Subsidence of cover sequences at Kawerau Geothermal Field, Taupo Volcanic Zone, New Zealand

Abstract

Subsidence occurring at geothermal fields requires monitoring, analysis, and understanding of the mechanisms in order to ensure that it does not affect field operations. This study utilised a broad range of techniques including spatial analysis, three-dimensional modelling, and the comparison of samples of the cover sequences to investigate the subsidence at Kawerau Geothermal Field. Subsidence at Kawerau is of concern because the Tasman Pulp and Paper Mill is located within the geothermal field and utilises machinery with small alignment tolerances that are sensitive to ground deformation. A probabilistic hazard analysis of Kawerau was completed and maps created indicating the potential for subsidence in the future. Spatial analysis of benchmark re-levelling surveys revealed two types of subsidence features: 1) field wide subsidence and 2) subsidence anomalies. Field wide subsidence, currently covering ~17 km2, is driven by thermal contraction of reservoir deep formations and/or compaction of the reservoir due to effective stress increases related to pressure drawdown. Four local subsidence anomalies each covering 150 – 400 m2 are likely driven by varying shallow processes. Two of these features, termed Bowls B and D, south of the Kawerau Geothermal Ltd. power station are the main focus of this thesis along with an assessment of the Tasman Mill site, its potential to develop an anomaly, and the mechanism of subsidence currently occurring across it. Three-dimensional modelling of the cover sequences to 750 m below relative level was completed in Leapfrog Geo using well logs from Kawerau. Modelling revealed an anomalous thickness of Tahuna Formation below Bowls B and D, and relatively uniform thicknesses across the mill site of other shallow formations. The anomalous thickness of Tahuna Formation was hypothesised as being responsible for the presence of the subsidence bowls by being more compressible than the overlying Caxton Formation which is thicker across the mill site while the Tahuna Formation is thinner. Alternative hypotheses were explored by mapping the relative level of the Matahina ignimbrite, thickness of the Caxton Formation, and distribution of brecciation. To test the main hypothesis, samples of Tahuna and Caxton formations were collected from the Kawerau Core Shed and tested for their physical properties and relative compressibility. XRD and thin section analysis was also completed on the samples. Tahuna Formation was found to have more than three times the porosity of the Caxton Formation and have smectite clays present. Using a method developed for testing the relative compressibility of weak rock the Tahuna Formation was found to generally be twice as compressible and elastic as the Caxton Formation when saturated. Samples of Recent alluvium from the mill site were also tested for their physical properties and found not to have the potential to contribute to subsidence across the mill site. However further investigation is required to confirm the mechanisms of Bowls B and D. A hazard analysis of Kawerau Geothermal Field found that the field has a low annual probability of being impacted by volcanic and volcanogenic, earthquake, and flooding events. Probabilities are calculated based on the reoccurrence intervals for each event. A hazard map for subsidence at Kawerau is also developed and outlines four zones of risk. Infrastructure at risk based on trends of subsidence is also analysed for its susceptibility to subsidence and mitigation methods discussed. The overall conclusion is that the geological conditions beneath the mill site are unlikely to form a local subsidence anomaly, and the mill site is largely unaffected by the field wide subsidence bowl. Ground tilt values are within mill machinery tolerances, and based on current trends the spatial extent of subsidence anomalies will remain approximately the same into the future.