A review of the Rotokawa Geothermal Field, New Zealand

McNamara, David D.; Sewell, Steven; Buscarlet, Etienne; Wallis, Irene C.

doi:10.1016/j.geothermics.2015.07.007

Cited by 54 publications

(67 citation statements)

References 34 publications

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“…This system is 14 km NE of Taupo and hosts >13 production wells as part of a large industrial complex, producing 24 MWe at the Rotokawa I power station and 140 MWe at the Nga Awa Purua power station. The 1.9-Ma Rotokawa andesite has a primary composition of medium-to low-K, calc-alkaline andesitic lavas; however, these lavas have undergone extensive propylitic hydrothermal alteration in a high-enthalpy hydrothermal system following their burial 0.34-0.30 Ma ago (Chambefort et al, 2014;McNamara et al, 2016). Prealteration mineralogy was predominantly composed of plagioclase and pyroxene phenocrysts in a microlitic groundmass consisting of plagioclase and titanomagnetite.…”

Section: Geological Context Of the Rotokawa Hydrothermal Systemmentioning

confidence: 99%

Increasing the Permeability of Hydrothermally Altered Andesite by Transitory Heating

Mordensky

Kennedy

Villeneuve

et al. 2019

Geochem Geophys Geosyst

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Changes in permeability can impact geological processes, geohazards, and geothermal energy production. In hydrothermal systems, high‐temperature heat sources drive fluid convection through the pore network of reservoir rocks. Additionally, thermal fluctuations may induce microfracturing and affect the mineralogical stability of the reservoir rock, thus modifying the fluid pathways and affecting permeability and strength. This study describes the results of thermal heating events lasting several hours on a “moderately altered” plagioclase‐clinochlore‐calcite‐quartz andesite and a “highly altered” plagioclase‐clinozoisite‐quartz‐clinochlore andesite from the Rotokawa Geothermal Field, New Zealand. We use a low thermal gradient (~1.2 °C/min) in an H2O‐saturated, 20‐MPa pressure environment to constrain changes in petrophysical properties associated with transitory thermal phenomena between 350 and 739 °C. As the treatment temperature increases, the mass reduces, while porosity and permeability increase. These effects were greater in the “moderately altered” andesite than in the “highly altered” andesite. Microfracturing is responsible for these changes at lower temperatures (e.g., ≤400 °C). At higher temperatures (e.g., >400 °C), microfracturing remains partially responsible for these rock property changes (e.g., higher permeability); however, these changes are also a product of clinochlore, quartz, and (when present) calcite reacting out of the altered andesite, and increasing porosity. We propose that at temperatures >400 °C, volumetric phase changes associated with heat‐driven reactions in a wet environment can contribute to microcracking and porosity/permeability changes. Our data support observations where high‐temperature conditions at the margins of magma bodies can be associated with substantial increased permeability and decreased strength.

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Section: Geological Context Of the Rotokawa Hydrothermal Systemmentioning

confidence: 99%

Increasing the Permeability of Hydrothermally Altered Andesite by Transitory Heating

Mordensky

Kennedy

Villeneuve

et al. 2019

Geochem Geophys Geosyst

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“…The BHTV logs were acquired with an ABI‐85, a high‐temperature logging tool able to operate at up to 300°C [ Ásmundsson et al , ]. The boreholes intersect several lithologies; here we focus on the andesitic formations which form the main part of the deep reservoir [ McNamara et al , ]. Fractures are dominantly steeply dipping (>60° dip) and striking parallel to the azimuth of maximal horizontal compressive stress (S Hmax ) which varies between NNE‐SSW and ENE‐WSW (Figure a), and reflects the regional normal faulting in the Taupo Rift [ Seebeck et al , ; McNamara et al , , ].…”

Section: Datamentioning

confidence: 99%

Statistical methods of fracture characterization using acoustic borehole televiewer log interpretation

et al. 2017

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Acoustic borehole televiewer (BHTV) logs provide measurements of fracture attributes (orientations, thickness, and spacing) at depth. Orientation, censoring, and truncation sampling biases similar to those described for one‐dimensional outcrop scanlines, and other logging or drilling artifacts specific to BHTV logs, can affect the interpretation of fracture attributes from BHTV logs. K‐means, fuzzy K‐means, and agglomerative clustering methods provide transparent means of separating fracture groups on the basis of their orientation. Fracture spacing is calculated for each of these fracture sets. Maximum likelihood estimation using truncated distributions permits the fitting of several probability distributions to the fracture attribute data sets within truncation limits, which can then be extrapolated over the entire range where they naturally occur. Akaike Information Criterion (AIC) and Schwartz Bayesian Criterion (SBC) statistical information criteria rank the distributions by how well they fit the data. We demonstrate these attribute analysis methods with a data set derived from three BHTV logs acquired from the high‐temperature Rotokawa geothermal field, New Zealand. Varying BHTV log quality reduces the number of input data points, but careful selection of the quality levels where fractures are deemed fully sampled increases the reliability of the analysis. Spacing data analysis comprising up to 300 data points and spanning three orders of magnitude can be approximated similarly well (similar AIC rankings) with several distributions. Several clustering configurations and probability distributions can often characterize the data at similar levels of statistical criteria. Thus, several scenarios should be considered when using BHTV log data to constrain numerical fracture models.

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“…Cores in the Rotokawa Geothermal Field display a variety in their fracture attributes (density, orientation, and infilling minerals), volcanic lithologies, and hydrothermal alteration (Figure ). Volcanic rocks sampled by the cores have been subject to pervasive alteration consisting of a propylitic‐style hydrothermal assemblage (quartz, wairakite, epidote, clinozoisite, illite, chlorite, adularia, albite, and calcite), with an intensity varying from weak to strong [ McNamara et al , , and references therein]. Average clast size of breccias (Figure c) is 26 ± 22 mm (one standard deviation).…”

Section: Datamentioning

confidence: 99%

“…To date, little work has been conducted on the fracture geometries and densities in these systems, and on the factors that control the fracture system architecture. The Rotokawa reservoir, TVZ, is a high‐temperature faulted reservoir hosted in andesitic lavas and breccias [ McNamara et al , , and references therein], where fractures are thought to be the major control on the flow of hot fluids [ McNamara et al , ]. The available BHTV logs, as well as cores and thin sections collected during the past 50 years, provide a rare opportunity to evaluate the fracture system organization in a hydrothermally altered, 1 km thick sequence of lava flows.…”

Section: Introductionmentioning

confidence: 99%

Evidence for tectonic, lithologic, and thermal controls on fracture system geometries in an andesitic high‐temperature geothermal field

et al. 2017

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Analysis of fracture orientation, spacing, and thickness from acoustic borehole televiewer (BHTV) logs and cores in the andesite‐hosted Rotokawa geothermal reservoir (New Zealand) highlights potential controls on the geometry of the fracture system. Cluster analysis of fracture orientations indicates four fracture sets. Probability distributions of fracture spacing and thickness measured on BHTV logs are estimated for each fracture set, using maximum likelihood estimations applied to truncated size distributions to account for sampling bias. Fracture spacing is dominantly lognormal, though two subordinate fracture sets have a power law spacing. This difference in spacing distributions may reflect the influence of the andesitic sequence stratification (lognormal) and tectonic faults (power law). Fracture thicknesses of 9–30 mm observed in BHTV logs, and 1–3 mm in cores, are interpreted to follow a power law. Fractures in thin sections (∼5 μm thick) do not fit this power law distribution, which, together with their orientation, reflect a change of controls on fracture thickness from uniform (such as thermal) controls at thin section scale to anisotropic (tectonic) at core and BHTV scales of observation. However, the ∼5% volumetric percentage of fractures within the rock at all three scales suggests a self‐similar behavior in 3‐D. Power law thickness distributions potentially associated with power law fluid flow rates, and increased connectivity where fracture sets intersect, may cause the large permeability variations that occur at hundred meter scales in the reservoir. The described fracture geometries can be incorporated into fracture and flow models to explore the roles of fracture connectivity, stress, and mineral precipitation/dissolution on permeability in such andesite‐hosted geothermal systems.

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A review of the Rotokawa Geothermal Field, New Zealand

Cited by 54 publications

References 34 publications

Increasing the Permeability of Hydrothermally Altered Andesite by Transitory Heating

Increasing the Permeability of Hydrothermally Altered Andesite by Transitory Heating

Statistical methods of fracture characterization using acoustic borehole televiewer log interpretation

Evidence for tectonic, lithologic, and thermal controls on fracture system geometries in an andesitic high‐temperature geothermal field

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