Upscaling permeability in anisotropic volcanic systems

Farquharson, Jamie; Wadsworth, Fabian B.

doi:10.31223/osf.io/xewqm

Cited by 5 publications

(8 citation statements)

References 86 publications

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“…We also note that even if tuffisites are relatively uncommon, their influence on the permeability of an otherwise impermeable magmatic plug can be very large. For example, a single permeable pathway within an large low‐permeability rock mass can increase the equivalent permeability of the system by many orders of magnitude, as discussed in, for example, Heap and Kennedy (), Farquharson et al (), and Farquharson and Wadsworth (). Finally, although the outgassing flux could be computed using either Darcy law (low Reynolds number) or the Forchheimer equation using the constraints of permeability provided herein, we note that while our determination of the porosity‐permeability relationship is valid locally, the depth‐dependent stress and the coupling between the evolving gas pressure and the sintering rates demands a full numerical solution (e.g., Michaut et al, ).…”

Section: Discussionmentioning

confidence: 99%

The Permeability Evolution of Tuffisites and Implications for Outgassing Through Dense Rhyolitic Magma

et al. 2019

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There is growing evidence that outgassing through transient fracture networks exerts an important control on conduit processes and explosive‐effusive activity during silicic eruptions. Indeed, the first modern observations of rhyolitic eruptions have revealed that degassed lava effusion may depend upon outgassing during simultaneous pyroclastic venting. The outgassing is thought to occur as gas and pyroclastic debris are discharged through shallow fracture networks within otherwise low‐permeability, conduit‐plugging lava domes. However, this discharge is only transient, as these fractures become clogged and eventually blocked by the accumulation and sintering of hot, melt‐rich pyroclastic debris, drastically reducing their permeability and creating particle‐filled tuffisites. In this study we present the first published permeability measurements for rhyolitic tuffisites, using samples from the recent rhyolitic eruptions at Chaitén (2008–2009) and Cordón Caulle (2011–2012) in Chile. To place constraints on tuffisite permeability evolution, we combine (1) laboratory measurements of the porosity and permeability of tuffisites that preserve different degrees of sintering, (2) theoretical estimates on grainsize‐ and temperature‐dependent sintering timescales, and (3) H2O diffusion constraints on pressure‐time paths. The inferred timescales of sintering‐driven tuffisite compaction and permeability loss, spanning seconds (in the case of compaction‐driven sintering) to hours (surface tension‐driven sintering), coincide with timescales of diffusive degassing into tuffisites, observed vent pulsations during hybrid rhyolitic activity (extrusive behavior coincident with intermittent explosions), and more broadly, timescales of pressurization accompanying silicic lava dome extrusion. We discuss herein the complex feedbacks between fracture opening, closing, and sintering and their role in outgassing rhyolite lavas and mediating hybrid explosive‐effusive activity.

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Section: Discussionmentioning

confidence: 99%

The Permeability Evolution of Tuffisites and Implications for Outgassing Through Dense Rhyolitic Magma

et al. 2019

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“…However, at the borehole scale, Durán, van Wijk, et al () show that elastic wave attenuation estimated over tens of meters of rocks is equivalent to arithmetically averaging attenuation values estimated over finer (few meter) scales. Farquharson and Wadsworth () propose different ways of upscaling laboratory permeability to the field in anisotropically fractured rocks. Although matrix porosity is thought to play a nominal role in fluid movement within liquid‐dominated geothermal reservoirs like Ngatamariki (Wallis et al, ), the methodology presented herein and the systematic patterns of porosity with lithology‐alteration can be applied to two‐phase or vapor‐dominated reservoirs.…”

Section: Discussionmentioning

confidence: 99%

Mineral Alteration and Fracture Influence on the Elastic Properties of Volcaniclastic Rocks

et al. 2019

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In geothermal environments, the physical properties of rocks, such as porosity and alteration, are highly variable and control the reservoir's elastic and hydraulic properties. Elastic wave velocity in volcaniclastic rocks and their relation to fractures, pore shapes, and mineral alteration is mostly unknown. We measure ultrasonic P and S wave speeds on volcanic rocks from the Ngatamariki Geothermal Reservoir, New Zealand. Data clustering of wave speed versus porosity and density allow us to classify lithotypes of variable propylitic and phyllic mineral alteration. Wave speeds increase first due to porosity reduction as a result of mineral alteration and welding and, second, due to the high elasticity of alteration minerals (epidote, chlorite, and carbonates). We model the rock porosity as composed of equant pores and oblate-spheroidal microfractures following an elastic effective medium model. For our samples, microfracture porosities range from 4% in the volcaniclastic tuffs to less than 1% in the ignimbrites, which we validate with pycnometer porosity data under effective pressure. We quantify that within one geological volcaniclastic formation, wave speeds vary up to 47% due to rock alteration and welding, while the effect of microfractures and changes in effective stress on wave speeds is secondary (up to 11%) but not negligible. From the experimental and numerical results we show that equant pores remain open at reservoir conditions (2,000 m) and can retain considerable porosity (10%). Our analysis has implications for microfracture and pore network characterization in geothermal reservoirs, in particular, in vapor-dominated systems where matrix porosity and permeability play a role.

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“…If we relax that assumption, we can add the scaled contribution from the host by (Ebigbo et al, ; Heap & Kennedy, )

(⟨⟩, k) = ϕ_{f} k_{f} + ϕ_{h} k_{h} = \frac{A_{f} k_{f} + A_{h} k_{h}}{A_{T}},

where k f is the total fracture permeability, ϕ h is the host surface fraction, k h is the host permeability, which we take to be isotropic, and A h is the cross‐sectional area of the host rock, such that A h = A T − A f . When the fractures are of nonequal width, the contributions from each width class can be summed and weighted by the area of that class, following solutions for directionally dependent permeability in anisotropic systems (Farquharson & Wadsworth, ). We note that this is different from the approach of Lamur et al () in which k h is added to k ′ without scaling the contributions.…”

Section: Scaling Approach For the Permeability Of Fractured Lavamentioning

confidence: 99%

“…For flow in the horizontal directions, there are two possibilities: in the x ‐direction, the hexagons are arranged in a “pointy topped” array and in the y ‐direction in a “flat topped” array. We can use the rotation matrix method (described in the context of volcanic rocks in Farquharson & Wadsworth, ) to scale the two‐dimensional tensorial description of the permeability of a hexagonal array in each mutually perpendicular horizontal direction, which we term

\overset{k}{true}

, as follows

\overset{k}{true} bold= bold-italicRk R^{- 1} = ([], \begin{matrix} k_{1} \cos^{2} θ + k_{2} \sin^{2} θ & ((), k_{1} - k_{2}) \cos θ \sin θ \\ ((), k_{1} - k_{2}) \cos θ \sin θ & k_{1} \sin^{2} θ + k_{2} \cos^{2} θ \end{matrix}),

where k is the permeability tensor prior to rotation and R is the rotation matrix, such that

boldk bold= ([], \begin{matrix} k_{1} & 0 \\ 0 & k_{2} \end{matrix}) bold, bold bold-italicR = ([], \begin{matrix} \cos θ & - \sin θ \\ \sin θ & \cos θ \end{matrix}) .

…”

Section: Scaling Approach For the Permeability Of Fractured Lavamentioning

confidence: 99%

“…However, the fractures formed by jointing are often wide and open—a result of both cooling contraction after column formation, and erosion during weathering—and not amenable to direct fluid‐flow measurements from which permeability can be obtained. In such scenarios, scaling for the permeability of large fractures is needed (Farquharson & Wadsworth, ; Heap & Kennedy, ; Zimmerman & Bodvarsson, ). Here, we develop a framework for the permeability of hexagonally jointed lavas as they cool.…”

Section: Introductionmentioning

confidence: 99%

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The Permeability of Columnar Jointed Lava

Vasseur

Wadsworth

2019

JGR Solid Earth

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Columnar jointed lava is an important facies in many geothermal reservoir systems. The permeability of jointed lavas is dominated by the contribution from fracture networks. We use a scaling for the permeability of a set of fractures in a solid or porous mass and extend this to arrays of hexagonal intercolumn fractures. To validate our analytical results, we create numerical domains with relevant geometries and extract system-scale permeability using the LBflow lattice-Boltzmann fluid flow simulation tool. Finally, we model the cooling contraction of columns to extend our results so that they predict the permeability with time after lava emplacement. Importantly, we use these results to estimate the range of permeabilities typical of columnar joints that form during cooling from high temperature and are preserved in the crust at moderate to low temperatures.Plain Language Summary Lava fractures when it cools, often forming long, hexagonal columns of rock, such as those at Giant's Causeway in Ireland. Cold fractured lavas such as these are also found throughout the Earth's crust and through which groundwater or geothermal fluids flow. We give a simple mathematical equation to predict how much and how quickly fluids flow through these rocks.

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Upscaling permeability in anisotropic volcanic systems

Cited by 5 publications

References 86 publications

The Permeability Evolution of Tuffisites and Implications for Outgassing Through Dense Rhyolitic Magma

The Permeability Evolution of Tuffisites and Implications for Outgassing Through Dense Rhyolitic Magma

Mineral Alteration and Fracture Influence on the Elastic Properties of Volcaniclastic Rocks

The Permeability of Columnar Jointed Lava

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