Abstract

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.

Highlights

  • Geothermal and hydrothermal systems are typically found in active volcanic environments [1,2,3,4], where fluid convection transfers heat and mass from the high-temperature subsurface (e.g., [5])

  • During the burial of fresh, surficial hyaloclastite in a reservoir, local pressures will increase causing the physical properties of the material to evolve upon exceeding the elastic limit (P∗), which prompts collapse of the porous network, resulting in reduced porosity and permeability

  • After increasing the effective pressure of the surficial hyaloclastite beyond P∗, it is observed that compaction alone does not recreate the physical and mechanical properties of subsurface hyaloclastites, as strength is not correspondingly increased (Q/P∗ is reduced)

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Summary

Introduction

Geothermal and hydrothermal systems are typically found in active volcanic environments [1,2,3,4], where fluid convection transfers heat and mass from the high-temperature subsurface (e.g., [5]). As magma underrooted systems can be intermittently volcanically active over long periods of time (i.e., ~1 Ma), it is common for the reservoir rock, hosting highenthalpy fluids, to be of volcanic origin. The products of such activity include highly variable, quench-fragmented glass, termed hyaloclastite (e.g., [11,12,13,14,15,16,17,18]). Hyaloclastite comprises a time- and temperaturedependent, variably indurated, and heterogeneous assortment of palagonite, hydrated glass [17], lithics, and crystal fragments. The common presence of clay phases in reservoir rocks can be mapped from the surface using electrical resistivity, providing information about the structure of the reservoir [23,24,25]

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