Developing the Acid Reservoir at Tiwi Geothermal Field, Philippines

The southwestern sector of the Tiwi Field - Bariis and South Kapipihan (SoKap) - has been developed since the late 1980s and many of the wells drilled in these areas proved to be acidic and could not commercially produce. However, successes in the redrilling of B8 in 2005 and drilling of the big-hole producer B11 in 2008 suggest that the strategy employed to avoid the acid-sulfate fluids worked so that commercial drilling can resume in this sector. These two wells were completed with deep cemented liners set at ~900 meters below sea level (mbsl) and are currently two of the biggest producers in the field. The deep cemented liner strategy proved to be an effective solution in isolating the shallow acid-sulfate fluids in Tiwi. Acid production has been encountered in 28 of the 156 wells drilled in the Tiwi Field, mostly in Bariis and SoKap. Eight of the 13 wells drilled in Bariis produced acidic fluids. These wells were completed with shallow production casing at ~500 mbsl. The shallow acid model describes the acid fluid as a product of the oxidation of H2S exsolved from either the reservoir or ascending steam as it encounters oxygenated groundwater and condenses at the top of the geothermal reservoir. The zone defined from 840 mbsl to 900 mbsl in Bariis and 700 mbsl to 910 mbsl in SoKap is considered to be the primary (or known) acid-sulfate fluid entries in wells B8, K30, K31 and K32. These zones correlate with advanced argillic (acid) alteration zones logged in nearby wells B1, B2 and K28 (including K30), although several studies suggest that the advanced argillic zones are relict. Several inferred acid-sulfate fluid entries were also used in combination with the known acid-sulfate fluid entries and acid alteration zones to define a broader interval of acid-sulfate fluid zone for Bariis (670 mbsl to 1040 mbsl) and for SoKap (700 mbsl to 1,080 mbsl). Furthermore, a recent sulfur isotope study also reveals that the Tiwi acid-sulfate fluid has a non-magmatic origin but is influenced by shallow processes. This then leads to the proposal to set the cemented liner at ~1,100 mbsl to avoid the acid fluids and this deep cemented liner strategy will be applied in future make-up well drilling campaigns in southwest Tiwi.