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X-ray micro-CT image data of heterogeneous sandstone
No full textRaw and processed tomographic image data of heterogeneous sandstone samples, supplementary to the manuscript entitled, 'Optimal X-ray micro-CT image based methods for porosity and permeability quantification in heterogeneous sandstones' published in Geophysical Journal International (2020) Vol. 223(2)</span
Characterising the permeability and structure of fluid-escape conduits in sedimentary basins - application to geological carbon sequestration
No full textIncreased greenhouse gas emissions entering the atmosphere and hydrosphere are causing changes to global climate. Geological carbon sequestration is a proven technology, used to reduce anthropogenic emissions from the atmosphere. However, there are concerns about the unintended migration of CO2 from sub-surface storage reservoirs. Fluid-escape structures, which act as conduits for pressure-driven fluids, are observed in sedimentary basins globally. These structures can extend over 500 m across and intrude vertically through kilometres of sedimentary overburden. The quantitative assessment and nature of fluid-escape conduits are currently poorly constrained. Here we sample and characterise analogous onshore field outcrop analogues in Panoche Hills, California and Varna, Bulgaria to complement the study of active structures in the Witch Ground Basin, Central North Sea. A key aim is to quantify permeability and determine the process mechanisms of fluid flow through focused fluid conduits. Here we generate an accurate, repeatable and upscalable 3D X-ray micro-computed tomography (XCT) image-based methodology workflow to calculate porosity, effective porosity, and permeability of fluid-escape conduit samples. During fieldwork and sampling, the geometry, distribution, physical interaction with host-rocks and 3D properties are determined. Given the large scale of the structures (>150 m wide), porosity and permeability transects are performed across the intrusions and their host sediments, to characterise natural variability and identify preferential fluid flow pathways.
Studies of sand intrusions in the Panoche Hills reveal permeability heterogeneity is largely controlled by silica cementation processes linked to the drainage of pore waters from silica-rich host rock sediments during intrusion formation. Sub-vertically orientated intrusions have reduced permeability due to the incorporation of clay and silt host rock sediments into the matrix of intrusions, and subsequent dewatering, grain compaction and silica Opal-A to Opal-CT transformation. Further, the investigation of fluid-escape structures in Varna and Panoche Hills highlights a three orders of magnitude reduction in permeability of unconsolidated host rock sediments caused by the transformation of methane gas by anaerobic oxidation to carbonates. The mapping of carbonate pipes revealed that focused fluid flow process mechanisms are not restricted to fine-grained host rock strata, and can occur in unconsolidated sediments.
High-resolution 2D and 3D seismic reflection data, integrated with sediment core data, are used to characterise the Scanner Pockmark Complex, Central North Sea. The study has revealed that gas flows vertically upwards through chimneys, directly observed as a series of interconnected fractures. Previous studies interpret seismic chimneys as vertical conduits in hydraulic connection to a single, deeper zone of overpressured fluid. Here we observe that fluid overpressure generation, leading to chimney and pockmark genesis, represents a complex system with gas-bearing zones at multiple depth intervals. When integrated into a multi-disciplinary approach, these findings can improve our understanding of focused fluid flow within the shallow overburden, for applications to shallow geohazard assessment and carbon sequestration
Assessing the carbon sequestration potential of basalt using X-ray micro-CT and rock mechanics
No full textMineral carbonation in basaltic rock provides a permanent storage solution for the mitigation of anthropogenic CO2 emissions in the atmosphere. 3D X-ray micro-CT (XCT) image analysis is applied to a core sample from the main basaltic reservoir of the CarbFix site in Iceland, which obtained a connected porosity of 2.05–8.76%, a reactive surface area of 0.10–0.33mm−1 and a larger vertical permeability (2.07×10−10m2) compared to horizontal permeability (5.10×10−11m2). The calculations suggest a CO2 storage capacity of 0.33 Gigatonnes at the CarbFix pilot site. The XCT results were compared to those obtained from a hydromechanical test applied to the same sample, during which permeability, electrical resistivity and volumetric deformation were measured under realistic reservoir pressure conditions. It was found that permeability is highly stress sensitive, dropping by two orders of magnitude for a −0.02% volumetric deformation change, equivalent to a mean pore diameter reduction of 5μm. This pore contraction was insufficient to explain such a permeability reduction according to the XCT analysis, unless combined with the effects of clay swelling and secondary mineral pore clogging. The findings provide important benchmark data for the future upscaling and optimisation of CO2 storage in basalt formations
Optimal X-ray micro-CT image based methods for porosity and permeability quantification in heterogeneous sandstones
Full text link3-D X-ray micro-CT (XCT) is a non-destructive 3-D imaging method, increasingly used for a wide range of applications in Earth Science. An optimal XCT image-processing workflow is derived here for accurate quantification of porosity and absolute permeability of heterogeneous sandstone samples using an assessment of key image acquisition and processing parameters: image resolution, segmentation method, representative elementary volume (REV) size and fluid-simulation method. XCT image-based calculations obtained for heterogeneous sandstones are compared to two homogeneous standards (Berea sandstone and a sphere pack), as well as to the results from physical laboratory measurements. An optimal XCT methodology obtains porosity and permeability results within ±2 per cent and vary by one order of magnitude around the direct physical measurements, respectively, achieved by incorporating the clay fraction and cement matrix (porous, impermeable components) to the pore-phase for porosity calculations and into the solid-phase for permeability calculations. Two stokes-flow finite element modelling (FEM) simulation methods, using a voxelized grid (Avizo) and tetrahedral mesh (Comsol) produce comparable results, and similarly show that a lower resolution scan (∼5 μm) is unable to resolve the smallest intergranular pores, causing an underestimation of porosity by ∼3.5 per cent. Downsampling the image-resolution post-segmentation (numerical coarsening) and pore network modelling both allow achieving of a REV size, whilst significantly reducing fluid simulation memory requirements. For the heterogeneous sandstones, REV size for permeability (≥1 mm3) is larger than for porosity (≥0.5 mm3) due to tortuosity of the fluid paths. This highlights that porosity should not be used as a reference REV for permeability calculations. The findings suggest that distinct image processing workflows for porosity and permeability would significantly enhance the accurate quantification of the two properties from XCT. </p
Experimental assessment of the stress-sensitivity of combined elastic and electrical anisotropy in shallow reservoir sandstones
Full text linkSeismic and electromagnetic properties are generally anisotropic, depending on the microscale rock fabric and the macroscale stress field. We have assessed the stress-dependent anisotropy of poorly consolidated (porosity of approximately 0.35) sandstones (broadly representative of shallow reservoirs) experimentally, combining ultrasonic (0.6 MHz P-wave velocity, VP, and attenuation 1/QP) and electrical resistivity measurements. We used three cores from an outcrop sandstone sample extracted at 0°, 45°, and 90° angles with respect to the visible geologic bedding plane and subjected them to unloading/loading cycles with variations of the confining (20–35 MPa) and pore (2–17 MPa) pressures. Our results indicate that stress field orientation, loading history, rock fabric, and the measurement scale, all affect the elastic and electrical anisotropies. Strong linear correlations (R2 > 0.9) between VP, 1/QP, and resistivity in the three considered directions suggest that the stress orientation similarly affects the elastic and electrical properties of poorly consolidated, high-porosity (shallow) sandstone reservoirs. However, resistivity is more sensitive to pore pressure changes (effective stress coefficients n > 1), whereas P-wave properties provide simultaneous information about the confining (from VP, with n slightly less than 1) and pore pressure (from 1/QP, with n slightly greater than 1) variations. We found n is also anisotropic for the three measured properties because a more intense and rapid grain rearrangement occurs when the stress field changes result from oblique stress orientations with respect to rock layering. Altogether, our results highlighted the potential of joint elastic-electrical stress-dependent anisotropy assessments to enhance the geomechanical interpretation of reservoirs during production or injection activities
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Geophysical early warning of salt precipitation during geological carbon sequestration
Full text linkSequestration of industrial carbon dioxide (CO2) in deep geological saline aquifers is needed to mitigate global greenhouse gas emissions; monitoring the mechanical integrity of reservoir formations is essential for effective and safe operations. Clogging of fluid transport pathways in rocks from CO2-induced salt precipitation reduces injectivity and potentially compromises the reservoir storage integrity through pore fluid pressure build-up. Here, we show that early warning of salt precipitation can be achieved through geophysical remote sensing. From elastic P- and S-wave velocity and electrical resistivity monitoring during controlled laboratory CO2 injection experiments into brine-saturated quartz-sandstone of high porosity (29%) and permeability (1660 mD), and X-ray CT imaging of pore-scale salt precipitation, we were able to observe, for the first time, how CO2-induced salt precipitation leads to detectable geophysical signatures. We inferred salt-induced rock changes from (i) strain changes, (ii) a permanent ~ 1.5% decrease in wave velocities, linking the geophysical signatures to salt volume fraction through geophysical models, and (iii) increases of porosity (by ~ 6%) and permeability (~ 7%). Despite over 10% salt saturation, no clogging effects were observed, which suggests salt precipitation could extend to large sub-surface regions without loss of CO2 injectivity into high porosity and permeability saline sandstone aquifers
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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