An estimate of the resource potential of New Zealand geothermal fields for power generation

Geothermics, Vol. 7, pp. 243-252. Pergamon Press Ltd. 1978. Printed in Great Britain

© CNR

0375-6506/78/0601-0243 $02.00/0

AN ESTIMATE OF THE RESOURCE POTENTIAL OF NEW ZEALAND GEOTHERMAL FIELDS FOR POWER GENERATION I. G. DONALDSON* and M. A. GRANTI" Abstract--The basic similarity between most of the New Zealand geothermal fields suggests that the exploited fields of Wairakei and Broadlands can be used as indicators of the potential of other fields as resources for steam for power production. Assuming adequate I£-rmeability will be o b t a ~ . . i n fields yet to be tested, the two parameters controlling this potential are areal extent (as defined by resistlvaty .survey) and temperature at depth. As most field temperatures are bracketed by Wairakei (270°C maxnnum) and Broadlands (3 i 0°C maximum), field potential per unit area should also be bracketed by the field potentials per unit area of these two fields, i.e. Walrakei at 10-11 MWe/km 2 and Broadlands at 13-14 MWe/km z. Based upon our present knowledge of the fields in question we may thus assess their proven, inferred and speculative reserves. Our totals for all fields of 450 MWe proven, 750 MWe inferred and 1300 MWe speculative suggests that New Zealand has some 1300-2500 MWe available from its geothermal resources should it desire to exploit these for electrical power. These figures can only be confirmed and improved by drilling and ultimately by exploitation. The most promising tool for a full assessment of a field potential, the reservoir model can only really be set up once the field has been exploited sufficiently to have been disturbed. In future cases this may only be the case once a power station has been established and has been operating for some time.

INTRODUCTION As New Zealand has begun a programme to evaluate its indigenous energy resources, it is extremely important that we obtain a realistic measure of our total geothermal resource as soon as possible. Although as a further step in the future we should consider our geothermal resources as heat energy resources, the general remoteness of our geothermal fields ~as up to the present precluded the transport of this heat as an attractive alternative. This assessment is thus directed purely at their potential as an electrical power generation resource. In carrying out any assessment of this nature we must also lean heavily on our present experience relating to the development of our geothermal reservoirs. Thus we cannot meaningfully go outside of the technology presently used in developed fields, i.e. present power plant operation and drilling levels must be assumed, although additional potential might be available from deeper drilling or from improvements in field and power plant operation. Previously applied techniques for the assessment of geothermal resource potential are somewhat limited. With a defined reservoir area, a defined drilling depth and some knowledge of field temperatures, it is not too difficult to obtain a measure of the total amount of heat energy contained in a geothermal reservoir. The fraction of this actually extractable and the reduced fraction actually available as electricity is however much more difficult to establish. In this paper we utilise our experience with models of our exploited fields to suggest potential outputs for other fields in this area and for the New Zealand system as a whole. We are aided by the fact that most New Zealand geothermal fields show strong similarities and are probably bracketed by the two main exploited fields, Wairakei and Broadlands. Temperatures are generally in the range 230-310°C and, with two or three possible exceptions, the fields are water- rather than vapour-dominated. Indications are that flow in these fields is fissure- or fracture-controlled rather than associated with bulk permeability. This complicates the task of forming conceptual or mathematical models of these fields as flow boundaries may not *Physics and Engineering Laboratory, Department of Scientific and Industrial Research, Lower Hutt, New Zealand. l"Appl/ed Mathematics Division, Department of Scientific and Industrial Research, Wellington, New Zealand. 243

244

I. G. Donaldson and kl. A. Grant

coincide with geological ones. Nonetheless, in most situations, the fissure and fracture pattern appears to be sufficiently heterogeneous on the gross scale that simple homogeneous models give adequate answers unless particularly detailed information is required. It further appears that, again with the exception of the two or-three possible vapour-dominated fields, capping structures are minor and have little consequential effect on the gross behaviour, and hence potential, of these fields. Thus, comparison between the various New Zealand fields is both sensible and logical in any resource study. RESOURCE EVALUATION For any evaluation of our geothermal resources it is convenient to divide our geothermal fields into four categories based upon the extent of our current knowledge of them. This enables us to consider our geothermal reserves in analogous fashion to coal and other mineral resources as proven, inferred and speculative. The categories are: (a) Known fields (proven reserves)--fields that have been exploited or so disturbed that we can reasonably accurately define the field capacity and response to exploitation. Fields in this category are Wairakei, Broadlands, Kawerau and Tauhara. (b)

Explored fields (inferred reserves)--fields into which deep test wells have been drilled and for which some permeability and potential production information is thus available. Fields here are Orakeikorako, Reporoa, Rotokawa, Waiotapu and Te Kopia.

(c)

Mapped fields--fields defined by a satisfactory resistivity survey, the technique that to date has proved most satisfactory for determining the areal extent of New Zealand's waterdominated geothermal reservoirs. Fields in this category include Mokai, Atiamuri, Tikitere-Taheke and Ngawha.

(d)

Identified fields--fields known mainly on account of surface manifestations. Deep temperatures may however have been estimated from the chemistry and some geological surveying will probably have been undertaken. Examples here are Ketetahi and Waimangu. The potential output of fields in these latter two categories must be largely speculative at this stage although the maintenance of surface discharge permits the inference of availability of at least a proportion of this energy. The location of various fields in the four categories is illustrated in the map (Fig. l).

Known fields (i) Wairakei. Wairakei is well-known internationally, data from it being used as a basis for development of detailed geothermal reservoir models by several groups in the U.S.A. as well as in New Zealand. It is a hot water field with temperatures of 235-260°C, a chloride content of 1500 g/m 3 and an initial gas content of 0"05~0. Modelling and assessment of this field is largely hindsight. The field was developed on a heuristic basis and this produced a satisfactory result. Later analyses all merely confirm that the station was the correct size. It would, in fact, be difficult to reach any other conclusion, given: (a) that the station has now run successfully for 20 yr; (b) that aquifer pressures throughout the reservoir have fallen about as much as we can comfortably accept. The project was designed to be built in three stages. Pressure drops after the completion of the second stage caused abandonment of the third. With time the rate of pressure drop has decayed and a quasi-steady state, some 26 bar below the original pressure level, is now being maintained. The most striking feature of Wairakei is its uniformity. Despite withdrawal in a limited borefield, pressures fell so uniformly across the field that the field can be characterised by a single

245

An Estimate oj the Resource Potential oJ New Zealand

36

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Fig. 1. The location of known New Zealand exploitable geothermal fields.

3,S

246

I. G. Donaldson and M. .4. Grunt

"'field pressure". This uniformity can only be due to direct communication between one part of the field and another and a layer of very high horizontal permeability has been conjectured. Whatever the reason, it simplifies the job of modelling the field's response to exploitation as other structural influences appear to be overridden by the influence of this particular feature. The basic picture of Wairakei is of a column of hot water rising vertically in an environment of cold water. This hot water boils near the surface to give a shallow two-phase zone at the top of the hot column. Exploitation draws hot fluid fairly uniformly from the top of the hot water region and from the two-phase zone. The fluid taken from the wells is thus supplied by three means: (a) drainage from the two-phase region into the hot water region; (b) a fall in level of the two phase/single-phase interface; (c) stimulated recharge of hot water from the same and greater depths, this water in turn being replaced by the gradual encroachment of cold water from outside. Most of the New Zealand models of the Wairakei reservoir are variants of the above simple model. Reflecting the field's simple behaviour, all of them give good fits to the pressure history. Only the trend curve analyses (Bolton and Wainwright, personal communications) are too simple to incorporate all features but even these indicate the continuing stability of the pressure drop at about the present level. Wainwright also argued heuristically from the observed pressure/digcharge history that the installation of additional low-pressure turbines would enable us to obtain the optimum power production from Wairakei. Although these turbines were not installed, his calculations appear correct since his heuristic calculations of pressure response give the same result as the more sophisticated later models. The earliest of the simple mathematical models is described by Marshall (1966). The latest, by McNabb (personal communication), uses analytic approximation to estimate the contributions from drainage of the two-phase region and recharge of hot water to give an intregro-differential equation relating pressure and discharge histories. This model gives a satisfactory description of field response and is consonant with other aspects of the field history (for example, temperature drops at depth caused by recharging water contracting temperature isotherms). All of these models suggest that increasing the rate of withdrawal from any part of the Wairakei reservoir must drop the pressure further and that any increases in output will either be extremely shortlived or result in such additional field pressure drops that power production will become virtually impossible to sustain. The more detailed models, mainly under development in the United States, are not likely to be much better than these simple models when it comes to predicting overall field response. When extended to three dimensions they should, however, describe local variations in behaviour and assist in optimising drilling procedures to sustain the established output. They are also likely to serve a more important role in less connected fields such as Broadlands. (ii) Tauhara. Adjacent to Wairakei and connected to that field by a narrow permeable neck, the Tauhara geothermal reservoir has not yet been exploited. Four deep wells have however been drilled to penetrate the Waiora aquifer, the main reservoir aquifer in Wairakei. These bores all show pressure drops a few bars less than those measured in Wairakei, the pressure drawdown taking a few years to diffuse from Wairakei to and across the Tauhara field. Despite this pressure drop it appears that little fluid has flowed from the reservoir to Wairakei. Certainly there is no chemical or temperature evidence to suggest a significant flow and Wooding (1978) has successfully modelled Tauhara behaviour by assuming the aquifer to be confined both vertically and horizontally. Thus any input to Wairakei from this area would only be due to decompression of the water and rock as the pressure declines. This model is being used to assess the likely effects of production and reinjection. The confined nature of the Ta uhara reservoir contrasts sharply with the uncapped behaviour of Wairakei and most other New Zealand fields.

An Estimate oJ" the Resource Potential oJ New Zealand

247

Based simply on the volume of the Waiora aquifer whose permeability is proven by the pressure spreading, 100 MWe may be taken as proven for this field. A larger exploitation would require permeability at greater depth. Reinjection would, however, probably be needed to protect Wairakei from additional drawdown.

(iii) Broadlands. The Broadlands field has now been thoroughly explored and tested and construction of the first stage I100 MWe) of the power station will start soon. Exploration began in 1966 and the initial scientific work indicated that a potential comparable with that of Wairakei might be expected. The field was discharged for 3 yr, then shut down in 1971. Drilling and testing has now been recommenced as a preliminary to station construction. That the field was discharged for 3 yr, then shut down, provides a near perfect experiment to test the reservoir. This shows behaviour startlingly different from Wairakei (Hitchcock and Bixley, 1976). Compared with Wairakei's uniformity Broadlands is divided into distinct zones. Of its 11 km 2 areal extent only 3--4 km 2, the Ohaki area, has good producing bores whose pressures communicate with each other. Even within this area however pressure transmission is slow (1 yr to travel 1 km) compared with Wairakei (less than ½ day to cross the field). Throughouf the remainder of the field, bores appear to be completely isolated and there are fewer good producers. Interestingly, the highest temperatures for depth and the best producer both exist outside the Ohaki area. Most of these features of the field were apparent by 1971. Wainwright (personal communication) at that time analysed the pressure drawdowns to show that drawdown was much greater than in Wairakei, that recharge was too small to measure, that the pressure response could not be attributed to expansion of compressed water, and hence that two-phase conditions were present in the well zone and that poor permeability must be restricting the flow. In part the difference between Broadlands and Wairakei is explained by the presence of gas. Broadlands deep water has a gas content of 2 - 3 ° and a temperature of 300-310°C. This fluid will boil down to depths in excess of 1"5 km and hence most exploitation presently occurs in the two-phase zone. The existence of gassy two-phase, rather than all-liquid, conditions explains the difference between Wairakei and the Ohaki section of the Broadlands field. It does not, however, explain the differences within the Broadlands field itself. These can only be due to geological or structural limitations. Two-phase conditions restrict the propagation of pressure and thus reduce the zone of influence of any bore. This will make the field behaviour much more sensitive to local geological and structural variations. The initial pressure response of the wells, i.e. a rapid drop, is misleading in this situation as a guide to ultimate behaviour. The field is gas-dominated and hence the initial response consists principally of degassing effects. Only when the effect of the gas is incorporated are reasonably reliable hydrological calculations possible. Grant (1977) has developed a gas-dominated model that is capable of assessing the response of the Ohaki section of this reservoir to exploitation. This model is a "lumped-parameter" model, treating the Ohaki area as a single homogeneous reservoir of porous rock, subject to discharge and recharge of water and gas. It predicts a capacity of about 100 MWe for this area. We cannot, however, treat this section in isolation, as flows from significant producing wells in other parts of the field may ultimately affect this output. Unfortunately, no quantitative model for the entire field is as yet established. A reasonable conceptual model is that the principal upflow at a depth of say 3-5 km is under the southeastern part of the field. This bifurcates with the bulk of the natural flow moving northwest to Ohaki and a lesser amount going straight up. The general flow pattern is then controlled by large contrasts in permeability, probably associated with fracturing at various depths and through various local structures. Significant variations in

248

I. G. Donaldson and :14. ~-I. Grant

chloride content at depth across the field further indicate the degree of areal variability in this Broadlands system. Wairakei's gross response can be described by a very simple model. In contrast a model giving a comparably good description of Broadlands needs to contain far more detail. Nonetheless, from the Ohaki model we have a proven output of 100 MWe and from producing wells outside Ohaki we have at least a further 20 MWe. With additional drilling in this latter area it is not unreasonable to infer a further 30 MWe. (iv) Kawerau. Kawerau has been exploited since the-late 1950s with the steam being used at Tasman Pulp and Paper Company mill at a rate roughly equivalent to 20 MWe. This withdrawal has not had a discernible effect on field pressures, although intrusions of cooler water have quenched some shallower wells. This experimentally demonstrates that the field has a capacity that considerably exceeds 20 Mwe. It also illustrates the restrictions on any reservoir modelling and assessment programme. We cannot prove the size of the exploitable resource in a geothermal field until we have discharged the field sufficiently to alter pressures throughout at least a significant section of the field. The past test of Kawerau--20 MWe discharge for nearly 20 yr---can only establish a lower bound to the field's capacity. In behaviour Kawerau shows some similarity to both Wairakei and Broadlands. The chemistry is similar and flow occurs in fractures. In contrast, however, Wairakei is very uniform with its boiling surface now at about 0.5 km. Its bores largely exploit the liquid zone and its gas content is virtually negligible. Broadlands is boiling to below 1'5 km, its bores exploit only two-phase conditions, and its gas content is 2-3°/~. Kawerau contains less gas and appears to boil over most of its area at about 1 kin. Past bores were restricted to the two-phase region and have shown the lack of connection typical of such conditions. Newer bores, including the powerful bore KA21, reach into the liquid zone. This places Kawerau in the middle of the Wairakei-Broadlands range, the range being characterised by increasing gas content and size of two-phase zone. Conceptually, we might expect bores of less than 1 km depth to be isolated but that deeper bores will tap an all liquid reservoir where flow and pressure propagation will not be impeded by the presence of the vapour phase. Other fields

Outside of these exploited and disturbed fields we can estimate potential only by analogy, based on experience. The evidence from other fields comes from drillholes, where these exist, and from surface features. Except for the two possible steam fields (Tokaanu and Ketetahi), and the much larger Ngawha field, this evidence all indicates that these fields all fall in most respects into the range already defined by the Wairakei-Kawerau-Broadlands experience. This suggests that given only three parameters for each field--field area, temperature and permeability--we may be able to estimate its power potential. Let us therefore look back at the likely power potential of our three exploited fields to see whether we may extrapolate this to other fields. THE POWER P O T E N T I A L OF THE EXPLOITED FIELDS As indicated above at about 150 MWe Wairakei is not far from its upper limit of exploitation. If the temperature did not vary over the field we would expect any local section of the field (if bounded areally) to be exploitable to a proportionate amount, i.e. for Wairakei with a maximum temperature of about 270°C and a defined (resistivity) area of about 15 km 2 we can suggest an exploitable potential of about 10-11 MWe/km 2.

An Estimate of the Resource Potential of New Zealand

249

For Broadlands modelling plus drilling again suggests a potential of about 150 MWe where this time the smaller area (11 km 2) is offset by a higher maximum temperature (310~C). In this case we might estimate an exploitable potential of 13-14 MWe/km 2. A more permeable overall field might push this figure a little higher. Our experience with Wairakei suggests that we might expect the limit withdrawal to be at least five times the quantity that can be withdrawn without disturbing the field. Thus from Kawerau a minimum potential of 100 MWe would appear reasonable. For an area of 8 km 2 this would mean an exploitable potential of 12-13 MWe/km 2. With a 290°C maximum temperature this fits in well with the Broadlands and Wairakei figures. With more general permeability than Broadlands and a possible area of 10 km 2, the 100 MWe might be increased by a speculative 30 MWe. The estimated power potentials of these and other New Zealand fields are listed in Table 1. Table

1. E s t i m a t e d

exploitable

geothermal

resources

Field

Area (kin a)

Max. Temp. l-C)

Proven"

W~rakei Tauhara Broadland s Kawerau

15 14--16 11 6-10

270 280 310 290

150 100 120 100

Wa~otapu-Reporoa

Inferred*

Speculative* 80

30 30

8-12 6-10 8-12

295 260 310 +

150 50 50

100 50 100

Tikiter~-Taheke Waimangu Te Kopia Mokai Atiamuri

12 12 5 17 3

270(71 270~?) 240 (?) (?1

75 50 20

75 10~ 20 170 30

Tokaanu-Waihi

4

Orakeikoeakn Rotokaua

steam

30-50

Nsawha Totals ,i i i

I00 25

steam

Ketetahi

J

i

260{71 [

i ill

i

470

i

200

500

625

1380 i

D

i

*MWe ~output from power station). Ante The power potemlal for some of the smaller field~ listed above have previously, been est,mated (N.Z. Geolog)cal Surv¢~ 1974l

THE POWER POTENTIAL OF OTHER WATER-DOMINATED FIELDS (i) Tauhara With an area of 14-16 km 2, a maximum temperature of 280~C, and proven adequate permeability, our area/temperature analysis would suggest a potential of 180 MWe. Wooding's (1978) sealed aquifer model gives a potential of 100 MWe. This will be taken as proven, the remaining 80 MWe, which will depend upon locating other permeable horizons, will be taken as speculative. (ii) Waiotapu-Reporoa Five shallow holes and two deep holes have been drilled at Waiotapu, one deep one has been drilled at Reporoa. The natural discharge at Waiotapu is about 550 MWt, i.e. a greater amount than that originally at Wairakei. This suggests that the surface area, 8-12 km 2, as indicated by the resistivity survey, may be underestimated. In fact there is some suggestion of horizontal flow with the field extending beneath the adjacent Rainbow Mountain. Well discharge water with 900-1600 g/m 3 chloride and a maximum temperature over 290~C has been measured. We might therefore infer an exploitable potential of some 150 MWe from this field. As Reporoa appears to be an extension of Waiotapu it cannot at this stage be treated independently. The chemistry is very similar but the temperatures are a bit lower. It may thus be downstream of Waiotapu (if the flow is horizontal) and hence any potential output must be

250

I.G. Donaldson and M. A. Grant

considered speculative. 100 MWe has been taken as a figure for this area and/or a Rainbow Mountain extension. (iii) Orakeikorako This field has a natural discharge of 340 MWt and an area of about 8-10 km z. So far four holes have been drilled with disappointing results in that temperatures are lower (260°C maximum) and little good permeability has been found. Only one bore gave any significant discharge (100 kg/s- 1) but this was not sustained. Assuming that good permeability could be found we might expect a total inferred exploitable potential of around 80-100 MWe. Due to its poor history, however, only 50 MWe of this has been included in the inferred category. (iv) Other fields m this category Our assessed proven, inferred, and speculative power potentials for all N.Z. fields in this category that might be exploited are given in Table 1 with areas and estimated maximum temperatures. Fields like Rotorua are omitted as exploitation of such areas is unlikely as they are of considerable tourist interest. Two fields, Atiamuri and Mokai, have only recently been surveyed and no temperatures are known for them. As, however, they will be drilled in the near future they have been included in the list and we have speculated their potential at a low 10 MWe/km 2. THE POWER POTENTIAL OF THE REMAINING FIELDS (i) The vapour-dominated(?) fields Both the Ketetahi and Tokaanu-Waihi fields appear to be quite small and hence the power potential per unit area chosen will not affect the totals greatly. A figure of 25 MWe/km 2, about twice that of the water-dominated fields, has therefore been taken as a speculative estimate. (ii) Ngawha Recent exploration suggests that the most promising unexploited New Zealand field is Ngawha. Situated outside our normal "geothermal belt" this field does not show much surface activity, probably on account of extensive surface clay beds. The resistivity survey (Macdonald et al., 1977), however, suggests an areal extent of 30-50 km 2 inside the 5 ohm-metre contour. The boundary is very poorly defined and the heat flow anomaly, covering some 180 km 2, suggests that the reservoir boundary may be much further out. A shallow (600 m) hole, drilled in 1966, produced about 5 t/h of high-enthalpy, high-gas fluid. Further investigation holes (1000-1500 m deep) are now being drilled with the first showing similar conditions to the 1966 hole. At this stage this field thus remains an enigma. With temperatures in the 240-270°C range and the large area, it appears a very attractive prospect. Indications from the relatively shallow drilling so far suggest, however, that the permeability is poor. The question thus arises as to whether we might still be going through the capping structures of a deeper vapour-dominated reservoir. The presence of chloride water in the surface discharge areas might be taken to argue against this. If the shallow low permeability structures are typical we may have a high-gas field with two-phase conditions, and hence even lower effective permeability, to a depth of several kilometres. In either event a significant heat anomaly exists and hopefully we will be able, in time, to exploit it. Taking a minimum area (30 kin2), a low 240°C temperature and reasonable permeability, we might infer a power potential of 300 MWe. With lower permeability, however, a figure of 200 MWe might be more reasonable. For a speculative estimate we have assumed a larger area, a

An Estimate of the Resource Potential o[ New Zealand

251

higher temperature and good permeability. If these conditions can be met an additional power potential of 500 MWe or more could well be anticipated. ADDITIONAL POWER POTENTIAL DUE TO TECHNOLOGICAL CHANGES The above estimates of power potential are heuristic. They are not derived by assessing the stored energy as X Megawatt-years and dividing by an acceptable lifetime. The factor limiting power station size is not the size of the total resourcemWairakei is expected to last for several more decades, but we cannot enlarge the station. Rather the limitations are the rate of fluid withdrawal, which is set by the acceptable drawdown, and the efficiency of conversion of the extracted energy to electrical energy, which is set by the power station design. Thus technological changes, such as deeper drilling (perhaps accompanied by hydro-fracturing), that permit a greater acceptable drawdown, and changes in power station design, that raise efficiency, will give larger potential power outputs. In the case of deeper drilling this increase could be quite marked as, if the permeability does not decrease with depth, the acceptable drawdown is roughly proportional to the depth of the bore's feeding fissures. The final estimate of power capacity of a field must thus be for a total (reservoir-bore-station) system. Reservoir analysis gives the limiting withdrawal rate given the drawdown; the bores define the acceptable drawdown; and the station controls the amount of power for a given mass flow. The power capacity is a composite of three factors: Power output = (reservoir performance) x (acceptable bore drawdown) x (station efficiency) (Reservoir performance = mass flow/unit drawdown) One of these factors, the reservoir performance, is beyond our control; the other two are not, and increases in power capacity will come from improvements in these. CONCLUSIONS Experience with the exploited New Zealand fields of Wairakei, Broadlands and Kawerau and with simple models of Wairakei and Broadlands has led to the conclusion that any energy-time concepts, such as assuming that a particular field may have a total power potential of X MWeyr, are not very meaningful. Rather we might expect, as with Wairakei, that an optimal energy production rate may be set for the field and that that rate may, with slow decay, continue for many years, possibly several times the planned lifetime of the power station equipment. Any increase in production over this optimal rate may only result in a brief surge of energy followed by a decay to a level only slightly above the optimal value. Associated with this would be a serious drop in field pressure (i.e. increase in drawdown). Although an areal definition of a geothermal reservoir and a representative measure of its bulk temperature give us a reasonable indication of the gross amount of heat contained in the reservoir, they neither give us an estimate of the extractable heat nor set any time-scale over which such extraction should be carried out. These two most pertinent factors, from the point of view of power engineers, depend strongly on both the field permeability characteristics and the nature of the fluid in the field (e.g. single- or two-phase, gassy or non-gassy). The system is also a dynamic one with the rate of withdrawal controlling the rate of drawdown and the likely useful lifetime of the field. Extrapolation of our modelling and field experience to New Zealand fields not so far exploited has led to the suggestion of likely (categorised in Table 1 as proven, inferred or speculative depending on our knowledge of the field) optimal power production rates for all potentially

252

I . G . Donaldson and :14..4. Grant

e x p l o i t a b l e fields k n o w n in this c o u n t r y at this time. In total we have s o m e 500 M W e proven, 600 M W e inferred a n d 1400 M W e speculative. This suggests that in time N e w Z e a l a n d could be injecting between 1100 a n d 2500 M W e into o u r electrical energy n e t w o r k from g e o t h e r m a l sources.

REFERENCES Grant, M. A. (1977) Bt~adlands, a gas-dominated syst,m. Geothermics 6, 1/2, 9-29. Hitchcock, G. and Bixley, P. (1976) Otzmrvations of thc effect of a thr~.year shutdown at Broadlands geothermal field, New Zcmland. Proc. 2nd U.N. Symposium on the Development and Use of Geothermal Resources, San Francisco. 3, 1657-1661. Macdonald, W. J. P., Dawson, G. B., Rayner, H. H. and Hewson, C. A. Y. (1977) Geophysical investigations of the Ngawha geothermal area. Geophysics Division, D.S.I.R., Report 130. Marshall, D. C. (1966) Preliminary theory of the Wairakei geothermal field. N.Z.J. Sci. 9, 3, 651-673. N.Z.C.~ologicai Survey (1974) Minerals of New Ztmland: Geothermal. N.Z. Geological Survey Report 38D. Wooding, R. A. (1979) Analysis of pressure drawdown measurements in the Wairakei-Tauhara geothermal area (in preparation).