A Case Study of Rock-Fluid Interaction in the Enhanced Geothermal System in Cooper Basin, South Australia

Abstract

This study was undertaken to observe mineral dissolution with replacing circulating fluid with fresh water every 24 hours. This was an attempt to accelerate the dissolution rate and to mimic the condition of a geothermal site when fresh water or treated water from a precipitation tank is reinjected to the fracture. The experiments were carried out in a titanium flow through cell for 1, 7, and 28 days at 250°C and 40 bars. Water analysis was performed using ICP-MS, and rock analyses were conducted using SEM, XRD and XRF. The experimental results revealed a linear correlation of mineral (element) dissolution at the early stages of the experiment. However at later stages, the mineral dissolution proceeds at a slower rate. This may have been caused by the exhaustion of a more soluble mineral phase in the sample. Therefore, this may cause the pore size in the fracture path size to enlarge. SEM observations showed evidence of etching of the mineral surfaces consistent with partial dissolution. SEM backscattered images reveals that mostly quartz phase (SiO₂) remains after 28 days of circulation. XRD results complement these finding, that quartz was stable throughout the experiment, and that the albite-feldspar (NaAlSi₃O₈) and microcline (KALSi₃O₈) in the rock had partially dissolved. As well, ICP-MS analysis of water samples confirmed that some mineral dissolution occurred. XRF study was used to generate an elemental mass balance. Determination of the dissolution kinetics of the various minerals phase is being undertaken.