Modelling of geysers

Abstract

Geysers that discharge water and steam intermittently to the atmosphere are one of the rarest natural phenomena associated with geothermal systems. Several approaches including laboratory experiments, field observations and mathematical and numerical modelling studies are used in the present study to explain the behaviour of geysers and the important parameters controlling the eruption of geysers. A particular study is made of three geysers at Rotorua geothermal field: Pohutu, Prince of Wales Feathers and Waikorohihi. The existing mathematical model (steinberg et al., 1981a) is studied and an improved mathematical model is developed to accommodate two-phase flow and the variation in fluid properties with temperature. Both the existing and the improved mathematical models are used to model Pohutu and are able to reproduce not only the interval between eruptions but also the durations of the cavern filling and the duration of the pre-play stage observed by the author on the 20th of August 1993. Fully transient numerical models, which include the eruption process itself, are developed using MULKOM and the AUTOUGH2 simulators and produce reasonably good agreement with the analytical solutions and experimental data. The model provides information about the processes inside the geyser system and models the surface discharge which cannot be modelled using the Steinberg type of model. A fully transient model for Pohutu, which is developed using the AUTOUGH2 simulator, is able to reproduce the behaviour observed by the author on the 20th of August 1993. The results of sensitivity studies show that of the three Rotorua geysers, the Feathers is the most sensitive to changes in the rate of the hot upflow from depths. Both the Feathers and Waikorohihi are more sensitive to temperature changes than Pohutu. Pohutu is currently a vigorous geyser with preliminary pulsating spring behaviour; large changes in the rate and temperature of the hot upflow would be required to stop it erupting. All geysers are sensitive to variations in the water level and temperature in Te Horu.