| Title | Brine Silica Management at Mighty River Power, New Zealand |
|---|---|
| Authors | Simon ADDISON, Kevin BROWN, Paul HIRTZ, Darryl GALLUP, Jeff WINICK, Farrell SIEGA, Toby GRESHAM |
| Year | 2015 |
| Conference | World Geothermal Congress |
| Keywords | silica, acid, pH-modification, Kawerau, Rotokawa, Ngatamariki, Mokai, Nga Awa Purua |
| Abstract | Mighty River Power (MRP) along with its joint-venture partners completed the Ngatamariki Power Station Project in 2013, following on the Nga Awa Purua Station Project in 2010 and the Kawerau Station Project in 2008. These plants all feature silica saturation indices (SSI’s) of more than 1.7 using plant design to reduce the risk of silica polymerization and deposition. This allows for additional energy extraction from the same amount of two-phase geothermal fluid. Through the design phase MRP has looked to form an optimal match between the plant and the geothermal reservoir that supplies it, with silica management forming a significant part. Each project made use of a small test-plant to assess various options to reduce the risks of silica deposition. Two of the recently completed projects are flash plants and make use of pH-modification via sulfuric acid addition to the geothermal brine to inhibit polymerization of silica. The third project uses binary technology and steam-condensate recombined with brine to reduce the pH and to dilute injection fluid, inhibiting the polymerization of silica. Despite the use of test-plant data and designing the plant for controlling silica polymerization, injectivity declines at one plant led to a requirement for well acid work-overs and a separate project to better understand the root causes of injectivity decline and risks of this at the other plants. Significant investigative work was conducted and a number of operational, plant and process changes were made. One of the key challenges with utilization of pH-modification plants is the need to balance corrosion risk alongside the risk of silica deposition, with appropriate materials being required in high corrosion areas. This paper discusses various silica management processes, designs and tools employed by MRP across its geothermal plants and some issues that have been identified and rectified. |