| Summary: | Kawerau is located in the Bay of Plenty on the north-east coast of the North Island, New Zealand. Kawerau is an active geothermal field where fluids have been extracted for energy use since the 1950’s when a pulp and paper mill was constructed due to the close proximity to forestry areas and the geothermal energy source. Kawerau has seen significant development in the last 10 years with the commissioning of a 100 MW geothermal power station by Mighty River Power in 2005. Kawerau is located on the south-western edge of the Rangitaiki Plains; these plains have been modified considerably over the last 125 years since the 1886 Tarawera eruption by both natural and anthropogenic mechanisms. Processes at work in the Kawerau area include active volcanism, rifting, fluvial processes, shallow and deep water extraction, anthropogenic river modification and diversion, and construction of buildings and factories.
Subsidence is an issue in geothermal and oil fields worldwide and Kawerau is no different. This research aims to determine the origin of localised subsidence features identified by levelling surveys within the Kawerau Geothermal Field. Ground subsidence surrounding the pulp and paper mill, geothermal power station and residential properties in Kawerau has been monitored with levelling surveys since the 1970’s. The potential effects of continued subsidence and tilt within this area could negatively affect the operation of the industry in the area, particularly the pulp and paper mill due to the sensitivity of the paper rollers to tilt. Subsidence in Kawerau occurs on two scales: the first is a large, field-wide subsidence feature, the second is a series of smaller, localised subsidence features which this thesis focuses on.
First, identifying the location and characterising the properties of historic river channels, as well as their response to human demand, such as land and water use has been the primary approach in determining the origin of subsidence features. This helped build a picture of how the area appeared 125 years ago and add to our understanding of the history and landscape of the Rangitaiki Plains.
Second, to determine the cause(s) and mechanism(s) of the subsidence in Kawerau, field and laboratory investigations were undertaken. Site investigations included geomorphological mapping, ground penetrating radar (GPR), electrical imaging, hand augering and face logging. Laboratory investigations included permeability testing, determination of Atterberg Limits, dispersion testing, grain size distributions, microscopy and allophane detection testing.
Aerial photograph and LiDAR interpretation as well as a literature review has shown the approximate location of where the Tarawera River used to flow before it was diverted to aid in the draining of the Rangitaiki Plains. In the approximate location of the old Tarawera River, the geophysical survey identified an extension of the Onepu fault. This fault may have influenced the original location of the Tarawera River by creating low points in the topography as the result of seismic events. The Tarawera River path was diverted to its current path in the early 1900s following the large outbreak flood from Lake Tarawera.
Basin wide subsidence at Kawerau has been attributed to geothermal fluid extraction and the resulting contraction and/or cooling of the reservoir. This has caused low rates of subsidence across the whole field. This subsidence is unlikely to cause any damage to surface features due to its low rate and low angles of tilt. Basin wide subsidence is not the focus of this thesis so is not covered in detail.
This thesis focuses on two main sites of subsidence. Site 1 lies between the mill site and the Mighty River Power geothermal power station. Site 2 lies in farm land to the north of the mill and the old air strip. The mechanisms controlling subsidence at these sites is believed to be acting independently of each other. Primary mechanisms of subsidence at Site 1 include indirect seismic activity, direct disturbance by construction, vibration, apparent subsidence, the influence of drainage through the site, and wetting and drying sequences associated with rainfall and the soak ponds immediately adjacent to Site 1. Subsidence at Site 2 is likely to be caused by direct seismic activity, indirect seismic activity, consolidation of sediment due to changes in the groundwater table, and the influence of perched water tables.
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