth extent, whereas the geophysical signature of a highly permeable fracture zone is more localized. Past hydrothermal circulation has enlarged the altered and porous zones around open Open Access© The Author(s) 2018. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creat iveco mmons .org/licen ses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.