Increasing the Permeability of Hydrothermally Altered Andesite by Transitory Heating

Abstract

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.

Thermal Liability of Hyaloclastite in the Krafla Geothermal Reservoir, Iceland: The Impact of Phyllosilicates on Permeability and Rock Strength

“…Deformation-induced macrofractures may induce a more substantial permeability increase [83,84], yet such damage is not imparted by thermal treatment alone. However, the impact of thermal fracturing on the broad range of volcanic lithologies is yet to be well constrained, as discussed in Heap et al [61], and very few studies focus on highly altered material and the impact of pressure on thermal stimulation [52].…”

“…This occurrence leads to a loss of material that results in the creation of porosity, thereby affecting the strength and permeability of the rock [49][50][51]. This is also true for incomplete reactions in rocks experiencing short excursions to high temperature [52]. In the case of palagonite undergoing a temperature increase, dehydration can begin below 200°C and result in greater mass loss than in many other clays [53].…”

Pressure Controlled Permeability in a Conduit Filled with Fractured Hydrothermal Breccia Reconstructed from Ballistics from Whakaari (White Island), New Zealand

“…Additionally, microcracking is a critical textural consequence of alteration related processes. Fractures in volcanic environments typically form in response to tectonic, intrusive, hydration, or thermal forces [44][45][46], however, they can also form in response to specific thermally-driven mineral reactions and drive porosity/permeability changes [47,48]. Cracks are an important textural consideration in volcanic conduits, and their influence on, for example, the physical properties of rocks may be highly dependent on lithology and confining pressure, particularly in altered volcanic rocks [37,46,[49][50][51][52].…”

Hydrothermal Alteration on Composite Volcanoes: Mineralogy, Hyperspectral Imaging, and Aeromagnetic Study of Mt Ruapehu, New Zealand

“…Hydrothermal alteration changes rock mechanical and geotechnical properties (del Potro & Hürlimann, 2009; Pola et al, 2014), promoting flank instability (Finn et al, 2001; Heap et al, 2015; John et al, 2008; López & Williams, 1993; Norini et al, 2020; Schaefer et al, 2015) and decreasing permeability, which can lead to phreatic eruptions (Mayer et al, 2017; Pardo et al, 2014). Hydrothermal alteration primarily reduces rock strength (del Potro & Hürlimann, 2009; Farquharson et al, 2019) and changes permeability (e.g., mineral precipitation into pores and cracks which decreases permeability) within the volcanic edifice (Mordensky et al, 2019), which may locally elevate pore pressure promoting edifice flank instabilities (Ball et al, 2018; Collard et al, 2020; Reid, 2004). Flank instabilities and the resulting mass flow events triggered by gravity, weather events, volcanic eruptions, magmatic intrusions, and earthquakes (Capra, 2006; Procter et al, 2014; Schaefer et al, 2018) can result in far‐reaching and potentially dangerous volcanic hazards downstream from volcanoes (Finn et al, 2001).…”