Fluid-Rock Interactions in Deep-Seated Geothermal Reservoirs

Abstract

Extending the depth of hydrothermal resources, and development of engineered reservoirs for Hot Dry Rock geothermal heat extraction, complement 'conventional' geothermal energy utilization, yet water-rock interaction processes and the physical character of induced/natural fracture systems in Deep-Seated Geothermal Reservoirs (DSGR) are poorly understood. Dissolution of granite and quartz, with pure water, has been investigated at sub- and supercritical conditions to evaluate deep geothermal reservoir dynamics. In terms of dissolution behavior of granite and quartz, the supercritical region may be subdivided into two apparent phases, comprising 'liquid-like' and 'vapor-like' regions. The solvent properties of supercritical fluid in the liquid-like region are similar to those of subcritical water, whilst dissolution reactions in the vapor-like supercritical state are weak.

SEM-Cathodoluminescence (SEM-CL) imaging techniques reveal chronological relationships between secondary (vein/replacement) quartz and (primary) rock-forming minerals, which aid differentiation of (multiple) hydrothermal events, and interpretation of physico-chemical changes in the active Te Kopia (New Zealand) geothermal system. SEM-CL, integrated with fluid inclusion/petrographic data, "fingerprints" quartz from specific alteration events, constrains the timing of fluid inclusion trapping and nature of fluid migration, which is important for understanding the pressure, temperature and chemical evolution of the hydrothermal system.