The Wairakei hydrothermal field under exploitation

Abstract

Heat flow through drillholes in the Wairakei hydrothermal field now equals the pre-existing natural heat escape, while this heat escape itself is little diminished. The total heat flow at surface has thus been doubled, and the source of this increase has an important bearing on the life of the field under exploitation.The drillholes tap the Waiora aquifer, in which the low density of the hot water gives rise to pseudo-artesian pressure, here designated thermo-artesian. There has been a general decline in this pressure, consequently cold water must be encroaching on the field, although drill hole temperatures show little evidence of this. Some of the increased heat flow must therefore come from heat stored in the rocks and ground water, and this is a wasting asset.The history of Wairakei's hot springs, and the very uniform physical and chemical conditions in the aquifer, indicate a long-established state of equilibrium. This is now being destroyed by exploitation, revealing at least one point, on the Waiora Fault, at which hot water is fed into the aquifer.. The feed channel is found to have so Iowan impedance that it may be possible to increase the rate of feed, by suitably placed holes, without excessive drawdown or exhaustion of storage. A limit to this increase will be set by the danger of blockage by calcite deposits; blockages can be cleared from the drillholes, but not from the feed channel.If the heat supply to the aquifer i,s thus increased, adjacent sections of th« Waiora Fault may then channel cold water into the field, and thus tend to curtail the utilization of stored heat.

Magnetic survey of the Wairakei hydrothermal field

“…It must be inferred, therefore, that the H 2 S content at these deeper levels is much lower than that in the aquifer. Of this, there is no direct evidence, but it is consistent with piezometric data, which show that at these levels cold recharge water flows in from the margins of the field (Studt, 1958).…”

“…The author has previously pointed out (Studt, 1958) that aqui fer pressures in the western sector may not be adequate for steam production. The importance of the magnetic map lies in the suggestion that deep~r holes might enter the ignimbrite in the neighbourhood of a feed channel, and that the hot water in this channel might be tapped at a significantly higher pressure than in the aquifer above.…”

The geochemistry of alkali metal ions in the Wairakei hydrothermal system

“…The present maps may be compared with the Wairakei isotherms at depths given by Banwell (1960) and the thermo-artesian pressure maps of Studt (1958).…”

“…The information now available from about 50 holes has enabled a consistent picture to be obtained of the temperature, pressure, and chemical distribution in the Wairakei hydrothermal system. Studt (1958) published the variations in artesian pressure with time and position in the hydrothermal field, while Banwell (1960) gave the comparable temperature distribution pattern.…”

“…Hole 37, drilled to 3,340 ft failed to strike the ignimbrite, which is downfaulted to the East (this hole is now partly filled in and has an open hole depth of 1,607 ft). In contrast many of the holes in the West reached the ignimbrite sheet at about 2,000 ft. A cross section of the field was given by Studt (1958).…”