73 results found
Ansari H, Joss L, Hwang J, et al., 2020, Supercritical adsorption in micro- and meso-porous carbons and its utilisation for textural characterisation, Microporous and Mesoporous Materials, Vol: 308, ISSN: 1387-1811
Understanding supercritical gas adsorption in porous carbons requires consistency between experimental measurements at representative conditions and theoretical adsorption models that correctly account for the solid’s textural properties. We have measured unary CO2 and CH4 adsorption isotherms on a commercial mesoporous carbon up to 25 MPa at 40 °C, 60 °C and 80 °C. The experimental data are successfully described using a model based on the lattice Density Functional Theory (DFT) that has been newly developed for cylindrical pores and used alongside Ar (87K) physisorption to extract the representative pore sizes of the adsorbent. The agreement between model and experiments also includes important thermodynamic parameters, such as Henry constants and the isosteric heat of adsorption. The general applicability of our integrated workflow is validated by extending the analysis to a comprehensive literature data set on a microporous activated carbon. This comparison reveals the distinct pore-filling behaviour in micro- and mesopores at supercritical conditions, and highlights the limitations associated with using slit-pore models for the characterisation of porous carbons with significant amounts of mesoporosity. The lattice DFT represents a departure from simple adsorption models, such as the Langmuir equation, which cannot capture pore size dependent adsorption behaviour, and a practical alternative to molecular simulations, which are computationally expensive to implement.
Pini R, Joss L, Hosseinzadeh Hejazi SA, 2020, Quantitative imaging of gas adsorption equilibrium and dynamics by X-ray Computed Tomography, Adsorption, ISSN: 0929-5607
Kurotori T, Zahasky C, Benson S, et al., 2020, Description of chemical transport in laboratory rock cores using the continuous random walk formalism, Water Resources Research, ISSN: 0043-1397
Nguyen HGT, Sims CM, Toman B, et al., 2020, A reference high-pressure CH(4)adsorption isotherm for zeolite Y: results of an interlaboratory study, ADSORPTION-JOURNAL OF THE INTERNATIONAL ADSORPTION SOCIETY, Pages: 1-14, ISSN: 0929-5607
This paper reports the results of an international interlaboratory study led by the National Institute of Standards and Technology (NIST) on the measurement of high-pressure surface excess methane adsorption isotherms on NIST Reference Material RM 8850 (Zeolite Y), at 25 °C up to 7.5 MPa. Twenty laboratories participated in the study and contributed over one-hundred adsorption isotherms of methane on Zeolite Y. From these data, an empirical reference equation was determined, along with a 95% uncertainty interval (Uk=2). By requiring participants to replicate a high-pressure reference isotherm for carbon dioxide adsorption on NIST Reference Material RM 8852 (ZSM-5), this interlaboratory study also demonstrated the usefulness of reference isotherms in evaluating the performance of high-pressure adsorption experiments.
Iruretagoyena D, Bikane K, Sunny N, et al., 2020, Enhanced selective adsorption desulfurization on CO2 and steam treated activated carbons: Equilibria and kinetics, Chemical Engineering Journal, Vol: 379, Pages: 1-11, ISSN: 1385-8947
Activated carbons (ACs) show great potential for selective adsorption removal of sulfur (SARS) from hydrocarbon fuels but require improvements in uptake and selectivity. Moreover, systematic equilibria and kinetic analyses of ACs for desulfurization are still lacking. This work examines the influence of modifying a commercial-grade activated carbon (AC) by CO2 and steam treatment for the selective adsorption removal of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) at 323 K. An untreated AC and a charcoal Norit carbon (CN) were used for comparative purposes. Physicochemical characterization of the samples was carried out by combining N2-physisorption, X-ray diffractometry, microscopy, thermogravimetric and infrared analyses. The steam and CO2 treated ACs exhibited higher sulfur uptakes than the untreated AC and CN samples. The steam treated AC appears to be especially effective to remove sulfur, showing a remarkable sulfur uptake (~24 mgS·gads−1 from a mixture of 1500 ppmw of DBT and 1500 ppm 4,6-DMDBT) due to an increased surface area and microporosity. The modified ACs showed similar capacities for both DBT and the sterically hindered 4,6-DMDBT molecules. In addition, they were found to be selective in the presence of sulfur-free aromatics and showed good multicycle stability. Compared to other adsorbents, the modified ACs exhibited relatively high adsorption capacities. The combination of batch and fixed bed measurements revealed that the adsorption sites of the samples are characterized as heterogeneous due to the better fit to the Freundlich isotherm. The kinetic breakthrough profiles were described by the linear driving force (LDF) model.
Hwang J, Pini R, 2019, Supercritical CO2 and CH4 uptake by illite-smectite clay minerals, Environmental Science & Technology, Vol: 53, Pages: 11588-11596, ISSN: 0013-936X
Clay minerals abound in sedimentary formations and the interaction of reservoir gases with their sub-micron features has direct relevance to many geo-energy applications. The quantification of gas uptake over a broad range of pressures is key towards assessing the significance of these physical interactions on enhancing storage capacity and gas recovery. We report a systematic investigation of the sorption properties of three source clay minerals – Na-rich montmorillonite (SWy-2), illite-smectite mixed layer (ISCz-1), and illite (IMt-2) – using CO2 and CH4 up to 30 MPa at 25 to 115 °C. The textural characterization of the clays by gas physisorption indicates that micropores are only partly accessible to N2 (77 K) and Ar (87 K), while larger uptakes are measured with CO2 (273 K) in the presence of illite. The supercritical excess sorption experiments confirm these findings, while revealing differences in uptake capacities that originate from the clay-specific pore size distribution. The Lattice Density Functional Theory (LDFT) model describes accurately the measured sorption isotherms by using a distribution of properly weighted slit pores and clay-specific solid-fluid interaction energies, which agree with isosteric heats of adsorption obtained experimentally. The model indicates that the maximum degree of pore occupancy is universal to the three clays and the two gases, and it depends solely on temperature, reaching values near unity at the critical temperature. These observations greatly support the model's predictive capability for estimating gas adsorption on clay-bearing rocks and sediments.
Wenning QC, Madonna C, Kurotori T, et al., 2019, Spatial mapping of fracture aperture changes with shear displacement using X-ray computerized tomography, Journal of Geophysical Research: Solid Earth, Vol: 124, Pages: 7320-7340, ISSN: 2169-9313
The shearing of fractures can be a significant source of permeability change by altering the distribution of void space within the fracture itself. Common methods to estimate the effects of shearing on properties, such as aperture, roughness, and connectivity are incapable of providing these observations in‐situ. Laboratory protocols are needed that enable measurements of the spatial structure of the fracture aperture field in the medium, non‐invasively. Here, we investigate changes in rough‐walled Brazilian‐induced tensile fracture aperture distribution with progressive shear displacement in Westerly granite and Carrara marble using a novel X‐ray transparent core‐holder. The so‐called calibration‐free missing attenuation method is applied to reconstruct highly‐resolved (sub‐millimeter) fracture aperture maps as a function of displacement (0 to 5.75 mm) in induced fractures. We observe that shearing increases the core‐averaged fracture aperture and significantly broadens the distribution of local values, mostly towards higher apertures. These effects are particularly strong in Westerly granite and may be the result of the higher initial roughness of its fracture surfaces. Also, while the correlation length of the aperture field increases in both parallel and perpendicular directions, significant anisotropy is developed in both samples with the progression of shearing. The results on Westerly granite provide a direct indication that fracture aperture remains largely unaffected until 1~mm of displacement is achieved, which is important when estimating permeability enhancement due to natural and induced shear displacement in faults.
Lin Q, Bijeljic B, Berg S, et al., 2019, Minimal surfaces in porous media: Pore-scale imaging of multiphase flow in an altered-wettability Bentheimer sandstone, Physical Review E, Vol: 99, Pages: 063105-1-063105-13, ISSN: 1539-3755
High-resolution x-ray imaging was used in combination with differential pressure measurements to measurerelative permeability and capillary pressure simultaneously during a steady-state waterflood experiment on asample of Bentheimer sandstone 51.6 mm long and 6.1 mm in diameter. After prolonged contact with crude oil toalter the surface wettability, a refined oil and formation brine were injected through the sample at a fixed total flowrate but in a sequence of increasing brine fractional flows. When the pressure across the system stabilized, x-raytomographic images were taken. The images were used to compute saturation, interfacial area, curvature, andcontact angle. From this information relative permeability and capillary pressure were determined as functionsof saturation. We compare our results with a previously published experiment under water-wet conditions. Theoil relative permeability was lower than in the water-wet case, although a smaller residual oil saturation, ofapproximately 0.11, was obtained, since the oil remained connected in layers in the altered wettability rock.The capillary pressure was slightly negative and 10 times smaller in magnitude than for the water-wet rock,and approximately constant over a wide range of intermediate saturation. The oil-brine interfacial area wasalso largely constant in this saturation range. The measured static contact angles had an average of 80◦ with astandard deviation of 17◦. We observed that the oil-brine interfaces were not flat, as may be expected for a verylow mean curvature, but had two approximately equal, but opposite, curvatures in orthogonal directions. Theseinterfaces were approximately minimal surfaces, which implies well-connected phases. Saddle-shaped menisciswept through the pore space at a constant capillary pressure and with an almost fixed area, removing most ofthe oil.
Hosseinzadeh Hejazi SA, Shah S, Pini R, 2019, Dynamic measurements of drainage capillary pressure curves in carbonate rocks, Chemical Engineering Science, Vol: 200, Pages: 268-284, ISSN: 1873-4405
The heterogeneity of rocks represents a challenge for interpreting and using outcomes from multiphase-flow experiments carried out on laboratory samples. While the capillary pressure–saturation function, , is known to vary spatially and cause local saturation development during immiscible displacements, its variation remains difficult to measure. This is particularly challenging for rocks with complex fabrics, such as carbonates. Here, we present a workflow for the dynamic measurement of core- and subcore-scale drainage curves in heterogeneous porous media. Multi-rate, two-phase core-flooding tests are conducted on three carbonate rocks with direct observations of local saturation data. The interpretation of the experiments is done by fitting the parameters of the curve, while describing both steady-state saturation and pressure profiles with a detailed one-dimensional model that accounts for the variation of subcore-scale properties in the direction of displacement. Workflow validation is achieved by means of synthetic data, thereby demonstrating the uniqueness of the solution of the resulting multi-objective optimisation problem. The model reproduces accurately experimental data on the three rocks and enables computing the effective core-scale curve in the limit of zero velocity, as it would be expected during a porous-plate experiment. The output of the proposed technique is however much richer and includes the relative curve that is universal and independent of the specific pattern of heterogeneity, in addition to a set of scaling factors. The latter describe the distribution of thecurves at the subcore-scale due to heterogeneity and form the statistical basis needed for upscaling studies.
Pini R, Joss L, 2019, See the unseen: applications of imaging techniques to study adsorption in microporous materials, Current Opinion in Chemical Engineering, Vol: 24, Pages: 37-44, ISSN: 2211-3398
Chemical processes that incorporate porous solids (adsorbents and catalysts) include transient and spatially localised phenomena. Their thorough understanding requires the development of experimental methods that enable in situ observations made under process conditions. Imaging techniques are providing an unprecedented level of detail in the study of adsorption, both in the gas and liquid phase, including spatiotemporal measurements of adsorption equilibrium, kinetics and dynamics in microporous solids. The available techniques range from microscopy to multidimensional spectroscopy and tomography, and enable the development of so-called digital workflows, where adsorbent properties can be computed from spatially distributed adsorption uptake curves and isotherms. The widespread applicability of these methods is expected to pave the way towards resolving the complex structure of adsorption systems, from nano-scale to macro-scale, while providing new fundamental understanding of adsorption processes operando.
Zahasky C, Kurotori T, Pini R, et al., 2019, Positron emission tomography in water resources and subsurface energy resources engineering research, Advances in Water Resources, Vol: 127, Pages: 39-52, ISSN: 0309-1708
Recent studies have demonstrated that positron emission tomography (PET) is a valuable tool for in-situ characterization of fluid transport in porous and fractured geologic media at the laboratory scale. While PET imaging is routinely used for clinical cancer diagnosis and preclinical medical research—and therefore imaging facilities are available at most research institutes—widespread adoption for applications in water resources and subsurface energy resources engineering have been limited by real and perceived challenges of working with this technique. In this study we discuss and address these challenges, and provide detailed analysis highlighting how positron emission tomography can complement and improve laboratory characterization of different subsurface fluid transport problems. The physics of PET are reviewed to provide a fundamental understanding of the sources of noise, resolution limits, and safety considerations. We then layout the methodology required to perform laboratory experiments imaged with PET, including a new protocol for radioactivity dosing optimization for imaging in geologic materials. Signal-to-noise and sensitivity analysis comparisons between PET and clinical X-ray computed tomography are performed to highlight how PET data can complement more traditional characterization methods, particularly for solute transport problems. Finally, prior work is critically reviewed and discussed to provide a better understanding of the strengths and weakness of PET and how to best utilize PET-derived data for future studies.
Kurotori T, Zahasky C, Hosseinzadeh Hejazi SA, et al., 2019, Measuring, imaging and modelling solute transport in a microporous limestone, Chemical Engineering Science, Vol: 196, Pages: 366-383, ISSN: 1873-4405
The analysis of dispersive flows in heterogeneous porous media is complicated by the appearance of anomalous transport. Novel laboratory protocols are needed to probe the mixing process by measuring the spatial structure of the concentration field in the medium. Here, we report on a systematic investigation of miscible displacements in a microporous limestone over the range of Péclet numbers, . Our approach combines pulse-tracer tests with the simultaneous imaging of the flow by Positron Emission Tomography (PET). Validation of the experimental protocol is achieved by means of control experiments on random beadpacks, as well as by comparing observations with both brine- and radio-tracers (labelled with 11C or 18F). The application of residence time distribution functions reveals mass transport limitations in the porous rock in the form of a characteristic flow-rate effect. Two transport models, namely the Advection Dispersion Equation (ADE) and the Multi-Rate Mass Transfer (MRMT) model, are thoroughly evaluated with both the experimental breakthrough curves and the internal concentration profiles. We observe that the dispersion coefficient scales linearly with the Péclet number for both porous systems. The tracer profiles acquired on the rock sample are successfully described upon application of the MRMT model that uses two representative grain sizes and a fraction of intra-granular pore space that is independent of the fluid velocity. The analysis of the PET images evidences the presence of macrodispersive spreading caused by subcore-scale heterogeneities, which contribute significantly to the value of the estimated core-scale dispersivity. This effect can be significantly reduced upon application of the ‘dispersion-echo’ technique, which enables decoupling the effects of spreading and mixing in heterogeneous porous media. These observations are likely to apply to any laboratory-scale rock sample and the approach presented here provides a
Joss L, Pini R, 2019, 3D mapping of gas physisorption for the spatial characterisation of nanoporous materials, ChemPhysChem, Vol: 20, Pages: 524-528, ISSN: 1439-4235
Nanoporous materials used in industrial applications (e.g., catalysis and separations) draw their functionality from properties at the nanoscale (1 – 10 Å). When shaped into a technical form these solids reveal spatial variations in the same properties over much larger length scales (1 µm – 1 cm). The multiscale characterization of these systems is impaired by the trade‐off between sample size and image resolution that is bound to the use of most imaging techniques. We show here the application of X‐ray computed tomography for the non‐invasive spatial characterization of a zeolite/activated carbon adsorbent bed across three orders of magnitude in scale. Through the unique combination of gas adsorption isotherms measured locally and their interpretation by physisorption analysis, we determine three‐dimensional maps of the specific surface area and micropore volume. We further use machine learning to identify and locate the materials within the packed bed. This novel ability to reveal the extent of heterogeneity in technical porous solids will enable a deeper understanding of their function in industrial reactors. Such developments are essential towards bridging the gap between material research and process design.
Liyanage R, Cen J, Krevor S, et al., 2019, Multidimensional observations of dissolution-driven convection in simple porous media using X-ray CT scanning, Transport in Porous Media, Vol: 126, Pages: 355-378, ISSN: 0169-3913
We present an experimental study of dissolution-driven convection in a three-dimensional porous medium formed from a dense random packing of glass beads. Measurements are conducted using the model fluid system MEG/water in the regime of Rayleigh numbers, Ra=2000−5000. X-ray computed tomography is applied to image the spatial and temporal evolution of the solute plume non-invasively. The tomograms are used to compute macroscopic quantities including the rate of dissolution and horizontally averaged concentration profiles, and enable the visualisation of the flow patterns that arise upon mixing at a spatial resolution of about (2×2×2)mm3. The latter highlights that under this Ra regime convection becomes truly three-dimensional with the emergence of characteristic patterns that closely resemble the dynamical flow structures produced by high-resolution numerical simulations reported in the literature. We observe that the mixing process evolves systematically through three stages, starting from pure diffusion, followed by convection-dominated and shutdown. A modified diffusion equation is applied to model the convective process with an onset time of convection that compares favourably with the literature data and an effective diffusion coefficient that is almost two orders of magnitude larger than the molecular diffusivity of the solute. The comparison of the experimental observations of convective mixing against their numerical counterparts of the purely diffusive scenario enables the estimation of a non-dimensional convective mass flux in terms of the Sherwood number, Sh=0.025Ra. We observe that the latter scales linearly with Ra, in agreement with both experimental and numerical studies on thermal convection over the same Ra regime.
Ansari H, Trusler M, Maitland G, et al., 2019, Characterisation of the Bowland shale porosity using N2 and CO2 adsorption
© 2019 European Association of Geoscientists and Engineers, EAGE. All Rights Reserved. The heterogeneous nature of shales arising from their sheer abundance in the natural world, presents a unique challenge to the field of characterisation. Shales vary significantly in terms of their age, composition and pore systems so the wide-range applicability of a study on just one shale is questionable. This uncertainty can be reduced through the use of analogous materials, such as pure carbons and clay minerals, which can make shale characterisation much more predictive. In this work, we characterise the Bowland shale in the UK. Three samples, taken at different depths, and of varying composition in terms of organic content and clay minerals, were studied. Characterisation was achieved using low pressure adsorption on N2 at 77K and CO2 at 273K. The results were complemented with the use of the same technique performed on pure components such as mesoporous carbon (representing the organic matter) and clay minerals. The pure material results are used to infer the independent contribution of the constituents to shale characteristics and can be used to build artificial isotherms. Results show that the shale composition is a key indicator of the pore space in shale and therefore adsorption capacity and gas storage potential.
Hwang J, Joss L, Pini R, 2019, Measuring and modelling supercritical adsorption of CO2 and CH4 on montmorillonite source clay, Microporous and Mesoporous Materials, Vol: 273, Pages: 107-121, ISSN: 1387-1811
The porosity of clay minerals is dominated by nanoscale pores that provide a large surface area for physical and chemical interactions with the surrounding fluids, including gas adsorption. Measuring gas adsorption at subsurface conditions is difficult, because elevated pressures are required and the interactions between the supercritical gas and the clay are relatively weak. Here, we report on the measurement of adsorption isotherms of CO2 and CH4 on the source clay Na-montmorillonite (SWy-2) at different temperatures (25–115°C) over a wide range of pressures (0.02–25 MPa). The experimental observations are thoroughly analysed by considering both net and excess adsorbed amounts, and by extracting adsorption metrics, such as the Henry's constants and enthalpy of adsorption. The results consistently indicate that SWy-2 favours adsorption of CO2 over CH4 with selectivity, . The experimental data are successfully described using a Lattice Density Functional Theory (LDFT) model. The adsorption energetics estimated by the model compare well with the experimentally obtained enthalpy of adsorption. It is further shown that even at the highest pressure the pore space of the clay is only partially filled and that the degree of saturation increases upon approaching the critical temperature of the gas. The ability of the LDFT model to reveal pore-dependent adsorption behaviours demonstrates its potential against empirical models, such as the Langmuir equation, which fail at capturing the complexities of supercritical gas adsorption at subsurface conditions.
Lin Q, Bijeljic B, Pini R, et al., 2018, Imaging and measurement of pore‐scale interfacial curvature to determine capillary pressure simultaneously with relative permeability, Water Resources Research, Vol: 54, Pages: 7046-7060, ISSN: 0043-1397
There are a number of challenges associated with the determination of relative permeability and capillary pressure. It is difficult to measure both parameters simultaneously on the same sample using conventional methods. Instead, separate measurements are made on different samples, usually with different flooding protocols. Hence, it is not certain that the pore structure and displacement processes used to determine relative permeability are the same as those when capillary pressure was measured. Moreover, at present, we do not use pore‐scale information from high‐resolution imaging to inform multiphase flow properties directly. We introduce a method using pore‐scale imaging to determine capillary pressure from local interfacial curvature. This, in combination with pressure drop measurements, allows both relative permeabilities and capillary pressure to be determined during steady state coinjection of two phases through the core. A steady state waterflood experiment was performed in a Bentheimer sandstone, where decalin and brine were simultaneously injected through the core at increasing brine fractional flows from 0 to 1. The local saturation and the curvature of the oil‐brine interface were determined. Using the Young‐Laplace law, the curvature was related to a local capillary pressure. There was a detectable gradient in both saturation and capillary pressure along the flow direction. The relative permeability was determined from the experimentally measured pressure drop and average saturation obtained by imaging. An analytical correction to the brine relative permeability could be made using the capillary pressure gradient. The results for both relative permeability and capillary pressure are consistent with previous literature measurements on larger samples.
Pini R, Krevor S, 2018, Laboratory studies to understand the controls on flow and transport for CO<inf>2</inf> storage, Science of Carbon Storage in Deep Saline Formations: Process Coupling across Time and Spatial Scales, Pages: 145-180, ISBN: 9780128127537
© 2019 Elsevier Inc. All rights reserved. We review outcomes from laboratory studies that target CO2 flow and transport in the subsurface across the scales from microns to meters. A key distinctive feature of the CO2/brine system is that it tends to be more capillary dominated than the oil/water system, because of the low viscosity of CO2 as compared to brine. This increases the importance of rock heterogeneity in governing fluid displacement and introduces challenges for researchers to make reliable measurements of characteristic transport (e.g., dispersivity) and multiphase flow properties (e.g., relative permeability and trapping curves). Accurate parameterization of rock heterogeneity can nowadays be achieved thanks to a more widespread use of imaging technology, such as X-ray Computed Tomography, coupled with numerical simulations. This enables identifying general rules for experiment design and to extend experiments to conditions (e.g., flow rates) prevalent in the subsurface, which would be otherwise not attainable in the laboratory. Data gaps still exist, particularly with respect to the characterization of mixed-wet systems, of the hysteretic behavior of capillary pressure and relative permeability curves and of the mixing process at subsurface conditions.
© Copyright 2018, Society of Petroleum Engineers At reservoir conditions, gas flow confined in submicron pores of shale falls within slip flow and transition flow regimes. Beyond the common instant equilibrium assumption, we believe that gas adsorption/ desorption on rough pore surfaces could be in non-equilibrium status when gas pressure keeps decreasing during production. We investigate the interplay of gas slip flow inside complex submicron-scale pores and gas adsorption/desorption kinetics on pore surfaces with computational fluid dynamics (CFD) under unsteady-state flow conditions. Different from previous studies, the gas adsorption/desorption is in non-equilibrium state, which is closer to real reservoir conditions. Given pore pressure Pp at time t, linear driving force model with gas desorption rate coefficient kd is applied to describe the difference between the equilibrium adsorption amount (calculated with adsorption isotherms) and the actual adsorption amount per unit pore surface area. Free gas flow inside 3D reconstructions of shale pore space is modeled by Navier-Stokes equations with Maxwell's first-order slip boundary conditions. To include gas contributions from desorption, extra source with strength equal to the gas desorption rate is added to the slip boundaries. Any type of adsorption isotherms can be incorporated into our CFD modeling. We investigate the coupling of slip flow and Langmuir adsorption isotherms for methane in 3D reconstructed pore space. We observe that not all of adsorbed gas measured in adsorption isotherms contribute to gas production. In our study the pore pressure, Pp, decreases along with time t. One significant finding is that there exists a key time point, tk, after which adsorbed gas starts desorbing off pore surfaces and the decreasing rate of pore pressure becomes smaller. The higher the gas desorption rate coefficient, kd, is, the earlier tk occurs. But the decreasing rate of pore pressure is no longer sensitive t
Joss L, Pini R, 2017, Digital Adsorption: 3D Imaging of Gas Adsorption Isotherms by X-ray Computed Tomography, The Journal of Physical Chemistry Part C: Nanomaterials and Interfaces, Vol: 121, Pages: 26903-26915, ISSN: 1932-7447
We report on a novel approach for the measurement of gas adsorption in microporous solids using X-ray computed tomography (CT) that we refer to as digital adsorption. Similar to conventional macroscopic methods, the proposed protocol combines observations with an inert and an adsorbing gas to produce equilibrium isotherms in terms of the truly measurable quantity in an adsorption experiment, namely the surface excess. Most significantly, X-ray CT allows probing the adsorption process in three dimensions, so as to build spatially-resolved adsorption isotherms with a resolution of approximately 10 mm3 within a fixed-bed column. Experiments have been carried out at 25 C and in the pressure range 1-30bar using CO2 on activated carbon, zeolite 13X and glass beads (as control material), and results are validated against literature data. A scaling approach was applied to analyze the whole population of measured adsorption isotherms (~7600), leading to single universal adsorption isotherm curves that are descriptive of all voxels for a given adsorbate-adsorbent system. By analyzing the adsorption heterogeneity at multiple length scales (1 mm3 to 1 cm3), packing heterogeneity was identified as the main contributor for the larger spatial variability in the adsorbed amount observed for the activated carbon rods as compared to zeolite pellets. We also show that this technique is readily applicable to a large spectrum of commercial porous solids, and that it can be further extended to weakly adsorbing materials with appropriate protocols that reduce measurement uncertainties. As such, the obtained results prove the feasibility of digital adsorption and highlight substantial opportunities for its wider use in the field of adsorptive characterization of porous solids.
Pini R, Benson SM, 2017, Capillary pressure heterogeneity and hysteresis for the supercritical CO2/water system in a sandstone, Advances in Water Resources, Vol: 108, Pages: 277-292, ISSN: 0309-1708
We report results from an experimental investigation on the hysteretic behaviour of the capillary pressure curve for the supercritical CO2-water system in a Berea Sandstone core. Previous observations have highlighted the importance of sub-core-scale capillary heterogeneity in developing local saturations during drainage; we show in this study that the same is true for the imbibition process. Spatially distributed drainage and imbibition scanning curves were obtained for mm-scale subsets of the rock sample non-invasively using X-ray CT imagery. Core- and sub-core scale measurements are well described using the Brooks-Corey formalism, which uses a linear trapping model to compute mobile saturations during imbibition. Capillary scaling yields two separate universal drainage and imbibition curves that are representative of the full sub-core scale data set. This enables accurate parameterisation of rock properties at the sub-core scale in terms of capillary scaling factors and permeability, which in turn serve as effective indicators of heterogeneity at the same scale even when hysteresis is a factor. As such, the proposed core-analysis workflow is quite general and provides the required information to populate numerical models that can be used to extend core-flooding experiments to conditions prevalent in the subsurface, which would be otherwise not attainable in the laboratory.
Liyanage R, Crawshaw, Krevor, et al., 2017, Multidimensional Imaging of Density Driven Convection in a Porous Medium, 13th International Conference on Greenhouse Gas Control Technologies, GHGT-13, Publisher: Elsevier, Pages: 4981-4985, ISSN: 1876-6102
Carbon dioxide (CO2) sequestration is a climate change mitigation technique which relies on residual and solubility trapping in injection locations with saline aquifers. The dissolution of CO2 into resident brines results in density-driven convection which further enhances the geological trapping potential. We report on the use of an analogue fluid pair to investigate density-driven convection in 3D in an unconsolidated bead pack. X-ray computed tomography (CT) is used to image density-driven convection in the opaque porous medium non-invasively. Two studies have been conducted that differ by the Rayleigh number (Ra) of the system, which in this study is changed by altering the maximum density difference of the fluid pair. We observe the same general mixing pattern in both studies. Initially, many high density fingers move downward through the bead pack and as time progresses these coalesce and form larger dominate flow paths. However, we also observe that a higher Rayleigh number leads to the denser plume moving faster towards the bottom of the system. Due to the finite size of the system, this in turn leads to early convective shut-down.
Trevisan L, Gonzalez-Nicolas A, Cihan A, et al., 2017, Experimental and modeling study of capillary/buoyancy-driven flow of surrogate CO2 through intermediate-scale sand tanks, 13th International Conference on Greenhouse Gas Control Technologies (GHGT), Publisher: ELSEVIER SCIENCE BV, Pages: 5032-5037, ISSN: 1876-6102
Trevisan L, Pini R, Cihan A, et al., 2017, Imaging and quantification of spreading and trapping of carbon dioxide in saline aquifers using meter-scale laboratory experiments, Water Resources Research, Vol: 53, Pages: 485-502, ISSN: 1944-7973
The role of capillary forces during buoyant migration of CO2 is critical towards plume immobilization within the post-injection phase of a geological carbon sequestration operation. However, the inherent heterogeneity of the subsurface makes it very challenging to evaluate the effects of capillary forces on the storage capacity of these formations and to assess in-situ plume evolution. To overcome the lack of accurate and continuous observations at the field scale and to mimic vertical migration and entrapment of realistic CO2 plumes in the presence of a background hydraulic gradient, we conducted two unique long-term experiments in a 2.44 m × 0.5 m tank. X-ray attenuation allowed measuring the evolution of a CO2-surrogate fluid saturation, thus providing direct insight into capillarity- and buoyancy-dominated flow processes occurring under successive drainage and imbibition conditions. The comparison of saturation distributions between two experimental campaigns suggests that layered-type heterogeneity plays an important role on non-wetting phase (NWP) migration and trapping, because it leads to (i) longer displacement times (3.6 months vs. 24 days) to reach stable trapping conditions, (ii) limited vertical migration of the plume (with center of mass at 39% vs. 55% of aquifer thickness), and (iii) immobilization of a larger fraction of injected NWP mass (67.2% vs. 51.5% of injected volume) as compared to the homogenous scenario. While these observations confirm once more the role of geological heterogeneity in controlling buoyant flows in the subsurface, they also highlight the importance of characterizing it at scales that are below seismic resolution (1-10 m).
Pini R, Vandehey NT, Druhan J, et al., 2016, Quantifying solute spreading and mixing in reservoir rocks using 3-D PET imaging, Journal of Fluid Mechanics, Vol: 796, Pages: 558-587, ISSN: 0022-1120
We report results of an experimental investigation into the effects of small-scale (mmcm)heterogeneities on solute spreading and mixing in a Berea Sandstone core. Pulsetracertests have been carried out in the regime Pe = 6 − 40 and are supplementedby a unique combination of two imaging techniques. X-ray CT is used to quantify subcorescale heterogeneities in terms of permeability contrasts at a spatial resolution ofabout 10 mm3, while [11C]PET is applied to image the spatial and temporal evolutionof the full tracer plume non-invasively. To account for both advective spreading andlocal (Fickian) mixing as driving mechanisms for solute transport, a streamtube model isapplied that is based on the 1D Advection Dispersion Equation. We refer to our modellingapproach as semi-deterministic, because the spatial arrangement of the streamtubes andthe corresponding solute travel times are known from the measured rock’s permeabilitymap, which required only small adjustments to match the measured tracer breakthroughcurve. The model reproduces the 3D PET measurements accurately by capturing thelarger-scale tracer plume deformation as well as sub-core scale mixing, while confirmingnegligible transverse dispersion over the scale of the experiment. We suggest that theobtained longitudinal dispersivity (0.10 ± 0.02 cm) is rock- rather than sample-specific,because of the ability of the model to decouple sub-core scale permeability heterogeneityeffects from those of local dispersion. As such, the approach presented here proves to bevery valuable, if not necessary, in the context of reservoir core analyses, because rocksamples can rarely be regarded as “uniformly heterogeneous”.
Hingerl FF, Yang F, Pini R, et al., 2016, Characterization of heterogeneity in the Heletz sandstone from core to pore scale and quantification of its impact on multi-phase flow, International Journal of Greenhouse Gas Control, Vol: 48, Pages: 69-83, ISSN: 1750-5836
Huo D, Pini R, Benson SM, 2016, A calibration-free approach for measuring fracture aperture distributions using X-ray computed tomography, Geosphere, Vol: 12, Pages: 558-571
© 2016 Geological Society of America. Various methods have been proposed to measure fracture aperture distributions, including X-ray computed tomography (CT) imaging, which has the advantage that it can be combined with dynamic flow experiments. In this paper, we present a calibration-free missing CT attenuation (CFMA) imaging method for measuring fracture apertures that avoids time-consuming calibration. In addition, this model does not assume a homogeneous matrix and thus provides a good estimate of fracture apertures even when rock properties are heterogeneous. The validity of the CFMA model is established by four approaches: Comparing apertures calculated with the conventional calibration- based method; evaluating model predictability at different scanner voxel sizes; comparing with calibration coefficients in the literature from a number of experiments with different rocks and X-ray scanners; and comparing aperture measurements for dry and wet scans. We analyze the systematic error and the random error introduced by rock heterogeneities and CT scanning and show that by averaging 5 replicate scans, we reduce the aperture measurement error to ~22 μm.
Pini R, 2016, On the adsorption properties of shale rocks, Pages: 22-26
© 2016, European Association of Geoscientists and Engineers. All rights reserved. The inherent complexity of shale rocks together with their relatively low adsorption capacity as compared to commercial adsorbents represent a new scientific and technical challenge in the study of adsorption at supercritical conditions. Some of these issues are discussed in this paper. The adsorption of CO2 on a sample of Eagle Ford shale has been measured at 50°C and up to 20 MPa, and a maximum adsorption capacity of 300 SCF/ton of dry (granulated) shale sample was obtained. The analysis has focused on the estimation of the density of adsorbed gas in the pores of the material, a parameter that is key to quantify the storage capacity of shale rocks. A wide range of values was obtained (0.3-0.8 g/cm3) depending on the assumed skeletal volume of the shale. Whether these variations in the adsorbed density are related to the distinct pore structure of the materials considered and/or to uncertainties associated to the experimental techniques requires more research work under different conditions. In this context, the design an experimental protocol to accurately quantify the inaccessible volume of shale would allow improving the reliability of storage capacity estimates in these rocks.
Trevisan L, Pini R, Cihan A, et al., 2015, Experimental analysis of spatial correlation effects on capillary trapping of supercritical CO2 at the intermediate laboratory scale in heterogeneous porous media, Water Resources Research, Vol: 51, Pages: 8791-8805, ISSN: 0043-1397
Several numerical studies have demonstrated that the heterogeneous nature of typical sedimentary formations can favorably dampen the accumulation of mobile CO2 phase underneath the caprock. Core flooding experiments have also shown that contrasts in capillary entry pressure can lead to buildup of nonwetting fluid phase (NWP) at interfaces between facies. Explicit representation of geological heterogeneity at the intermediate (cm-to-m) scale is a powerful approach to identify the key mechanisms that control multiphase flow dynamics in porous media. The ability to carefully control flow regime and permeability contrast at a scale that is relevant to CO2 plume dynamics in saline formations offers valuable information to understand immiscible displacement processes and provides a benchmark for mathematical models. To provide insight into the impact of capillary heterogeneity on flow dynamics and trapping efficiency of supercritical CO2 under successive drainage and imbibition conditions, we present an experimental investigation conducted in a synthetic sand reservoir. By mimicking the interplay of governing forces at reservoir conditions via application of surrogate fluids, we performed three immiscible displacement experiments to observe the entrapment of NWP in heterogeneous porous media. Capillary trapping performance is evaluated for each scenario through spatial and temporal variations of NWP saturation; for this reason we adopted X-ray attenuation to precisely measure phase saturation throughout the flow domain and apply spatial moment analysis. The sweeping performance of two different permeability fields with comparable variance but distinct spatial correlation was compared against a homogeneous base case with equivalent mean permeability by means of spatial moment analysis.
Pini R, Madonna C, 2015, Moving across scales: a quantitative assessment of X-ray CT to measure the porosity of rocks, Journal of Porous Materials, Vol: 23, Pages: 325-338, ISSN: 1573-4854
We apply multidimensional X-ray CT to quantify the porosity of Berea Sandstone by using both medical- and synchrotron-based X-ray radiation, so as to produce images of the same sample with mm- and micron-resolution, respectively. Three different samples are used and the obtained tomograms are compared by considering the spatial distribution of porosity values for the range of voxel sizes 0.25-16 mm3. The agreement between the two independent techniques is assessed by means of the concordance correlation coefficient. Statistically significant correlations are found for each sample up to the maximum resolution of the medical CT scanner, i.e. for images with a voxel size of (0.5x0.5x1) mm3. The direct comparison of images obtained by medical- and synchrotron-based X-ray radiation has a dual benefit. First, it objectively informs the segmentation step required for the binarization of the high-resolution synchrotron images that is otherwise prone to operator bias; in this context, the applicability of the proposed workflow is demonstrated with two widely applied locally adaptive thresholding algorithms, namely the hysteresis and the watershed methods. Secondly, once this calibration has occurred, the coupling of the two techniques allows analyzing porosity heterogeneity across a range of length-scales that spans over more than eight orders of magnitudes. We anticipate that the ability to perform a true multi-scale experiment may represent the required point of departure for developing up-scaling approaches that capture the inherently complex heterogeneity of rocks.
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