Unmanned arial system imaging and refined geologic modelling of the Waiwera geothermal Reservoir

The utilization of geothermal reservoirs as alternative energy source is becoming increasingly important worldwide. Details of rock properties, structures, heat transfer and resulting interactions are the basis for the implementation of a sustainable reservoir management, but are often not well enough understood. The investigated warm water reservoir in Waiwera, New Zealand, has been known for many centuries. Triggered by overproduction in the third quarter of the 20th century, the reservoir pressure dropped significantly and in the 1970s the natural seeps on the beach dried up [1]. However, the shutdown of the main user's pumps (Waiwera Thermal Pools) in 2018 led to renewed temporary and location-specific artesian activity. The question now is whether the seeps on the beach will also reappear?

Hydrogeological models are the basis for a sustainable management of groundwater resources. The key point for the Waiwera reservoir is the amount of geothermal water which is permanently available. However, models are also used to describe the current hydraulic and thermal situation of the study area [2].

An expedition was carried out in 2019 to investigate the artesian activity of the reservoir, which has been observed again since 2018, and to build a new geological model. For the first time, thermal cameras carried by unmanned aerial systems (UAS) show the emergence of warm water at the beach and photogrammetric analyses carried out allow structural and lithological mapping on exposed cliffs where localized thermal anomalies were identified for the first time. The Waitemata formation found there is considered as analogue of the reservoir rock and thus serves for an improved understanding of the subsurface reservoir properties. The analyses show individual water and heat conducting lithologies and thus provide details about geological units that also constitute the geothermal reservoir at depth.

Based on the field exploration and the associated structural interpretations, a geological and thermal 3D model is now available for the first time, which will be employed to improve calibration of the hydraulic conditions of the warm water reservoir. Further, the model will be applied in the context of a sustainable reservoir management to clarify the question about the natural seeps on the beach. The reappearance of artesian activity in the Waiwera area due to significant adaptation of production rates is unique but the improved understanding of the interaction between rock properties, existing structures and heat transfer will also enable other reservoirs to be better understood.

[1] Kühn M., Stöfen H. (2005) A reactive flow model of the geothermal reservoir Waiwera, New Zealand. Hydrogeology Journal 13, 606-626

[2] Kühn M., Altmannsberger C., Hens C. (2016) Waiwera’s warm water reservoir – What is the significance of models? Grundwasser 21, 107-117