THE FIRES OF NGATOROIRANGI

Te Ao Hou, May 1957, Page 37

THE FIRES OF NGATOROIRANGI by E. G. Schwimmer Suddenly the ordinary vegetation comes to an end; a smell of sulphur; the ground covered with a white crust. Nothing grows on it; it feels warm underfoot. A notice warns visitors not to proceed without a guide. The narrow path is occasionally crossed by a crack or interrupted by an irregular hole where the silica eaten ground has collapsed into an unplumbable cavity. Near the path, there are larger holes out of which steam is curling up continuously; from others fountains of hot water spurt forth into the air at regular intervals. Elsewhere, dark mud is slowly bubbling at enormous heat like simmering porridge. There are also lakes embedded in the brittle silicified ground, some of them lightly steaming, coloured pink or green or dark blue. Places such as these are among the most popular tourist haunts in New Zealand; there are seventeen of them along the 150 mile stretch of the Rotorua-Taupo volcanic zone. Romantic names have been invented such as Witches' Cauldron (for one of the mudpools) and Bridal Veil (for a geyser). One pond of delightfully warm water, overhung by large trees, is known as the Honeymoon pool. Charming though such names are, the visitor feels a primative awe at the immense forces roaring and bubbling at him derived from the steaming magma deep below. It may seem surprising that the Maoris who first landed in New Zealand some 600 years ago were not frightened away from these threatening places. On the contrary, they soon developed a linking for them and settled in their immediate neighbourhood. They tell of a priest Ngatoroi-

rangi, who came from Polynesia to New Zealand on one of the canoes. Ngatoroirangi, nearly dying with cold in the unfamiliar and less kindly climate, called on the goddess of fire to rescue him. She flew to him and on the places where she rested today's thermal phenomena are found. Ngatoro was revived by the fires brought by the goddess and ever since the Maori people have used the hot pools for cooking, washing and bathing. Many Maoris are working on the geothermal project at Wairakei, most of them as labourers, while some have been given important jobs on the drilling gangs. Two newcomers are Tarawa Whiu and Tom Te Karu, both from Reporoa. The European settlers, too, saw how helpful the steam could be to them. The remarkable fact was discovered that in many places appreciable amounts of steam or hot water could be tapped by drilling a shallow well. Particularly in Rotorua, hundreds of householders have sunk bores in their own back yards and laundries and so obtained a cheap hot water and central heating system. Now and then the earth has rebelled and a laundry or outhouse has been blown sky-high, but the people have not given up this way of cutting their fuel bills. Hundreds of other uses of the steam have been suggested or tried; from heating glasshouses and dehydrating eggs to recovering sulphur and making salt from sea water. The most vital potentiality of all, the generating of electric power, was not seriously considered until recently, because people used to think that the hydro-resources of New Zealand were virtually inexhaustible. The truth, not fully known until 1949, was different. While the South Island can depend on its rivers for electric power for many years to come, in the North Island the end of suitable locations for hydro-stations is in sight, with the demand for electricity continuing to rise as rapidly as ever. What previously had been the dream of idealists, now became an urgent demand; a group of scientists and engineers were sent to investigate the hidden stores of geothermal steam in the volcanic plateau, carry out a programme of drilling and report to the government whether an economic electric power supply could be obtained. When drilling started, the “cores” of rock cut out by the drilling bits were brought up to earth so that geologists could study them. From this study it gradually became known what went on underneath the earth and what was the history of the geothermal region. The top layer of the area is a pumice rock called pumice breccia which is from 1,000 to 4,000 feet thick. This pumice breccia is a highly porous layer. Ground water has penetrated all the voids and it is this ground water that is being heated by the steam deep below. At a temperature of perhaps 1000 deg. C, it rises through fissures to penetrate, and then makes contact with the ground water seeping down towards it. When this water surges up to the surface of the earth and the heavy pressure on it is removed, a portion of it is turned to vapour. For this reason steam and water both emerge from the well-heads. Although small flows of this hot water probably occur throughout the volcanic region, quantities suitable for economic development are only present in the thermal areas. Geologists have mapped a great number of fault lines in the region, invisible on the ground, but appearing on aerial photographs as straight lines like fences. Practically all the visible thermal activity is related to these active fault lines which are thought to carry the hot

This young Wairakei worker has found a practical use for geothermal steam: with an ingenious contrivance of copper wire, steam issuing from the bowels of the earth at enormous pressure is made to boil the billy, then gushes out into the air. (NPS PHOTOGRAPH) water and steam to the surface. Developing the steam resources has become a matter of striking them at depth. The location chosen for exploratory drilling was Wairakei, near Lake Taupo. Previously known only for its tourist hotel, attracting guests to the picturesque thermal phenomena, by 1952 Wairakei produced enough steam from shallow bores to provide adequate supply for a 20 megawatt power station. The exploratory step was over. The establishment was enlarged and placed in the charge of a special project engineer, Mr A. L. C. Fooks. Mr Fooks, like most of the other engineers at Wairakei, had gained his experience on the building of the hydro-electric dams, New Zealand's biggest engineering projects. He was supplied with two rotary rigs much bigger than the water-drilling equipment used previously. It was soon confirmed that deep bores (1500–2000 feet) were generally more productive. Now steam began to gush out in prodigious quantities. Wellheads continued roaring day and night, making so much noise that workers lost their sense of equilibrium. Silencers were designed, reducing the pressure of the steam. Drilling in the loose pumice soil presented its own peculiar difficulties. Blowouts and blockages occurred easily during the early investigations. To the expert, these little sticks tell the whole story of drilling at Wairakei. Each of the sticks represents a bore. The marks on the sticks show the temperature at every level and the type of rock that is found in the bores. From these sticks scientists can deduce at the glance what is going on thousands of feet beneath the surface of the earth. (PHOTO: PETER BLANC)

Most of the trouble is due to the really high temperatures found at shallow depths; considerable steam or hot water pockets are not unusual at 400 to 600 feet. Here the pressure of the steam easily exceeds the weight of the drilling mud in the bore, leading to a blowout. Experienced drillers know how to shield such cavities with a cement mixture hard enough to resist the pressure. Because of the risk, it is usual to consolidate the area surrounding the pit by pouring grouting under pressure into little holes all around it so that the whole earth up to 100 feet deep is filled and hardened with cement. A power house is now being built to produce 69 megawatt, utilizing the steam only. The government is considering putting in further turbines bringing production up to 200–250 megawatts. Not only the steam, but also the hot water from the bores will be utilized, for this water (which constitutes the larger part of the bore output) will produce a vast quantity of steam when pressure is reduced. An important part of geothermal development is the measurement of output Below: A typical drilling site showing the elaborate precautions that are made to prevent blowouts (prevention equipment, cementing of the earth surrounding the bore). PHOTO: PETER BLANC and the careful keeping of detailed records. Some bores decline in output over a period probably due to cementing of the feeding fault zone by silica and other minerals carried in the water. So far there is no sign of the supply of hot water being overdrawn. Nobody knows what will happen when the hot water stored in the rocks is taken out. Will the flow of hot magmatic steam increase or is the flow constant and less than the present rate of steam extraction? Even in the least favourable case, the known reserves of stored heat at Wairakei will last for many years. It is also likely that there are other thermal areas in the graben where large supplies can be tapped. For instance, the mills of the Tasman Pulp and Paper Company, built near a known fault line at Kawerau, will probably ultimately have a geothermal power-plant of at least 20 megawatts, while in the near future geothermal steam is to be used for kiln drying of timber. Wairakei drilling crews have also started shallow drilling for a second geothermal steam station at Waiotapu, 20 miles from Rotorua. Will this solve the North Island's power problems? It is too early to say just what the final capacity of the volcanic plateau will be and New Zealand is not slowing down its efforts to bring power to the North Island by more orthodox means. It seems however that the cost of geothermal power generation will compete with that from hydro-electric dams, which alone justifies this new effort of man to exploit some strange resources below the surface of the earth.