Imperial College London

DrRonnyPini

Faculty of EngineeringDepartment of Chemical Engineering

Reader in Chemical Engineering
 
 
 
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Contact

 

+44 (0)20 7594 7518r.pini Website

 
 
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Location

 

415ACE ExtensionSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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89 results found

Kuusela P, Pour-Ghaz M, Pini R, Voss A, Seppanen Aet al., 2021, Imaging of reactive transport in fractured cement-based materials with X-ray CT, Cement and Concrete Composites, Vol: 124, Pages: 1-12, ISSN: 0958-9465

The need to improve the understanding of the properties of cement-based materials calls for the development of tools for visualizing and quantifying chemicalreactions and flows of fluids within them. In this paper, we report the results ofan experimental study where a sample of fractured cement-paste was subjectedto injection of fluids (krypton, CO2 and water) and imaged simultaneously byX-ray computed tomography (CT). Initial porosity of the sample was estimatedusing a subtraction method based on CT scans taken initially and during krypton injection. The CT reconstructions were segmented to visualize crack patterns and fluid flow in three-dimensions and to quantify the evolution of porosityduring the experiment. The results show that CT captures the formation of acarbonate phase in the sample during CO2 injection, and the flow of water inthe fractured media. We quantify the reduction of porosity resulting from thecarbonation reaction. We observe that the newly formed carbonated layer impedes water flow and, locally, can lead to crack healing. The results demonstratethe ability of CT to image reactive transport in cement-based materials, andsupport the feasibility of this imaging tool for their characterization.

Journal article

Ansari H, Rietmann E, Joss L, Trusler J, Maitland G, Pini Ret al., 2021, A shortcut pressure swing adsorption analogue model to estimate gas-in-place and CO2 storage potential of gas shales, Fuel: the science and technology of fuel and energy, Vol: 301, Pages: 1-13, ISSN: 0016-2361

Natural gas extraction from shale formations has experienced a rapid growth in recent years, but the low recovery observed in many field operations demonstrates that the development of this energy resource is far from being optimal. The ambiguity in procedures that account for gas adsorption in Gas-in-Place calculations represents an important element of uncertainty. Here, we present a methodology to compute gas production curves based on quantities that are directly accessed experimentally, so as to correctly account for the usable pore-space in shale. We observe that adsorption does not necessarily sustain a larger gas production compared to a non-adsorbing reservoir with the same porosity. By analysing the entire production curve, from initial to abandonment pressure, we unravel the role of the excess adsorption isotherm in driving this behaviour. To evaluate scenarios of improved recovery by means of gas injection, we develop a proxy reservoir model that exploits the concept of Pressure Swing Adsorption used in industrial gas separation operations. The model has three stages (Injection/Soak/Production) and is used to compare scenarios with cyclic injection of CO2 or N2. The results show that partial pressure and competitive adsorption enhance gas production in complementary ways, and reveal the important trade-off between CH4 recovery and CO2 storage. In this context, this proxy model represents a useful to tool to explore strategies that optimise these quantities without compromising the purity of the produced stream, as the latter may introduce a heavy economic burden on the operation.

Journal article

Fan D, Pini R, Striolo A, 2021, A seemingly universal particle kinetic distribution in porous media, Applied Physics Letters, Vol: 119, Pages: 1-6, ISSN: 0003-6951

We study many-particle transport in randomly jammed packing of spheres at different particle Péclet numbers (𝑃𝑒∗). We demonstrate that a modified Nakagami-m function describes particle velocity probability distributions when particle deposition occurs. We assess the universality of said function through comparison against Lagrangian simulations of various particle types as well as experimental data from the literature. We construe the function's physical meaning as its ability to explain particle deposition in terms of 𝑃𝑒∗ and the competition between distributions of energy barriers for particle release and particles' diffusive energy.

Journal article

Ma L, Fauchille A-L, Ansari H, Chandler M, Ashby PD, Taylor KG, Pini R, Lee PDet al., 2021, Linking multi-scale 3D microstructure to potential enhanced natural gas recovery and subsurface CO2 storage for Bowland Shale, UK, Energy and Environmental Science, Vol: 14, Pages: 4481-4498, ISSN: 1754-5692

Injection of CO2 into shale reservoirs to enhance gas recovery and simultaneously sequester greenhouse gases is a potential contributor towards the carbon-neutral target. It offers a low-carbon, low-cost, low-waste and large-scale solution during energy transition period. A precondition to efficient gas storage and flow is a sound understanding of how the shale’s micro-scale impacts on these phenomena. However, the heterogeneous and complex nature of shales limits the understanding of microstructure and pore systems, making feasibility analysis challenging. This study qualitatively and quantitatively investigates the Bowland shale microstructure in 3D at five length scales: artificial fractures at 10-100 µm scale, matrix fabric at 1-10 µm-scale, individual mineral grains and organic matter particles at 100 nm- 1 µm scale, macropores and micro-cracks at 10-100 nm scale and organic matter and mineral pores at 1-10 nm-scale. For each feature, the volume fraction variations along the bedding normal orientation, the fractal dimensions and the degrees of anisotropy were analysed at all corresponding scales for a multi-scale heterogeneity analysis. The results are combined with other bulk laboratory measurements, including supercritical CO2 and CH4 adsorption at reservoir conditions, pressure-dependent permeability and nitrogen adsorption pore size distribution, to perform a comprehensive analysis on the storage space and flow pathways. A cross-scale pore size distribution, ranging from 2 nm to 3 µm, was calculated with quantified microstructure. The cumulative porosity is calculated to be 8%. The cumulative surface area is 17.6 m2/g. A model of CH4 and CO2 flow pathways and storage with quantified microstructure is presented and discussed. The feasibility of simultaneously enhanced gas recovery and subsurface CO2 storage in Bowland shale, the largest shale gas potential formation in the UK, was assessed based using multi-scale microstructure

Journal article

Savulescu GC, Rücker M, Scanziani A, Pini R, Georgiadis A, Luckham PFet al., 2021, Atomic force microscopy for the characterisation of pinning effects of seawater micro-droplets in n-decane on a calcite surface, Journal of Colloid and Interface Science, Vol: 592, Pages: 397-404, ISSN: 0021-9797

Hypothesis: Roughness is an important parameter in applications where wetting needs to be characterized. Micro-computed tomography is commonly used to characterize wetting in porous media but the main limitation of this approach is the incapacity to identify nanoscale roughness. Atomic force microscopy, AFM, however, has been used to characterize the topography of surfaces down to the molecular scale. Here we investigate the potential of using AFM to characterize wetting behavior at the nanoscale.Experiments: Droplets of water on cleaved calcite under decane were imaged using quantitative imaging QI atomic force microscopy where a force-distance curve is obtained at every pixel.Findings: When the AFM tip passed through the water droplet surface, an attraction was observed due to capillary effects, such that the thickness of the water film was estimated and hence the profile of the droplet obtained. This enables parameters such as the contact angle and contact angle distribution to be obtained at a nanometer scale. The contact angles around the 3-phase contact line are found to be quasi-symmetrically distributed between 10–30°. A correlation between the height profile of the surface and contact angle distribution demonstrates a quasi-proportional relationship between roughness on the calcite surface and contact angle.

Journal article

Eckel A-M, Pini R, 2021, Spreading and mixing during solutal convection in uniform porous media with application to geologic CO2 storage, Physics of Fluids, Vol: 33, ISSN: 1070-6631

Convective dissolution in saline aquifers is expected to positively impact subsurface storage of carbon dioxide (CO2) by accelerating its dissolution rate into reservoir brines. By largely focusing on the dissolution flux, previous studies lack a systematic evaluation of the mixing process following CO2 emplacement, including a quantitative analysis at conditions representative of subsurface traps (Rayleigh number, Ra≤1 000). Here, we investigate solutal convection numerically in a two-dimensional uniform porous medium in the regime Ra=100−10 000. The macroscopic evolution of the convective process is characterized by means of fundamental macroscopic measures of mixing that use the local spatial structure of the solute concentration field. It is shown that the intensity of segregation closely mimics the evolution of the in situ convective pattern arising from the stretching and merging of downwelling plumes. The spreading length and the dilution index both confirm that the mixing process accelerates over time (t) with a power law scaling (∝tα) that transitions from diffusive (α=0.5) to superdiffusive mixing (α≥1) irrespective of Ra. This transition time scales τon∝Ra−2 and is used as a measure of the onset time of convection. The dilution index indicates that the time needed to reach close-to-complete mixing reduces linearly with Ra. On the contrary, the non-dimensional mass flux, expressed in terms of the Sherwood number, Sh, reveals a natural logarithmic scaling for Ra≤2 500.

Journal article

Kurotori T, Pini R, 2021, A general capillary equilibrium model to describe drainage experiments in heterogeneous laboratory rock cores, Advances in Water Resources, Vol: 152, Pages: 1-12, ISSN: 0309-1708

Macroscopic observations of two-phase flow in porous rocks are largely affected by the heterogeneity in continuum properties at length scales smaller than a typical laboratory sample. The ability to discriminate among therock properties at the origin of the heterogeneity is key to the development of numerical models to be used forprediction. Here, we present a capillary equilibrium model that represents spatial heterogeneity in dual-porosityporous media in terms of the capillary entry pressure, 1∕𝛼, and the irreducible wetting phase saturation, 𝑆ir. Bothparameters are used to scale local capillary pressure curves by using three-dimensional imagery acquired duringmulti-rate gas/liquid drainage displacements. We verify the proposed approach by considering the case studyof a dual-porosity limestone core and use the spatial variation in 𝑆ir as proxy for microporosity heterogeneity.The latter places potentially next-to-leading order controls on the observed fluid saturation distribution, whichis strongly correlated to the distribution of 1∕𝛼. While microporosity is by and large uniform at the observationscale on the order of 0.1 cm3, the spatial correlation of 1∕𝛼 is on the order of 1 cm and is therefore not statisticallyrepresented in the volume of typical laboratory core samples.

Journal article

Tian T, Hou J, Ansari H, Xiong Y, L'Hermitte A, Danaci D, Pini R, Petit Cet al., 2021, Mechanically stable structured porous boron nitride with high volumetric adsorption capacity, JOURNAL OF MATERIALS CHEMISTRY A, Vol: 9, Pages: 13366-13373, ISSN: 2050-7488

Journal article

Pini R, Ansari H, Hwang J, 2021, Measurement and interpretation of unary supercritical gas adsorption isotherms in micro-mesoporous solids, Adsorption, Vol: 27, Pages: 659-671, ISSN: 0929-5607

Gas adsorption at high pressures in porous solids is commonly quantified in terms of the excess amount adsorbed. Despite the wide spectrum of adsorbent morphologies available, the analysis of excess adsorption isotherms has mostly focused on microporous materials and the role of mesoporosity remains largely unexplored. Here, we present supercritical CO2 adsorption isotherms measured at 𝑇=308 K in the pressure range 𝑝=0.02−21 MPa on three adsorbents with distinct fractions of microporosity, 𝜙2, namely a microporous metal-organic framework (𝜙2=70%), a micro-mesoporous zeolite (𝜙2=38%) and a mesoporous carbon (𝜙2<0.1%). The results are compared systematically in terms of excess and net adsorption relative to two distinct reference states–the space filled with gas in the presence/absence of adsorbent–that are defined from two separate experiments using helium as the probing gas. We discuss the inherent difficulties in extracting from the supercritical adsorption isotherms quantitative information on the properties of the adsorbed phase (its density or volume), because of the nonuniform distribution of the latter within and across the different classes of pore sizes. Yet, the data clearly reveal pore-size dependent adsorption behaviour, which can be used to identify characteristic types of isotherm and to complement the information obtained using the more traditional textural analysis by physisorption.

Journal article

Gardas RL, Kofke DA, Pini R, Sadowski G, Schwarz CE, Siepmann JI, Wu Jet al., 2021, Historical Perspective of the Journal of Chemical & Engineering Data's Published Topics, 1956-2020, JOURNAL OF CHEMICAL AND ENGINEERING DATA, Vol: 66, Pages: 1555-1556, ISSN: 0021-9568

Journal article

Hwang J, Pini R, 2021, Enhanced sorption of supercritical CO2 and CH4 in the hydrated interlayer pores of smectite, Langmuir: the ACS journal of surfaces and colloids, Vol: 37, Pages: 3778-3788, ISSN: 0743-7463

Understanding the long-term confinement of supercritical fluids in the clay pores of subsurface rocks is important for many geo-energy technologies, including geological CO2 storage. However, the adsorption properties of hydrated clay minerals remain largely uncertain because competitive adsorption experiments of supercritical fluids in the presence of water are difficult. Here, we report on the sorption properties of four source clay minerals—Ca-rich montmorillonite (STx-1b), Na-rich montmorillonite (SWy-2), illite–smectite mixed layer (ISCz-1), and illite (IMt-2)—for water at 20 °C up to relative humidity of 0.9. The measurements unveil the unsuitability of physisorption analysis by N2 (at 77 K) and Ar (at 87 K) gases to quantify the textural properties of clays because of their inability to probe the interlayers. We further measure the sorption of CO2 and CH4 on swelling STx-1b and nonswelling IMt-2, both in the absence (dehydrated at 200 °C) and the presence of sub-1W preadsorbed water (following dehydration) up to 170 bar at 50 °C. We observe enhanced sorption of CO2 and CH4 in STx-1b (50 and 65% increase at 30 bar relative to dry STx-1b, respectively), while their adsorption on IMt-2 remains unchanged, indicating the absence of competition with water. By describing the supercritical adsorption isotherms on hydrated STx-1b with the lattice density functional theory model, we estimate that the pore volume has expanded by approximately 6% through the formation of sub-nanometer pore space. By presenting a systematic approach of quantifying the smectite clay mineral’s hydrated state, this study provides an explanation for the conflicting literature observations of gas uptake capacities in the presence of water.

Journal article

Burridge HC, Pini R, Shah SMK, Reynolds TPS, Wu G, Shah DU, Scherman OA, Ramage MH, Linden PFet al., 2021, Identifying Efficient Transport Pathways in Early-Wood Timber: Insights from 3D X-ray CT Imaging of Softwood in the Presence of Flow (vol 136, pg 813, 2021), TRANSPORT IN POROUS MEDIA, Vol: 137, Pages: 799-800, ISSN: 0169-3913

Journal article

Wenning QC, Madonna C, Kurotori T, Petrini C, Hwang J, Zappone A, Wiemer S, Giardini D, Pini Ret al., 2021, Chemo-Mechanical Coupling in Fractured Shale With Water and Hydrocarbon Flow, GEOPHYSICAL RESEARCH LETTERS, Vol: 48, ISSN: 0094-8276

Journal article

Siepmann JI, Gardas RL, Kofke DA, Pini R, Sadowski G, Schwarz CE, Wu Jet al., 2021, Journal of Chemical & Engineering Data: Why Change the Cover Page?, JOURNAL OF CHEMICAL AND ENGINEERING DATA, Vol: 66, Pages: 859-860, ISSN: 0021-9568

Journal article

Siepmann JI, Gardas RL, Kofke DA, Pini R, Sadowski G, Schwarz CE, Wu Jet al., 2021, Journal of Chemical & Engineering Data: An Update from the Editorial Team, JOURNAL OF CHEMICAL AND ENGINEERING DATA, Vol: 66, Pages: 1-2, ISSN: 0021-9568

Journal article

Burridge HC, Pini R, Shah SMK, Reynolds TPS, Wu G, Shah DU, Scherman OA, Ramage MH, Linden PFet al., 2021, Identifying efficient transport pathways in early-wood timber: insights from 3D X-ray CT imaging of softwood in the presence of flow, Transport in Porous Media, Vol: 136, Pages: 813-830, ISSN: 0169-3913

Wider use of timber has the potential to greatly reduce the embodied carbon of construction. Improved chemical treatment could help overcome some of the barriers to wider application of timber, by furthering the durability and/or mechanical properties of this natural material. Improving timber treatment by treating the whole volume of a piece of timber, or tailored sections thereof, requires sound understanding and validated modelling of the natural paths for fluid flow through wood. In this study we carry out a robust analysis of three-dimensional X-ray CT measurements on kiln-dried softwood in the presence of flow and identify small portions of early-wood which are uniquely capable of transporting fluids—herein ‘efficient transport pathways’. We successfully model the effects of these pathways on the liquid uptake by timber by introducing a spatial variability in the amount of aspiration of the bordered pits following kiln drying. The model demonstrates that fluid advances along these efficient transport paths between 10 and 30 times faster than in the remainder of the timber. Identifying these efficient transport pathways offers scope to improve and extend the degree to which timber properties are enhanced at an industrial scale through processes to impregnate timber.

Journal article

Ansari H, Joss L, Hwang J, Trusler J, Maitland G, Pini Ret 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.

Journal article

Pini R, Joss L, Hosseinzadeh Hejazi SA, 2020, Quantitative imaging of gas adsorption equilibrium and dynamics by X-ray Computed Tomography, Adsorption, Vol: 27, Pages: 801-818, ISSN: 0929-5607

We present the development and application of X-ray Computed Tomography (CT) for the determination of the adsorption properties of microporous adsorbents and the study of breakthrough experiments in a laboratory fixed-bed adsorption column. Using the model system CO2/helium on activated carbon, equilibrium and dynamic adsorption/desorption measurements by X-ray CT are described, and the results are successfully compared to those obtained from conventional methods, including the application of a one-dimensional dynamic column breakthrough model. The study demonstrates the practical feasibility of applying X-ray CT to measure internal and transient concentration profiles in adsorbent systems on the length-scales from a single adsorbent pellet to a packed column.

Journal article

Kurotori T, Zahasky C, Benson S, Pini Ret al., 2020, Description of chemical transport in laboratory rock cores using the continuous random walk formalism, Water Resources Research, Vol: 56, ISSN: 0043-1397

We investigate chemical transport in laboratory rock cores using unidirectional pulse tracer experiments. Breakthrough curves (BTCs) measured at various flow rates in one sandstone and twocarbonate samples are interpreted using the one-dimensional Continuous Time Random Walk (CTRW) formulation with a truncated power law (TPL) model. Within the same framework, we evaluate additionalmemory functions to consider the Advection-Dispersion Equation (ADE) and its extension to describe mass exchange between mobile and immobile solute phases (Single-Rate Mass Transfer model, SRMT). Toprovide physical constraints to the models, parameters are identified that do not depend on the flow rate. While the ADE fails systematically at describing the effluent profiles for the carbonates, the SRMT andTPL formulations provide excellent fits to the measurements. They both yield a linear correlation between the dispersion coefficient and the Péclet number (DL Pe for 10 < (Pe) < 100), and the longitudinal dispersivity is found to be significantly larger than the equivalent grain diameter, De. The BTCs of the carbonate rocks show clear signs of nonequilibrium effects. While the SRMT model explicitly accounts for the presence of microporous regions (up to 30% of the total pore space), in the TPL formulation the time scales of both advective and diffusive processes (t1(Pe) and t2) are associated with two characteristic heterogeneity length scales (d and l, respectively). We observed that l  2.5 × De and that anomalous transport arises when ld (1). In this context, the SRMT and TPL formulations provide consistent, yet complementary, insight into the nature of anomalous transport in laboratory rock cores.

Journal article

Nguyen HGT, Sims CM, Toman B, Horn J, van Zee RD, Thommes M, Ahmad R, Denayer JFM, Baron GV, Napolitano E, Bielewski M, Mangano E, Brandani S, Broom DP, Benham MJ, Dailly A, Dreisbach F, Edubilli S, Gumma S, Moellmer J, Lange M, Tian M, Mays TJ, Shigeoka T, Yamakita S, Hakuman M, Nakada Y, Nakai K, Hwang J, Pini R, Jiang H, Ebner AD, Nicholson MA, Ritter JA, Farrando-Perez J, Cuadrado-Collados C, Silvestre-Albero J, Tampaxis C, Steriotis T, Rimnacova D, Svabova M, Vorokhta M, Wang H, Bovens E, Heymans N, De Weireld Get 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, Vol: 26, Pages: 1253-1266, 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.

Journal article

Iruretagoyena D, Bikane K, Sunny N, Lu H, Kazarian SG, Chadwick D, Pini R, Shah Net 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.

Journal article

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.

Journal article

Wenning QC, Madonna C, Kurotori T, Pini Ret 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.

Journal article

Lin Q, Bijeljic B, Berg S, Pini R, Blunt MJ, Krevor Set 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.

Journal article

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.

Journal article

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.

Journal article

Zahasky C, Kurotori T, Pini R, Benson Set 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.

Journal article

Kurotori T, Zahasky C, Hosseinzadeh Hejazi SA, Shah S, Benson S, Pini Ret 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

Journal article

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.

Journal article

Liyanage R, Cen J, Krevor S, Crawshaw J, Pini Ret 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.

Journal article

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