scholarly journals Fluid substitution in shaley sediment using effective porosity

Geophysics ◽  
2007 ◽  
Vol 72 (3) ◽  
pp. O1-O8 ◽  
Author(s):  
Jack Dvorkin ◽  
Gary Mavko ◽  
Boris Gurevich
Keyword(s):  
Elastic Modulus ◽  
Pore Space ◽  
Total Porosity ◽  
Effective Porosity ◽  
Fluid Substitution ◽  
Mineral Phase ◽  
Hydrocarbon Production ◽  
System A ◽  
Alternative Method ◽  
Water Saturated

The traditional method of fluid substitution in porous rock requires the total porosity and the elastic modulus of the mineral phase as input and assumes that the fluid reaches instantaneous hydraulic equilibrium throughout the pore space. This assumption may not be appropriate for shaley sediment because of the low permeability of shale and the resulting immobility of the water in it. To address this problem, we propose an alternative method that uses effective porosity instead of total porosity. Effective porosity is lower than total porosity if porous shale is present in the system. A new, composite mineral phase is introduced, which includes the porous water-saturated shale together with the nonporous minerals and whose elastic modulus is an average of those of its components, including the porous shale. This alternative method increases the sensitivity of the elastic properties of sediment-to-pore-fluid changes and therefore may be used as a physics-based theoretical tool to better explain and interpret seismic data during exploration as well as variations in seismic response as hydrocarbon production progresses.

A physical model for porosity reduction in sandstones

Geophysics ◽  
1998 ◽  
Vol 63 (2) ◽  
pp. 454-459 ◽  
Author(s):  
Doron Gal ◽  
Jack Dvorkin ◽  
Amos Nur
Keyword(s):  
Elastic Moduli ◽  
Pore Space ◽  
Total Porosity ◽  
Effective Porosity ◽  
High Porosity ◽  
Formation Factor ◽  
Archie’S Law ◽  
Unconsolidated Sands ◽  
Porosity Reduction ◽  
Reduction Mechanisms

The experimental elastic moduli‐porosity trends for clean sandstones can be described by the modified upper Hashin‐Shtrikman (MUHS) bound. One geometrical (but not necessarily geological) realization is: as porosity decreases, the number of the pores stays the same and each pore shrinks while maintaining its shape. This concept of uniform porosity reduction implies that permeability is proportional to the effective porosity squared, and that formation factor is proportional to the inverse of the effective porosity. The effective porosity here refers to the part of the pore‐space that dominates fluid flow. The proposed relations for permeability and formation factor agree well with the experimentally observed values. These laws are different from the often used forms of the Kozeny‐Carman equation and Archie’s law, where permeability is proportional to the total porosity cubed and formation factor is proportional to the inverse of the total porosity squared, respectively. We suggest that the uniform porosity reduction concept be used in consolidated rocks with porosities below 0.3. The transition from high‐porosity unconsolidated sands to consolidated sandstones can be described by the cementation theory: the MUHS moduli‐porosity curves connect with those predicted by the cementation theory at the porosity of about 0.3. This scheme is not appropriate for modeling other porosity reduction mechanisms such as glass bead sintering because, during sintering, the pores do not maintain their shapes, rather they gradually evolve to rounder, stiffer pores.

Probing Confining Geometries with Molecular Diffusion: A Revisited Analysis of NMR-PGSE Experiments

2000 ◽  
Vol 651 ◽  
Author(s):  
Stéphane Rodts ◽  
Pierre Levitz
Keyword(s):  
Transport Process ◽  
Molecular Diffusion ◽  
Pore Space ◽  
The Self ◽  
Length Scale ◽  
Self Diffusion ◽  
System A ◽  
Water Saturated ◽  
Short Time ◽  
Theoretical Results

AbstractWe present an interpretation of the self diffusion propagator for a molecular fluid confined in a porous medium. Breaking with conventional routes of interpretation, we focus in reciprocal space on the time dependence of the propagator at a fixed wave vector q. New theoretical results are reported, as well as NMR measurements on a water-saturated packing of glass beads and on a stack of plastic platelets with rough surfaces. It is shown that at least three time regimes may be distinguished, characteristic of new ways in which the propagator is affected by the geometry of the system: a short-time exponential regime of almost unrestricted diffusion, a pseudo-exponential regime probing the transport process across the material at the length scale λ=2π/q, and an algebraic regime at long times with exponent -d/2, where d is the dimensionality of connected parts of the pore space.

Fluid substitution for thinly interbedded sand-shale sequences using the mesh method

Geophysics ◽  
2016 ◽  
Vol 81 (6) ◽  
pp. D599-D609 ◽  
Author(s):  
Piyapa Dejtrakulwong ◽  
Gary Mavko
Keyword(s):  
Pore Space ◽  
Linear Trend ◽  
Rock Physics ◽  
Normal Incidence ◽  
Effective Porosity ◽  
P Wave ◽  
Measurement Scale ◽  
Fluid Substitution ◽  
Conventional Procedure ◽  
Substitution Procedure

Gassmann’s fluid substitution model is intended for a monomineralic, homogeneous porous rock, in which pore pressures induced by applied loads can equilibrate throughout the pore space. These assumptions are violated when Gassmann’s equations are applied to measurements that represent effective medium averages over subresolution layers of alternating sand and shale. The conventional procedure for treating this problem has been to first downscale; i.e., estimate the properties of the fine-scale sand and shale endmembers from the coarse-scale measurements, apply Gassmann’s fluid substitution to the sand only, and then Backus average back to the original scale. This procedure, however, is very sensitive to errors in estimated sand fraction and shale properties and becomes particularly unstable at small sand fractions. A new method for fluid substitution combines rock-physics models for dispersed and interbedded sand-shale systems, which are often approximated with Reuss or lower Hashin-Shtrikman interpolations between endmembers quartz mineral, clean sand, and shale. When expressed as P-wave compliance versus porosity, these trends become approximately linear. The Backus average of the normal incidence P-wave compliance of thinly layered mixtures of various sand-shale facies is also a linear trend with porosity. As a result, the upscaled fluid substitution change of compliance of any point within the dispersed or layered sand-shale system is approximately proportional to the fluid-substituted change of compliance of the clean sand endmember, scaled by the ratio of effective porosity to clean sand porosity. The result is a fluid substitution procedure that operates directly at the measurement scale, without the need to downscale the measurements, while still changing fluid in the sand layers only.

Resolving lithostratigraphic complexities in the Crayfish Group, Otway Basin using chemostratigraphy

2021 ◽  
Vol 61 (2) ◽  
pp. 588
Author(s):  
Betina Bendall ◽  
Anne Forbes ◽  
Tony Hill
Keyword(s):  
Continental Margin ◽  
Structural Complexity ◽  
Significant Part ◽  
Rift System ◽  
Lacustrine Environment ◽  
Hydrocarbon Production ◽  
Inorganic Geochemistry ◽  
System A ◽  
Structural Relationships ◽  
Complex Structural

The Otway Basin comprises a significant part of the eastern Australian Southern Rift System, a divergent passive continental margin formed during the Cretaceous separation of the Australian and Antarctic continents. Early rifting activity resulted in the development of many half grabens within the Otway Basin, which are largely infilled by sediments of the Casterton Formation and Crayfish Group. Despite over 20 years of exploration and hydrocarbon production from these units however, their lithostratigraphic characterisation and nomenclature remain ambiguous, with structural complexity and prevalent lateral facies changes leading to confusion in their basin-wide correlation. Deposited in a largely non-marine, fluvial/lacustrine environment, repeating cycles of sandstones and shales of the Crayfish Group can be difficult to resolve using petrology, palynology and wireline log data. The use of chemostratigraphy is favoured as an investigative tool in this situation since changes in provenance, lithic composition, facies, weathering and diagenesis are reflected in the mineralogy of the sediments, resulting in variations in their inorganic geochemistry. Uniform sedimentary successions can thus potentially be differentiated into unique sequences and packages based on their characteristic geochemistry, aiding in the resolution of complex structural relationships and facies changes. In this study, we present new inorganic geochemistry data for four key wells in the South Australian (SA) Penola Trough and interpret the geochemistry data consistent with, and building on, the chemostratigraphic schema of Forbes et al. to demonstrate its utility and robustness. We then undertake inter-well wireline log correlations across the SA Penola Trough using the wells with chemostratigraphic data as controls.

A rare free silica-bearing micrometeorite

2021 ◽  
Author(s):  
Dafilgo Fernandes
Keyword(s):  
Mineral Assemblage ◽  
Frictional Heating ◽  
Upper Atmosphere ◽  
Carbonaceous Chondrites ◽  
Mineral Phase ◽  
Unique Type ◽  
Free Silica ◽  
System A ◽  
The Earth ◽  
Cosmic Spherules

<p>Extraterrestrial dust that reaches the Earth’s surface has shown to represent the diverse types of samples from different precursors, namely, asteroid complexes and cometary bodies from the solar system. A substantial amount of this dust that strikes the upper atmosphere is believed to have been lost due to frictional heating with air molecules. Cosmic spherules that are melted particles are some of the widely recognized micrometeorites that survived this catastrophic entry process; however, their primordial characteristics are altered from their precursors making it difficult to identify the precursors. An individual peculiar spherule MS-I35-P204 recovered from the Antarctica blue ice has been identified. The spherule has been segregated using magnetic separation method, mounted in epoxy, and examined using SEM, subsequently analysed under electron microprobe. It is surrounded by a thin magnetite rim, and also holds a single kamacite bead that protrudes out at its top. The interior mineralogy mostly constitutes of a bulk pyroxene normative glass (MnO>2wt%) with several vesicles. The rare mineral phase is a skeletal aggregate of free silica, bearing Fe nuggets embedded in a glass. An isolated narrow lath of forsterite appears to be chondritic and is observed as relict grain that is associated with an anomalous low Ca pyroxene (MnO ~1.3 wt%, FeO~13 wt%). Earlier, free silica has been reported in some chondritic meteorites particularly the Enstatite and Ordinary group, and also in some carbonaceous chondrites such as CM, CR, CH, and K. It profoundly forms a pod that encloses the ferromagnesian silicate in silica-bearing chondrules. The unusual mineral assemblage seen in this spherule thereby appears to constrain probably the unique type of its contributor which need to be studied.</p>

Numerical analysis of soil microstructure reveals conditions for natural attenuation in aged tar oil contaminated soil

2021 ◽  
Author(s):  
Pavel Ivanov ◽  
Karin Eusterhues ◽  
Kai Uwe Totsche
Keyword(s):  
Soil Solution ◽  
Contaminated Soil ◽  
Natural Attenuation ◽  
Pore Space ◽  
Total Porosity ◽  
Control Soil ◽  
Soil Microstructure ◽  
Cell Counts ◽  
Good Protection ◽  
Tar Oil

<p>Understanding of ongoing biogeochemical processes (natural attenuation) within contaminated soils is crucial for the development of plausible remediation strategies. We studied a tar oil contaminated soil with weak grass vegetation at a former manufactured gas plant site in Germany. Despite of the apparent toxicity (the soil contained up to 120 g kg<sup>-1</sup> petroleum hydrocarbons, 26 g kg<sup>-1</sup> toxic metals, and 100 mg kg<sup>-1</sup> polycyclic aromatic hydrocarbons), the contaminated layers have 3-5 times as much cell counts as an uncontaminated control soil nearby. To test, if the geometry of the pore space provides favourable living space for microorganisms, we applied scanning electron microscopy to the thin sections and calculated on sets of 15 images per layer three specific Minkowski functionals, connected to soil total porosity, interface, and hydraulic parameters.</p><p>Our investigation showed that the uncontaminated control soil has a relatively low porosity of 15-20 %, of which 50-70 % is comprised of small (< 15 µm) pores. These pores are poorly connected and show high distances between them (mean distance to the next pore 10 µm). The dominating habitats in the control soil are therefore created by small pores. They provide good protection from predators and desiccation, but input of dissolved organic C and removal of metabolic products are diffusion limited. Coarser pores (>15 µm) provide less space (< 50 % of total porosity) and solid surface area (< 20 %), are prone to desiccation and offer less protection from predators. However, they serve as preferential flow paths for the soil solution (input of nutrients) and are well aerated, therefore we expect the microbial activity in them to appear in “hot moments”, i.e. after rain events.</p><p>All layers of the contaminated profile have higher porosities (20-70 %) than the control. Coarse pores comprise 83-90 % of total pore area and create 34-52 % of total interface. Pores are also more connected and tortuous than in the control soil, which implies a better aeration and circulation of soil solution. The loops of pore channels may retain soil solution and be therefore preferably populated with microorganisms. The small (< 15 µm) pores comprise less than 17 % of total porosity but represent a substantial proportion of the interface (48-66 % vs 82-91 % in control). In the uppermost layer of the contaminated profile, such pores occur in plant residues, are close to the largest pores (mean distance to the next pore 4 µm) and therefore, along with good protection, are supplied with air, water, and non-tar C. In the middle of the profile, the small pores, presumably constantly filled with water, are located within dense tar pieces remote from the neighbouring pores (mean distance to the next pore 22 µm), and therefore, with hindered aeration and no supply of non-tar C, may create anaerobic domains of tar attenuation.</p><p>Our results show that the contaminated soil offers more favourable conditions for microorganisms than the control soil, probably because the hydrocarbons provide suitable energy and nutrition sources and a beneficial pore space geometry.</p>

Textural and mineralogical controls on temperature dependent SIP behavior during freezing and thawing

2021 ◽  
Author(s):  
Jonas K. Limbrock ◽  
Maximilian Weigand ◽  
Andreas Kemna
Keyword(s):  
Freezing Point ◽  
Pore Space ◽  
Thermal Characterization ◽  
Complex Frequency ◽  
Freezing And Thawing ◽  
Temperature Dependent ◽  
Measured Resistance ◽  
Ice Content ◽  
Water Saturated ◽  
Geoelectrical Methods

<p>Geoelectrical methods are increasingly being used for non-invasive characterization and monitoring of permafrost sites, since the electrical properties are sensitive to the phase change of liquid to frozen water. Here, electrical resistivity tomography (ERT) is most commonly applied, using resistivity as a proxy for various quantities, such as temperature or ice content. However, it is still challenging to distinguish between air and ice in the pore space of the rock based on resistivity alone due to their similarly low electrical conductivity. Meanwhile, geoelectrical methods that utilize electrical polarization effects to characterize permafrost are also being explored. For example, the usage of the spectral induced polarization (SIP) method, in which the complex, frequency-dependent impedance is measured, can reduce ambiguities in the subsurface conduction properties, considering the SIP signature of ice. These measurements seem to be suitable for the quantification of ice content (and thus the differentiation of ice and air), and for the improved thermal characterization of alpine permafrost sites. However, to improve the interpretation of SIP measurements, it is necessary to understand in more detail the electrical conduction and polarization properties as a function of temperature, ice content, texture, and mineralogy under frozen and partially frozen conditions.</p><p>In the study presented here, electrical impedance was measured continuously using SIP in the frequency range of 10 mHz to 45 kHz on various water-saturated solid rock and loose sediment samples during controlled freeze-thaw cycles (+20°C to -40°C). These measurements were performed on rock samples from different alpine permafrost sites with different mineralogical compositions and textures. For all samples, the resistance (impedance magnitude) shows a similar temperature dependence, with increasing resistance for decreasing temperature. Also, hysteresis between freezing and thawing behavior is observed for all measurements. During freezing, a jump within the temperature-dependent resistance is observed, suggesting a lowering of the freezing point to a critical temperature where an abrupt transition from liquid water to ice occurs. During thawing, on the other hand, there is a continuous decrease in the measured resistance, suggesting a continuous thawing of the sample. The spectra of impedance phase, which is a measure for the polarization, exhibit the same qualitative, well-known temperature-dependent relaxation behaviour of ice at higher frequencies (1 kHz - 45 kHz), with variations in shape and strength for different rock texture and mineralogy. At lower frequencies (1 Hz - 1 kHz), a polarization with a weak frequency dependence is observed in the unfrozen state of the samples. We interpret this response as membrane polarization, which likewise depends on the texture as well as on the mineralogy of the respective sample. This polarization response partially vanishes during freezing. Overall, the investigated SIP spectra do not only show a dependence on texture and mineralogy, but mainly a dependence on the presence of ice in the sample as well as temperature. This indicates the possibility of a thermal characterization, as well as a determination of the ice content, of permafrost rocks using SIP.</p>

Porosity Volumetrics and Pore Typing

2014 ◽  
Author(s):  
John H. Doveton
Keyword(s):  
Pore Volume ◽  
Pore Space ◽  
Bound Water ◽  
Primary Objective ◽  
Effective Porosity ◽  
Void Space ◽  
Face Centered ◽  
Petrophysical Logs ◽  
Saline Formation ◽  
Many Core

The primary objective of porosity estimations based on measurements made either from petrophysical logs or core is the volume of pore space within the rock, given simply by the equation: . . . Φ = Vp/Vb . . . The Greek letter, phi, is the standard symbol for porosity and is expressed in this equation as the ratio of the volume of void space (Vp) to the bulk volume of the rock (Vb). The simplest concepts of porosity are generally explained in terms of the packing of spheres as the sum of the pore volume of the space between the spheres. There are five basic arrangements of uniform-sized spheres that can be constructed: simple cubic, orthorhombic, double-nested, face-centered cubic, and rhombohedral packing (Hook, 2003). Each has a geometrically defined pore volume that represents an upper limit for granular rocks whose constituent grains have a variety of sizes and shapes and whose pore volumes have been reduced by compaction and diagenetic cements. This intergranular model is a useful starting point for the characterization of pores in clastic rocks and will be considered first, before reviewing the additional complexities of pore geometry introduced by dissolution in carbonate rocks. The solid framework of a sandstone consists of a nonconductive “matrix” dominated by quartz, but commonly with accessory nonconductive minerals, and conductive clay minerals, whose electrical properties are caused by cation exchange with ions in saline formation water. It is important to distinguish between connected and unconnected pores, as well as larger pores that sustain fluid movement in contrast to smaller pores filled with capillary-bound water. A graphic presentation of these components is widely used in the petrophysical literature as a reference basis to disentangle terminology that can be confusing and contradictory. In particular, the term “effective porosity” has different meanings that vary from one technical discipline to another. In their review of porosity terms, Wu and Berg (2003) concluded that many core analysts considered all porosity to be effective, log analysts excluded clay-bound water, while petroleum engineers excluded both clay-bound and capillary-bound from porosity consideration, thereby restricting effective porosity to pores occupied by mobile fluids.

THE STAG OILFIELD

1996 ◽  
Vol 36 (1) ◽  
pp. 130 ◽  
Author(s):  
J. Crowley ◽  
E.S. Collins
Keyword(s):  
Water Saturation ◽  
Vertical Section ◽  
Far East ◽  
Total Porosity ◽  
Analysis Data ◽  
Effective Porosity ◽  
Seismic Survey ◽  
North West ◽  
The North ◽  
Southeastern Margin

The Stag Oilfield is located approximately 65 km northwest of Dampier and 25 km southwest of the Wandoo Oilfield near the southeastern margin of the Dampier Sub-basin, on the North West Shelf of Western Australia,.The Stag-1 discovery well was funded by Apache Energy Ltd (formerly Hadson Energy Ltd), Santos Ltd and Globex Far East in June 1993 under a farmin agreement with BHP Petroleum Pty Ltd, Norcen International Ltd and Phillips Australian Oil Co. The well intersected a gross oil column of 15.5 m within the Lower Cretaceous M. australis Sandstone. The oil column intersected at Stag-1 was thicker than the pre-drill mapped structural closure.A 3D seismic survey was acquired over the Stag area in November 1993 to define the size and extent of the accumulation. Following processing and interpretation of the data, an exploration and appraisal program was undertaken. The appraisal wells confirmed that the oil column exceeds mapped structural closure and that there is a stratigraphic component to the trapping mechanism. Two of the appraisal wells were tested; Stag-2 flowed 1050 BOPD from a 5 m vertical section and Stag-6 flowed at 6300 BOPD on pump from a 1030 m horizontal section.Evaluation of the well data indicates the M. australis Sandstone at the Stag Oilfield is genetically related to the reservoir section at the Wandoo Oilfield. The reservoir consists of bioturbated glauconitic subarkose and is interpreted to represent deposition that occurred on a quiescent broad marine shelf. Quantitative evaluation of the oil-in-place has been hampered by the effects of glauconite on wireline log, routine and special core analysis data. Petrophysical evaluation indicates that core porosities and water saturations derived from capillary pressure measurements more closely match total porosity and total water saturation than effective porosity and effective water saturation.A development plan is currently being prepared and additional appraisal drilling in the field is expected.

Sign in / Sign up

Export Citation Format

Share Document