1,721,015 research outputs found
Multiphase flow for CO2 injection in saline aquifers and oil reservoirs: multi-scale observations
Full text linkGeologic CO2 storage has been identified as key to avoiding dangerous climate change. Carbon dioxide is also injected into oil fields to enhance oil recovery (EOR). The aim of EOR is to enhance recovery by reducing the residual or remaining oil inside the pore space of reservoir rocks while the aim of carbon storage is to maximize CO2 trapping. The processes governing trapping are fundamentally rooted in the wettability or capillarity of the system. The wettability of CO2-brine-rock systems will have a major impact on the management of carbon sequestration in subsurface geological formations. Therefore, understanding the wetting properties of storage sites and its impact on multiphase flow properties is crucial. In this work, three aspects of wettability were investigated: The impact of reservoir conditions on the capillarity and multiphase flow of CO2-brine-sandstone systems, the impact of fractional wettability on capillary trapping of CO2 in mixed-wet carbonate rocks and the physical basis of capillary trapping behaviour in mixed-wet systems using pore scale observations.
This thesis sought to resolve an outstanding uncertainty about the effective wetting properties of CO2-brine rock systems, characteristic of CO2 storage in saline aquifers. Contact angle measurement studies have reported sensitivity in wetting behaviour of CO2-brine-sandstone systems to pressure, temperature, and brine salinity. We report observations of the impact of reservoir conditions on the capillary pressure characteristic curve and relative permeability of a single Berea sandstone during drainage, CO2 displacing brine, through effects on the wetting state. Eight reservoir condition drainage capillary pressure characteristic curves were measured using CO2 and brine in a single fired Berea sandstone at pressures (5–20 MPa), temperatures (25–50 oC), and ionic strengths (0–5 mol kg-1 NaCl). A ninth measurement using a N2-water system provided a benchmark for capillarity with a strongly water wet system. The capillary pressure curves from each of the tests were found to be similar to the N2-water curve when scaled by the interfacial tension. Reservoir conditions were not found to have a significant impact on the capillary strength of the CO2-brine system during drainage through a variation in the wetting state. Two steady-state relative permeability measurements with CO2 and brine and one with N2 and brine similarly show little variation between conditions, consistent with the observation that the CO2-brine-sandstone system is water wetting and multiphase flow properties invariant across a wide range of reservoir conditions.
While saline aquifers make up the majority of the storage capacity, storage in oil reservoirs dominate the portfolio of existing projects due to favourable economics when combined with EOR. Observations and modelling have shown how capillary trapping leads to the immobilization of CO2 in saline aquifers, enhancing the security and capacity of storage. There are, however, no observations of trapping in rocks with a mixed-wet-state characteristic of hydrocarbon-bearing carbonate reservoirs, where most of the oil is reserved. Here, we report the observations of residually trapped CO2 in mixed-wet systems. The motivation of the work is to understand the residual trapping process underpinning the safety of geologic CO2 storage as it pertains to CO2 injection into oil fields. We found that residual trapping of supercritical CO2 in a limestone altered to a mixed-wet state with oil was significantly less than trapping in the unaltered rock. In unaltered samples, the trapping of CO2 and N2 were indistinguishable, with a maximum residual saturation of 24%. After the alteration of the wetting state, the trapping of N2 was reduced, with a maximum residual saturation of 19%. The trapping of CO2 was reduced even further, with a maximum residual saturation of 15%. Best-fit Land-model constants shifted from C = 1.73 in the water-wet rock to C = 2.82 for N2 and C = 4.11 for the CO2 in the mixed-wet rock.
We investigated the physical basis of this weakened trapping using pore scale observations of supercritical CO2 in mixed-wet carbonates. This is the first multi-scale study directly tying the underlying pore scale displacement physics to the residual trapping property used in flow modelling conventionally measured at larger scales. These are also the first observations at the pore scales of supercritical CO2 trapped in mixed wet rocks characteristic of oil field carbonates. In situ measurements of contact angles showed that CO2 varied from non-wetting to wetting throughout the pore space, with contact angles ranging 25o < θ< 127o; in contrast, N2 was non-wetting with a smaller range in contact angle 24o < θ< 68o. Observations of trapped ganglia morphology showed that this wettability allowed CO2 to create large, connected, ganglia by inhabiting small pores in mixed-wet rocks. The connected ganglia persisted after three pore volumes of brine injection, facilitating the desaturation that leads to decreased trapping relative to water-wet systems.
The result is a uniquely comprehensive and self-consistent dataset showing how the pore scale effects of wettability alteration lead to large scale decreases in trapping with significant relevance for CO2 storage in oil fields. The results indicate that plume migration will be less constrained by capillary trapping for CO2 storage projects using oil fields compared with those for saline aquifers. The work provides a significant enhancement in our knowledge of CO2 injection into oil fields for enhanced oil recovery and storage – the geologic unit currently dominating the deployment of the first mover generation of industrial scale storage projects globally.Open Acces
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
Characterisation of layer mechanism in immiscible three-phase flow of water-wet and mixed-wet rocks
No full textThe drainage layer is an oil layer that sits between gas and water in three-phase fluid systems. Flow in the layer is an important recovery mechanism responsible for reducing the residual oil saturation during immiscible gas injection in porous media. In sandpack and rock systems, where visual observations were previously impossible, the occurrence of drainage layer has long been disputed. In rocks where the drainage layer has only been indirectly observed, the drainage layer was inferred to have occurred when the oil relative permeability curve has a quadratic form with respect to oil saturation.
This study utilized a high-resolution X-ray microtomography technique to investigate drainage layers in both water-wet and mixed-wet sandstone rocks, arising from the simultaneous flow of three fluid phases: oil, water, and gas. Our experimental methodology requires no contrast agent to be added to the oil phase; hence, any potential issues posed by the addition of contrast agents (i.e., Iododecane) were avoided. We introduce a new image processing recipe whereby small features such as a thin oil layer can be effectively extracted and segmented from 3D microtomography images.
We performed a series of displacement tests on sandstone rock, whereby water first displaced oil, followed by gas, and then chase water injections. We studied the amount of oil and gas remaining as well as their respective morphologies and fluid configurations at the end of each displacement stage. The pore occupancy, residual saturation, fluid–fluid curvature, interfacial area, Euler characteristic, and specific surface area of the remaining fluid phases were investigated to elucidate the displacement mechanisms, such as double-drainage and flow in layers. Simultaneously, we studied the thickness of layers that formed in the system. We identified two methods to examine the contribution of layer flow from thickness information: the first was from the analysis of specific surface area of fluid in clusters, and the second was from the frequency distribution of thickness histogram.
The results show that in a water-wet system, the drainage layer has a major role in reducing the residual oil saturation to a level approaching zero. In contrast, in a mixed-wet system, the drainage layer is discontinuous, attributed to the discontinuity of water layer in the oil-wet pores. Since the drainage layer requires a simultaneous presence of continuous gas and water phase to form, the discontinuity in water phase contributes to the discontinuity of drainage layer. Therefore, a continuous drainage layer in mixed-wet rock is only possible in small pores that exhibit water-wetness.
In oil-wet pores of mixed-wet rocks, the piston-like flow of water during waterflooding is akin to the water flow in an oil-wet rock. The piston-like displacement of water initiates two important occurrences; the water trapping and continuous flow of oil layer, called the wettability layer. The water trapping occurs when there is a substantial increase in the thickness of wettability layer which triggers the snap-off at pore throats. Additionally, the wettability order of oil wet pores (of mixed-wet rocks) is oil-gas-water from most to least wetting thus justifying the presence of water trapping in the pore centers. Meanwhile, the wettability layer is thick and continuous and contributes considerably to reducing the residual oil saturation. The injection of gas does not disrupt the flow of oil via wettability layer; however, the flow of wettability layer is highly sensitive to the level of water saturation in the pores.
Finally, in exploratory work, flow in layers was modeled as an incompressible liquid flow through a pipe with four different cross-sectional shapes (sphere, cuboid, cylinder, and prism) whose dimensions were determined by matching the specific surface area of layers. Using computational fluid dynamics (CFD) to solve Navier-Stokes equations, we predicted the permeability of layers, thus avoiding assumptions from the use of a simplified Kozeny-Carman equation. The result matches well to the hydraulic conductance empirical model calculated from Ransohoff and Radke (1988). The relationship between specific surface area and hydraulic conductance was developed to simplify the requirement for solving the flow on a complex geometry of fluid-connected pathways. The comparison with Corey’s correlation shows that the generated relative permeability for layer flow matches at low residual oil saturation with the relative permeability modelled as a power-law with exponent 2.0. Using the workflow presented here, we can rapidly estimate the relative permeability of three-phase flow in rocks without requiring input from a pressure transducer. The model however requires further validation.Open Acces
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
Pore-scale heterogeneity in the mineral distribution and reactive surface area of permeable rocks
Full text linkThis work was carried out within the Qatar Carbonates and Carbon Storage Research Centre to improve the characterisation of carbonate reservoirs. 3D images obtained using X-ray micro-tomography were used to characterise heterogeneity in surface area in one sandstone and five carbonate rocks. Surface area measured from X-ray imagery were 1 to 2 orders of magnitude lower than nitrogen BET measurements. Roughness factor, i.e. the ratio of BET surface area to X-ray based surface area, was correlated to the presence of clay or microporosity. Comparing statistical distributions of surface area to those in published modelling studies showed that the common practice of leaving surface area and pore volume uncorrelated in a pore led to unrealistic combinations of surface area and pore volume. In Berea sandstone, constraining ratios of surface area to pore volume to a range of values between that of quartz-lined and five times that of clay-lined spheres appeared sufficient. Statistical analysis suggest that at 600 micrometre, the observations do not yet form a representative elementary volume. The development of dual-energy CT for phase identification is presented. Kaolinite, plagioclase, biotite, quartz, orthoclase, albite, dolomite, calcite, pyrite, magnetite, and hematite were scanned. The pairs: plagioclase-orthoclase, albite-quartz, biotite-orthoclase, and hematite-magnetite were not distinguished. Discrimination was possible for all other combinations. In Berea sandstone, kaolinite-smectite, quartz-albite, alkali feldspar-plagioclase, and pyrite-rutile were identified separately. Dual-energy CT has better contrast or capacity for larger samples than single-energy CT. Finally, an adsorption isotherm was measured from batch experiments with quartz and aqueous caesium chloride. 0.755 M of caesium remained in solution while 0.995 mole of caesium was adsorbed per square metre of quartz. Adsorption was expected to increase average attenuation seven-fold and contribute to 86% of the average attenuation in the image. CT images were collected of an equivalent column experiment with quartz powder and aqueous caesium chloride. However, adsorbed caesium was measured to contribute less than 1% of the average attenuation. This work has provided new capabilities to characterise pore-scale mineral and surface area heterogeneity and improve our understanding of CO2 storage in carbonates.Open Acces
Wettability characterisation of sandstone and carbonate rocks using X-ray micro-CT imaging
Full text linkThe macroscopic movement of subsurface fluids involved in CO2 storage, groundwater, and petroleum engineering applications is controlled by interfacial forces in the pores of rocks, micrometre to millimetre in length scale. Recent advances in physics based models of these systems has arisen from approaches simulating flow through a digital representation of the complex pore structure. X-ray microcomputed tomography (X-ray micro-CT) has emerged as the leading technology for creating these digital models and for uncovering the fundamental mechanisms governing multiphase transport in porous media. However, further progress in predicting multiphase flow in porous rocks is limited by a lack of approaches to characterising the spatial distribution of the wetting state within the pore structure, particularly when this wetting state is heterogeneous. This thesis focuses on wettability characterisation by means of X-ray micro-CT imaging. In particular, our work addresses the challenge of describing the wetting state of porous rocks, when it is non-uniform in space.
The characterisation of the wettability of such systems constitutes a challenge for the approaches currently available in literature, either from a theoretical or a practical point of view. The first part of this thesis presents a sensitivity analysis of the estimates of some multiphase fluid and solid properties of interest to image processing. This is a required step to better understanding the factors to take into account for developing a new approach to wettability characterisation which is both flexible in application and reliable. We assess the sensitivity of porosity, permeability, specific surface area, in situ contact angle measurements, fluid-fluid interfacial curvature measurements and mineral composition to processing choices. We compare the results obtained upon the employment of two processing pipelines: non-local means filtering followed by watershed segmentation; segmentation by a manually trained random forest classifier. Single-phase flow permeability, in situ contact angle measurements and mineral-to-pore total surface area are the most sensitive properties, as a result of the sensitivity to processing of the phase boundary identification task. Porosity, interfacial fluid-fluid curvature and specific (intensive) mineral descriptors are robust to processing. The sensitivity of the property estimates increases with the complexity of its definition and its relationship to boundary shape.
Following these results, we propose a novel approach to wettability characterisation rooted in the analysis of the relative fluid coverage of rock surfaces. We present a thermodynamic model explaining why changes in wettability determine a change in the extent of fluid-solid interfacial area. We thus develop a first workflow to wettability characterisation by analysing relative fluid coverage of rock surfaces as a function of fluid saturation. A first test of the approach on a water-wet and an altered wettability single-mineralogy sandstone (Bentheimer) rocks confirms its viability. In two multi-mineralogical (Berea) samples, one water-wet and the other altered with crude oil, the analysis of fluid surface coverage after imbibition revealed mineral specific wetting preferences only in the altered system. Clays and calcite preferentially alter to an oil-wet state, leading to mixed wettability in the rock.
In the final part of the thesis, an improved workflow to wettability characterisation consisting in the analysis of relative fluid coverage on a pore-by-pore basis is introduced. We demonstrate the approach with brine-oil fluid pairs on rocks with distinct lithologies (sandstone and carbonate) and wetting states (water-wet, intermediate-wet and mixed-wet). The distinct characteristics of the different wetting states encountered are identified. Finally, a quantitative measure of wetting at a pore level is proposed. This wetting index is then employed to build 3D wetting maps. These maps may be used as a deterministic input to pore scale modelling workflows and applied to all multiphase flow problems in porous media ranging from soil science to fuel cells.Open Acces
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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