Filtern
Dokumenttyp
- Zeitschriftenartikel (2)
- Beitrag zu einem Tagungsband (2)
- Vortrag (1)
- Posterpräsentation (1)
Schlagworte
- µ-CT (6) (entfernen)
Organisationseinheit der BAM
Eingeladener Vortrag
- nein (1)
Relaxation phenomena observed in the electrical low-frequency range (approximately 1 mHz-10 kHz) of natural porous media like sandstones is often assumed to be directly related to the dominant (modal) pore throat sizes measured, for instance, with mercury intrusion porosimetry. Attempts to establish a universally valid relationship between pore size and peak Spectral Induced Polarization (SIP) relaxation time have failed, considering sandstones from very different origins and featuring great variations in textural and chemical compositions as well as in geometrical pore space properties. In addition working with characteristic relaxation times determined in Cole-Cole or Debye decomposition fits to build the relationship have not been successful. In particular, samples with narrow pore throats are often characterized by long SIP relaxation times corresponding to long “characteristic length scales” in these media, assuming that the diffusion coefficients along the electrical double layer were constant. Based on these observations, three different types of SIP relaxation can be distinguished. We present a new way of assessing complex pore spaces of very different sandstones in a multi-methodical approach to combine the benefits of mercury intrusion porosimetry, micro-computed tomography, and nuclear magnetic resonance. In this way, we achieve much deeper insight into the pore space due to the different resolutions and sensitivities of the applied methods to both pore constrictions (throats) and wide pores (pore bodies). We experimentally quantify pore aspect ratios and volume distributions within the two pore regions. We clearly observe systematic differences between three SIP relaxation types identified previously and can attribute the SIP peak relaxation times to measured characteristic length scales within our materials. We highlight selected results for a total of nine sandstones. It seems that SIP relaxation behavior depends on the size difference of the narrow pore throats to the wide pore bodies, which increases from SIP Type 1 to Type 3.
We investigate the pore space of rock samples with respect to different petrophysical parameters using various methods, which provide data on pore size distributions, including micro computed tomography (µ-CT), mercury Intrusion porosimetry (MIP), nuclear magnetic resonance (NMR), and spectral-induced polarization (SIP). The resulting cumulative distributions of pore volume as a function of pore size are compared. Considering that the methods differ with
regard to their limits of resolution, a multiple-length-scale characterization of the pore space is proposed, that is based on a combination of the results from all of these methods.
The approach is demonstrated using samples of Bentheimer and Röttbacher sandstone. Additionally, we compare the potential of SIP to provide a pore size distribution with other commonly used methods (MIP, NMR). The limits of Resolution of SIP depend on the usable frequency range (between 0.002 and 100 Hz). The methods with similar Resolution show a similar behavior of the cumulative pore volume distribution in the verlapping pore size range. We assume that µ-CT and NMR provide the pore body size while MIP and SIP characterize the pore throat size. Our study Shows that a good agreement between the pore radius distributions can only be achieved if the curves are adjusted considering the resolution and pore volume in the relevant range of pore radii. The MIP curve with the widest range in Resolution should be used as reference.
Other than commonly assumed the relaxation times observed in the electrical low-frequency range (1 mHz – 40 kHz) of natural porous media like sandstones and tuff stones cannot be directly related to the dominant (modal) pore throat sizes, measured (e.g.) with mercury intrusion porosimetry (MIP). Working with a great variety of sandstones from very different origins and featuring great variations in textural and chemical compositions as well as in geometrical pore space properties, we observed that particularly samples with narrow pore throats were characterized by long (low-frequency) relaxations. These, however, can (following the current theories) be rather explained by long “characteristic length scales” in these media or low diffusion coefficients along the electrical double layer. However, there is no straightforward way (or single approved method) of getting reliable numbers for properties such as the lengths of pore throats, the diameter and length of the wide pores and their respective distributions. Consequently we follow a multi-methodical approach and combine the benefits of MIP, micro-computed tomography (µ-CT) and nuclear magnetic resonance (NMR) to achieve much deeper insight due to the different resolutions and sensitivities to either pore constrictions (throats) or wide pores. This helps us to understand, whether the observed electrical relaxation phenomena actually depend on geometric length scales or rather on other properties such as chemical composition, clay content, clay type or cation exchange capacity. Our poster showcases selected results of a systematic study on texturally and petrophysically very different sandstones.
Die elektrischen Eigenschaften im Niederfrequenzbereich der Spektral Induzierten Polarisation (SIP) von silikatisch gebundenen, porösen Medien werden maßgeblich durch der Porenstruktur, der Art des Sättigungsfluids, der geochemischen Zusammensetzung und den Oberflächeneigenschaften (wie z.Bsp. der Oberflächenladung oder der Kationenaustauschkapazität) bestimmt. Einige Autoren konnten an Sandsteinen mit geringer Variation der petrophysikalischen Eigenschaften zeigen, dass die SIP-Relaxationszeit maßgeblich von der Porenhalsradienverteilung - ermittelt durch Quecksilberporosimetrie (MIP) - bestimmt wird. Diese Beobachtungen bezogen sich jedoch auf Probensätze mit dominanten Porenhalsgrößen (Ddom) im Bereich von 10 µm bis 100 µm. Betrachtet man heterogenere Datensätze von Sandsteinen mit vollkommen unterschiedlicher Herkunft und einer breiten Variation dominanter Porenhalsgrößen (z. Bsp. über vier Dekaden mit Ddom von 0.01 µm bis 100 µm), ist der Zusammenhang zwischen SIP-Relaxationszeit und MIP-Porenhalsgröße deutlich schwächer. Stattdessen beobachtet man auch für Proben mit engen Porenhälsen (Ddom < 5-10 µm) häufig relativ lange Relaxationszeiten (bzw. niederfrequente SIP-Relaxationen).
Um die Porensysteme in derartigen Medien besser zu charakterisieren, führen wir zusätzlich Untersuchungen mit µ-CT, Gassorption (BET), Quecksilberporosimetrie (MIP) und Nuklear-Magnetischer Resonanz (NMR) durch und versuchen, die verfahrensspezifisch und auflösungsbedingt unterschiedlichen Ergebnisse zu einer Gesamtaussage zusammenzuführen. In unserem Beitrag zeigen wir ausgewählte Ergebnisse einer systematischen Studie an insgesamt 16 Sandsteinen und 3 Tuffsteinen. Ziel dieser Arbeit ist es, einerseits durch die Methodenkombination Porenräume allgemein ganzheitlicher abzubilden, andererseits die beobachteten SIP-Relaxationszeiten so gut wie möglich charakteristischen Längen zuordnen zu können. Dies ist ein wichtiger Beitrag zur Klärung der Frage, ob die SIP-Relaxationszeit überhaupt maßgeblich von geometrischen Porenraumeigenschaften bestimmt wird oder ggf. von anderen Eigenschaften wie der chemischen Zusammensetzung (z.B. Tongehalt, Tonart) abhängt.
We investigate the pore space of rock samples with respect to different petrophysical parameters using various methods, which provide data upon pore size distributions, including micro computed tomography (μ-CT), mercury intrusion porosimetry (MIP), nuclear magnetic resonance (NMR), and spectral induced polarization (SIP). The resulting cumulative distributions of pore volume as a function of pore size are compared. Considering that the methods differ with regard to their limits of resolution, a multiple length scale characterization of the pore space geometry is proposed, that is based on a combination of the results from all of these methods. The findings of this approach are compared and discussed by using Bentheimer sandstone. Additionally, we compare the potential of SIP to provide a pore size distribution with other commonly used methods (MIP, NMR). The limits of resolution of SIP depend on the usable frequency range (between 0.002 and 100 Hz). The methods with similar resolution show a similar behavior of the cumulative pore volume distribution in the overlapping pore size range. The methods μ-CT and NMR provide the pore body size while MIP and SIP characterize the pore throat size. Using this difference, the average pore body to throat ratio is determined to be about three for the Bentheimer sandstone.
Our study shows that a good agreement between the pore radii distributions can only be achieved if the curves are adjusted considering the resolution and pore volume in the relevant range of pore radii. The MIP curve with the widest range in resolution should be used as reference
Within this study we have shown, that spectral induced polarization is a reliable method for the enhanced characterization of the Fontainebleau sandstone in general, and for its related stratigraphical units in particular. Due to its high sensitivity towards pore network and pore surface changes, different stratigraphical units can be clearly differentiated and
probably even classified. We have observed a good correlation between the maximum of the SIP phase shift and the dominant pore throat radius for this rock type, as it has been reported for others sandstones before [20].