Geochemical Modelling of the High-Temperature Mahanagdong Geothermal Field, Leyte, Philippines

Abstract

The high-temperature and liquid-dominated Mahanagdong geothermal field has supplied steam since 1997 to power plants with total installed capacity of 180 MWe. A geochemical assessment of the field is presented based on analytical data of fluids sampled at the wellheads of 26 wet-steam wells. The pH of the liquid samples ranges from 3 to 8 as measured on-site. Analyses of the water samples include major and minor elements. With the aid of speciation programs, the analytical data were used to model individual species activities in the initial aquifer fluids that feed the wells. The modelling indicates that excess discharge enthalpy of wells is mostly caused by phase segregation of the vapour and liquid phases in producing aquifers. The modelled aquifer fluid compositions were used to assess how closely equilibrium is approached between solution and various minerals. At inferred Mahanagdong aquifer temperatures (250-300°C), the concentrations of H2S and H2 in the initial aquifer fluids, assuming they are purely liquid, are somewhat higher than those at equilibrium with hydrothermal mineral assemblages, one of which incorporates grossular, pyrite, magnetite and wollastonite, and the other hematite, magnetite and pyrite. The equilibrium constant for both buffers is very similar. The observed distribution of the data points for the gases is attributed to the presence of equilibrium vapour in the aquifer fluid. The concentrations of H2,aq show more scatter. Aquifer fluid concentrations of CO2,aq are slightly above equilibrium curve for both of the assemblages considered (czo+cal+qtz+gro and czo+cal+qtz+gro+pre). However, variation in the composition of the solid-solution minerals may also contribute as well as departure from the model selected to calculate the gas concentration in the initial aquifer fluid. The aquifer liquid is in close saturation with various calcium-bearing, Fe-sulphide and Fe-oxide minerals but is significantly undersaturated with fluorite, grossular and wollastonite, all of which are rare at Mahanagdong. Departure from equilibrium for the Fischer-Tropsch and NH3-N2-H2 reactions is high. To move the system towards equilibrium, H2 concentrations need to increase, or in the case of the Fischer-Tropsch reaction, CH4 must decrease.Initial aquifer vapour fractions were derived assuming equilibrium between H2,aq and the gro+mag+qtz+epi+wol assemblage at the chosen mineral composition. Selecting the hem+mag mineral assemblage will give similar results. H2Saq concentrations, considering equilibrium vapour fraction in the initial aquifer fluid, are significantly above the equilibrium curves for the gro+pyr+mag+qtz+epi+wol or the hem+mag mineral assemblages. However for CO2,aq, they closely approach equilibrium with either of the two mineral assemblages considered. Derived aquifer vapour fractions are highest in the upflow region (~4%) and in the collapse area (1-3%). The aquifer fluids east of the upflow region, which are at ~300°C, have equilibrium vapour fractions of ~1%. This possible extension of the upflow area is suggested by systematics of the rare alkali analyses. Earlier data and those produced for this study both indicate maximum vapour loss in the western peripheral wells and vapour gain in wells when brine injection was dispersed farther from the production zone. The most mobile elements in Mahanagdong fluids are Cl, As, Na, S, Rb, K, Li and Br. Ti and Al have the lowest mobility, both in neutral pH and acidic fluids. The acidic discharges have high metal content. Zn and Mg are probably mobilized from the reservoir rocks whereas Fe, Mn, Pb, Cu, Ni and Co in acidic waters are likely to come, at least partly, from well casing material. The only chemical differences between acidic and neutral pH waters, apart from pH and the mentioned metals, are higher levels of SO4 and Mg in the former. The modelled aquifer pH (~4.5) of two acidic samples is lower by about 1 unit compared to the other aquifer fluids. The acidity at the surface of the Cl-SO4-type waters is mainly caused by the dissociation of HSO4- as the fluids cool by depressurization boiling when they flow from the aquifer to the wellhead. The molar ratios of H2S and SO4 in the aquifer fluids, if they still retain the signal of their magmatic sources, suggest that the disproportionation of SO2 at subcritical conditions and hydrolysis of native sulphur contribute to acidity of fluids discharged in some wells.