Cooling and aging of silica saturated geothermal water as a means of minimizing and controlling silica scaling prior to disposal by injection has been trialled and utilised at many geothermal fields. These show that although scaling may be reduced, it is typically insufficient to be an effective means of control with the risk of scaling in reinjection wells and prematurely reducing the permeability of the surrounding aquifer.We describe small scale field experiments undertaken at the Wairakei and Ohaaki Geothermal Fields (New Zealand) to measure the silica scaling potential of cooled separated geothermal water and to assess whether it could be safely disposed of by injection.The Wairakei experiments showed that rapid cooling followed by aging for several days successfully suppressed silica scaling. Based on this result Contact Energy Ltd undertook a large scale outfield injection trial of cold (30 degrees C) geothermal water where 270 t/hr of separated geothermal water containing total silica of similar to 730 mg/kg was injected for 10 months. The injectivity of the wells did not show any indications of reduced permeability due to deposition of silica either in the well bore or in the formation. This was the first major large scale injection of cold geothermal water undertaken in New Zealand. Subsequently, after the commissioning of the Te Mihi Power Station in 2013, the injection of geothermal water diluted (average 42%) by steam condensate has continued to date.In contrast, at Ohaaki, scaling observed in rapidly cooled separated water aged for 30 min at 80 degrees C (total silica 840 mg/kg) suggested that the injection of this fluid would result in impairment at or close to the well bore. Therefore, no subsequent field injection trial was undertaken at Ohaaki.

The Rotorua Geothermal Field (RGF) is a unique example of a geothermal system that has been managed intensively to both obtain energy in a sustainable manner and to preserve the surface features and their intrinsic value. The field underwent an extensive bore closure programme in the 1980s. Exploitation today is characterised by a reduced number of shallow bores (140 consented bores and an additional 42 with down hole heat exchanges) with limits set on use by a management plan designed and monitored by the Bay of Plenty Regional Council. The RGF has a wide range of uses, values and differing significance to the Rotorua community, including cultural values, economic benefits, energy source and a tourism driver.A collection of research and monitoring activities are presented in this paper. We summarise the current management regime, surface feature trends and results of chemical research, repeated heat flow surveys at Whakarewarewa and representative temperature-contour maps of the geothermal resource. These data and results show that the composition of the primary deep fluids have changed little overtime, while marked physical changes have occurred at surface features; a mix of positive recovery signs, along with many complex exceptions to those trends are seen. The use of modern numerical modelling methodology, using bore temperature records, geology and chemical data allow for improved modelling of the system. (c) 2015 Elsevier Ltd. All rights reserved.

From April 2010 to February 2011, CO2 flux surveys were performed on Lake Rotomahana, New Zealand. The area has been hydrothermally active with fumaroles and sublacustrine hydrothermal activity before and since the eruption of Mt Tarawera in 1886. The total CO2 emission from the lake calculated by sequential Gaussian simulation is 549 ± 72 t d−1. Two different mechanisms of degassing, diffusion through the water‐air interface and bubbling, are distinguished using a graphical statistical approach. The carbon dioxide budget calculated for the lake confirms that the main source of CO2 to the atmosphere is by diffusion covering 94.5% of the lake area (mean CO2 flux 25 g m−2 d−1) and to a lesser extent, bubbling (mean CO2 flux 1297 g m−2 d−1). Mapping of the CO2 flux over the entire lake, including over lake floor vents detected during the survey, correlates with eruption craters formed during the 1886 eruption. These surveys also follow regional tectonic patterns present in the southeastern sector of Lake Rotomahana suggesting a deep magmatic source (∼10 km) for CO2 and different pathways for the gas to escape to the surface. The values of δ13CCO2 (−2.88 and −2.39‰) confirm the magmatic origin of CO2.

Shallow warm water resources associated with low enthalpy geothermal systems are often difficult to explore using geophysical techniques, mainly because the warm water creates an insufficient physical change from the host rocks to be easily detectable. In addition, often the system also has a limited or narrow size. However, appropriate use of geophysical techniques can still help the exploration and further investigation of low enthalpy geothermal resources. We present case studies on the use of geophysical techniques for shallow warm water explorations over a variety of settings in New Zealand (mostly in the North Island) with variable degrees of success.A simple and direct method for the exploration of warm water systems is shallow temperature measurements. In some New Zealand examples, measurements of near surface temperatures helped to trace the extent of deeper thermal water.The gravity method was utilised as a structural technique for the exploration of some warm water systems in New Zealand. Our case studies show the technique can be useful in identifying basement depths and tracing fault systems associated with the occurrence of hot springs.Direct current (DC) ground resistivity measurements using a variety of electrode arrays have been the most common method for the exploration of low enthalpy geothermal resources in New Zealand. The technique can be used to detect the extent of shallow warm waters that are more electrically conductive than the surrounding cold groundwater. Ground resistivity investigations using the electromagnetic (EM) techniques of audio magnetotellurics(AMT or shallow MT), controlled source audio magnetotellurics (CSAMT) and transient electromagnetic (TEM) methods have also been used. Highly conductive clays of thermal or sedimentary origin often limit the penetration depth of the resistivity techniques and can create some interpretation difficulties. Interpretation of resistivity anomalies needs to be treated in a site specific manner.

After 40 years of investigations silica scaling is qualitatively well understood, but reliable predictions of scaling under typical power station operating conditions are still not possible. In this paper we review a few selected field silica scaling experiments undertaken primarily in New Zealand, which have involved measuring scaling rates. The aim is to provide insights into what can be learnt from such studies and to show that there is much more to understand about the scaling process. Aging of water to allow silica polymerization has been shown to reduce scaling potential. Icelandic researchers have demonstrated that rapid cooling and dilution are very effective at suppressing deposition. We describe work underway at Wairakei to determine the effectiveness of this technique for New Zealand waters.