Huge untapped geothermal reserves in New Volcanic Zone and Ngawha) or within sedimentary basins, metamorphic

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    Huge untapped geothermal reserves in New Zealand

    A. G. Reyes a.reyes@gns.cri.nz

    GNS-Science 1 Fairway Drive, Avalon, Lower Hutt, New Zealand

    New Zealand is one big geothermal system with pockets of subsurface high temperatures where heat can be mined economically from fluids and/or rock. However, there is difficulty in truly estimating the overall geothermal potential of a region especially where most of the heat reserves are “locked” in deep low-permeability rock formations that only nascent or yet-to-be developed technology can harness for economic use, e.g., as in Enhanced Geothermal Systems (EGS). The main objectives of this study are to propose a grading system for heat reserves for geothermal purposes, document the conventional and unconventional sources of geothermal energy

    in New Zealand, and roughly estimate the potential heat reserves in both conventional and unconventional sources.

    At present, New Zealand has 600 MWe capacity of geothermal power installed in Ngawha and five geothermal systems in the Taupo Volcanic Zone, with a further 1000 MWe within the Taupo Volcanic Zone, alone, that remain untapped (http://www.eeca.govt.nz, 2011).

    Keywords: conventional, unconventional, New Zealand, heat grade

    Figure 1: A geothermal grading system for conventional and unconventional sources of geothermal energy in New Zealand for direct use and power generation and their general temperatures, source depths, permeability, heat grade and volume of circulating fluids. Also shown is the estimated potential energy from each source. Technological difficulties render some sources uneconomical at present. ORC= Organic Rankine Cycle; NE= Natural Energy engine (from Reyes, 2007a,b).

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    Heat grading system A geothermal resource is a volume of rock where heat can be economically harnessed for conversion to power or for direct utilization. Heat is mined from fluids circulating in the rock, or in the case of geothermal ground source heat pumps, directly from the ground or circulating groundwater. Because of technological advances in harnessing heat, e.g., heat pumps (Lund and Freeston, 2001, Yasukawa and Takasugi, 2003) and even improvements in drilling and well maintenance techniques (e.g., Lund, 2007) in the last 20 years, exploitable geothermal temperatures have been extended to as low as

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    The term “low-grade heat” in unconventional geothermal systems refers to the economic extractability of heat and not to temperature. As shown in Figure 1, unconventional sources of geothermal energy can have temperatures up to >350 oC. The low-grade heat of some unconventional geothermal systems stems from insufficient permeability to induce flow, low fluid volume circulating in the rock formation and/or location at depths >3500 m, requiring unconventional or experimental techniques such as EGS (Engineered or Enhanced Geothermal Systems (Lund, 2007) to harness. Thus, in most cases, unconventional geothermal sources are not economically viable at present, except probably for cases where heat can be extracted by heat pumps at relatively shallow depths; or

    made more accessible for extraction by pre- existing structures, for example, such as abandoned oil and gas wells and underground mineral and coal mines.

    Hot spring systems- a conventional source of geothermal energy To evaluate the potential for low-temperature geothermal utilization in New Zealand, a survey of low-temperature thermal spring systems outside the high-temperature Taupo Volcanic Zone and Ngawha was carried out by GNS-Science (Reyes, 2007a, Reyes et al, 2010).

    In this paper a mineral spring is considered thermal when its discharge temperature is 4 oC above the annual ambient air temperature (Reyes

    Figure 3: Variations in fluid compositions of discharge waters from mineral springs located in 18 tectono-geographic regions in New Zealand and estimated subsurface temperatures based on the K/Na geothermometer. NAFS- North Alpine Fault System, MFS= Marlborough Fault System, FAFS = Fiordland Alpine Fault System, P.= Prism (adapted from Reyes et al, 2010).

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    et al, 2011). Discharge temperatures from thermal springs range from 17 oC in the Cascade Terraces in Fiordland in South Island, where annual ambient air temperature is about 12 oC, to local boiling (98 oC to 100 oC) in the Taupo Volcanic Zone. The highest spring discharge temperature in low-temperature geothermal systems, outside the Taupo Volcanic Zone and Ngawha, is 87 oC with a median value of 50 oC (Figure 2). Apart from hot springs, another 12 major areas of warm water upflow were discovered during drilling of wells for domestic or industrial use and during exploration for coal, oil and gas (in blind thermal regions).

    Thermal mineral spring occurrences in New Zealand are clustered in 18 different tectono- geographic regions characterised by specific fluid chemical and isotopic compositions attendant to the different tectonic settings as summarised in Figure 3.

    Subsurface temperatures estimated from the K/Na ratio of spring waters range from 250 oC in the Taupo Volcanic Zone and Ngawha. Measured well temperatures are as high as >320 oC in the Taupo Volcanic Zone and Ngawha (Thain et al, 2006).

    In the low-temperature thermal spring systems, outside the Taupo Volcanic Zone and Ngawha, the highest subsurface temperatures occur in Northland and the Coromandel Peninsula in the North Island; and in the Northern Alpine Fault System (NAFS) in the South Island where K/Na ratios indicate 200–250 oC at depth (Figure 3).

    Because of differences in heat sources (ranging from magmatic to conductive heating of deeply circulating waters) and variations in the permeability of tectonic structures, the estimated annual flow of thermal waters and subsurface temperatures vary widely in the low-temperature hot spring systems.

    The available energy from the TVZ surface springs is at least 7500 MWt (Bibby et al, 1995) or about 237,000 TJ, more than 200 x the energy from low-temperature hot spring systems in the North and South Islands where the minimum estimated energy is only about 33 MWt (1042 TJ) in the North Island and 1.5 MWt (47 TJ) in the South Island.

    Unconventional sources of geothermal energy In estimating the heat in place for unconventional sources of geothermal energy, a porosity of 0.01 is used for hot rock and 1.0 for flooded

    underground mines where heated flood waters fill mine caverns.

    Hot rock

    Apparently, estimates of the geothermal potential of hot rock of a region are fraught with assumptions, and in the end, could merely be a futile unrealistic academic exercise. In attempts to estimate the geothermal potential of hot rock from near surface to about 4000 m several assumptions were made: (1) recovery is 1%, (2) 70 % of the country is available for heat extraction (about 185,000 km2) at shallow depths (

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    The use of abandoned hydrocarbon wells, with bottom hole temperatures ranging from about 30oC to 170oC, for direct heat utilisation and power generation could add, at least, another 6,000 TJ to the geothermal energy potential of New Zealand. The energy would be harnessed from shallow depths using ground source heat pumps for space heating and heating of domestic water (Figure 4). Twenty wells have been identified in Taranaki and the Hikurangi Accretionary Prism (Figure 3) which, if converted, could potentially be used for geothermal power generation using a binary system.

    Heated water in abandoned underground mines

    There are about 20 abandoned underground coal mines and at least 10 underground mineral (gold and/or base metals) mines that have been evaluated (Reyes, 2007a).

    Estimated and measured temperatures of waters accumulated in sealed parts of coal mines range from 18oC to 26oC at depths of 80m to 300m. Projected temperatures are higher in mineral mines at 19oC to 35oC because of deeper levels (as deep as 700 to 850m) and also because some are located in high heat flow areas in the Coromandel Peninsula and the South Island (Figure 2) where the thermal gradient is 35 to 40oC/km. Very rough estimates of the volume of heated water in the abandoned mines yield an estimate of about 4000 TJ of potential geothermal energy from old abandoned underground mines. Again, like geothermal production from abandoned hydrocarbon wells, there are technical and non-technical factors to be considered in

    using mines as sources of geothermal energy, including safety, distance from populated areas environmental considerations, long-term sustainability and economic viability. However one coal company in the Waikato region of the North Island has shown interest in using heated waters (as high as 26oC) discharging from orifices from abandoned sealed mines as a source of alternative energy.

    Summary and conclusions Conventional sources of geothermal energy with high grade heat reserves have an energy potential of at least 265 PJ, mostly occurring in the Taupo Volcanic Zone. However, only about 5- 10% of this energy is being used at present. There is also a wide range of unconventional sources of geothermal energy with enormous heat potential (Figure 1) but

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