Assessment of Boiling Processes and Initial Steam Fractions of the Mindanao Geothermal Production Field, Philippines

Abstract

The different sources of steam production were quantified using the H2S-H2 gas pair from geochemical data obtained from the geothermal wells in Mindanao Geothermal Production Field, Philippines. The calculation is based on the method developed by Arnorsson in 1990 in evaluating the boiling processes and initial steam fractions of geothermal reservoirs. The calculations showed that more than 50 % of the steam produced by the geothermal field is attributed to enhanced vaporization due to heat flow from the rock to the boiling water. The production wells affected by the re-injection returns coming from Matingao and Kullay injection sinks have minimal steam produced through enhanced vaporization and the greater part of the steam production mainly resulted to plain adiabatic boiling. The process indicates significant cooling at the reservoir rock in the Marbel production sector brought by the persistent inflow of cooler injected fluids. The wells drilled in Sandawa sector, the hottest region and postulated upflow zone of the geothermal field, reveal higher % steam produced by enhanced vaporization varying from 40 to as high as 80. The vapor dominated and high enthalpy wells of the field are by products of enhanced vaporization and phase segregation of steam from water. The phase segregation is very evident to vapor dominated wells wherein the steam concentrates in the discharge since the water component is left behind in the reservoir due to faster mobility of vapor than the liquid phase. The steam inventory calculation validates that the geothermal system is composed of two reservoir types: the liquid saturated in Marbel production sector and the highly two-phase in the Sandawa production sector.