Evaluation of Chemical Equilibria of Calcite Deposition in a Geothermal Well at Northern Negros Geothermal Project, Philippines

Abstract

Calcite deposition in geothermal wells is commonly expressed as:

Ca++ + 2HCO3- = CaCO3(s) + H2CO3 (or dissolved CO2
)
For calcite supersaturated reservoir fluids, at constant reservoir and boiling temperatures, deposition was triggered by boiling and release of dissolved CO2. This paper evaluates this equilibrium based on gas and water samples collected in one of the calciting wells, PT2D, in the Northern Negros Geothermal Project. This type of study was possible because water and gas data were available as the well's massflow and wellhead pressure declined due to calcite deposition.

Through the series of time-plots of chemical species' activities related to calcite, and comparison of Gibb's Free energy from each postulated equilibria, calcite deposition was likely through dissociation of

CaHCO3+ : CaHCO3+ = CaCO3(s) + H+

In this reaction, deposition of calcite from supersaturated reservoir fluids was possible in the wellbore because there was an increase in reservoir temperature during discharge, as manifested by an increase in quartz geothermometer (Tqtz) with time. It is not clear whether calcite deposition was caused by mixing of the supersaturated fluids and the hotter up-flowing fluids or by heating of the reservoir fluids in the wellbore.