2.5-D discrete-dual-porosity model for simulating geoelectrical experiments in fractured rock

Sanz, V. Caballero; Roubinet, Delphine; Demirel, S.; Irving, James

doi:10.1093/gji/ggx080

Cited by 6 publications

(7 citation statements)

References 36 publications

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“…These methods are used to infer rock porosity from in-situ measurements relying on laboratory-scale petrophysical relationships (e.g., [13][14][15]), but also to obtain vertically distributed information on reservoir content and heterogeneities along boreholes (e.g., [16][17][18]). As demonstrated by several studies, when electrical measurements are acquired along the Earth's surface with various electrode spacing, they also provide information on the presence and properties of fractures at several depths (e.g., [19][20][21]). …”

Section: Introductionmentioning

confidence: 99%

Relating Topological and Electrical Properties of Fractured Porous Media: Insights into the Characterization of Rock Fracturing

2018

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Numerous laboratory and field experiments suggest that electrical properties of fractured rocks may provide critical information regarding the topological properties of the underlying fracture networks. However, a lack of numerical studies dedicated to realistic fractured media prevents us from assessing, in a systematic manner, the relationships between electrical and topological properties in complex domains for which a representative elementary volume may not exist. To address this issue, we conduct an extended numerical analysis over a large range of realistic fractured porous media with an explicit description of the fractures that takes into account the fracture-matrix interactions. Our work shows that the fracture density determines the suitability of Archie's law for describing effective electrical properties with complex behavior associated with small fracture densities. In particular, for fracture networks at the percolation threshold surrounded by a low-porosity matrix, the effective petrophysical relationships are impacted by the assumed fracture-length distribution and the exchange of electric current between the fractures and surrounding matrix. These results help in understanding experimental observations that were difficult to explain so far, suggesting that the effective electrical properties of fractured rock may be used to obtain insights into the properties of their geological structures.

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Section: Introductionmentioning

confidence: 99%

Relating Topological and Electrical Properties of Fractured Porous Media: Insights into the Characterization of Rock Fracturing

2018

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“…Indeed, all of the studies cited above have focused on either the impact of multiple fractures or fracture networks on the bulk resistivity characteristics of the subsurface, or the influence of fracture zones on ERT data. Recent numerical modeling results, however, suggest that, despite the well-known limits in resolution of ERT methods from a tomographic inverse standpoint [42][43][44], even a single millimeter-scale fracture in the near-surface can significantly influence the distribution of subsurface electric potential [45], which in some cases can produce a strong anomaly in ER profiling data [46]. A systematic study of the magnitude of changes in such data, corresponding to different fracture configurations, is therefore necessary to evaluate whether, and under what conditions, ER data might be used to characterize individual fractures.…”

Section: Introductionmentioning

confidence: 99%

“…In this work, we explore whether surface-based ER measurements may be used to recover information about fractures in near-surface bedrock aquifers. To this end, we conduct a detailed numerical analysis using a highly efficient and accurate 2.5D modeling approach that is based on an explicit representation of the fractures, with no limit to the complexity of the considered fracture distribution [45,46]. After presenting the methodological background for our study, we show the results of ER profiling over a single buried fracture considering various values for the fracture length, dip angle, aperture, and electrical resistivity, as well as different values for the overburden thickness and resistivity.…”

Section: Introductionmentioning

confidence: 99%

Characterizing Near-Surface Fractured-Rock Aquifers: Insights Provided by the Numerical Analysis of Electrical Resistivity Experiments

Demirel

Roubinet

Irving

et al. 2018

Water

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Fractured-rock aquifers represent an important part of the groundwater that is used for domestic, agricultural, and industrial purposes. In these natural systems, the presence and properties of fractures control both the quantity and quality of water extracted, meaning that knowledge about the fractures is critical for effective water resource management. Here, we explore through numerical modeling whether electrical resistivity (ER) geophysical measurements, acquired from the Earth’s surface, may potentially be used to identify and provide information about shallow bedrock fractures. To this end, we conduct a systematic numerical modeling study whereby we evaluate the effect of a single buried fracture on ER-profiling data, examining how the corresponding anomaly changes as a function of the fracture and domain characteristics. Two standard electrode configurations, the Wenner-Schlumberger (WS) and dipole-dipole (DD) arrays, are considered in our analysis, with three different spacing factors. Depending on the considered electrode array, we find that the fracture dip angle and length will impact the resistivity anomaly curves differently, with the WS array being better adapted for distinguishing between sub-horizontal and sub-vertical fractures, but the DD array leading to larger overall anomaly magnitudes. We also find that, unsurprisingly, the magnitude of the resistivity anomaly, and thus fracture detectability, is strongly affected by the depth of overburden and its electrical resistivity, as well as the fracture aperture and contrast between the fracture and bedrock resistivities. Further research into the electrical properties of fractures, both above and below the water table, is deemed necessary.

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“…The primary reason for this has been the absence of tools for numerically modelling electric current flow in fractured media. Whereas fluid flow can be rather easily examined because the rock matrix is often ignored on the basis that it is effectively impervious (Neuman 2005;Maillot et al 2016;Cvetkovic 2017), this is not the case for the electrical conductivity where the matrix typically plays an important role in the conduction of electric current (Roubinet & Irving 2014;Roubinet et al 2016;Caballero Sanz et al 2017;Beskardes & Weiss 2018). As a result, modelling approaches that explicitly account for both the fractures and matrix, as well as interactions between these domains, are required.…”

Section: Introductionmentioning

confidence: 99%

Comparison of REV size and tensor characteristics for the electrical and hydraulic conductivities in fractured rock

Demirel

Irving

Roubinet

2018

Geophysical Journal International

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The representative elementary volume (REV) is a critically important concept in fractured rock investigations as it tells us at what scale the fractured domain can be represented by an anisotropic tensor as opposed to requiring the details of each individual fracture for modelling purposes. Whereas the REV size and corresponding tensor characteristics for the hydraulic conductivity (K) in fractured rock have been the subject of numerous previous investigations, no studies to date have focused on the electrical conductivity (σ). This is despite the fact that geoelectrical measurements are arguably the most popular means of geophysically investigating fractured rock, typically via azimuthal resistivity surveying where the observed electrical anisotropy is commonly used to infer hydraulic characteristics. In this paper, we attempt to fill this void and present a systematic numerical study of the impacts of changes in fracturenetwork properties on the REV size and equivalent tensor characteristics for both the electrical and hydraulic conductivities. We employ a combined statistical and numerical approach where the size of the REV is estimated from the conductivity variability observed across multiple stochastic fracture-network realizations for various domain sizes. Two important differences between fluid and electric current flow in fractured media are found to lead to significant differences in the REV size and tensor characteristics for σ and K; these are the greater importance of the matrix in the electrical case and the single power instead of cubic dependence of electric current flow upon aperture. Specifically, the REV for the electrical conductivity will always be smaller than that for the hydraulic conductivity, and the corresponding equivalent tensor will exhibit less anisotropy, often with notably different principal orientations. These findings are of key importance for the eventual interpretation of geoelectrical measurements in fractured rock, where we conclude that extreme caution must be taken when attempting to make the link to hydraulic properties.

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2.5-D discrete-dual-porosity model for simulating geoelectrical experiments in fractured rock

Cited by 6 publications

References 36 publications

Relating Topological and Electrical Properties of Fractured Porous Media: Insights into the Characterization of Rock Fracturing

Relating Topological and Electrical Properties of Fractured Porous Media: Insights into the Characterization of Rock Fracturing

Characterizing Near-Surface Fractured-Rock Aquifers: Insights Provided by the Numerical Analysis of Electrical Resistivity Experiments

Comparison of REV size and tensor characteristics for the electrical and hydraulic conductivities in fractured rock

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