Numerical modelling of Pohutu geyser, Rotorua, New Zealand

Abstract

The Rotorua geothermal field (RGF) in the North Island of New Zealand is renowned for an abundance of natural geothermal manifestations and contains one of New Zealand's last remaining areas of major geyser activity at Whakarewarewa. Close proximity of the geothermal resource to a population centre and ease of access for end-users resulted in intensive drilling and fluid abstraction from shallow bores for domestic and commercial usage from the 1950s onwards. Increasing concern about the effect of geothermal fluid withdrawal on the activity of springs and geysers led to the establishment of the Rotorua Geothermal Monitoring Programme (RGMP) in 1982. By 1986, aquifer pressures declined to the lowest levels since the monitoring programme began, and to prevent further deterioration of spring and geyser activity, a Bore Closure Programme was enforced. By 1988, the programme contributed to a 75% decrease in net withdrawal and an immediate increase in reservoir pressures was observed. During the ensuing years recovery of some surface features has also been observed. Thus, although the geysers at Rotorua, New Zealand have shown some natural variation in their behaviour, they have also been significantly affected by human interference. The aim of this study is to quantify the past response of geyser activity to human-induced changes in the state of the Rotorua geothermal reservoir and to provide a tool for predicting the future behaviour of the geysers. The study is based on a simple computer model of a geyser that consists of a chamber linked to cold recharge and deeper hot recharge. The chamber also has an outlet to the surface through a narrow channel. The TOUGH2 simulator is used to carry out many numerical experiments to determine how parameters such as the size of chamber, cold recharge pressure, hot recharge pressure and permeability of the channel affect whether or not the model produces geysering behaviour and how they affect the period of the eruptions. The model successfully matches the observations that: (i) geysering ceases if hot recharge diminishes, (ii) the frequency of eruption increases if hot recharge increases and (iii) if hot recharge increases enough, then geysering turns into continual spouting.