Title: 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+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 area
has characteristics of an upflow as 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.