| Abstract |
The Mmdanao 1 geothermal field is located on the northwestern flanks of Mt. Apo, Cotabato province in the island of Mindanao, Philippines. Four of the wells drilled in the Sandawa Collapse will be producing for the second stage development of a 52 W e power plant. The scope of this study is to assess water and gas compositions from the Sandawa production wells and to interpret the results in relation to the reservoir conditions and fluid mineral equilibria. The evaluation of water compositions from the Sandawa production wells show that there are two types of fluids discharged, the near neutral to neutral pH chloride water and the mixed chloride sulfate water. The C1-waters comprise the fluids residing at reservoir depths which have a C1 content of -6600 mgkg. The eqwlibration temperalure based on the Na-K-Mg geothermometer at the reservoir is 320░C which is in accordance with the measured downhole temperatures. Evaluation of gas compositions indicate that the fluids have undergone continuous degassing from deeper horizons to the present reservoir depths tapped by the wells. The degassing is presently occurring as shown by the two-phase conditions exhibited by the well discharges. The degassed characteristics of the fluids are reflected in gas equilibration, resulting in vapor losses in TM-1, TM-2 and KN-3 to as much as 5 percent. The gases are assessed to be unequilibrated based on the C02/Ar and HdAr ratios. Gas geothermometers predicted a variable range of temperatures from 21O-32l0C, however, the temperatures estimated by carbon dioxide-argon geothermometer closely agree with measured downhole temperatures. Aqueous speciation calculations using the computer program WATCH shows that the fluids kom the wells (TM-1, TM-2, KN-3) are saturated with respect to anhydrite and undersaturated with calcite at reservoir conditions. With respect to the cationic components, the fluids are found to be primarily in equilibrium with K-mica at the reservoir, however, the fluids approaching TM-2 conditions are also in equilibrium with albite, wairakite, K-feldspar and calcite. Owing to the low C q content of the fluids, other calc-silicates may form readily. Hydrothermal alteration mineralogy from these wells indicate wairakite, adularia and calcite are found to exist in the rocks at the reservoir. Moreover, sulfide reactions do not influence hydrogen-hydrogen sulfide equilibria but arise as a result of the redox state of the fluids. This results in the stability of pyrite plus other sulfides in the system. |