Extending PHREEQCs capabilities to high temperature, salinity and pressure environments
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Research of subsurface environments targeted for unconventional gas resources, CO2 sequestration and geothermal energy generation, where high temperatures, pressures and salinities usually prevail, are driving the need for extended capabilities of geochemical modelling codes. This presentation will describe the recent changes that were made to PHREEQC to extend its applicability to higher temperatures and pressures. The databases distributed with PHREEQC may give widely different results for concentrated solutions, and only the database that uses Pitzer’s interaction coefficients provides both correct solubilities and mean activity coefficients at high salinities. The applicability of this database for predicting mineral precipitation was extended by fitting interaction coefficients to solubility data at high temperatures. At depths of 1km or more, the pressure dependence of equilibrium constants becomes significant and must be taken into account in geochemical calculations. It is shown how the pressure-dependence can be calculated from apparent molar volumes as a function of temperature, pressure, and ionic strength, and how the complicated changes of the partial molar volumes of the water molecules was accounted for. Finally, PHREEQC’s extended capabilities now also allow for fugacity coefficients for CO2 to be obtained reliably. This talk will describe the abovementioned changes and provide examples of applications, and discuss the applicability of the new code within the Australian context.
Tony Appelo has been a professor at the Free University in Amsterdam and is since 1998 the director of his own consulting firm. His interest is to develop hydrochemistry into a quantitative science in which computer models help to unravel complicated reactions in laboratory experiments and field studies. He has co-authored the popular textbook “Geochemistry, groundwater and pollution” of which a 2nd, completely updated and rewritten edition was published in 2005, and wrote numerous papers in international journals, e.g. on ion exchange during fresh/salt water displacements in laboratory columns and aquifers, redox reactions with pyrite and manganese oxides, diffusion and transport in clay rocks and clay minerals, and more. He co-develops the computer program PHREEQC to the practical tool that has made it the present-day standard in hydrogeochemical computing and gives courses in using and applying this code in Holland and abroad.