Application of SOLVEQ in Evaluating the Dosing-rate of NaOH and H2SO4 Treatment of Geothermal fluids

Abstract

A method was developed using SOLVEQ geochemical program in evaluating the amount of base (NaOH) and acid (H2SO4) added to thermal fluids. In this method, pH is modified, as the charge balance is re-adjusted to compensate for the given pH. The adjustment of the charge balance, using a pre-selected ion, makes this method useful in computing the required dosage of base (such as NaOH) or acid (such as HCl or H2SO4) to thermal fluids to attain a desired pH.

For addition of base, Na ion (assuming addition as NaOH = Na+ + OH-) is added as the charge balancer while for acid addition, Cl- is added as the balancer (assuming addition as HCl = H+ + Cl-). For acid like H2SO4, one can also use Cl ion as the balancer, provided its adjusted mole is divided by two by the concept neutralization-equivalent-weight or simply the equivalent of H2SO4 to HCl: there are two equivalents per mole of H2SO4 per mole of HCl.

Based from the results and comparison with actual field trials, differences highlighted the intricacy of the chemical equilibria involved in the procedure. Although measurements of pH's at laboratory condition remain the only method of choice in evaluating the required dosages, the method shows the following sensitivity of the treatment: for NaOH, fluxes in condensate flow and the role of Eh are essential parameters to monitor effectiveness of the treatment, while for H2SO4, the line-temperature dictates the strength of acidity, hence the amount of dosage.