| Title | Sulfide Scales Downhole in Well RN-22 Reykjanes High-temperature Area |
|---|---|
| Authors | Vigdis HARDARDOTTIR, Nicole HURTIG |
| Year | 2020 |
| Conference | World Geothermal Congress |
| Keywords | sulfides,scale, downhole,Reykjanes,high-temperature,pipelines,Iceland,geothermal |
| Abstract | Reykjanes is a high-temperature (270-310°C) seawater dominated geothermal area located on the southern tip of the Reykjanes peninsula SW-Iceland where the Atlantic Ridge comes on shore. Between 2006 and 2013, output maximum capacity dropped from 14 to 2 MWe in well RN22 as a result of scale formation and a decrease in pressure. Several attempts were made to clean the well from scales. Early attempts only reached down to a depth of ~670 m and showed relatively minor scaling. In 2013, scales were collected from the liner at 679–1647 m depth. Significant scales formed at a depth of ~1100 m depth where boiling is intense. Material collected from the bottom of the pipe is mainly composed of rounded fragments of sulfides, clusters of sulfides, mainly sphalerite, and rounded rock samples indicating that the material had fallen from the liner above. The material collected from the bottom of the pipe (liner, No. 79) amounts to ~0.48 m3, i.e. ~2000 kg. Calculated from the volume of the scales in the pipelines the thickness is ~1 mm if spread evenly over the 1100 m casing string. However, scales precipitated most where the boiling is intense (~1100 m depth), and in areas where pressure decreases. Another possibility is also intense precipitation of sulfides in the host-rock where the main feed points at ~1100 m are in the well. The GEM-selektor numerical simulation package was used to estimate the amount of minerals that can precipitate from the cooling ascending fluid and complement observations from well RN-22. Numerical simulations of cooling and decompression followed the PT fluid pathway observed at well RN-22 (320°C and 120 bar to 220°C and 23 bar) using an average downhole fluid composition. The equilibrated downhole fluid precipitates bornite, chalcopyrite and sphalerite upon cooling and decompression. Below 250°C and 38 bar, sulfide precipitation increases by one order of magnitude, which corresponds to observations from well RN-22. A total amount of to 0.86 g sulfides precipitate per 100 kg fluid. Mass balance calculations were used to scale the numerical simulations in terms of potential kilograms which can precipitate during geothermal energy production. In 2006 and 2007, production was at 1.9 and 3.1 M tones of water. Based on these production numbers and numerical simulations using the average downhole fluid we calculated that potential sulfide precipitation (16000-26000 kg) might be an order of magnitude higher than material collected in pipe No. 79. |