Pore fabric geometry inferred from magnetic and acoustic anisotropies in rocks with various mineralogy, permeability and porosity

Robion, Philippe; David, Christian; Dautriat, Jérémie; Colombier, J.C.; Zinsmeister, L.; Collin, Pierre‐Yves

doi:10.1016/j.tecto.2014.03.029

Cited by 21 publications

(59 citation statements)

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“…Since the grains in sandstones are typically orientated such that their pore major axis is sub-parallel to bedding (Louis et al 2003;Benson et al 2005;Robion et al 2014;Griffiths et al 2017;Farrell and Healy 2017), a higher permeability parallel to bedding is perhaps expected. Although we do not provide an average orientation for the pore or grain major axes of the Buntsandstein sandstones studied herein, we do note that their average grain aspect ratios are within the range 1.54-1.76 (Table 1).…”

Section: Matrix Permeability Of the Buntsandsteinmentioning

confidence: 99%

Microstructural and petrophysical properties of the Permo-Triassic sandstones (Buntsandstein) from the Soultz-sous-Forêts geothermal site (France)

et al. 2017

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Geothermal projects in the Upper Rhine Graben aim to harness thermal anomalies that have arisen due to hydrothermal circulation within the granitic basement and the overlying Permo-Triassic sedimentary units. We present here a systematic microstructural, mineralogical, and petrophysical characterisation of the lowermost unit of this Permo-Triassic sedimentary succession-the Buntsandstein-sampled from exploration well EPS-1 at the Soultz-sous-Forêts geothermal site (France). Twelve depths were sampled (from 1008 to 1414 m) and cylindrical cores were prepared perpendicular and parallel to bedding. These cores were described in terms of their microstructure, grain size and shape, specific surface area, pore size and pore throat size distribution, mineral content, porosity, P-wave velocity, and permeability. The Buntsandstein sandstones are predominantly feldspathic sandstones, often characterised by pores filled or partially filled (with clays (R3 illite-smectite), dolomite, siderite, and barite) as a consequence of diagenesis, tectonics, and the circulation of hydrothermal fluids. The porosity, dry P-wave velocity, and permeability of these sandstones vary from ~ 0.03 to 0.2, ~ 2.5 to 4.5 km s −1 , and ~ 10 −18 to 10 −13 m 2 , respectively. Our data show that P-wave velocity decreases and permeability increases as porosity increases. P-wave velocities are significantly higher when measured parallel to bedding (by about 10 to 25%), and that saturation with water increases P-wave velocity (by about 5 to 50%, depending on sample orientation). The pervasive pore-filling precipitation has significantly reduced the permeabilities of the Buntsandstein sandstones, which are orders of magnitude less permeable than similarly porous unaltered sandstone. We also find that their permeability can be up to an order of magnitude more permeable when measured parallel to bedding than perpendicular to bedding. Although Buntsandstein units with low matrix permeabilities (as low as ~ 10 −18 m 2 ) require macroscopic fractures to attain the high permeability required to sustain regional hydrothermal circulation, matrix permeability is important for units with low fracture densities and high matrix permeabilities. We anticipate that these data will aid future fluid flow modelling and seismic investigations at geothermal sites within the Upper Rhine Graben. Open Access© The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

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Section: Matrix Permeability Of the Buntsandsteinmentioning

confidence: 99%

Microstructural and petrophysical properties of the Permo-Triassic sandstones (Buntsandstein) from the Soultz-sous-Forêts geothermal site (France)

et al. 2017

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“…Traditional pore characterization methods such as X-ray tomography face trade-offs between sample size and resolution, and generate large amounts of data that need to be processed (Cnudde andBoone, 2013, Landis andKeane, 2010). For applications that require characterization of the average pore fabric, MPFs provide a promising alternative in that they describe the average pore fabric as a single second-order tensor, measured on a representative sample volume, and potentially capturing pores down to 10 nm, without being affected by mineral and grain boundary properties unlike seismic anisotropy (Robion et al, 2014, Almqvist et al, 2011, Pfleiderer and Halls, 1990, Benson et al, 2003.…”

Section: Introductionmentioning

confidence: 99%

“…Additionally, MPFs were compared to other measures of pore space anisotropy, e.g. anisotropy of elastic properties or electrical conductivity (Louis et al, 2005, Robion et al, 2014, Benson et al, 2003, Nabawy et al, 2009. Although reported empirical relationships for fabric orientation are similar for all studies (maximum susceptibility indicating the average pore elongation direction and maximum permeability), there is a large variability in reported relationships between MPF anisotropy degree and pore aspect ratio or degree of permeability anisotropy (Fig.…”

Section: Introductionmentioning

confidence: 99%

Explaining the large variability in empirical relationships between magnetic pore fabrics and pore space properties

Biedermann

Pugnetti

Zhou

2021

Geophysical Journal International

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Summary The magnetic anisotropy exhibited by ferrofluid-impregnated samples serves as a proxy for their pore fabrics, and is therefore known as magnetic pore fabric. Empirically, the orientation of the maximum susceptibility indicates the average pore elongation direction, and predicts the preferred flow direction. Further, correlations exist between the degree and shape of magnetic anisotropy and the pores’ axial ratio and shape, and between the degrees of magnetic and permeability anisotropies. Despite its potential, the method has been rarely used, likely because the large variability in reported empirical relationships compromises interpretation. Recent work identified an additional contribution of distribution anisotropy, related to the arrangement of the pores, and a strong dependence of anisotropy parameters on the ferrofluid type and concentration, partly explaining the variability. Here, an additional effect is shown; the effective susceptibility of the ferrofluid depends on the measurement frequency, so that the resulting anisotropy depends on measurement conditions. Using synthetic samples with known void geometry and ferrofluids with known susceptibility (4.04 SI and 1.38 SI for EMG705 and EMG909, respectively), magnetic measurements at frequencies from 500 Hz to 512 kHz are compared to numerical predictions. Measurements show a strong frequency-dependence, especially for EMG705, leading to large discrepancies between measured and calculated anisotropy degrees. We also observe artefacts related to the interaction of ferrofluid with its seal, and the aggregation of particles over time. The results presented here provide the basis for a robust and quantitative interpretation of magnetic pore fabrics in future studies, and allow for re-interpretation of previous results provided that the ferrofluid properties and measurement conditions are known. We recommend that experimental settings are selected to ensure a high intrinsic susceptibility of the fluid, and that the effective susceptibility of the fluid at measurement conditions is reported in future studies.

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“…Anisotropy of magnetic susceptibility (AMS) of ferrofluid‐impregnated samples has been proposed as a fast and efficient method to assess the average 3‐D structure of connected pores (Parés et al, ; Pfleiderer & Halls, , ). MPF measurements can capture pores down to 10 nm (Parés et al, ; Robion et al, ), and because AMS characterizes the full second‐order tensor, no a priori knowledge on the fabric orientation is necessary. The few studies that exist on MPF found promising empirical relationships between the average pore orientation and the AMS of ferrofluid‐impregnated samples (Jones et al, ; Pfleiderer & Halls, , ), or between permeability anisotropy and MPF (Benson et al, ; Hailwood et al, ; Louis et al, ; Pfleiderer & Halls, ; Figure ).…”

Section: Introductionmentioning

confidence: 99%

Magnetic Pore Fabrics: The Role of Shape and Distribution Anisotropy in Defining the Magnetic Anisotropy of Ferrofluid‐Impregnated Samples

Biedermann

2019

Geochem Geophys Geosyst

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Pore fabrics define physical properties of a rock, such as permeability and elasticity, both of which are important to many geological, hydrological, and environmental applications. Minerals and hence pores are often preferentially aligned, leading to anisotropy of physical properties and preferred flow directions. Preferred flow paths are defined by the shape and arrangement of pores, and a characterization of this pore fabric forms the basis for prediction of fluid flow directions. Magnetic pore fabrics (MPFs), that is, magnetic anisotropy measurements on ferrofluid-impregnated samples, are a promising and fast way to characterize the pore fabric of connected pores in 3-D, while analyzing a large number of pores with sizes down to 10 nm, without the need for any a priori knowledge about fabric orientation. Empirical relationships suggest that the MPF is related to the pore shape and orientation and approximates permeability anisotropy. This study uses models including shape and distribution anisotropy to better understand and quantify MPFs, using simple pore shapes and pore assemblies measured in previous studies. The results obtained in this study show that (1) shape anisotropy reliably predicts the MPF of single pores, (2) both shape and distribution anisotropy are needed to predict MPFs of pore assemblies, and (3) the anisotropy parameters P, L, and F are affected by the intrinsic susceptibility of the ferrofluid in addition to pore geometry. These findings can help explain some of the variability in empirical relationships and are an important step toward a quantitative understanding and application of MPFs in geological and environmental studies.Plain Language Summary To produce clean drinking water, use geothermal energy, or control contamination, it is necessary to understand how fluids flow underground. They have to find their way from pore to pore. As soon as pores are elongated or flattened, fluids can move more easily and thus faster in some directions than others. It is desirable to predict such preferred flow directions, and a good description of the pore space is needed to do so. Many methods exist to characterize the pore space, and one of these is based on the directional dependence of magnetic properties of samples, whose pores have been filled with strongly magnetic fluid. The method is efficient and promising, but unfortunately, it is not well understood how the magnetic data reflects the details of the pore space. The models developed here help define and quantify the factors controlling the observable magnetic properties. This understanding will make the method more applicable and useful in geothermal, hydrological, and environmental applications.

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Pore fabric geometry inferred from magnetic and acoustic anisotropies in rocks with various mineralogy, permeability and porosity

Cited by 21 publications

References 30 publications

Microstructural and petrophysical properties of the Permo-Triassic sandstones (Buntsandstein) from the Soultz-sous-Forêts geothermal site (France)

Microstructural and petrophysical properties of the Permo-Triassic sandstones (Buntsandstein) from the Soultz-sous-Forêts geothermal site (France)

Explaining the large variability in empirical relationships between magnetic pore fabrics and pore space properties

Magnetic Pore Fabrics: The Role of Shape and Distribution Anisotropy in Defining the Magnetic Anisotropy of Ferrofluid‐Impregnated Samples

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