Thermo-physical rock properties and the impact of advancing hydrothermal alteration — A case study from the Tauhara geothermal field, New Zealand

Mielke, Philipp; Nehler, Mathias; Bignall, Greg; Sass, Ingo

doi:10.1016/j.jvolgeores.2015.04.007

Cited by 49 publications

(37 citation statements)

References 32 publications

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“…Triaxial testing of igneous rocks produces cohesion ( c ) of up to 10,000 kPa, but the cohesion of highly altered rocks can be as low as 10 kPa (del Potro & Hürlimann, ; Reid et al, , ; Watters & Delahaut, ; Watters et al, ) (Figure b). Fractured but unaltered Mount St. Helens dacite yields internal friction angle ( φ ) values of 27° to 35° (Reid et al, ; Figure c), whereas other studies report internal friction angles of around 35° for unaltered volcanic rock (Apuani et al, ; Moon et al, ; Pola et al, ; Reid et al, ; Watters et al, ) and as low as 13° for altered rock (del Potro & Hürlimann, ; Mielke et al, ; Watters et al, ; Wyering et al, ). We simulate altered layers with a constant cohesion ( c ) of 200 kPa and internal friction angle ( φ ) of 20° (Figures b and c, Table ) to represent reduced mechanical strength due to argillic alteration processes.…”

Section: Modeling Methods and Input Parametersmentioning

confidence: 98%

“…Rocks that have undergone extensive argillic alteration tend to be less dense than unaltered rocks. Further, hydrothermal alteration tends to be concentrated in brecciated or pyroclastic deposits that have a relatively low primary (pre‐alteration) density (del Potro & Hürlimann, , ; Mielke et al, ). In the simulations, densities of 2,500 and 1,700 kg/m 3 were assigned to unaltered and altered rocks, respectively (Table ), consistent with recent studies of rock material properties (Mielke et al, ; Pola et al, ; Siratovich et al, ; Wyering et al, ).…”

Section: Modeling Methods and Input Parametersmentioning

confidence: 99%

“…Further, hydrothermal alteration tends to be concentrated in brecciated or pyroclastic deposits that have a relatively low primary (pre‐alteration) density (del Potro & Hürlimann, , ; Mielke et al, ). In the simulations, densities of 2,500 and 1,700 kg/m 3 were assigned to unaltered and altered rocks, respectively (Table ), consistent with recent studies of rock material properties (Mielke et al, ; Pola et al, ; Siratovich et al, ; Wyering et al, ). Saturated unit weights ( γ sat ), which were used in slope stability assessments, are defined by the following:

γ_{sat} = \frac{((), ((), ρ_{w} g) ((), \frac{ρ_{r}}{ρ_{w}})) + ((), ((), ρ_{w} g) ((), \frac{ϕ}{1 - ϕ})) S}{1 + ((), \frac{ϕ}{1 - ϕ})}

where ρ r is the density of the rock (either unaltered or altered) and S is the volumetric saturation ( S = 1 for saturated rocks).…”

Section: Modeling Methods and Input Parametersmentioning

confidence: 99%

“…Porosity and permeability of altered and unaltered volcanic rocks vary widely (Figure a), but altered rocks tend to have lower permeabilities (Mielke et al, ; Siratovich et al, ; Wyering et al, ). We use porosity and permeability values in our simulations similar to those invoked in several previous modeling studies of volcanic terrane (Aizawa et al, ; Hicks et al, ; Hurwitz et al, ; Ingebritsen et al, , ; Reid, ).…”

Section: Modeling Methods and Input Parametersmentioning

confidence: 99%

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Combining Multiphase Groundwater Flow and Slope Stability Models to Assess Stratovolcano Flank Collapse in the Cascade Range

et al. 2018

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Hydrothermal alteration can create low‐permeability zones, potentially resulting in elevated pore‐fluid pressures, within a volcanic edifice. Strength reduction by rock alteration and high pore‐fluid pressures have been suggested as a mechanism for edifice flank instability. Here we combine numerical models of multiphase heat transport and groundwater flow with a slope‐stability code that incorporates three‐dimensional distributions of strength and pore‐water pressure to address the following questions: (1) What permeability distributions and contrasts produce elevated pore‐fluid pressures in a stratovolcano? (2) What are the effects of these elevated pressures on flank stability? (3) Finally, what are the effects of magma intrusion on potential flank failure in an edifice? Simulation results show that under a range of plausible parameters, water tables in a stratovolcano can be elevated or perched. These elevated water tables result in universally lower stability (lower factor of safety) compared with equivalent dry edifices, indicating a higher likelihood of flank collapse. Low‐permeability (<1 × 10−17 m2) layers such as altered pyroclastic deposits or breccias can result in locally saturated regions (perched water) and lower factors of safety near the ground surface but may actually reduce liquid water saturation and pore pressures in the core of the edifice and thus may favor small, shallow collapses over larger, deeper collapses. Magma intrusion into the base of the edifice increases pore‐fluid pressures and decreases the factor of safety. However, the shear strength of edifice rocks also exerts a significant control on stability, so both mechanical properties and pore‐fluid pressures are important for stability assessments.

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Section: Modeling Methods and Input Parametersmentioning

confidence: 98%

Section: Modeling Methods and Input Parametersmentioning

confidence: 99%

γ_{sat} = \frac{((), ((), ρ_{w} g) ((), \frac{ρ_{r}}{ρ_{w}})) + ((), ((), ρ_{w} g) ((), \frac{ϕ}{1 - ϕ})) S}{1 + ((), \frac{ϕ}{1 - ϕ})}

where ρ r is the density of the rock (either unaltered or altered) and S is the volumetric saturation ( S = 1 for saturated rocks).…”

Section: Modeling Methods and Input Parametersmentioning

confidence: 99%

Section: Modeling Methods and Input Parametersmentioning

confidence: 99%

See 2 more Smart Citations

Combining Multiphase Groundwater Flow and Slope Stability Models to Assess Stratovolcano Flank Collapse in the Cascade Range

et al. 2018

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“…As reported from field studies and simulations, porosity and permeability changes due to precipitation and dissolution of minerals as silica, quartz, anhydrite, gypsum and calcite can occur when exploiting geothermal reservoirs (Libbey and Williams-Jones 2016;McNamara et al 2016;Mielke et al 2015;Mroczek et al 2000;Pape et al 2005;Sonnenthal et al 2005;Taron and Elsworth 2009;Wagner et al 2005;White and Mroczek 1998;Xu et al 2009). These fluid-rock interactions can alter and possibly clog flow paths, potentially affecting the performance of the geothermal plant significantly.…”

Section: Introductionmentioning

confidence: 95%

Effective Behavior Near Clogging in Upscaled Equations for Non-isothermal Reactive Porous Media Flow

Bringedal

Kumar

2017

Transp Porous Med

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For a non-isothermal reactive flow process, effective properties such as permeability and heat conductivity change as the underlying pore structure evolves. We investigate changes of the effective properties for a two-dimensional periodic porous medium as the grain geometry changes. We consider specific grain shapes and study the evolution by solving the cell problems numerically for an upscaled model derived in Bringedal et al (2016). In particular, we focus on the limit behavior near clogging. The effective heat conductivities are compared to common porosity-weighted volume averaging approximations and we find that geometric averages perform better than arithmetic and harmonic for isotropic media, while the optimal choice for anisotropic media depends on the degree and direction of the anisotropy. An approximate analytical expression is found to perform well for the isotropic effective heat conductivity. The permeability is compared to some commonly used approaches focusing on the limiting behavior near clogging, where a fitted power law is found to behave reasonably well. The resulting macroscale equations are tested on a case where the geochemical reactions cause pore clogging and a corresponding change in the flow and transport behavior at Darcy scale. As pores clog the flow paths shift away, while heat conduction increases in regions with lower porosity.

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Stratigraphic reconstruction of the Víti breccia at Krafla volcano (Iceland): insights into pre-eruptive conditions priming explosive eruptions in geothermal areas

Montanaro

Mortensen²,

Weisenberger

et al. 2021

Bull Volcanol

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Krafla central volcano in Iceland has experienced numerous basaltic fissure eruptions through its history, the most recent examples being the Mývatn (1724‒1729) and Krafla Fires (1975–1984). The Mývatn Fires opened with a steam-driven eruption that produced the Víti crater. A magmatic intrusion has been inferred as the trigger perturbing the geothermal field hosting Víti, but the cause(s) of the explosive response remain uncertain. Here, we present a detailed stratigraphic reconstruction of the breccia erupted from Víti crater, characterize the lithologies involved in the explosions, reconstruct the pre-eruptive setting, fingerprint the eruption trigger and source depth, and reveal the eruption mechanisms. Our results suggest that the Víti eruption can be classified as a magmatic-hydrothermal type and that it was a complex event with three eruption phases. The injection of rhyolite below a pre-existing convecting hydrothermal system likely triggered the Víti eruption. Heating and pressurization of shallow geothermal fluid initiated disruption of a scoria cone “cap” via an initial series of small explosions involving a pre-existing altered weak zone, with ejection of fragments from at least 60-m depth. This event was superseded by larger, broader, and dominantly shallow explosions (~ 200 m depth) driven by decompression of hydrothermal fluids within highly porous, poorly compacted tuffaceous hyaloclastite. This second phase was triggered when pressurized fluids broke through the scoria cone complex “cap”. At the same time, deep-rooted explosions (~ 1-km depth) began to feed the eruption with large inputs of fragmented rhyolitic juvenile and host rock from a deeper zone. Shallow explosions enlarging the crater dominated the final phase. Our results indicate that at Krafla, as in similar geological contexts, shallow and thin hyaloclastite sequences hosting hot geothermal fluids and capped by low-permeability lithologies (e.g. altered scoria cone complex and/or massive, thick lava flow sequence) are susceptible to explosive failure in the case of shallow magmatic intrusion(s).

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Thermo-physical rock properties and the impact of advancing hydrothermal alteration — A case study from the Tauhara geothermal field, New Zealand

Cited by 49 publications

References 32 publications

Combining Multiphase Groundwater Flow and Slope Stability Models to Assess Stratovolcano Flank Collapse in the Cascade Range

Combining Multiphase Groundwater Flow and Slope Stability Models to Assess Stratovolcano Flank Collapse in the Cascade Range

Effective Behavior Near Clogging in Upscaled Equations for Non-isothermal Reactive Porous Media Flow

Stratigraphic reconstruction of the Víti breccia at Krafla volcano (Iceland): insights into pre-eruptive conditions priming explosive eruptions in geothermal areas

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