Subsidence of cover sequences at Kawerau Geothermal Field, Taupo Volcanic Zone, New Zealand A thesis submitted in partial fulfilment of the requirements for the degree of Master of Science in Hazard and Disaster Management at the University of Canterbury by Scott David Kelly Department of Geological Sciences, University of Canterbury, Christchurch, New Zealand 2015 i Table of Contents Table of Contents ...................................................................................................................... i List of Figures .......................................................................................................................... iv List of Tables .......................................................................................................................... vii Abstract .................................................................................................................................... ix Acknowledgements ................................................................................................................. xi 1 Introduction ...................................................................................................................... 1 1.1 Regional Geological Setting........................................................................................ 2 1.2 Kawerau Geothermal Field ......................................................................................... 5 1.2.1 Wells and KGL Power Station ............................................................................. 6 1.2.2 Geology ................................................................................................................ 9 1.2.3 Permeability ....................................................................................................... 15 1.2.4 Surface Geology................................................................................................. 15 1.2.5 Surface Thermal Features .................................................................................. 16 1.3 Scope ......................................................................................................................... 17 1.4 Methodology ............................................................................................................. 17 1.5 Thesis Structure ......................................................................................................... 19 2 A Review of Production Induced Subsidence .............................................................. 21 2.1 Wilmington Oil Field, California, United States....................................................... 21 2.2 Cerro Prieto Geothermal Field, Mexico .................................................................... 22 2.3 Reykjanes Peninsula, Iceland .................................................................................... 23 2.4 New Zealand Geothermal Fields ............................................................................... 24 2.4.1 Ohaaki Geothermal Field ................................................................................... 25 2.4.2 Wairakei-Tauhara Geothermal System .............................................................. 25 2.5 Conclusions ............................................................................................................... 28 3 Spatial Analysis of Subsidence using Benchmarks ..................................................... 29 3.1 Regional Subsidence ................................................................................................. 31 3.2 Field Wide Subsidence .............................................................................................. 32 ii 3.3 Localised Subsidence Features.................................................................................. 33 3.4 Rates of Subsidence .................................................................................................. 35 3.5 Mill Site Subsidence.................................................................................................. 39 3.5.1 Norske Skog Tasman Paper Mill ....................................................................... 41 3.5.2 Carter Holt Harvey Structures ........................................................................... 41 3.6 Subsidence Compared with Monitoring Data ........................................................... 44 3.7 Conclusions ............................................................................................................... 50 4 Modelling of Cover Sequences ...................................................................................... 51 4.1 Workflow Methods ................................................................................................... 52 4.2 Cover Sequences Model ............................................................................................ 54 4.2.1 Geological Interpretation ................................................................................... 60 4.2.2 Proposed Mechanisms of Subsidence Anomalies.............................................. 61 4.2.3 Brecciation ......................................................................................................... 62 4.3 Geotechnical Investigation ........................................................................................ 66 4.4 Conclusions ............................................................................................................... 67 5 Properties of the Shallow Cover Sequences................................................................. 69 5.1 Methods ..................................................................................................................... 69 5.1.1 Sample Description ............................................................................................ 69 5.1.2 Physical Properties ............................................................................................. 71 5.1.3 Thin Section Analysis ........................................................................................ 72 5.1.4 X-Ray Diffraction (XRD) and Laser sizing ....................................................... 72 5.1.5 Compressibility .................................................................................................. 73 5.2 Results ....................................................................................................................... 76 5.1.1 Physical Properties ............................................................................................. 76 5.2.1 Thin Section Analysis ........................................................................................ 78 5.2.2 X-Ray Diffraction (XRD) .................................................................................. 81 5.2.3 Laser Sizing ....................................................................................................... 83 5.2.4 Compressibility .................................................................................................. 84 5.3 Conclusions ............................................................................................................... 90 iii 6 Hazard Analysis of Kawerau Geothermal Field ......................................................... 93 6.1 Introduction ............................................................................................................... 93 6.2 Earthquake ................................................................................................................. 93 6.2.1 1987 Edgecumbe Earthquake ............................................................................ 94 6.3 Volcanic .................................................................................................................... 95 6.4 Flooding .................................................................................................................. 100 6.5 Tsunami ................................................................................................................... 102 6.6 Subsidence ............................................................................................................... 103 6.7 Probabilistic Hazard Analysis ................................................................................. 106 6.8 Conclusions ............................................................................................................. 106 7 Discussion and Management of subsidence at Kawerau .......................................... 109 7.1 Interpretation and Implications of Key Results....................................................... 109 7.1.1 Spatial Analysis of Subsidence ........................................................................ 109 7.1.2 Modelling of Cover Sequences ........................................................................ 111 7.1.3 Properties of Cover Sequences ........................................................................ 113 7.2 Management of Subsidence Bowls B and D ........................................................... 114 7.3 Management of Subsidence across Tasman Mill Site ............................................. 120 7.4 Mechanisms of Field Wide Subsidence .................................................................. 123 7.5 Future Subsidence ................................................................................................... 124 7.6 Considerations for Future Development and Exploitation of Kawerau