Mitigation of Corrosion in an Acid Producing Well at Tiwi Geothermal Field

Abstract

Acid-sulfate fluids are present in most exploited and explored geothermal areas in the Philippines. Existing acid wells are mostly shutin due to their high corrosion rate despite proximity to operating power plants. At least 80 MW of proven generation is currently unavailable in producing fields due to acidsulfate fluids, and the potential generation &om acid-sulfate fluids nationwide could easily exceed 300 MW. These estimates highlight the current potential value of acid-mitigation technology to the Philippines in terms of indigenous energy production, but development of cost-effective corrosion mitigation technology is needed to realize this value.

The Bariis-8 production well at the Tiwi geothermal field produces acid-sulfate brine exhibiting a pH as low as 3. Wellhead piping experiences significant corrosion if produced as is. Short-tern tests demonstrated that corrosion maybe mitigated by neutralizing the acidic brine with sodium hydroxide, or by addition of filming amine-type corrosion inhibitors. Commercial production of the well has been achieved by injecting dilute NaOH solution into the well at a depth of nearly f,OOQ m employing high alloy capillary tubing and specially-designed injection nozzle system. Although wellhead corrosion can be mitigated by injecting NaOH solution just below the master valve, deep injection into twophase fluid has been utilized to prevent casing corrosion. The pH of the brine at the wellhead was increased from -3 to -5. Corrosion of the wellhead and surface piping decreased by more than 50%. Corrosion mitigation of Bariis-8 contributes 7 MWe of additional power to the Tiwi field output. The success of the corrosion mitigation effort is attributed to an interdisciplinary team approach to design, testing, and implementation of the system. Resulting design features include alloy capillary tubing, lubricator, tubing injection stinger, packoff assembly, injection nozzle configuration, caustic solution strength, duplex pumping system, distributive control system, and on-line monitoring equipment have all contributed to system function.