Added: Lebaron Stocking - Date: 14.08.2021 06:49 - Views: 11134 - Clicks: 4395
There is currently a debate within the SIP community about the characteristic textural length scale controlling relaxation time of consolidated porous media. One idea is that the relaxation time is dominated by the pore throat size distribution or more specifically the modal pore throat size as determined in mercury intrusion capillary pressure tests. Recently new studies on inverting pore size distributions from SIP data were published implying that the relaxation mechanisms and controlling length scale are well understood.
In contrast new analytical model studies based on the Marshall-Madden membrane polarization theory suggested that two relaxation processes might compete: the one along the short narrow pore the throat with one across the wider pore in case the narrow pores become relatively long. This paper presents a first systematically focused study into the relationship of pore throat sizes and SIP relaxation times. The generality of predicted trends is investigated across a wide range of materials differing considerably in chemical composition, specific surface and pore space characteristics.
Three different groups of relaxation behaviors can be clearly distinguished. The different behaviors are related to clay content and type, carbonate content, size of the grains and the wide pores in the samples. Induced polarization is the ability of a porous medium to store electrical charges at low frequency Revil et al.
In geophysical applications measurements can either be performed in the time domain or in the frequency domain spectral induced polarization or SIP. For SIP measurements an alternating harmonic current is applied and the resulting voltage al recorded. The complex impedance shows in many cases a characteristic dispersive behavior and different models are used to fit the data, common are for example Debye decomposition, Cole—Cole distribution or others Nordsiek and Weller,Cole and Cole, The characteristic length scale though, controlling the observed polarization, is as yet a highly controversial subject e.
Revil et al. Scott and Barker were the first to find a correlation between the pore throat size determined from mercury intrusion tests and the SIP relaxation time. Later Kruschwitz et al. Particularly sandstones having small pore throats exhibit slow relaxation times, sometimes decades lower than what would be expected from the pore size distribution. Other researcher groups argue that the length of the pore is the characteristic scale governing the diffusion based relaxation process Titov et al. Very recent approaches tried to use SIP as a porosimetry tool showing a successful recovery of the pore throat size distribution using a Warburg decomposition of SIP spectra for selected samples Florsch et al.
Although this approach might seem as a door opener for non-destructive pore size distribution estimations, it is inevitable that the underlying polarization phenomena and controlling length scales need to be fully understood in order to apply such algorithms successfully and reliably.
As the diameter of pore-throats is intimately related to the permeability and capillary suction or freeze—thaw behavior of porous media Scott and Barker, and Bloomfield et al. A common theory to describe induced polarization in porous media is the membrane polarization Marshall and Madden,which assumes that IP atures are caused by polarization mechanisms in the electrical double layer EDL at the mineral—fluid interface e. Schwarz,Vinegar and Waxman, Both parts of the EDL the Stern layer, which describes charges directly sorbed to the mineral surface, and the diffuse layer, which represents ions that are linked to the surface by Coulombic interactions, become polarized.
A more comprehensive description of the processes can be found in Leroy and Revil In the nSNP model the relaxation time is controlled by the length of the narrow pore pore throat whereas in the LNP model it is dominated by the length of the wide pore. A clear experimental support and Looking to dominate a throat analysis based explanation, why either one or the other relaxation Looking to dominate a throat has yet to be provided. Kruschwitz et al. Two ideas were discussed in these experimental studies. One mechanism could be that narrow pathways interconnect and form long narrow pores along which diffusion occurs at long relaxation time scales.
But in all experimental studies so far the of involved narrow pore sandstones was pretty small. This fact has been the starting point for this more systematically focused investigation presented here. The authors of this work have measured complex resistivity spectra of a variety of sandstones and other porous natural and artificial building materials with a wide range of different pore size distributions, specific surfaces and clay contents.
The diversity and size of this data set are considered most qualified to generally prove general relationships and try clustering or unclustering of sample characteristics, SIP behaviors and derive reasons.
In the Theory section the basic relaxation principles are described and the samples and methods are presented. The used data base combines own and data from nine other sources, in total a of samples are involved, all showing peak relaxation behavior. In the next section the are presented and three groups of SIP relaxation behaviors differentiated. The SIP findings are interpreted and discussed in terms of textural properties, chemical composition as well as other sample characteristics. In the last section the findings are summarized. Electrical polarization at low frequencies mHz to kHz is usually carried out in four-electrode measuring cells either in time-domain or frequency-domain mode Kemna et al.
For time-domain measurements, the voltage decay with time is measured after the current injection is stopped. In frequency-domain mode, a phase-shifted voltage relative to an injected alternating current is measured.
By injecting current at a range of frequencies a spectrum of measured impedance may be obtained. Spectral induced polarization measurements were carried out on a variety of own, new samples: 19 sandstones, 14 building materials including 4 tuff stones, 4 bricks, 4 aerated concretes, 1 artificial sand-limestone and 1 natural sand-limestone Opuka from the Czech Republic.
The samples were either cylindrical core samples measuring 20 mm in diameter and 70 mm in length or rectangular cores measuring 20 mm in height and width and 70 mm in length. As has been stated earlier in Kruschwitz et al. The dashed line marks the best exponential log—log linear fit for the data that follow the predicted power-law correlation between relaxation time and D dom. In the following discussion section we will examine features like clay content and pore space characteristics like specific surface area, formation factor and others.
All of which are known to influence SIP behavior.
Also new considerations of an extended MICP data analysis using the extrusion information are presented. All these approaches have the one aim to understand why we observe three different types of SIP relaxation behavior and what are the common features of the samples belonging to.
The spectral induced polarization behavior of a series of own sandstones and common building materials has been presented together with data from the literature and partly unpublished data from other working groups. By compiling this diverse data basis it was the main aim to bring together quite diverse materials from geological different areas and with different textural properties. Especially a great range of dominant pore throat sizes and specific surface areas was important in order to.
D dom dominant or modal pore throat size. The support of Angela Ehling is deeply acknowledged who provided plenty of information on the properties of the used sandstones. Herbert Wiggenhauser is thanked for giving me the opportunity to investigate and elaborate the use of SIP for non-destructive testing in civil engineering. The mechanical properties of soils play a crucial role in site assessment for construction and infrastructure. Standard geotechnical methods, such as compressive strength tests, quantify the mechanical properties of soils, but these methods have low spatiotemporal resolution and may involve disruption of existing infrastructure.
In contrast, the complex conductivity geophysical method can provide information on spatiotemporal changes in the subsurface in a minimally invasive manner. We investigated complex conductivity atures resulting from soil deformation and failure during an unconfined compression test. This soil sample underwent a constant and slow rate of compression. Soil stress, strain, effluent volume, along with the frequency dependent real and imaginary parts of the complex conductivity were recorded over distinct time intervals.
The first experiment focused on the sensitivity of complex conductivity to soil failure. Imaginary conductivity equivalent to surface conductivity abruptly decreased at the failure point similar to the decrease in stress compared to the real conductivity al.
The square of the dominant geophysical length scale L determined from the complex conductivity spectra related to pore size exhibits an inverse linear dependence on compression. The second experiment focused on the complex conductivity during shearing beyond failure. In this case, the imaginary or surface conductivity closely tracked changes in the sample stress. In both experiments, imaginary or surface conductivity is highly sensitive to changes caused by rearrangement of soil structure under stress Looking to dominate a throat.
In contrast, the real conductivity is minimally sensitive, and the electrolytic conductivity is insensitive to these changes. Our findings indicate that complex conductivity is capable of tracking mechanical changes of soils under stress and during failure foremost through the surface conductivity. A range of theories exists to describe the physical origin of individual relaxation phenomena that add up to the measured SIP spectra in ore-free rocks.
A review of physical pore-scale theories can be found in Kruschwitz et al.
One approach states that electrolytic interface polarization from different ion mobility and diffusion processes causing a charge buildup in nonmet allically mineralized rocks. Carbonate formations still present a challenging target with regard to reservoir characterization as they possess a wide range of pore types and sizes as well as interconnected and isolated pore space. The aims of the study were to combine these pieces of information to characterize the pore space of mud-dominated and grain-dominated Looking to dominate a throat rocks and to investigate potential correlations between pore space properties and SIP-derived parameters.
For validation of the NMR-derived r b distributions, scanning electron microscope images were used for the computation of image-derived pore size distributions. The SIP data were interpreted in terms of characteristic features for oomoldic and micritic samples. The indicate that grain-dominated carbonates possess comparatively large radii r b and r t and low S porwhich seems to go along with sharp phase peaks i.
The mud-dominated carbonates, in contrast, possess relatively small radii r b and r t and high S por which is obviously associated with broad phase peaks i. Consequently, the of this study contribute to a better understanding of the petrophysical properties of carbonate rocks as a function of their pore space characteristics. Debye relaxation, as applied on porous media, is related to the mineral grain charge transport properties, and could provide additional information on the rock - mineral matrix.
Using DD models, the relaxation time distribution RTD can be retrieved, from which the grain and pore size distribution can be estimated Florsch et al.Looking to dominate a throat
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