Laboratory-scale study of fluid-rock interaction in the enhanced geothermal systems in Cooper Basin, South Australia

Abstract

A study of fluid-rock interaction is essential to determine the chemical changes and mineral alteration in Enhanced geothermal system. Preliminary mineralogical investigation and geothermal experiments have been performed to investigate the hydrothermal alteration of the Habanero 3 well in the Cooper Basin, South Australia. Samples of drill cuttings from a borehole 5 km deep were reacted with pure water (reverse osmosis treated) in a titanium geothermal cell at 250°C and approximately 45 bar. Fluid and rock samples were analysed prior to, and after circulation of the water through crushed samples of the rock (100 - 200 μm diameter) for 1, 2, 4, 7, 14 and 28 days. Water analyses were undertaken using inductively coupled plasma mass spectrometry (ICP-MS) and heteropoly blue method tor silica analysis, and rock analyses were conducted using scanning electron microscopy (SEM), x-ray diffraction (XRD) and x-ray fluorescence (XRF). Experimental results showed that mineral dissolution was rapid in the early stages of the experiment. This may be a consequence of the dissolution of smaller rock particles and dissolution of more soluble mineral phases. SEM observations showed evidence of etching of the mineral surfaces consistent with partial dissolution. SEM backscattered images reveals that the quartz phase (SiO2) had little or no alteration after 28 days of circulation. XRF and XRD results complement this finding, that quartz was most stable throughout the experiment, and that the albite-feldspar (NaAlSi 3O8) and microcline (KAlSi3O8) in the rock had partially dissolved. As well, ICP-MS analysis of water samples confirmed that some mineral dissolution occurred. Determination of the dissolution kinetics of the various minerals phase is being undertaken.