Geochemical reactions can induce significant changes of rock reservoir porosity and permeability via mineral dissolution and precipitation processes, affecting the long-term fluid behaviour within various geological sys-tems. The understanding and quantification of these reactions rely on field and experimental studies and on the predictions of reactive transport models.The present study was aimed at assessing the extent to which current geochemical models integrating avail-able mineral dissolution/precipitation rate equations can reproduce the experimental data obtained from 4 to 17 -day long hydrothermal alteration experiments of a muscovite-biotite granite and, thus, help provide an accurate description of the evolution of geothermal systems within granitic reservoirs. The experiments were conducted at a constant temperature of 180 degrees C and over an aqueous fluid pH range of 2 to 8.5, using both mixed-flow and static batch reactors. Modelled major element (K, Al, Si, Ca, and Mg) concentrations were generally in satis-factory agreement with the corresponding measured elemental fluxes - the differences between modelled and experimental values were generally within a factor of 5 - and the predicted identity and mass of formed sec-ondary phases were consistent with the microscopic observations of the reacted solids. However, larger differ-ences between measured and modelled element concentrations were observed when significant amounts of secondary phases formed, notably at pH 2 to 3, and for longer-term batch experiments. Much of this concen-tration difference stems from the underestimation of the amounts of Al-phases formed at acid to near-neutral pH. Although an idealized rock composition was considered, the observed mismatch between model calculations and experimental data can be attributed to inadequate mineral precipitation reaction rates and a poor description of reactive surface areas in existing geochemical modelling codes. More accurate quantification of precipitation kinetics, including nucleation and growth, and improved descriptions of the temporal change of mineral surface area would enhance the predictive capabilities of reactive transport models and benefit, particularly, the efforts aimed at increasing the sustainability of EGS reservoirs.
A combined experimental and modelling study of granite hydrothermal alteration
Saldi G.
;
2023
Abstract
Geochemical reactions can induce significant changes of rock reservoir porosity and permeability via mineral dissolution and precipitation processes, affecting the long-term fluid behaviour within various geological sys-tems. The understanding and quantification of these reactions rely on field and experimental studies and on the predictions of reactive transport models.The present study was aimed at assessing the extent to which current geochemical models integrating avail-able mineral dissolution/precipitation rate equations can reproduce the experimental data obtained from 4 to 17 -day long hydrothermal alteration experiments of a muscovite-biotite granite and, thus, help provide an accurate description of the evolution of geothermal systems within granitic reservoirs. The experiments were conducted at a constant temperature of 180 degrees C and over an aqueous fluid pH range of 2 to 8.5, using both mixed-flow and static batch reactors. Modelled major element (K, Al, Si, Ca, and Mg) concentrations were generally in satis-factory agreement with the corresponding measured elemental fluxes - the differences between modelled and experimental values were generally within a factor of 5 - and the predicted identity and mass of formed sec-ondary phases were consistent with the microscopic observations of the reacted solids. However, larger differ-ences between measured and modelled element concentrations were observed when significant amounts of secondary phases formed, notably at pH 2 to 3, and for longer-term batch experiments. Much of this concen-tration difference stems from the underestimation of the amounts of Al-phases formed at acid to near-neutral pH. Although an idealized rock composition was considered, the observed mismatch between model calculations and experimental data can be attributed to inadequate mineral precipitation reaction rates and a poor description of reactive surface areas in existing geochemical modelling codes. More accurate quantification of precipitation kinetics, including nucleation and growth, and improved descriptions of the temporal change of mineral surface area would enhance the predictive capabilities of reactive transport models and benefit, particularly, the efforts aimed at increasing the sustainability of EGS reservoirs.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
