| Title | Prevention and Dissolution of Anhydrite Deposit Using Chemical Agents for Production Wells at the Mori Geothermal Field, Hokkaido, Japan |
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| Authors | Daisuke Fukuda, Masato Watanabe, Koutaro Maetou, Fumiaki Arai, Makoto Ohta, Kazuhiko Odagiri, and Yoshinobu Matsumoto |
| Year | 2011 |
| Conference | Western Pacific Regional Branch |
| Keywords | scale, anhydrite, calcite, smectite, inhibition, polyacrylate, geothermal, Mori, Japan |
| Abstract | Deposition of scale minerals, such as calcite, anhydrite, and smectite (Mg-silicate), has been found in production wells in the Mori geothermal field. The most serious decline in steam production was caused by calcite deposition occurring around a boiling depth in wellbores. The calcite deposition has been inhibited by injection of a sodium polyacrylate solution as a scale inhibitor several hundred meters below the boiling depth through downhole tubing. Deposition of anhydrite, which occurred at a greater depth than that of calcite, also reduced the production of deep wells having shallow and deep feed zones. We concluded from results of well loggings and mineralogical studies on scale samples that the anhydrite deposition was induced by a water entering from the shallow feed zone, and that the polyacrylate solution could prevent anhydrite deposition. The injection point of the inhibitor solution was accordingly lowered below the shallow feed zone, and this setting of the injection has been highly effective against both of the calcite and anhydrite depositions. In addition to the prevention method, chemical dissolution of anhydrite deposit was conducted in a production well. A chelating agent, a sodium salt of EDTA, was injected as a dissolving agent in the operation of chemical dissolution. This operation effectively dissolved the deposit and provided a recovery of the production with much less cost than a mechanical workover. |