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Australian Geothermal Energy Conference 2011

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Huge untapped geothermal reserves in New Zealand

A. G. Reyes [email protected]

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 <10 oC to as high as about 350 oC. In these systems, natural permeability and the presence of circulating fluids can range from high to nearly nil (Figure 1). Conventional geothermal systems contain large quantities of hot water or steam that can be tapped economically from permeable zones at <3500 m for direct use of power generation. These systems usually discharge from hot springs and fumaroles on the surface. Conventional sources of geothermal energy for power generation and direct heat utilisation in New Zealand occur in (1) high-temperature geothermal systems of the Taupo Volcanic Zone

and in Ngawha, Northland (including waste water from power generation) and (2) about 140 low-temperature thermal spring systems (with one or more springs) outside these two regions. In conventional geothermal systems, heat resources are high grade because heat is effectively transferred to exploitable depths by large volumes of fluids circulating in permeable zones. High-grade heat can be harnessed economically from circulating fluids using conventional extraction and energy conversion techniques as shown in Figure 1.

Unconventional geothermal sources include any source of heat outside hot spring systems where permeability and fluid flow may be marginal and heat low-grade. These resources include conductive systems at the edges of high temperature geothermal systems (e.g., Taupo Volcanic Zone and Ngawha) or within sedimentary basins, metamorphic and igneous terranes including hot rock at >3500 m. In some of these geological settings, heat may be more readily available because of conductively-heated waters circulating in abandoned oil and gas wells or accumulating in abandoned underground mineral and coal mines.

Figure 2: Discharge temperatures of mineral springs in New Zealand. Heat flow contours are from Allis et al, 1998. Because of the low annual ambient air temperature in some South Island sites, springs with discharge temperatures <20 oC are sometimes considered thermal; but not in the North island where ambient temperatures are higher (adapted from Reyes, 2007a.).


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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 <109 oC inTaranaki, Wanganui, Taieri and coastal Canterbury to >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 (<250 m) and at deep levels (3) the composition of the rock remains the same from surface to about 4000 m and thus the heat capacity and density of the rock are constant, (4) the country is divided into two major regions: low heat flow (<70 mW/m2; thermal gradient is <33 oC/km; Figure 2) and high heat flow (>70 mW/m2; thermal gradient is >33 oC/km), and (5) the base temperature all over the area of interest is 15 oC. Thus, at shallow depths (<250 m), the estimated heat energy is about 8,000 PJ and at deep levels, >1,000 EJ (Figure 1). Thus, the heat calculations merely prove the enormous geothermal potential of the ground beneath our feet.

Heated water in abandoned hydrocarbon wells

Abandoned hydrocarbon wells and underground mineral and coal mines provide access to conductively heated waters at depth.

There are 349 onshore abandoned hydrocarbon wells in New Zealand that might potentially be harnessed for geothermal energy for direct use of heat, power production and development as pseudo hot springs for tourism (Reyes 2007b). Well depths range from 17 m to 5065 m vertical. Estimated bottom hole temperatures range from ambient (12 oC to 18 oC) to 172 oC. The K/Na ratios of waters in some oil and gas wells indicate subsurface temperatures of 180 + 20 oC.

Of these wells 40% are located in Taranaki (Figure 3), the only producer of oil, gas and condensate in New Zealand at present. The rest of the wells are distributed in the North and South Island sedimentary basins. Although the requisite temperature may be present in abandoned hydrocarbon wells for a wide range of geothermal energy uses, there are many geoscientific, technical and non-technical problems to be considered before these wells could be used for geothermal power generation or cogeneration (Reyes, 2007b). Waters from an abandoned hydrocarbon well in Taranaki are presently being used for a bathing complex and to produce mineral waters.


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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 <0.1 PJ of these are being harnessed at present. Thus, more than 90% of New Zealand’s geothermal reserves remain unexploited at present.

References

Allis, R.G., Funnell, R.H., and Zhan, X., 1998, From basins to mountains and back again, NZ basin evolution since 10 Ma: Proceedings 9th International Symposium on Water–Rock Interaction, A.A. Balkema, Rotterdam, New Zealand, pp. 3–9.

Bibby, H.M., Caldwell, T.G., Davey, F.J., and Webb, T.H., 1995, Geophysical evidence on the structure of the Taupo Volcanic Zone and its hydrothermal circulation: Journal of Volcanology and Geothermal Research, v. 68, p. 29–58.

Figure 4: Cross plot of depth (m) and estimated bottom hole temperatures (oC) of abandoned offshore and onshore hydrocarbon wells in New Zealand and possible uses of thermal fluids (adapted from Lindal, 1973; Reyes, 2007b).


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Lindal, B., 1973, Industrial and other applications of geothermal energy. In: Geothermal Energy: Review of Research and Development, Paris, UNESCO, LC No. 72-97138, pp.135-148.

Lund, J.W., 2007, Characteristics, development and utilization of geothermal resources: Geo-Heat Centre Quarterly Bulletin, Oregon Institute of Technology, v. 28(2), pp. 1–9.

Lund, J.W. and Freeston, D., 2001, World-wide direct uses of geothermal energy 2000, Geothermics, v. 30, pp. 29-68.

Reyes, A.G., 2007a, A preliminary evaluation of sources of geothermal energy for direct use: GNS-Science Report 2007/16, 42 p.

Reyes, A.G., 2007b, Abandoned oil and gas wells- a reconnaissance study of an

unconventional geothermal resource: GNS-Science report 2007/23, 41 p.

Reyes, A.G., Christenson, B.W. and Faure, K., 2010, Sources of solutes and heat in low-enthalpy mineral waters and their relation to tectonic setting, New Zealand: Journal of Volcanology and Geothermal Research, v. 192, p. 117-141.

Thain, I., Reyes, A.G. and Hunt, T., 2006, A practical guide to exploiting low temperature geothermal resources: GNS-Science Report 2006/09.

Yasukawa, K. and Takasugi, S., 2003, Present status of underground thermal utilization in Japan: Geothermics, v. 2, pp. 609-618.