| Abstract |
Heat from the earth is an inexhaustible and ubiquitous resource generated mainly by pervasive radioactivity and enhanced where tectonic, structural and hydrological conditions converge to focus heat- and fluid-transfer. Localised heat may also be generated by exothermic chemical processes or by shearing along large faults, albeit ephemeral. Low-enthalpy heat can also be human generated e.g., urban heat islands and excess heat from industrial manufacturing. Heat becomes a geothermal resource only when it is economically accessible and technologically recoverable. The enthalpy of fluids, the main commodity in a geothermal resource, becomes a critical factor only when development goes beyond ground source heat pumps. The heat reserves down to 3.5 km in about 120,000km2 of onshore New Zealand, outside the main active volcanic regions where thermal gradients are only 30-35oC/km, are estimated to be about 36.5 x 1014 PJ/a. A miniscule amount of this tremendous stored heat can be accessed and recovered at shallow depths via ground source heat pumps. However, the main foci of this paper are low-enthalpy sources in New Zealand where heat is transferred by fluids including more than 750 low-temperature thermal and non-thermal spring systems outside active volcanic zones, hundreds of abandoned underground coal and mineral mines, more than 400 abandoned petroleum wells, and thousands of shallow domestic, industrial and mine exploration drill holes. Drillholes and mines can hold conductively-heated aqueous solutions or act as a focus for advecting hot thermal fluids from depth. Low-enthalpy springs in New Zealand with surface discharges of ambient to 90oC are widely spread in regions of magmatism (Pleistocene arc-volcanism, Recent intraplate volcanism and post-Miocene mantle diapirism), rapid crustal uplift, rapid upflow of hot metamorphic or subduction fluids from great depths along permeable faults and normal heat gradients in metamorphic terrain and uplifted sedimentary basins. Subsurface temperatures vary from 55-250oC, based on fluid geothermometry, with minimum estimated accessible heat ( more than 20oC) of 20PJ/a and recoverable heat of 2PJ/a. Because of the dearth of uncontaminated fluid samples from abandoned underground mines and wells, bottomhole temperatures, based on measured data or estimated from the heat flow map of New Zealand, are often used instead of fluid geothermometry to determine subsurface fluid temperatures. Thus, at a minimum, underground mine and well temperatures vary from ambient to 175oC. Accessible heat, estimated from minimum flowrates, are 8.6PJ/a (for 400 wells) and 0.8 PJ/a (for 58 mines) or recoverable heat of 0.86PJ/a and 0.08 PJ/a, respectively. Although there is still a widespread perception in New Zealand to associate geothermal energy with thermal springs and volcanic regions, the onshore landmass can be viewed as one big geothermal system where ground source heat pumps and heat exchangers can be universally employed to extract heat from shallow ground or water reservoirs for low-temperature direct heat applications; and deeper heat sources accessed via thermal spring discharges, wells and underground mines. |