Natural state geothermal reservoir modelling: Mineralogical and geochemical evolution perspective

Abstract

Reactive transport modelling plays a crucial role in unravelling the dynamics of geothermal reservoirs as they evolve. By utilising these models, we gain a deeper understanding of the temporal progression of mineral alteration and its influence on the reservoir's characteristics.

This study was designed to understand the mineralogical and geochemical evolutionary process of conventional geothermal reservoirs. An idealised 3D large-scale numerical model was set up to comply with the prevailing conditions in a two-phase liquid-dominated geothermal reservoir with near-neutral pH and a temperature of up to 350 °C. The models were established to assess the long-term fluid-rock interaction in tholeiitic basalt-hosted geothermal systems, including the effects induced by the natural influx of CO₂, H₂S and H₂ from the magmatic body heat source of the theoretical reservoir. As part of the study, gold, which is associated with hydrothermal systems, was included in the geochemical system to investigate ore deposition processes.

Our reactive transport study results indicate that basalt glass, olivine, augite and the primary plagioclase altered to form secondary albite, chlorite, grossular, epidote, quartz, zeolites and carbonates after 100,000 years. Due to mineral alteration, reservoir porosity and permeability are expected to decrease with time. A near-stable state was achieved based on the temperature profile and a comparative analysis of geochemical results over time. Gold in the basalt was shown to dissolve at depth and re-deposit at shallower locations, resulting in high concentration regions.