Pore Fabric Research Articles

Seismic wave dispersion and attenuation resulting from multiscale wave-induced fluid flow in partially saturated porous media

SUMMARY Sedimentary rocks are typical heterogeneous porous media induced by fluid patches and pore fabric. It is well acknowledged that the wave-induced fluid flow (WIFF) at different scales will cause seismic wave dispersion and attenuation in a wide frequency range. Consequently, modelling wave dispersion and attenuation due to multiscale WIFF is of significance for reservoir characterization from multiscale geophysical measurement and interpretation. In this study, the multiscale WIFF in partially saturated porous media, including global, 3-D mesoscopic and squirt flows, are investigated. And we derive the wave equations by introducing the Rayleigh's spherical bubble oscillation and the porous grain models into Santos poroelasticity theory. Numerical simulations demonstrate that the multiscale model can interpret the transition of rock states as frequency increases and capture the broad-band seismic wave dispersion and attenuation characteristics, which are directly associated with the heterogeneity scale. Besides, the multiscale model can be degraded to a single- or dual-scale model under specific parameters. We validate the proposed model with board-band experimental data of partially saturated sandstones, confirming its comprehensive characterization of velocity dispersion and attenuation over a wider range of frequencies. Moreover, the model successfully interprets the unrelaxed state of partially saturated rock at ultrasound frequencies.

Physical and Chemical Stability of Nanoparticles in Ferrofluid Before and After Impregnation: Implications for Magnetic Pore Fabric Studies

AbstractMagnetic pore fabrics (MPFs) are a promising approach to explore the 3D pore space of rocks. There exist empirical relationships between the orientation, degree, and shape of MPF with pore space geometry and permeability anisotropy. Nevertheless, the precise nature of these relationships remains elusive. A common assumption in MPF studies is that ferrofluid uniformly fills the pore space, establishing a constant nanoparticle density and magnetic properties in space and time. However, this assumption is challenged by observations of particle sedimentation and time‐dependent magnetic properties, which render the quantitative interpretation of MPFs challenging. This study explores the physical and chemical stability of ferrofluid, mobility of particles after impregnation, and the magnetic properties of impregnated rocks over time. Water‐based ferrofluids are physically more stable than oil‐based ferrofluids, though with higher magnetic variability. In impregnated rock, magnetic nanoparticles display a certain degree of mobility, resulting in changes in MPF degree and shape. When ferrofluid is mixed with epoxy, particles are less mobile, though some changes were observed both during polymerization and aging. The susceptibility of ferrofluid‐epoxy mixtures is lower than for ferrofluid and carrier liquid at the same concentration. Interestingly, the susceptibility of impregnated rock increases over time, regardless of the ferrofluid used for the impregnation. Understanding and controlling these processes will enhance the reliability of MPF interpretations and increase the applicability of the method.

Characterization of pore space in Permo-Triassic sandstone from SW-Germany using the anisotropy of magnetic susceptibility

Pore space in siliciclastic rocks is one of the most important petrophysical properties in geothermal and hydrocarbon reservoir rock characterization. We used the anisotropy of magnetic susceptibility (AMS) of ferrofluid-impregnated Permo-Triassic sandstones of different Buntsandstein and Rotliegend facies as a proxy for pore space anisotropy and preferred flow direction as a case study for reservoir characterization. We compared the calculated ferrofluid porosity (2–21%) with He porosity (2–26%) and permeability (0.002–214 mD) and described the sediment microstructure using petrographic point-counting analysis. For water- and oil-based ferrofluid impregnation, we observed a positive correlation with He porosity and mass and susceptibility impregnation efficiency were used to control the quality of the impregnation process. Triaxial to oblate magnetic rock fabrics were mostly mimicked by the magnetic pore fabrics, except for some of the water-based ferrofluid impregnated samples, where magnetic ellipsoid shapes changed from oblate to prolate. AMS of the unimpregnated sandstones reflects well defined primary sedimentary to diagenetic fabrics with grain imbrication and cross bedding along with more laminated sedimentary structures. Deviation in ferrofluid-impregnated AMS axes orientation can be related either to the low anisotropy < 1.07 in sandstones from the Lower and Upper Buntsandstein, or the low impregnation efficiency. The mimicry is mostly better when the magnetic susceptibility of the sandstone is higher due to a higher concentration of phyllosilicates while micro-porosity is controlled by the clay fabric. A comparison of sediment petrography with magnetic pore fabrics suggests that the pore space is controlled by the bedding of the sandstones with mostly no preferred flow direction within the bedding plane.Graphical

Toward Better Understanding of the Ferrofluid Impregnation Process and Potential Artefacts—A Prerequisite for Reliable Interpretation of Magnetic Pore Fabrics

AbstractMagnetic pore fabrics (MPFs) serve as efficient proxy for pore alignment and preferential flow directions. Anisotropy of magnetic susceptibility of impregnated samples reflects the shape and arrangement of ferrofluid‐filled pores. Reliable interpretation requires that the entire connected pore space is reached by ferrofluid. This study investigates impregnation efficiencies of various methods (standard vacuum, pressure injection, magnetic flowthrough, and resin impregnation) and ferrofluids (water‐based versus oil‐based) on sandstone and calcarenite with different wettabilities. Mass‐based and susceptibility‐based impregnation efficiencies further evaluate the influence of impregnation time and flow rate. Because directional impregnation may introduce artificial fabrics, mutually perpendicular cores were measured, and subsamples served to capture spatial variability. Impregnation efficiencies vary between &lt;1% and &gt;100%, with resin impregnation and pressure injection using oil‐based ferrofluids being least efficient. Oil‐based ferrofluid generally leads to lower susceptibility‐based compared to mass‐based impregnation efficiencies, suggesting particle filtering. Water‐based ferrofluids show larger ranges of susceptibility‐based compared to mass‐based impregnation efficiencies. In particular, impregnation efficiencies &gt;100% exclusively occur for susceptibility‐based impregnation efficiencies of water‐based ferrofluid, using either standard vacuum or pressure injection methods. MPFs after magnetic flowthrough impregnation reflect the impregnation direction rather than pore fabric. A major challenge is to apply enough force to reach sufficiently high impregnation efficiencies without creating MPFs that do not represent pore fabrics. While we cannot define a best practice for ferrofluid impregnation yet, this study improves our understanding of the impregnation process, and provides experimental procedures to identify artifacts.

Quantitative Comparison of 3D Pore Space Properties With Magnetic Pore Fabrics—Testing the Ability of Magnetic Methods to Predict Pore Fabrics in Rocks

AbstractPore fabrics characterize the anisotropy of pore space in rocks and influence the direction of fluid flow. This is important in reservoir characterization, and petroleum and geothermal energy exploitation. X‐ray computed micro‐tomography (XRCT) is commonly used to analyze pore fabrics, but limited by the micron‐scale resolution for representative 1‐inch rock cores. The magnetic pore fabric (MPF) method has been proposed to capture pores down to 10 nm. Although empirical relationships between MPF and pore space properties or permeability anisotropy are available, their application is compromised by large variability. This study integrates He pycnometry and XRCT‐derived pore space models with MPFs, and provides a quantitative comparison for calcarenite (∼50 vol% porosity and complex pore structure), and molasse sandstone (10%–30% porosity and relatively homogeneous pore fabrics). The preferred orientation of pores obtained from XRCT is described by a total shape ellipsoid, calculated by summing the second‐order tensors reflecting the best‐fit ellipsoids of individual pores. This ellipsoid is then compared to the MPF magnitude ellipsoid in terms of fabric orientation, degree and shape of anisotropy. The MPF and total shape ellipsoids are generally coaxial. The MPF has a smaller anisotropy degree than the total shape ellipsoid, and their relationship depends on the ferrofluid properties. The anisotropy shapes show large variability. Nevertheless, the good agreement of principal directions in most samples makes MPFs a valuable and efficient complementary tool to analyze a large number of samples, in combination with XRCT on selected samples, for a field‐scale pore space characterization.

Magnetic Properties of Ferrofluid Change Over Time: Implications for Magnetic Pore Fabric Studies

AbstractThe anisotropy of magnetic susceptibility of ferrofluid‐impregnated samples is an efficient and powerful proxy for pore space anisotropy and preferred flow directions. One of the main assumptions in pore fabric studies is that all pores &gt;10–20 nm are homogeneously filled with ferrofluid, and that the ferrofluid has constant properties throughout the pore space and over time. If only part of the pore space is filled, this can lead to artifacts. Additionally, because magnetic anisotropy of a given pore space depends on fluid susceptibility, quantitative interpretations may be affected by the interval between impregnation and measurement time, or by the age of the ferrofluid during impregnation, unless fluid properties remain constant. A careful investigation of the time variation of ferrofluid properties and magnetic pore fabrics in synthetic and natural samples shows time‐dependence of susceptibility and hysteresis properties, related to dissolution of particle surfactants, and deterioration of colloidal stability. The latter leads to particle aggregation and sedimentation, which changes the anisotropy properties, and also affects impregnation behavior. Natural samples impregnated by oil‐based ferrofluid also experience a 2.5–3‐fold increase in mean susceptibility, with important consequences for the susceptibility‐based determination of impregnation efficiency. Based on our results, we recommend that ferrofluid properties are determined at the time of impregnation, and that samples are measured shortly after impregnation.

Microfabric evolution of coral sand foundations during seismic liquefaction using 3D images

Industrial micro X-ray computed tomography (μCT) is applied to link the macroscopic shaking table tests with the microscopic sand particle–pore structure evolution. Based on 3D image reconstruction, the variation in the particle–pore fabric of coral sand during earthquakes was studied. Moreover, the special evolution of coral sand was revealed through a comparative experiment using Fujian sand. The results show that the variation in the coordination number of the coral sand is positively correlated with the excess pore pressure ratio. The uniformity of the pore diameter decreases after earthquakes, and the reduction degree of the coral sand is smaller than that of Fujian sand. This result is related to the irregular shape and larger surface friction coefficient of coral sand particles. This study provides a deeper understanding of the microscopic evolution of coral sand liquefaction.

Ferrofluid Impregnation Efficiency and Its Spatial Variability in Natural and Synthetic Porous Media: Implications for Magnetic Pore Fabric Studies

Magnetic pore fabrics (MPF) are an efficient way to characterize pore space anisotropy, i.e., the average pore shape and orientation. They are determined by impregnating rocks with ferrofluid and then measuring their magnetic anisotropy. Obtaining even impregnation of the entire pore space is key for reliable results, and a major challenge in MPF studies. Here, impregnation efficiency and its spatial variability are systematically tested for natural (wood, rock) and synthetic (gel) samples, using oil- and water-based ferrofluids, and comparing various impregnation methods: percolation, standard vacuum impregnation, flowthrough vacuum impregnation, immersion, diffusion, and diffusion assisted by magnetic forcing. Seemingly best impregnation was achieved by standard vacuum impregnation and oil-based ferrofluid (76%), and percolation (53%) on rock samples; however, sub-sampling revealed inhomogeneous distribution of the fluid within the samples. Flowthrough vacuum impregnation yielded slightly lower bulk impregnation efficiencies, but more homogeneous distribution of the fluid. Magnetically assisted diffusion led to faster impregnation in gel samples, but appeared to be hindered in rocks by particle aggregation. This suggests that processes other than the mechanical transport of nanoparticles in the pore space need to be taken into account, including potential interactions between the ferrofluid and rock, particle aggregation and filtering. Our results indicate that bulk measurements are not sufficient to assess impregnation efficiency. Since spatial variation of impregnation efficiency may affect MPF orientation, degree and shape, impregnation efficiency should be tested on sub-samples prior to MPF interpretation.

Investigating the Aeroacoustic Properties of Porous Fabrics

The aeroacoustic properties of porous fabrics are investigated experimentally with the goal of finding a fabric that serves as an improved interface between wind tunnel flow and quiescent conditions. A total number of eight porous fabrics were investigated, namely, four glass fiber fabrics, two plain-weave Kevlar fabrics, and two modified plain Kevlar fabrics with their pores irregularly clogged. Two custom-made rigs were used to quantify the transmission loss (TL) and self-noise of all fabrics. The pores were found to serve as a low-resistance gateway for sound to pass through, hence enabling a low TL. The TL was found to increase with decreasing open area ratio (OAR), whereas other fabric properties had a minor impact on TL. The thread density was found to be a primary factor in determining the frequency range of porous fabrics’ self-noise, with the OAR potentially playing a secondary role in the self-noise levels. Fabrics with irregular pore distribution showed a more broadband self-noise signature associated with lower frequencies compared to fabrics with periodic pore patterns. Overall, fabrics with an irregular pore distribution or fabrics with increased thread density were identified as two potential ways to obtain superior aeroacoustic behavior compared to commonly used Kevlar fabrics.

Pore and Mineral Fabrics Control the Elastic Wave Velocities of Metapelite With Implications for Subduction Zone Tomography

AbstractSeismic tomography is used to infer compositional variations and environmental conditions in active subduction zones. In some regions, low P‐wave and S‐wave velocities and high are documented near and along the plate boundary, and are interpreted to reflect high pore fluid pressures in metasedimentary rocks. However, there are limited experimental data with which to constrain the in‐situ wave speeds of metasedimentary rocks, particularly under elevated pore pressure. To determine whether the velocities of metasedimentary rocks are consistent with low wave speeds and high , we measured the ultrasonic velocities of the Orocopia schist, a greenschist facies metapelite, in the laboratory. Velocities were measured under dry and saturated conditions, at effective pressures between 1 and 136 MPa, parallel and normal to foliation, and at room temperature. We find that, under fluid‐saturated conditions, the velocities of the Orocopia schist and are consistent with anomalous values in subduction zones ( km/s, km/s, and ) at effective pressures less than 10 MPa and normal to foliation. We used differential effective medium models and microscopy to quantify the contributions of mineralogy, mineral fabric, porosity, and pore shape to , , and their anisotropies. We find that, although mineralogy and mineral anisotropy impart some control on the wave velocities, aligned, small aspect ratio pores (0.005–0.03) are required to explain the measured velocities and high . The low and and high in subduction zones can be explained by phyllosilicate‐rich metapelitic rocks under near‐lithostatic pore pressure.

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

SUMMARYThe magnetic anisotropy exhibited by ferrofluid-impregnated samples serves as a proxy for their pore fabrics, and is therefore known as magnetic pore fabric (MPF). 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 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 MPFs 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.

Effect of porewater salinity on compression behaviors and hydraulic conductivity of soft marine clay

This study investigates the effect of porewater salinity on the compression behaviors of soft marine clay with multi-mineral compositions. A series of one-dimension compression tests were conducted with clays at different porewater salinity. The results indicate that the compression index decreases with increasing porewater salinity and can be described by a second-order polynomial equation, where the influence of porewater salinity is limited as the salinity increases to a threshold value. The normalized void indexes are taken from both ICL and EICL curves. It is believed that the compression behaviors of soft marine clay depend on the bentonite content. Hydraulic conductivity was back calculated based on empirical equations and is found to increase with increasing porewater salinity. Hydraulic conductivity is dominated by the inter-aggregated voids for soft marine clay at relatively high porewater salinity, and the influence of porewater salinity on the inter-aggregated voids of soil is limited at high salinity. A conceptual framework was presented to understand the changes in macroscopic soil behaviors with respect to the particles and pore fabrics of soil under the effect of porewater salinity.

FinIrrSDA: A 3‐D Model for Magnetic Shape and Distribution Anisotropy of Finite Irregular Arrangements of Particles With Different Sizes, Geometries, and Orientations

AbstractThe magnetic anisotropy carried by strongly magnetic particles such as magnetite or ferrofluid‐filled pores is generally composed of shape anisotropy and distribution anisotropy. Their relative importance in rocks depends on numerous factors and has been discussed controversially. A major challenge in estimating their contributions so far has been that models for distribution anisotropy only exist for regular arrangements of equal particles along lines or in planes. Because magnetite grains or pores in rocks display wide ranges of orientations, shapes, and sizes in generally irregular arrangements, new models are needed to describe distribution anisotropy for more realistic grain and pore assemblies. The model presented in this study, FinIrrSDA, calculates shape and distribution anisotropy for finite irregular assemblies of unequal particles with different orientations. Input parameters are provided as a table with x, y, and z coordinates of the particle centers and the lengths and orientations of the major, intermediate, and minor axes of best fit ellipsoids. The model output consists of two susceptibility tensors: (1) the shape anisotropy tensor and (2) the total tensor combining shape and distribution anisotropies. FinIrrSDA can be applied to a wide range of input data sets, including known structures of synthetic samples, particle analyses from tomography data, and, subject to certain assumptions, 2‐D images. The model will hopefully increase our understanding of the interplay between shape and distribution anisotropies in natural rocks and facilitate future interpretations of both the magnetic anisotropy carried by magnetite grains and magnetic pore fabrics.

Hydrothermal dolomite reservoirs in a fault system and the factors controlling reservoir formation-A case study of Lower Paleozoic carbonate reservoirs in the Gucheng area, Tarim Basin

The widespread hydrothermal activity in the Lower Paleozoic carbonate reservoirs of the Tarim Basin has received much attention. The discovery of hydrothermal dolomite reservoirs indicates that hydrothermal alteration may have a positive effect on reservoir development. However, the factors controlling hydrothermal dolomite reservoirs and their pore development characteristics are difficult to recognize and interpret. Based on the lithology, petrology, pore fabrics and geochemical characteristics of the Lower Paleozoic carbonate reservoirs in the Gucheng area, this research discloses that the origin of the carbonate reservoirs is related to alteration by hydrothermal brine. Further, hydrothermal dolomite reservoirs are strongly heterogeneous in different fault zone structures. This research divides the fault structures into fault cores (type 1) and damage zones (types 2 and 3). Type 1 is characterized by many breccias cemented by saddle dolomite, locally developed vugs or fractures in the cores, low density (1.7–2.8 g/cm3) and resistance (22.4–1075 Ω) in the log response, and shows abundant black-spotted vugs and sinusoidal fractures in formation microscanner images (FMI). Due to poor connectivity and high heterogeneity (permeability: 3.24 to 0.01 mD, average 0.59 mD; porosity: 5.1 to 0.7%, average 1.8%), type 1 hydrothermal dolomite reservoirs show both positive and negative hydrothermal fluid contributions. In the damage zones, especially in the areas near fault cores, type 2 is rich in fracture-pore systems in both the cores and FMI and is characterized by serrated fluctuations in acoustic waves and densities (2.4–2.8 g/cm3). Type2 performs well in terms of physical properties (permeability: 8.06 to 0.011 mD, average 0.837 mD; porosity: 6.6 to 0.65%, average 2.5%), indicating potentially favorable areas for the development of hydrothermal dolomite reservoirs. At the distal end of a damage zone or in poorly developed areas, type 3 features filling by hydrothermal products or higher densification (permeability: 0.77 to 0.0003 mD, average 0.125 mD; porosity: 1.6 to 0.2%, average 0.2%). The well logging response of type3, which has a density of around 2.8 g/cm3 and flat acoustic wave curves, is similar to that of the host rock. The differences in the development of hydrothermal dolomite reservoirs among diverse fault zones suggest that the damage zone (type 2) may be a potential area for hydrothermal dolomite reservoirs in the fault system. This study illustrates a model in which the hydrothermal fluid dissolves host rock in the proximal area and precipitates it at the distal end of the damage zone. The hydrothermal dolomite reservoir characteristics of different fault zones are also further interpreted and described. Therefore, different locations within a fault zone need to be considered and distinguished in studies of the factors controlling hydrothermal dolomite reservoirs in fault systems. This view may provide new ideas regarding whether hydrothermal activity in the Tarim Basin plays a positive role in the formation of reservoirs and offers new suggestions for the exploration of hydrothermal dolomite reservoirs in fault systems.

The Birgitta Field, Block 22/19a, UK North Sea

Abstract Birgitta Field was discovered by well 22/19-1 which encountered a 230 ft gas–condensate column in Triassic Skagerrak Formation and tested at a combined rate of 38.3 MMscfgd and 3750 bcpd. The tilted-fault block trap forms the crest of a Triassic ‘pod’ or mini-basin formed by salt withdrawal during Triassic extension, further rotated and eroded during Jurassic extension. Field extent is supported by apparent seismic hydrocarbon indications. An early oil charge was likely converted to condensate by Plio-Pleistocene gas influx and rapid burial, whilst an underlying palaeoresidual gas column reflects some trap leakage. Birgitta typifies certain Triassic reservoir characteristics in this part of the Central North Sea. The thick, relatively high net:gross reservoir comprises moderate to poorly sorted, sub-lithic to sub-arkosic sandstones deposited in a dryland braided fluvial system. Pore-lining chlorite overgrowths dominate the pore fabric, reducing pore throat sizes and contributing to appreciable levels of non-effective micro-porosity and hence elevated water saturation. Key petrophysical challenges are the accurate determination of effective porosity and water saturation. Birgitta approaches high pressure–high temperature conditions and illustrates some of the challenges of progressing small, unappraised field tie-backs. These include resource uncertainty, compartmentalization risk, infrastructure access and marginal economics. Evaluated for development several times, Birgitta presently remains undeveloped.

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

AbstractPore 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.

Impacts of microfacies type on reservoir quality and pore fabric anisotropy of the Nubia sandstone in the central Eastern Desert, Egypt

The Nubia sandstone has a high potential for water and hydrocarbons exploration in its subsurface extensions. The present work aims to study the Nubia sandstone at Gebel Duwi in the central Eastern Desert to characterize its lithostratigraphic setting, microfacies types, and its petrophysical properties. In addition, impacts of mineral composition and diagenetic history of the Nubia sandstone on its storage and deliverability capacities and on its hydraulic and electric pore fabrics will be estimated. The Nubia sandstone is subdivided into three informal members: a lower trough cross‐bedded, coarse‐grained with little mud braided fluvial member; a middle planar cross‐bedded sandstone with ferruginous siltstone intercalations of meandered fluvial member; and an upper gypsiferous coarsening upward clastic coastal to deltaic sediments. The microfacies analysis indicated that six microfacies are present: quartz arenite, subarkose, arkose, sublitharenite, ferruginous greywacke, and ferruginous siltstone microfacies. Several types of diagenetic processes affected the Nubia sandstone; cementation, compaction, and pressure solution are the main reservoir quality‐reducing factors, whereas fracturing, dissolution, and leaching out are the most important reservoir quality‐enhancing factors. For the present study, helium and water porosities, air permeability, and electric resistivities (for plugs drilled in three perpendicular directions for each block sample) were measured. Petrophysically, the studied Nubia sandstone can be summed up into three petrophysical facies: Facies‐1 (quartz arenite and subarkose), Facies‐2 (ferruginous subarkose to greywacke), and Facies‐3 (sublitharenite and arkose). The slight ferruginous and clay content of the second facies has slightly increased the grain density values and reduced its storage capacity and deliverability. Porosity (∅He) values which are measured by the helium injection for the first and third facies samples are good to excellent (14.6 ≤ ∅He ≤ 28.9%), whereas their permeability (k) values are good to excellent (106 ≤ k ≤ 2231 md). The second facies is characterized by relatively less petrophysical potential than the other two facies (av. ∅He = 19.2% and av. k = 557 md). The hydraulic pore anisotropy (λk) of the studied three petrophysical facies is isotropic to moderately anisotropic (1.02 ≤ λk ≤ 1.96), whereas the electric pore anisotropy (λE) is less intensive (1.11 ≤ λE ≤ 1.61).

First investigation of quartz and calcite shape fabrics in strained shales by means of X-ray tomography

We document the evolution of the 3D fabric of shale along a km-long strain gradient in the Jaca basin (Southern Pyrenees, Spain). With respect to the distance from a thrust, samples were collected in the cleavage-free domain, at the onset of the pencil-cleavage domain, within the pencil-cleavage domain and within the slaty-cleavage domain. By combining high resolution X-ray computed tomography (XCT) and energy dispersive X-ray spectroscopy (EDS), the morphology and the Shape Preferred Orientation (SPO) of thousands of quartz grains, calcite grains and pores was studied. In the least deformed samples, quartz and calcite display mean foliation parallel to bedding with comparable dispersion. In the pencil-cleavage domain, quartz foliation still follows the bedding while calcite foliation is mostly governed by cleavage. In the slaty-cleavage domain, calcite shows a much better organization, with foliation parallel to cleavage, mimicking closely the pore fabric. By contrast, the quartz shape fabric is much less defined, scattered between bedding and cleavage planes. This suggests that quartz grain act as rigid marker in the ductile matrix, while calcite grain orientation is governed by dissolution-precipitation processes.

Microstructural evolution and texture analysis of magnesium phosphate cement

AbstractThree‐dimensional quantitative image analysis from synchrotron X‐ray microcomputed tomography indicated a coarsening of the microstructure of magnesium potassium phosphate cements driven by crystallization of K‐struvite from the first amorphous product. Porosity and pore surface area increased because of the progressive build‐up of a network of elongated/tabular crystal domains, with density higher than the amorphous. The known increase in strength with time is thought to occur thanks to the overwhelming contribution of a developing interlocked lath‐shaped microstructure. Combined X‐ray and neutron diffraction texture analysis indicated that at least a fraction of K‐struvite nucleates at the surface of MgO grains, suggesting the intervention of more than one crystallization mechanism. The detected weak texture, compatible with a nearly random orientation of crystallites, and the isotropic pore fabric, are beneficial with respect to crack propagation.

Pore Fabric Anisotropy of the Cambrian–Ordovician Nubia Sandstone in the Onshore Gulf of Suez, Egypt: A Surface Outcrop Analog

Integrated studies on pore fabric anisotropy have increased the general understanding of fluid flow patterns through reservoir rocks. In this study, pore anisotropy was studied based on measuring permeability and formation resistivity factors in vertical and horizontal directions for 130 plug samples from a total of 65 oriented block samples. These samples were representatively collected for the Nubia sandstones C and D in the western southern onshore of the Gulf of Suez. In addition, the porosity was measured using two techniques, namely water and helium injection. The effective pore radius r35 and the pore radius of the displacement pressure (rdp) were also measured. Petrographic studies of some representative thin sections and scanning electron microscope studies (SEM) were applied to study the mineral compositions of the studied samples to declare the most important porosity-reducing and porosity-enhancing diagenetic factors. The results show that the studied samples can be categorized into three rock types (RRTs), namely quartz arenite (RRT1), quartz wacke (RRT2) and mudstone (RRT3). The best storage and flow capacity was assigned to the RRT1 samples, whereas the least quality was assigned to the RRT3 samples. This could be attributed to wide pore throat distributions and the anisotropy of pore spaces due to the presence of a vertical subsidiary fracture system that dominated in many samples. This system was enhanced by introducing authigenic kaolinite as pore-filling clay minerals, causing the reduction in vertical permeability but supporting the vertical electric current flow. For the RRT3 samples, this vertical micro-fracture system was reduced by silica cementation which caused the reduction in both fluid and electric current flow. The pore anisotropy of most of the studied samples is in the range of slight anisotropy with some exceptions in the RRT3 samples which are characterized by moderate anisotropy.