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TRANSACTIONS,VOl. 9 PART II, August 1985 Geothermal Resources Council -

SHALLOW REINJECTION INVESTIGATIONS AT ,

Brendan R Berry and Barry N Denton

Ministry of Works and Development, Wairakei, New Zealand

ABSTRACT In 1952 the field, known then as the Onepu Springs, was chosen as the site for a pulp and Shallow reinjection into identified permeable paper mill by the Tasman Pulp and Paper Company. zones above the producing Kawerau geothermal An associated town, Kawerau, grew adjacent to field in New Zealand is being considered for the mill. the disposal of waste separated geothermal fluids. Investigations to date indicate that Since 1952 some thirty wells have been drilled this may be an economic and viable means of within the field which covers approximately reducing contamination of the Tarawera River. 12 haand encompasses the pulp and paper mill Further testing and an examination of alter- site. Four of these wells, two on each side of natives will be required before reinjection the Tarawera River, are currently used to supply can be adopted as a long term means of geo- 195 tonnes per hour of geothermal steam for thermal waste disposal. direct and indirect process use and electricity generation in the mill. Between 500 and 700 INTRODUCTION tonnes per hour of separated water, the by- product of the steam supply operation, is The Kawerau geothermal field is located disposed of via a series of open channels to towards the north eastern end of the Taupo the Tarawera River. Volcanic Zone in the North Island of New Zealand. (See figure 1). Some 22 km from The layout of the field is shown in figure 2. the ocean, the field straddles the Tarawera River. GEOLOGY

The geology of the Kawerau geothermal field consists of a series of volcanic and sedimentary beds including breccias, ash, mudstone, ignim- brites and andesites overlying a Mesozoic qrey- wacke basement which is faulted in a NE trending pattern with possible NW trending cross faulting.

A current model of the field has deep upflows originating at the base of the 80Om high volcanic centre, located some 4 km SSE of the field, and flowing in a north westerly direction across the field.

In the south and south easterly portion of the field high temperatures (3OO+OC) are encountered in the greywacke basement although little per- meability has been found in wells drilled in this area. Further north in the field basement rock temperatures are lower (260-280") and production is obtained from the overlying volcanics and sedimentary beds. A cross section of the field is shown in figure 3.

Figure 1 : Locality Map - North Island of New Zealand. Berry, et a1

\ I \ \ KAWERW - TOWNSHIP \

Figure 2 : General Layout of the Kawerau Geothermal Field.

PLANT

Geothermal fluid is taken via two phase pipe- lines from the production wellheads to flash (separati0n)plants where it is separated at approximately 8 bg.

The steam is reticulated through 2 steam mains to the Tasman Pulp and Paper Company mill where it is used directly, to operate log kickers, in shatter sprays and combustion air heating, and for timber drying, and indirectly to produce clean process steam in heat exchangers and to evaporate spent cooking liquor from kraft pulp. Steam is also passed through a 10 MW back pressure turbine to generate electricity - the turbine output being dependent on the amount of steam available (Wilson, 1974 ).

The steam is supplied to the mill on a contract basis at an indexed price. Disposal of the steam condensate and non-condensable gases in the steam is the responsibility of the .

The water obtained from the initial separation -Fim : Hypothetical structure in northern - at each flash plant is passed through a central Kawerau Geothermal Field. Stratigraphy silencer, where it is flashed to atmospheric simplified. Equal Horizontal and Vertical Scales pressure,and then discharged by open channel (from Nairn, 1982). to the Tarawera River.

288 Berry, et a1

WATER DISPOSAL would exceed that allowed under the water right.

The release of waste geothermal water to the ALTERNATIVES FOR WATER DISPOSAL Tarawera River is covered by a water right which limits the dissolved hydrogen sulphide discharged A number of alternatives exist which should allow to 1.5 grams/second and the heat discharged to the waste fluid to be disposed of in a manner 75.4 MW above OOC. In addition the water right that would satisfy water right requirements when requires that investigations be carried out into disposal quantities increase up to 1280 tonnes pbssible reinjection of this waste fluid and into per hour. The two broad categories of these possible treatment of the fluid in the Tasman alternatives are firstly, to treat the fluid aeration lagoons which treat effluent from the prior to discharging it to the Tarawera River pulp mill. and secondly to cease disposing of fluid to the river and reinject it into the geothermal The limited duration of the water right granted reservoir. and the conditions imposed including those above, reflect the desire to protect and improve the In the first category was the requirement of the water quality of the Tarawera River. The water right that investigations be carried out conditions imposed are designed to reduce the into the possibility of treating the geothermal Biochemical Oxygen Demand (B.O.D.) loading of wastes in the pulp mill aeration lagoons before the river which, at certain times of the year, discharge to the Tarawera River. This was shown reaches the current maximum tolerable level. during tests to be not advantageous and has not The B.O.D. loading is affected by the H,S been pursued although it may yet be reconsidered. concentration which consumes oxygen as an inorganic reducing agent and by the heat which In the same category is a system of hold up reduces the saturation concentration of dissolved tanks and chemical treatment to remove the heat, oxygen in the water. dissolved H,S and some of the toxic elements from the water. As yet untried at Kawerau, this The other facet of the discharge of geothermal treatment is unlikely to be economically viable. fluid is the effect of toxic materials such as heavy metals, horon, lithium and arsenic. In In the second category was the requirement of the addition to having a detrimental effect on water right that investigations be carried out aquatic life these constituents could prevent into the possibility of reinjecting waste fluids the use of the Tarawera River downstream of back into the ground. Kawerau for irrigation and frost control of sensitive crops such as kiwifruit. The problem in selecting any reinjection solution for waste disposal is to delineate the area and The type and concentration of chemicals dis- depth at which to reinject. Objectives of a charged to the Tarawera River from the flash successful reinjection operation are that it plants is as follows: must minimise possible detrimental effects on the resource such as temperature decline, hydro- Chemic a 1 concentration (ppm) thermal eruptions and pollution of the ground water whilst maximising possible beneficial Na 300-1000 effects such as reservoir pressure maintenance, K 50-170 reduced ground subsidence and improved heat B 17-60 extraction from the reservoir. Li 3.7-8.8 As 1.5-2.1 Deep reinjection has the advantage of maintaining H9 0.5-4.4 x deep reservoir pressures, however, the cost of a "3 1.2-2.7 deep reinjection system with expensive deep wells, H,S 6-17 high pressure pipelines, pumps and pumping energy s10, 360-1000 charges, could make geothermal steam a marginally c1 500-1400 economic energy source at Kawerau. The testing HCO, 80-190 required to find the combination of reinjection s04 19-36 area and depth to minimise detrimental effects and maximise benefits would be both extremely The minimum flow in the Tarawera River has been time consuming and expensive. taken as 20 cubic metres per second but may be less under severe drought conditions. The Shallow injection at Kawerau has the advantage current steam supply level of 195 tonnes per of being attractive from both an economic and hour results in 660 tonnes per hour of waste logistical point of view. A permeable shallow water being disposed of into the Tarawera River. aquifer is known to exist in the centre of the A planned increase to 250-270 tonnes per hour of field and the postulated hydrology suggests that steam in the immediate future would increase the shallow reinjection in this area would be bene- quantity of waste water for disposal to 900 ficial. Disadvantages of shallow reinjection tonnes per hour and future planned increases are increased risk of hydrothermal eruption, would increase the waste water for disposal up possible contamination of shallow ground,water to 1280 tonnes per hour. At this level of fluid and pollution of the Tarawera River, and disposal both dissolved H,S and heat discharged possible rapid return to adjacent deep production

289 Berry, et a1 wells. Measurements in the monitor wells and in the original wells (in 1984) show that temperatures SUBSIDENCE are still near boiling point for depth even after 30 years of field exploitation. Subsidence in the Kawerau Geothermal Field is very small compared to other New Zealand fields, If the shallow reinjection testing is successful, such as Wairakei and Broadlands. The subsidence wells M1, M3 and M4 would be excellent rein- comprises two superimposed components : a 10 km a jection wells because of their high injectivity. area due to compaction (approx 8 mm/yr) at pro- duction depths (500-9OOm) and a 1 km2 area of The testing programme consists of a series of local subsidence (approx 14 mm/yr) due to com- interference and radioactive tracer tests. The paction within 20Om of the ground surface first test was a tracer test in September/October (Allis, 1982). 1984. Continuous injection of hot water into well M1 from the KA21/27 flashplant weirbox was Reinjection whether shallow or deep is expected carried out from late August until the end of to reduce the rate of subsidence by maintaining October 1984 with a three curie dose of radio- reservoir pressures. A significant fall in active tracer (Iodine 131) being injected into reservoir pressures may cause an increase in M1 on 3 September. Over the period that sampling the rate of subsidence at Kawerau. for tracer returns was undertaken approximately 50,000 tonnes of water was injected into well M1. REINJECTION TESTING A combination of continuous monitoring with A number of the early wells drilled 300-6OOm deep scintillation detectors and chemical processing in the area known as the 'old Tasman Borefield' of daily fluid samples obtained from monitored failed after approximately 5 years of production wells and the Tarawera River, was used to check due to falling feedwater temperatures with time for tracer returns. No indications of returns and increasing dilution of produced fluids by were detected by either method in any of the groundwater, (Dench, 1962) (Sheppard, 1978). This locations monitored. (McCabe & Barry, 1984). evidence of 'cold' downflows is supported by the geology of the area which lacks any rock type This result was surprising because of the close which would form a full or partial cap to the proximity of M1 to the other M wells (250-500 reservoir. Alternatively this 'cold' downflow metres) and to wells with deeper permeable may be entering the deeper production levels in levels (40Om, 65Om depth). At Wairakei similar the area via the incompletely cemented annulii tracer tests have shown returns from over 1 km of the original production well casings. (Syms in less than 10 days. et al, 1982). The second test was a combined interference and Reinjection in shallow permeable levels in the radioactive tracer test which commenced in 'old Tasman Borefield' was considered unlikely February 1985. Continuous injection of hot to be more detrimental than the effect of these water into I(AM3 from the KA21/27 flashplant downflows and possibly even beneficial by weirbox commenced on the same day as a 5 curies reducing the amount of cold groundwater drawn dose of radioactive tracer (Iodine 131) was into the reservoir. injected into the well.

It was decided to evaluate shallow reinjection Monitoring for tracer returns was carried out by ahead of deep reinjection primarily because of chemical processing of fluid samples obtained the existing cool downflow within the 'old Tasman from selected wells and the Tarawera River. As Borefield.' Other reasons were the close prox- of 5.3.85 no returns had been detected in any of imity to the KA21/27 flashplant which is the the wells monitored. Pressure monitoring for largest in the field, the relatively dense layout the interference test was by either quartz of existing wells for monitoring purposes, the pressure gauge where the well being monitored demonstrated permeability (from original drilling had water to the wellhead or by float activated records) of the shallow (80-110m) depths, the variable voltage device on wells with water relative speed and economy of shallow reinjection levels below the wellhead. testing and the more cost effective solution it offered if successful. If this test in well M3 is successful (i.e. no tracer returns in other wells) a medium term Four monitor wells wcrc drilled into the shallow injection test of betweer. 6 and 12 months permeability to enable pressure changes during duration into well M3 is proposed using 15OOC interference testing, possible tracer returns wssurised water direct from a separator. During during tracer tests and temperatures to be this test pressure monitoring and checks for any monitored. All were completed with 6-5/8" temperature effects on surrounding wells will casing and 44" slotted liner in 54'' open hole. be carried out. Well data is shown in Table 1.

290 Berry, et a1

Well No. Drilled Depth Casing Shoe Permeable Temperature at Permeable Injectivity (m) Depth (m) Depth (m) Level ("C) 1 t/h/b I I M1 147 74 85 144 60 M2 139 61 130 118 25 M3 129 69 95 152 250 M4 110 76 85 144 60

Table 1 : Bore Data Summary for Kawerau Monitor Wells

CONCLUSIONS REFERENCES

Investigations to date have indicated that Allis R.G., 1982, Interpretation of the shallow reinjection of separated geothermal Subsidence at Kawerau, New Zealand water at modest pressures may be a viable and Department of Scientific and Industrial economic means of disposal for part or all of Research - Geothermal Circular RGA-5. the waste water at Kawerau. This will have the effect of reducihg or eliminating the contam- Dench N.D., 1962, Reconditioning of Steam Bores ination of the Tarawera River due to separated at Kawerau, N.Z. Engineering, 17(10) 353. geothermal water. Further testing is required before shallow reinjection can be adopted as a McCabe W.J., and Barry B., 1984, Kawerau Tracer means of long term disposal of waste fluids. Tests 1984, New Zealand Department of Scientific and Industrial Research - ACKNOWLEDGEMENT Geothermal Circular WJMcC-6.

The authors acknowledge the permission of the Nairn I.A., 1982, Geology of Kawerau Geothermal. Commissioner of Works and Development and the Field (Mk 11) : Results of Drilling 1977- Secretary for Energy to publish this paper. 1982, New Zealand Department of Scientific The Ministry of Works and Development and Industrial Research - Geothermal operate the Kawerau Geothermal Field on behalf Circular IAN-4. of the Oil & Gas Division, Ministry of Energy. Sheppard D.S., 1978, Chemistry of Kawerau Geothermal Waters, New Zealand Department of Scientific and Industrial Research - Chemistry Division - Open File Report.

Syms M.C., Syms P.H. and Bixley P.F., Interpretation of Flow Measurements in Geothermal Wells Without Caliper Data, The Log Analyst, March-April 1982, p34-45.

Wilson R.D., 1974, Use of Geothermal Energy at Tasman Pulp and Paper Company Ltd, New Zealand., Proceedings of Inter- national Conference on Geothermal Energy for Industrial, Agricultural and Commer- cial - Residential Uses; Klamath Falls.

291