| Abstract: |
The Soultz geothermal site located in the Rhine graben (eastern France), currently operates one active production well (GPK-2) and several reinjection wells drilled down to 5 km depth into the fractured granite basement. The geothermal fluid with a high salinity (100 g/l) is produced at 160°C. The brine is pumped by a Line Shaft Pump (LSP), whose technology was adapted to slim-hole diameter conditions. Since 2008, several LSP technologies have been tested and huge improvements were made in order to face many technical challenges. The first LSP was supplied by an Icelandic company and was installed in Soultz in May 2008. It was based on standard column assembly, with water-lubricated Teflon line shaft bearings. The pumps themselves were standard cast iron units of 8” outside diameter, running at 2900 rpm, with an extra allowance to compensate for shaft axial load and thermal extension. Since its start-up in June 2008, the LSP assembly has been removed and reinstalled six times, due to a series of different operational and technical failures. The first failure, in August 2008, was caused by carbonate scaling inside the shaft’s enclosing tube, as local domestic water was used to lubricate. Carbonate rapidly scaled and tied up the bearing lubrication flow, leading to a break-down of the line shaft. An attempt was done to avoid carbonate scaling by using a reverse osmosis module in order to demineralise the domestic water before the injection into the lubrication system. Subsequent failures were all related to damage due to abrasion and corrosion. Each time the pump assembly was removed from the production well, considerable wear and damage have been observed on both rotating and stationary parts. Both impellers and stator bowls were highly corroded. Unfortunately, the supplier of the first generation of pumps could not offer a technical solution adapted to the Soultz well conditions. Thus, thanks to the support of the German government (BGR/BMU), a new 8” outside diameter pump was developed, based on the technology of a German supplier and derived from one of their existing submersible pumps. The new pump was built in a different material and with a larger axial shaft extension allowance in order to better manage the operational conditions. It was built up in only 3 months and tested at full speed on December 2012 in an 18 m deep test well. The new LSP was installed in January 2013 and operated continuously for nearly six months. In July 2013, the pump was then stopped and the whole LSP column was immediately removed from the well. The water-lubricated line shaft assembly and the pump components underwent detailed inspections, which led to a better understanding of the pump, particularly with respect to the pump metallurgy and with the lifetime of the Teflon bearings. These results have provided useful data for further development of a slim-hole line shaft pump adapted to the Soultz well conditions: slim well diameter and deviation to linearity, high temperature and the typical chemistry of the Rhine graben brine. |