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

Abstract

<jats:p>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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M1"><mml:msup><mml:mrow><mml:mi>P</mml:mi></mml:mrow><mml:mrow><mml:mo>∗</mml:mo></mml:mrow></mml:msup></mml:math>, 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, &gt;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.</jats:p>

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

“…The resultant strengths measured in all test types display a porosity control, irrespective of the stress field experienced (Figure 7(a)), which follows the common porosity-strength trend (Figures 7(b) and 7(c)) for a range of igneous rock types, regardless of TT [7,11,63,87,88]. The treatment temperature has a strong impact on porosity (Figure 4(c)), thereby further influencing mechanical compaction; however, comparison with strength and Young's modulus data collected from subsurface samples [63] shows that temperature alone cannot explain the mechanical changes occurring within the reservoir, instilling the roles of compaction and alteration on strength changes. In part, this is due to the opposing influences of temperature and compaction on porosity; however, in concert, increased temperature will create additional pore space that may enable more complete compaction and densification to occur at depth [63].…”

“…The treatment temperature has a strong impact on porosity (Figure 4(c)), thereby further influencing mechanical compaction; however, comparison with strength and Young's modulus data collected from subsurface samples [63] shows that temperature alone cannot explain the mechanical changes occurring within the reservoir, instilling the roles of compaction and alteration on strength changes. In part, this is due to the opposing influences of temperature and compaction on porosity; however, in concert, increased temperature will create additional pore space that may enable more complete compaction and densification to occur at depth [63]. The change in style, from a dominantly brittle to ductile failure, may be explained by a change in geometry of the pore space (e.g., Figures 3(b) and 3(r)), as dehydration increases the connectivity and irregularity of the desiccated pore network [89].…”

Using acoustic velocities and microimaging to probe microstructural changes caused by thermal shocking of tight rocks

“…This expansion and contraction can generate what are called, 'thermal cracks' (Pearson, 1941;Johnson and Parsons, 1944;Heap et al, 2014). Thermal cracking is prevalent in geothermal reservoirs and is particularly pertinent to geothermal energy production (Siratovich et al, 2015;Eggertsson et al, 2020). Conventional geothermal systems (CGS) exist in reservoirs with naturally high permeability (k) that allows for water to circulate through pore spaces, heat up, and turn to steam, which is then produced to rotate a turbine and generate power (U.S. Department of Energy, 2019).…”

Structural Controls on Basin- and Crustal-Scale Fluid Flow and Resulting Mineral Reactions

“…This, together with other alteration processes and compaction, as well as precipitation of other mineral phases, results in changes in their mechanical properties and porosity and permeability distribution. Eggertsson et al [36] examine the effects of compaction on the petrophysical and mechanical evolution of hyaloclastites from an active geothermal system of the Krafla volcano in northeast Iceland. They compare experimental results of yield points and porosity and permeability evolution of hyaloclastite samples collected from the surface with those from subsurface drill core.…”

“…The results show that subsurface samples display higher strengths due to their lower porosity and permeability. Eggertsson et al [36] conclude that burialinduced compaction cannot alone account for the physical and mechanical properties of hyaloclastites of the Krafla volcano subsurface geothermal reservoir. By examining samples with optical and electron microscopy, they suggest that pore networks were modified by mineral precipitation and alteration associated with the flow of high-temperature fluids, resulting in rock strengthening.…”