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