Scaling and Corrosion Control in a High Chloride Geothermal Brine

Abstract

The Lihir geothermal wells typically produce two phase fluid with neutral to slightly alkaline brine. The brines are high in dissolved salts. Silica saturation levels can lead to scaling in separation plant, in reinjection wells and in the injection formation. Control of silica scaling in the high chloride waters by acidification to delay polymerisation was considered to present an unacceptably high risk of corrosion of carbon steels used for process piping and steam separation plant as well as reinjection wells. Increasing the pH was seen as a viable alternative solution to increase the silica solubility at selected reinjection temperatures and avoid the risks identified for acid corrosion. A pilot plant designed to give controlled reduction of brine temperature in a small bore heat exchanger was used to test carbon steel corrosion rates under simulated heat exchange and reinjection conditions. The results obtained showed silica could be kept in solution under the selected conditions. However, onset of calcite scaling was observed in the test plant as a result of the well mix used for testing and the test arrangement that appeared to promote flashing. A low and acceptable corrosion rate of carbon steel was measured for the alkaline pH conditions tested, this being potentially due to the favorable pH, or the formation of thin protective silica scales or calcite scaling. Standby corrosion resulting from residual chlorides present after shutdown led to onset of localized corrosion of the order 0.1 mm on the corrosion monitors used.