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This institution reserves the right to refuse to accept a copying order if, in its judgment, fulfillment of the order would involve violation of copyright law. TRANSACTIONS,VOl. 9 PART II, August 1985 Geothermal Resources Council - SHALLOW REINJECTION INVESTIGATIONS AT KAWERAU, NEW ZEALAND 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 Edgecumbe 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 Tasman mill. 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.