The development and application of the alteration strength index equation

Wyering, L.D.; Villeneuve, Marlène; Wallis, Irene C.; Siratovich, Paul; Kennedy, Ben; Gravley, Darren M.

doi:10.1016/j.enggeo.2015.10.003

Cited by 21 publications

(6 citation statements)

References 71 publications

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“…In our study, these fluctuations have been attributed to the variations in primary lithological textures and microstructure as well as secondary alteration, dissolution, and re-precipitation. Other studies of burial diagenesis in geothermal systems have observed similar changes in mechanical properties with depth (Tewhey 1977;Stimac et al 2004;Mielke 2009; Dillinger et al 2014;Wyering et al 2014Wyering et al , 2015. This shows that while the primary textures of the deposited lithologies must contribute to the effects of burial diagenesis by reducing porosity and increasing density, the hydrothermal alteration plays a much larger role in controlling the physical properties.…”

Section: Effect Of Increased Depth On Physical Propertiesmentioning

confidence: 55%

Matrix permeability of reservoir rocks, Ngatamariki geothermal field, Taupo Volcanic Zone, New Zealand

et al. 2018

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The Taupo Volcanic Zone (TVZ) hosts 23 geothermal fields, seven of which are currently utilised for power generation. Ngatamariki geothermal field (NGF) is one of the latest geothermal power generation developments in New Zealand (commissioned in 2013), located approximately 15 km north of Taupo. Samples of reservoir rocks were taken from the Tahorakuri Formation and Ngatamariki Intrusive Complex, from five wells at the NGF at depths ranging from 1354 to 3284 m. The samples were categorised according to whether their microstructure was pore or microfracture dominated. Image analysis of thin sections impregnated with an epoxy fluorescent dye was used to characterise and quantify the porosity structures and their physical properties were measured in the laboratory. Our results show that the physical properties of the samples correspond to the relative dominance of microfractures compared to pores. Microfracture-dominated samples have low connected porosity and permeability, and the permeability decreases sharply in response to increasing confining pressure. The pore-dominated samples have high connected porosity and permeability, and lower permeability decrease in response to increasing confining pressure. Samples with both microfractures and pores have a wide range of porosity and relatively high permeability that is moderately sensitive to confining pressure. A general trend of decreasing connected porosity and permeability associated with increasing dry bulk density and sonic velocity occurs with depth; however, variations in these parameters are more closely related to changes in lithology and processes such as dissolution and secondary veining and re-crystallisation. This study provides the first broad matrix permeability characterisation of rocks from depth at Ngatamariki, providing inputs for modelling of the geothermal system. We conclude that the complex response of permeability to confining pressure is in part due to the intricate dissolution, veining, and recrystallization textures of many of these rocks that lead to a wide variety of pore shapes and sizes. While the laboratory results are relevant only to similar rocks in the Taupo Volcanic Zone, the relationships they highlight are applicable to other geothermal fields, as well as rock mechanic applications to, for example, aspects of volcanology, landslide stabilisation, mining, and tunnelling at depth.

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Section: Effect Of Increased Depth On Physical Propertiesmentioning

confidence: 55%

Matrix permeability of reservoir rocks, Ngatamariki geothermal field, Taupo Volcanic Zone, New Zealand

et al. 2018

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“…The data presented in this study show that porosity increases as treatment temperature increases (Figure and Table ). Generally, an increase in porosity is associated with a decrease in the strength of hydrothermally altered rock (e.g., Wyering et al, , ; Mordensky et al, ). Weakening intact rock (i.e., rock at the sample scale) will affect the behavior of the intact rock around the wellbore, for example, contributing to changes in drilling performance (as shown in Wyering et al, ).…”

Section: Discussionmentioning

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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“…Thus, they experience elevated pressures, high temperatures, and corrosive fluids [26,[37][38][39][40]. Such extreme conditions may promote compaction [27,[41][42][43][44], precipitation of secondary mineral phases [26,[45][46][47], and variable degrees of alteration [43,48], modifying the mechanical properties and the permeable-porous network through which fluids circulate. Previous mechanical studies of porous rock compaction [41,49,50] have characterised rock strength by evaluating yield curves to identify the stress conditions where permanent inelastic deformation may occur.…”

Section: Introductionmentioning

confidence: 99%

Compaction of Hyaloclastite from the Active Geothermal System at Krafla Volcano, Iceland

et al. 2020

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Hyaloclastites commonly form high-quality reservoir rocks in volcanic geothermal provinces. Here, we investigated the effects of confinement due to burial following prolonged accumulation of eruptive products on the physical and mechanical evolution of surficial and subsurface (depths of 70 m, 556 m, and 732 m) hyaloclastites from Krafla volcano, Iceland. Upon loading in a hydrostatic cell, the porosity and permeability of the surficial hyaloclastite decreased linearly with mean effective stress, as pores and cracks closed due to elastic (recoverable) compaction up to 22-24 MPa (equivalent to ~1.3 km depth in the reservoir). Beyond this mean effective stress, denoted as P∗, we observed accelerated porosity and permeability reduction with increasing confinement, as the rock underwent permanent inelastic compaction. In comparison, the porosity and permeability of the subsurface core samples were less sensitive to mean effective stress, decreasing linearly with increasing confinement as the samples compacted elastically within the conditions tested (to 40 MPa). Although the surficial material underwent permanent, destructive compaction, it maintained higher porosity and permeability than the subsurface hyaloclastites throughout the experiments. We constrained the evolution of yield curves of the hyaloclastites, subjected to different effective mean stresses in a triaxial press. Surficial hyaloclastites underwent a brittle-ductile transition at an effective mean stress of ~10.5 MPa, and peak strength (differential stress) reached 13 MPa. When loaded to effective mean stresses of 33 and 40 MPa, the rocks compacted, producing new yield curves with a brittle-ductile transition at ~12.5 and ~19 MPa, respectively, but showed limited strength increase. In comparison, the subsurface samples were found to be much stronger, displaying higher strengths and brittle-ductile transitions at higher effective mean stresses (i.e., 37.5 MPa for 70 m sample, >75 MPa for 556 m, and 68.5 MPa for 732 m) that correspond to their lower porosities and permeabilities. Thus, we conclude that compaction upon burial alone is insufficient to explain the physical and mechanical properties of the subsurface hyaloclastites present in the reservoir at Krafla volcano. Mineralogical alteration, quantified using SEM-EDS, is invoked to explain the further reduction of porosity and increase in strength of the hyaloclastite in the active geothermal system at Krafla.

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The development and application of the alteration strength index equation

Cited by 21 publications

References 71 publications

Matrix permeability of reservoir rocks, Ngatamariki geothermal field, Taupo Volcanic Zone, New Zealand

Matrix permeability of reservoir rocks, Ngatamariki geothermal field, Taupo Volcanic Zone, New Zealand

Increasing the Permeability of Hydrothermally Altered Andesite by Transitory Heating

Compaction of Hyaloclastite from the Active Geothermal System at Krafla Volcano, Iceland

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