69 Proceedings 18th NZ Geothermal Workshop 1996

PRECURSORY CHANGES TO NATURAL THERMAL FEATURES DURING TESTING OF THE AND BROADLANDS-OHAAKI FIELDS

R.B. GLOVER AND T.M. HUNT

Wairakei Research Centre, IGNS, Taupo, NZ

SUMMARY - Exploitation of geothermal fields in has led to moderate to severe, but localised, effects ' on natural thermal features, such as geysers and hot springs. These effects have been caused primarily by decreases in reservoir pressure associated with mass withdrawal and net mass loss.

Small, but now clearly identifiable, precursors for some of these effects were observed during pre-production testhg at Wairakei and Broadlands fields, but their significance was not recognised at that time. Precursors of changes to ~turalthem features were: decline in flow rate from springs, decline in chloride content and temperature of emerging fluids, and an increase in the eruption period of geysers.

1. INTRODUCTION any precursory signs (changes) during discharge testing of exploration wells in the early stages of development. In the last decade, concern has mounted about the This may allow developments to proceed, at least environmental impacts resulting from the development through a testing stage, afrer which rational and of energy resources and all new power generation informed decisions could be made as to whether, and proposals are now subject to intense environmental how, full-scale development could be done with scrutiny. Geothermal is generally regarded as a benign minimum impact on such features. energy source; particularly when compared to nuclear, and oil. However, experience shows that there can Questions about precursors that need answers are: be environmental problems associated with its 0 what are they? exploitation. These problems are becoming of 0 what is their magnitude? increasing concern, and to an extent which may be 0 how soon can they be identified? limiting developments. A recent example of this 0 do they occur everywhere in the field? concern has been the rejection of a Resource Consent 0 do they always occur? application for test drilling of the Reporoa Field. (At 0 in what order do they occur? time of writing, an appeal is being made to the Planning 0 how much testing is needed? Tribunal). A major reason for the rejection was that no 0 are any reversible? guarantee could be given there would not be any effects We also need to know what does not change; Le. what on natural features within the field, or on the features in are not good precursory indicators. neighbouring Waiotapu Field which are a major tourist attraction. Answers to all these questions are beyond the scope of this paper. Here we present historical observations History shows that hiding or ignoring such problems can made during discharge testing of the Wairakei and be counterproductive to development of an industry Broadlands (OW) fields to show what changes because it may lead to a loss of confidence in that occmed, and their magnitude. We hope, at a later industry by the public, regulatory, and financial sectors. date, to address the other questions about precursors. If our aim is to further the use of , then possible environmental effects should be clearly Surprisingly, there has been little previous identified, and countexmeasures devised and adopted to documentation of the changes which have occurred in avoid or minimise their impact. Techniques have these fields. The only significant published papers have already been developed to avoid or minimize most of been Glover (1977), who describes mainly the chemical the impacts associated with drilling operations and gas changes at Wairakei, and Thompson (1957) who records discharges during production (Brown, 1995). physical changes (up to 1955). The flow rate, chemistry and temperature data presented here have However, in the public perception, the effects on been taken from the fXes of the Institute of Geological natural physical features, such as geysers and hot and Nuclear Sciences (inherited from the foxmer DSIR) springs, are of major concern. One step in addressing and of Downer Energy Services Ltd (formerly Ministry this concern is to establish whether such effects have of Works). 70

3. PRESSURE CHANGES

0 Prior to development of the fields, the reservoirs were Broa dla nds liquiddominated with fluid generally at or near boiling point for depth. At Wairakei, a 2-phase zone existed in the upper part of the reservoir (Grant and Home, 1980). Over-lying each reservoir is a zone of cold ground- water, locally heated by fluids escaping upwards to supply natural thermal features at the surface.

W At Wairakei, the withdrawal during the Test Discharge I]L -20 Period resulted in deep-liquid pressures decreasing by 3 v) about 3 bar (0.3 MPa). However, this value must be v) W treated with caution because some of the data were not I]Le obtained by direct down-hole measurements but calculated from well head pressures in wells standing -30 11111111 50 55 60 65 70 75 80 85 90 95 shut and full of water (Grant and Home, 1980). During YEAR the early stages of production (1960s), large pressure decreases extended across most of the field leading to the expansion (both vertical and horizontal) of the 2- Fig. 1: Changes in deep liquid pressures at Wairakei phase zone, followed by the foxmation of a vapour- and Broadlands fields. Data from ECNZ (1990), Hunt dominated region in the upper part of this zone. By the (1995), and Clotworthy et aZ., (1995). mid-1970s deep-liquid pressures had settled at about 25 bar (2.5 MPa) below pre-production values (Fig. 1). 2. FIELD DEVELOPMENT FIXSTORIES At Broadlands, deep-liquid pressures decreased by about At Wairakei, exploratory drilling began in 1950 and 69 15 bar during the Test Discharge Period, but recovered prospecting holes had been drilled and test discharged by about 10 bar in the Recovery Period before by December 1958. During this "Test Discharge decreasing again after production began (Fig. 1). Period", mass withdrawal increased to about 20 Mt/yr. Considering the test periods at each field, the pressure The Wairakei (original installed capacity changes at Broadlands were much greater (15 bar) than 192 MWe) was progressively commissioned from at Wairakei (3 bar), despite the mass withdrawal rates November 1958 to October 1964, during which time the at Broadlands being smaller than at Wairakei (10, 20 annual mass withdrawal rose from about 20 to 75 Mt/yr, respectively). Mt/yr, after which it declined and has remained at about 45 Mt/yr since 1975. The time since November 1958 4. ENVIRONMENTALEFFECTS is referred to as the "Production Period". All the fluid withdrawnhas been discharged into the nearby No environmental effects were foreseen during the River or into the atmosphere (except for about 5 Mt planning and initial development of Wairakei Power reinjected during tests). Scheme. At that time (early 1950s) the environment was not such an imporfant consideration (relative to the At Broadlands, drilling began in 1965, and by 1971,25 demand for energy) and Environmental Impact Reports deep wells had been drilled. From the middle of 1967 were not required. Indeed, the technology for until the start of 1972, test discharges were conducted measuring many environmental parameters was still in during which time the annual mass withdrawal increased its infancy. Furthermore, the conceptual models to about 10 Mt/yr. During this "Test Discharge Period" prevalent at that time for geothermal systems, were that all the fluid withdrawn was discharged into the Whto the fluid obtained from the bores was of deep, magmatic River or the atmosphere; there was no reinjection. In origin, and not closely related to Sufface or near-surface the following 16 years, a further 18 holes were drilled geothermal fluids (Wilson, 1976). It was envisaged that but no extensive testing was done; the average mass any fluid withdrawn from the upper part of the reservoir discharge was only 1.5 Mt/yr and did not exceed 3.5 would be rapidly replaced by hot fluid from deeper in Mt/yr. This time is known as the "Recovery Period". the reservoir, and that the natural features would be Commissioning of the (1 16 MWe unaffected (Wilson, 1976). Most of the natural features installed capacity) began in August 1988 and was were in Geyser Valley, about 0.5 - 1 lan distant from completed in November 1989. Mass withdrawal rose to the production wells, so it was thought that mass 16.2 Mt in 1990, and has remained at similar values withdrawal from depths of less than 0.5 lan would not since then. Since commissioning, most of the waste affect the features. Despite reports of diminishing fluid has been reinjected (mainly around the periphery thermal activity and other environmental changes in the of the production areas) and net mass loss has been mid-l950s, it was not clear at that time whether the about 6 Mt/yr. changes were a result of the test discharges or merely 71

natural variations. However, while the Wairakei Power Prior to exploitation the Broadlands Field had few Station was being commissioned it became apparent that ~turalthermal features (cf. Wairakei); the largest and severe environmental effects were happening, but by most significant feature being the Ohaaki Pool, a boiling that time it was too late to modify the development pool with a surface area of about 850 m2. This pool has programme. cultural significance for the local Maori people, and is noted for its beautiful fretted sinter lip and surrounding Information obtained from Wairakei during the 1960s sinter apron. Prior to test drilling the pool generally and 1970s led to a better understanding of the overflowed. During the Test Discharge Period the flow relationship between Sufface features and the geothermal rate decreased until it ceased overflowing, and then the reservoir. It is now recognised that the thermal features water level fell. At the start of the Recovery Period the were fed by fluids escaping from the upper part of the water level in the pool rose, but it did not begin to reservoir, along faults or fissures. Withdrawal of fluid overflow again until 1981. Overflow stopped and the from the upper part of the reservoir, through production level in the pool again fell when production began in bores, and concomitant pressure decrease led to a 1988, and it is now kept full and overflowing by the decrease (with time) in the amount of this fluid feeding addition of bore water. the features (Glover, 1977; Allis, 1981). 6. PRECURSORY CHANGES During the planning stages of the Ohaaki Power Scheme, it was recognised that environmental effects Measurements made during the Test Discharge and might occur, and an Environmental Impact Report was early part of the Production Periods, at both fields, prepared. In this report (NZED, 1977) the effects of show that the main precursors of the decline in natural chemical and gas discharges, noise, and thermal thermal features were: pollution on the climate, ~turalwaterways, flora and fauna, were assessed and steps taken to mitigate the 6.1 Decrease in flow rate from spMgs and pools effects. The possible effects of exploitation on ~tural thermal features were not mentioned in the Impact Measurements show that there were large decreases in Report, despite a fall in water level in the Ohaaki Pool the flow rate from many springs and pools in Geyser during the Test Discharge Period and the changes which Valley at Wairakei during the Test Discharge Period. were known to have occurred at Wairakei. Examples are shown in Figure 2. At Waitangi Pool (SP55) in Nov. 1953 the outflow rate was about 1.2 Us, 5. DECLINE OF NATURAL THERMAL which decreased to about 0.2 l/s in late 1957. Another FEATURES example is Spring 29: in Nov. 1952 this discharged periodically, but in October 1953 the periodicity ceased, Prior to development, Wairakei was a major tourist and the rate of discharge steadily declined until April attraction noted for a wide variety of natural the& 1954 when the discharge ceased (Thompson, 1957). features which included geysers, fumaroles, hot springs, The water level then decreased until it was 1.5 m below hot pools, and sinter slopes (Herbert, 1921; Wilson, the edge, at which point measurements could no longer 1976). Most of these features were located in two be made (Fig. 2). These changes occurred as a result adjacent valleys: Geyser Valley (Wairakei Stream) and of pressure drop of less than 3 bar (0.3 MPa) in the Waiora Valley (Kiriohinekei Stream). Exploratory reservoir beneath the main production borefield. drilling began in the Waiora Valley, and it was here that most production wells were located; no wells have been Prior to exploitation the Ohaaki Pool generally drilled in the Geyser Valley. Names and numbers for overflowed with a flow rate of about 9 I/s (Lloyd, the features follow that of Thompson (1957). 1957). During the Test Discharge Period at Broadlands, the outflow rate from the Pool decreased when nearby Regular observations and measurements (flow rate, wells were discharged, recovered when the discharges chloride content and temperature) of selected thermal stopped, and decreased again when the bores were features did not begin until November 1952, after reopened (Fig. 3). This effect was also noticed during exploratory drilling and well discharges had begun. small discharge tests in 1976, during the Recovery Initially, the effects of mass withdrawal on the ~tural Period (Grant, 1982). features were small and isolated, and were thought at that time to be caused by natural climatic variations. 6.2 Decrease in chloride content of springs This was, in part, because the data showed that although some features changed during the testing period, others Prior to exploitation, fluids in the upper part of the did not show any change. For many people the rapid Wairakei reservoir had a chloride content of about 47.5 decline of the thermal features in Geyser Valley mM/1(1680 ppm, 265"C, enthalpy 1160 kJ/kg; Brown following the large increase in mass withdrawal after et al., 1988) which, after adiabatic steam loss, would commissioning came as a surprise. have a content of about 70.9 mM/1 (2506 ppm, 99OC) 72

n 111 3.0 2 1.0 I SP 18 W 1 SP 29

n u) 2 2.0 W I II -00.5 k E 1bo W 3 0 -1.0 cl ns W 0.0 -1.5 50 55 60 65 50 55 60 65 YEAR YEAR n u) 1.5 1.5 1 - 2- WAITANGI POOL SP 113 I - 8 (SP 55) 0,I n El u) .- 0.5 -I 2 1.0 - 3 - u) I Pool 0 \ 8 overflowing 0.0 ------...--..-....- n 5 a Woter level 0.5 - &-0.5 I

2 ns -1.0 - W I 0.0 -1.5 I I I 1

Fig. 2: Changes in flow rate and water level of hot springs and pools in Geyser Valley at Wairakei. Note the rapid changes with time during the Test Discharge Period.

n Ul 500 W5 lo 11957 OHMKI POOL 400

300

200

100

0 66 67 68 69 70 YEAR

Fig. 3: Changes in flow rate and water level (solid dots) of the Ohaaki Pool and output fkom nearby bores (line), at Broadlands. Note the decrease in flow rate when nearby bores were open and discharging, and the increase when these bores were shut. Scatter in the data is largely caused by atmospheric pressure variations. 73

W 8 I 5O 1 W I I SP 18 DRAGONS MOUTH 50140 I 40 GEYSER (SP 38) - I x I I I € I € I Production i I 30 - - +II W Production W CII I 0 ('L I E 20 - -378 20 I 9 =iI \ B 10- I Testing I I I I J n!VI I I I I VI I I I I 50 55 60 65 70 50 55 60 65 70 YEAR YEAR

50 1 50 I. WAlTANGl POOL 1=' GREAT WAIRAKEI II(SP c40 55) c40 GEYSER (SP 59) 1 I 4 I I I I Production -11 I Production' -m I W .I .?!, .?!, :I \ : ICIIQ.. I I I . 5 10 I 12010 I m I2*l Testing I =a 5 Testing I i I I 0 I I I 1 50 55 60 65 70 YEAR

I

40

-30 2o DANCING ROCK . 4 GEYSER (SP 190) 0 a d I .. I Testing Production Testing 1 Production I VI I I I I Oi 50 55 60 65 70 50 55 60 65 70 YEAR YEAR

Fig. 4: Changes in chloride content of hot springs in Geyser Valley at Wairakei. Note that some springs responded quickly to the test discharges, but for others the changes did not occur until after production began. 74 at the surface. Most fluids emerging fiom natural /' features at Wairakei had a chloride content of about 45 - I'

50 mM/1, indicating some dilution by warm (150°C) 1.' \\ 1.5 3- I\ - near-surface groundwaters containing about 8.5 mM/1 n X (300 ppm) chloride (Brown et al., 1988). As the deep- U liquid pressures declined, so too did the chloride content P of emerging waters, indicating increased dilution of the 2 E I' upflowing geothermal fluids by groundwater. e Many springs in Geyser Valley showed rapid decreases z 2- w in chloride content during the Test Discharge and early part of the Production Periods (Fig. 4). The largest sI (measured) decreases in the Test Discharge Period were 0 at Springs 18 and 38 (Dragon's Mouth Geyser), where the chloride content declined from about 50 mM/1 in 1951 to about 20 mM/l in 1957 (Fig. 4); Le. a decrease 1 1-1 of more than 50%. In general, the highest 50 51 52 53 54 55 (topographically) springs showed the earliest change. YEAR Springs which were at lower elevations, and had larger flow rates, had the smallest change during the Test Discharge Period; for example in Champagne Cauldron (Tuhuatahia; Spring 97) the chloride decreased by only F'ig. 5: Changes in eruption period (TlTo) of geysers about 20% during the Test Discharge Period (Fig. 4). in Geyser Valley at Wairakei during the Test Discharge However, during the early 1960s the chloride content Period. (a) Great Wairakei Geyser (data from Grant et decreased fiom about 40 mM/1 to about 15 mM/1 in 3 al., 1985); (b) Bridal Veil Geyser (data from Y-. Thompson, 1957). Periods are normalised to To = 12.5 hr for Great Wairakei, and 39 min. for Bridal At Broadlands, there was little change in chloride Veil. Broken line shows monthly running total of content of the Ohaaki Pool during the Test Discharge rainfall in previous 12 months. Period: prior to testing it was 29.4 - 32.0 mM/1 (av. 8 samples = 30.6 f 0.9), and during testing it was 29.1 - 29.7 mM/l (av. 7 samples = 29.4 f 0.3). Such data 6). However, the temperatures of some other features led Hochstein and Henrys (1 988) to the conclusion that in Geyser Valley showed little change: these included there was a large subsurface outflow from the pool. Rainbow Pool (SP197) and Ocean Geyser (SP198) (Fig. 6). These features maintained temperatures near 63 Increase in eruption period of geysers boig, while flow rates decreased significantly, because the upflowing geothermal fluids were diluted by Little quantitative data are available about the decline of warm (> 100OC) groundwater. the geysers at Wairakei. It is known that the eruption period (time between start of successive eruptions) of At Broadlands, there are fewer data. Prior to testing, two geysers increased during the Test Discharge Period, the temperature of the Ohaaki Pool varied by up to 25", befbre geysering ceased. The eruption period of Bridal but it is clear that during the initial part of the Test Veil Geyser (Spring 199) increased from about 38 min. Discharge Period (when the overflow rate decreased) in Nov. 1952, to about 55 min. in Dec. 1953, to about that temperatures fell to about 65"C, although they 65 min. in Dec. 1954 (Fig. 5). Another example is the subsequently recovered (Fig. 7). During the Recovery Great Wairakei Geyser (Spring 59): during the Test Period, temperatures in the pool fluctuated in response Discharge Period the eruption period increased fiom to small test discharges from nearby wells. about 12 to more than 30 ius, before the feature stopped geysering in 1954 (Fig. 5). Comparison of the eruption Data from both fields suggests that the absence of period data with rainfslll measurements (Fig. 5) shows temperature changes in a single hot spring or pool may that the increases in period were not caused by a not be a reliable indication that there will be no future decrease in rainfall. Similarly, the reductions in flow environmental impact. rate from springs could not have been caused by changes in rainfall. 7. CONCLUSIONS

6.4 Decrease in temperature of springs and pools Moderate to severe impacts on natural thermal features occurred at Wairakei and Broadlands fields, Some hot springs and pools at Wairakei showed caused primarily by exploitation-induced pressure temperature declines of up to 30°C during the Test decreases in the upper parts of the reservoirs. Discharge Period: these included SP18 and SP178 (Fig. 75

SP 18 SP 178 g 90- g 90- U U U - w 8 w 80- I w 80 - I a I .rn 2 z .I d 70- I 70 - w I I [5 I I 60 - t-w 6o 1 Testing I Produ’ction Testing I Production ‘ P 4 I I I 1 50 I I I 1 50 I I I i 50 55 60 65 70 50 55 60 65 70 YEAR YEAR

100 n - *. 8 .I 0 - I I g 90- I g 90 W I U 801 U I RAINBOW I POOL OCEAN GEYSER w 80- I a I (SP 197) I (SP 198) I 70: I ; I $ I B I w . I w I [5 I e I I I w 6o E 60 t-= 1 Testing Production P Testing I Production I I I I 50 I I I 1 50 55 60 65 70 YEAR YEAR Fig. 6: Changes in temperature of fluid fiom hot springs and pools in Geyser Valley at Wairakei during the Test Discharge Period and early part of the Production Period.

Pre-test

60 65 70 85 YEAR

Fig. 7: Changes in temperature (solid symbols) of Ohaaki Pool at Broadlands. Solid lines indicate total mass discharge rate from nearby bores. 76

The extent and severity of the impacts were Glover, R.B. 1977. Chemical and physical changes at dependent on local conditions. Geyser Valley, Wairakei and their relationship to changes in borefield performance. NzDSIR Bulletin Small, but now clearly identifiable, precursors 218 19-26. occurred during pre-production testing, but their significance was not recognised at that time. Glover, R.B., Hedenquist, J.W. 1989. A brief history of chemical exploration at OM-Broadlands. Proc. The most significant precursory signals of future 11th hZ Geothermal Workshop: 73-79. impacts were: Grant, M.A. 1982. The interaction of the Ohaaki decline in flow rate from natural features - in reservoir with surrounding fluid. DSIR Geotheml some cases the flow stopped within 2 years; Circular MAG 37, (unpub).

decline in chloride content of spring waters - Grant, M.A., Home, R.N. 1980. The initial state and decreases of up to 60% occurred during response to exploitation of the Wairakei Geothermal testing; Field. GRC Transactions 4: 333-336.

increase in the eruption period of geysers - in Grant, M.A., McGuiness, M.J., Dalziel, S.B., two cases, these doubled over a 2-yr interval; Razali, Y., O'Sullivan, M.J. 1985. A model of Rotorua Geothermal Field and springs. "Rotorua decline in temperature of emerging fluids - Geothermal Field, Technical Report of the some decreased by up to 20°C over a 5-yr Geothexmal Monitoring Programme 1982-1985". period. NZ Ministry of Energy. 522 p.

ACKNOWLEDGEMENTS Herbert, A.S. 1921. % hot springs of New ZeaW. H.K. Lewis & Co., London, 284 p. We thank Environment Waikato and Works Geothermal for providing data, and H M Bibby, G B Dawson, and Hochstein, M.P., Henrys, S. 1988. Assessment of total J T McLeod for reviewing the manuscript. Funding heat and mass transfer of a large hot pool using the was provided by the NZ Foundation for Research, balance method. Proc. Intemationul Symposium on Science and Technology (Geothermal Utilisation and Geothermal Energy (Kumamto and Beppu, Jap): Environment Programme). 97-101.

9. REFERENCES Hunt, T.M. 1995. Microgravity measurements at Wairakei Geothermal Field, New Zealand; a review Allis, R.G. 1981. Changes in heat flow associated with of 30 years data (1961-1991). Proc. World exploitation of the Wairakei Geothermal Field, New Geothermal Congress 1995: 863-868. zealand. Nz J Geology & G~OP~YS~CS,24 1-19. Lloyd, E.F. 1957. Ohaki Hot Spring. DSLR Brown, K.L. 1995. Environmental aspects of Geothermal Circular EE2, Unpub. geothermal development. WGC 1995 Pre-Congress Course Handbook. International Geothermal NZED, 1977. Environmental Impact Report for the Association. 145 p. Broadlands Development. NZ Electricity Department, 172 p. Brown, K.L., Henley, R.W., Glover, R.B., Mroczek, E.K., Plum, H. 1988. Dilution and boiling in the Thompson, G.E.K. 1957. Some physical measurements Wairakei field due to exploitation 1959-1982. Proc. inthe Wairakei-Taupo areas. NZDSLR Bulletin 123: 10th hZ Geothermal Workshop: 263-268. 81-96.

Clotworthy, A., Lovelock, B., Carey, B. 1995. Wilson, S.H. 1976. Possible conflict between the Operational history of the OWGeothermal Field, interests of tourism and geothermal power New Zealand. Proc. World Geothermal Congress development. Proc. 2nd UN Symposium on 1995: 1797-1802. Development and Use of Geothermal Resources: 2457-2466. ECNZ 1990. Water right applications and impact assessment - Wairakei Geothermal Power Station, Electricorp Production. 230 p.