| Abstract |
The ECOGI Company develops a project of deep geothermal energy located in the Rittershoffen site, in Alsace, in the Upper Rhine Graben area, 50 km north of Strasbourg and 10 km east from the Soultz EGS site, in order to produce steam for the industrial drying of starch from a well doublet (injection and production wells). The first geothermal well, GRT-1, with a vertical depth of about 2,550 m (drilled length of about 2,580 m) was drilled in 2012, and tested and successfully stimulated in 2013. The second well, GRT-2, with a vertical depth of about 2,750 m (drilled length of about 3,196 m) and distant from GRT-1 from approximately 1.2 km at the bottom-hole, was recently completed in July 2014. The open-hole section is made of Triassic clastic sandstones and fractured crystalline basement. On the request of the ESG Company and under ECOGI funding, a chemical tracer test using fluorescein and 2,7-naphthalene disulfonate (2,7-nds), associated with a short-term circulation loop between the wells GRT-1 and GRT-2, was conducted over a period of 25 days in 2014. During this test, about 60,000 m3 of fluid were discharged from GRT-2 with a mean temperature close to 162°C and 54,000 m3 of fluid were injected into GRT-1 with a mean temperature close to 40°C. Only fluorescein was analyzed both on site and in laboratory. As expected, the 2,7-nds results obtained in laboratory were the most reliable and relevant ones for data quantitative interpretation of this tracer test. The tracer detection in the GRT-2 fluid (corresponding to a volume of about 27,000 m3 of fluid discharged) occurred at about 12 days after tracer injection. The maximum signal of 2,7-nds was not reached during the production time of 25 days, after about 60,000 m3 of fluid discharged from GRT-2. The concentration values of both tracers analyzed in the fluid discharged from GRT-2 remained relatively low (< 15 μg/l for fluorescein and < 40 μg/l for 2,7-nds). Consequently, after 25 days of production, the recovery rates for fluorescein and 2,7-nds were very low, close to 0.06 and 0.2% in mass, respectively. If we compare these tracing results with those obtained in the Soultz EGS site, we can conclude that the hydraulic connections between GRT-1 and GRT-2 are much poorer than all those observed between the Soultz wells. |