| Title | LONG TERM USE OF PRODUCTION WELLS WITH ACIDIC DISCHARGE AT MINDANAO-2 POWER STATION, MINDANAO GEOTHERMAL PRODUCTION FIELD (MGPF), PHILIPPINES |
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| Authors | James B. Nogara, Roel S. Alincastre and Rosella G. Dulce |
| Year | 2002 |
| Conference | Stanford Geothermal Workshop |
| Keywords | Mindanao-2, Philippines |
| Abstract | Mindanao-2 power plant is the second geothermal power station in Mindanao Geothermal Production Field operated by PNOC-EDC. Mindanao-2 was commissioned in June 1999 with a base load capacity of 52 MWe. The plant is a dual (high pressure-low pressure) flash type. High-pressure steam is provided by nine production wells piped to two central separator stations, while the low-pressure steam is supplied from second flash of separated water from the adjacent Mindanao-1 power plant. Two production wells used by Mindanao-2 discharge acid NaCl + SO4= fluids with pH ranging from 3.2 to 5.5 at 25?C. Steam production from these two wells contributes about 35% of total high-pressure steam production. Separated water from the high-pressure steam-liquid separators used by Mindanao-2 is also acidic with pH ranging from 3.8 to 5.8. After two years of utilization of the acidic wells there were no significant thinning observed in the surface piping and wellhead facilities of Mindanao-2 based on UT (ultrasonic test) measurements and visual inspections. Scales consisting of magnetite and amorphous silica are deposited in the inner surface of the casing from the fluid. These scales are also found in the re-injection piping system and produced by the acidic separated water. Although the silica content of the separated water exceeds saturation level by more than 45% voluminous amorphous silica scales were not observed in the re-injection pipelines. The acidity of brine significantly delayed the onset of silica polymerization effectively preventing scale formation in the injection well and pipelines. However, significant erosion/corrosion was experienced in the piping system when the separated water was discharged to the atmosphere. The experience gained from the use of acid wells in Mindanao-2 is encouraging and may eventually result in the utilization of other acid wells currently not used in the field. |