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Pre- and post-stimulation characterization of geothermal well GRT-1, Rittershoffen, France : insights from acoustic image logs of hard fractured rock

Pre- and post-stimulation characterization of geothermal well GRT-1, Rittershoffen, France : insights from acoustic image logs of hard fractured rock

Auteurs : Vidal, J., Genter, A. et Schmittbuhl, J.
Journal : Geophysical Journal International, Marine geosciences and applied geophysics
doi : 10.1093/gji/ggw181

Geothermal well GRT-1 (Rittershoffen, Alsace) was drilled in 2012. Its open-hole section (extending down to a depth of 2.6 km) penetrated fractured sandstones and granite. In 2013, the well was subjected to Thermal, Chemical and Hydraulic (TCH) stimulation, which improved the injectivity index five-fold. The goal of the study was to assess the impact of the stimulation by comparing pre- and post-stimulation logs and well-logging (temperature [T] log) and mud-logging data. This comparison revealed modifications of almost all the natural fractures. However, not all of these fractures are associated with permeability enhancement, and the post-stimulation T logs are important for characterizing this enhancement. Chemical alteration due to mechanical erosion at the tops and bottoms of the fractures was observed in the sandstones. These zones display indications of very small new permeability after the TCH stimulation. Because a major fault zone caved extensively where it crosses the borehole, it was not imaged in the acoustic logs. However, this originally permeable zone was enhanced as demonstrated by the T logs. Based on the natural injectivity of this fault zone, hydraulic erosion and thermal microcracking of its internal quartz veins are associated with this permeability enhancement. Although local changes in the borehole wall observed in the acoustic images cannot be directly linked to the improved injectivity index, the comparison of the acoustic image logs allows for identification of fracture zones impacted by the TCH stimulation.

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11 mai 2016