Publications
25 results found
Gago PA, Madrid MA, Boettcher S, et al., 2023, Effect of bevelled silo outlet in the flow rate during discharge, Powder Technology, Vol: 428, ISSN: 0032-5910
We investigate the effect of a bevelled (or slanted) outlet on the discharge rate of mono-sized spheres from a quasi-two-dimensional silo, using the discrete element method. In contrast to hopper discharges, where the bevelling is across the entire base of the container, we study a bevelled opening that is significantly smaller than the silo width and in which the slanting is limited to half a sphere diameter at the boundary of the outlet. We show that the bevelling increases the flow rate comparably to the inclination in hopper walls. Using Beverloo’s model, we relate this increase in rate to what we define as the ‘effective opening’ of the silo and analyse the velocity profiles associated with the discharges. We show that different openings, having effectively the same discharge rates, give rise to distinctly different internal dynamics in the silo. These results have the potential to aid industrial processes by fine-tuning and improving control of silo discharges, with a minimal impact on silo design, thus significantly reducing production and handling costs.
Gago P, Konstantinou C, Biscontin G, et al., 2022, A numerical characterisation of unconfined strength of weakly consolidated granular packs and its effect on fluid-driven fracture behaviour, Rock Mechanics and Rock Engineering, Vol: 55, Pages: 4565-4575, ISSN: 0723-2632
Soft or weakly consolidated sand refers to porous materials composed of particles (or grains) weakly held together to form a solid but that can be easily broken when subjected to stress. These materials do not behave as conventional brittle, linear elastic materials and the transition between these two regimes cannot usually be described using poro-elastic models. Furthermore, conventional geotechnical sampling techniques often result in the destruction of the cementation and recovery of sufficient intact core is, therefore, difficult. This paper studies a numerical model that allows us to introduce weak consolidation in granular packs. The model, based on the LIGGGHTS open source project, simply adds an attractive contribution to particles in contact. This simple model allows us to reproduce key elements of the behaviour of the stress observed in compacted sands and clay, as well as in poorly consolidated sandstones. The paper finishes by inspecting the effect of different consolidation levels in fluid-driven fracture behaviour. Numerical results are compared qualitatively against experimental results on bio-cemented sandstones.
Gago P, Boettcher S, 2022, Density fluctuations in granular piles traversing the glass transition: A grain-scale characterization via the internal energy, Science Advances, Vol: 8, Pages: 1-7, ISSN: 2375-2548
The transition into a glassy state of a tapped granular pile is explored in detail using extensivemolecular dynamics simulations. We measure the density and density fluctuations of the ensembleof mechanically stable configurations reached after the energy induced by the perturbation hasdissipated. We show that the peak in density fluctuations concurs with the density undergoingthe transition. We find that different horizontal sub-regions (“layers”) along the height of the piletraverse the transition in a similar manner but at distinct tap intensities, demonstrating that at agiven intensity certain regions of the same pile may respond in a glassy manner while others remainequilibrated. To address this phenomenon, we supplement the conventional approach based purelyon properties of the static configurations with investigations of the grain-scale dynamics, inducedby a tap, by which the energy is transmitted throughout the pile. We find that the effective energythat particles dissipate is a function of each particle’s location in the pile and, moreover, that itsvalue plays a distinctive role in the transformation between configurations. This internal energyprovides a “temperature-like” parameter that allows us to align the transition into the glassy statefor all layers, as well as different annealing schedules, at a critical value.
Gago PA, Boettcher S, 2021, Density fluctuations in granular piles traversing the glass transition: A grain-scale characterization of the transition via the internal energy, Publisher: arXiv
The transition into a glassy state of a tapped granular pile is explored indetail using extensive molecular dynamics simulations. We measure the densityand density fluctuations of the ensemble of mechanically stable configurationsreached after the energy induced by the perturbation has dissipated. We showthat the peak in density fluctuations concurs with the density undergoing thetransition. We find that different horizontal sub-regions ("layers") along theheight of the pile traverse the transition in a similar manner but at distincttap intensities, demonstrating that at a given intensity certain regions of thesame pile may respond glassy while others remain equilibrated. To address thisphenomenon, we supplement the conventional approach based purely on propertiesof the static configurations with investigations of the grain-scale dynamics,induced by a tap, by which the energy is transmitted throughout the pile. Wefind that the effective energy that particles dissipate is a function of theparticles location in the pile and, moreover, that its value plays adistinctive role in the transformation between configurations. This internalenergy provides a "temperature-like" parameter that allows us to align thetransition into the glassy state for all layers as well as different annealingschedules at a critical value.
Gago PA, Konstantinou C, Biscontin G, et al., 2021, A numerical characterisation of unconfined strength of weakly consolidated granular packs and its effect on fluid-driven fracture behaviour, Publisher: arXiv
Soft or weakly-consolidated sand refers to porous materials composed ofparticles (or grains) weakly held together to form a solid but that can beeasily broken when subjected to stress. These materials do not behave asconventional brittle, linear elastic materials and the transition between thesetwo regimes cannot usually be described using poro-elastic models. Furthermore,conventional geotechnical sampling techniques often result in the destructionof the cementation and recovery of sufficient intact core is, therefore,difficult. This paper studies a numerical model that allows us to introduceweak consolidation in granular packs. The model, based on the LIGGGHTS opensource project, simply adds an attractive contribution to particles in contact.This simple model allow us to reproduce key elements of the behaviour of thestress observed in compacted sands and clay, as well as in poorly consolidatedsandstones. The paper finishes by inspecting the effect of differentconsolidation levels in fluid-driven fracture behaviour. Numerical results arecompared against experimental results on bio-cemented sandstones.
Boettcher S, Gago PA, Sibani P, 2021, Extremal fluctuations are essential for relaxation in complex energy landscapes, Publisher: arXiv
We determine the importance of extreme, record-sized events on thenon-equilibrium relaxation ("aging") after a sudden quench into the glassyphase. Here, we directly measure the impact of extreme events on the evolvingsystem in Monte Carlo simulations of Ising spin glasses and ferromagnetsundergoing quenches into either a low or high-temperature phase. Our resultsshow that, if we bar the attainment of new record-high energy fluctuations (byexplicitly imposing a "lid" on the fluctuation spectrum), further relaxation inthe low-temperature glassy phase is impeded markedly while in all other phasessuch a lid actually accelerates the relaxation process. Such rare recordevents, emerging naturally in the sequence of ordinary fluctuations of anyrelaxing system, thus prove to be key in activating the aging dynamics, as hasbeen argued for systems like spin glasses, superconductors, gels, colloids, andgranular piles. This dynamics of records succeeds in explaining the logarithmicdecay of the free energy and the memory effects encoded in the scaling oftwo-time correlation functions of those aging systems. These findings areinterpreted through the interplay of fluctuations and generic features of thehierarchical, complex energy landscape of glassy systems.
Gago PA, Boettcher S, 2020, Universal features of annealing and aging in compaction of granular piles., Proceedings of the National Academy of Sciences of USA, Vol: 117, Pages: 33072-33076, ISSN: 0027-8424
This paper links the nonequilibrium glassy relaxation behavior of otherwise athermal granular materials to those of thermally activated glasses. Thus, it demonstrates a much wider universality among complex glassy materials out of equilibrium. Our three-dimensional molecular dynamics simulations, fully incorporating friction and inelastic collisions, are designed to reproduce experimental behavior of tapped granular piles. A simple theory based on a dynamics of records explains why the typical phenomenology of annealing and aging after a quench should extend to such granular matter, activated by taps, beyond the more familiar realm of polymers, colloids, and magnetic materials that all exhibit thermal fluctuations.
Wanjura C, Gago P, Matsushima T, et al., 2020, Structural evolution of granular systems: theory, Granular Matter, Vol: 22, ISSN: 1434-5021
A general theory is developed for the evolution of the cell order (CO) distribution in planar granular systems. Dynamic equations are constructed and solved in closed form for several examples: systems under compression; dilation of very dense systems; and the general approach to steady state. We find that all the steady states are stable and that they satisfy a detailed balance-like condition when the CO≤6. Illustrative numerical solutions of the evolution are shown. Our theoretical results are validated against an extensive simulation of a sheared system. The formalism can be readily extended to other structural characteristics, paving the way to a general theory of structural organisation of granular systems.
Gago P, Konstantinou C, Biscontin G, et al., 2020, Stress inhomogeneity effect on fluid-induced fracture behaviour into weakly consolidated granular systems, Physical Review E: Statistical, Nonlinear, and Soft Matter Physics, Vol: 102, ISSN: 1539-3755
We study the effect of stress inhomogeneity on the behavior of fluid-driven fracture development in weakly consolidated granular systems. Using numerical models we investigate the change in fracture growth rate and fracture pattern structure in unconsolidated granular packs (also referred to as soft-sands) as a function of the change in the confining stresses applied to the system. Soft-sands do not usually behave like brittle, linear elastic materials, and as a consequence, poroelastic models are often not applicable to describe their behavior. By making a distinction between “cohesive” and “compressive” grain-grain contact forces depending on their magnitude, we propose an expression that describes the fluid opening pressure as a function of the mean value and the standard deviation of the “compressive stress” distribution. We also show that the standard deviation of this distribution can be related with the extent to which fracture “branches” reach into the material.
Gago P, Raeini A, King P, 2020, A spatially resolved fluid-solid interaction model for dense granular packs/Soft-Sand., Advances in Water Resources, Vol: 136, Pages: 1-7, ISSN: 0309-1708
Fluid flow through dense granular packs or soft sands can be described as a Darcy’ s flow for low injection rates, as the friction between grain-grain and grain-walls dominate the solid system behaviour. For high injection rates, fluid forces can generate grain displacement forming flow channels or “fractures”, which in turn modify local properties within the system, such as permeability and stress distribution. Due to this kind of “self organized” behaviour, a spatially resolved model for these interactions is required to capture the dynamics of these systems. In this work, we present a resolved model based on the approach taken by the CFDEM open source project which uses LIGGGHTS – a discrete elements method (DEM)– to model the granular behaviour and OpenFoam finite volume library for computational fluid dynamics (CFD), to simulate the fluid behaviour. The capabilities provided by the DEM engine allows the properties of the solid phase, such as inter-grain cohesion and solid confinement stress to be controlled. In this work the original solver provided by the CFDEM project was modified so as to deal with dense granular packs more effectively. Advantages of the approach presented are that it does not require external “scaling parameters” to reproduce well known properties of porous materials and that it inherits the performance provided by the CFDEM project. The model is validated by reproducing the well-known properties of static porous materials, such as its permeability as a function of the system porosity, and by calculating the drag coefficient for a sphere resting inside a uniform flow. Finally, we present fracture patterns obtained when modelling water injection into a Hele-Shaw cell, filled with a dense granular pack.
Gago P, King P, Wieladek K, 2020, Fluid-induced fracture into weakly consolidated sand: Impact of confining stress on initialization pressure, Physical Review E: Statistical, Nonlinear, and Soft Matter Physics, Vol: 101, Pages: 012907-1-012907-6, ISSN: 1539-3755
This paper studies the process of fluid injection driven fractures in granular packs where particles are held together by external confining stresses and weak intergrain cohesion. We investigate the process of fracture formations in soft sand confined into a radial Hele-Shaw cell. Two main regimes are well known for fluid injection in soft sand. For low fluid injection pressures it behaves as a solid porous material while for high enough injection pressures grain rearrangement takes place. Grain rearrangements lead to the formation of fluid channels or “fractures,” the structure and geometry of which depend on the material and fluid properties. Due to macroscopic grain displacements and the predominant role of dissipative frictional forces in granular system dynamics, these materials do not behave as conventional brittle, linear elastic materials and the transition between these two regimes cannot usually be described using poroelastic models. In this work we investigate the change in the minimum fluid pressure required to start grain mobilization as a function of the confining stresses applied to the system using a spatially resolved computational fluid dynamics–discrete element method numerical model. We show that this change is proportional to the applied stress when the confining stresses can be regarded as uniformly distributed among the particles in the system. A preliminary analytical expression for this change is presented.
Gago PA, King P, Muggeridge A, 2018, Fractal growth model for estimating breakthrough time and sweep efficiency when waterflooding geologically heterogeneous rocks, Physical Review Applied, Vol: 10, ISSN: 2331-7019
We describe a fast method for estimating flow through a porous medium with a heterogeneous permeability distribution. The main application is to contaminant transport in aquifers and recovery of oil by waterflooding, where such geological heterogeneities can result in regions of bypassed contaminants or oil. The extent of this bypassing is normally assessed by a numerical flow simulation that can take many hours of computer time. Ideally the impact of uncertainty in the geological description is then evaluated by the performing of many such simulations using different realizations of the permeability distribution. Obviously, a proper Monte Carlo evaluation may be impossible when the flow simulations are so computationally intensive. Consequently, methods from statistical mechanics, such as percolation theory and random walkers (such as diffusion-limited aggregation), have been proposed; however, these methods are limited to geological heterogeneities where the correlation lengths are smaller than the system size or to continuous permeability distributions. Here we describe a growth model that can be used to estimate the breakthrough time of the water (and hence the sweep efficiency) in most types of geologically heterogeneous rocks. We show how the model gives good estimates of the breakthrough time of water at the production well in a fraction of the time needed to perform a full flow simulation.
Masihi M, Gago P, King P, 2016, Estimation of the Effective Permeability of Heterogeneous Porous Media by Using Percolation Concepts, Transport in Porous Media, Vol: 114, Pages: 169-199, ISSN: 1573-1634
In this paper we present new methods to estimate the effective permeability (k_eff) of heterogeneous porous media with a wide distribution of permeabilities and various underlying structures, using percolation concepts. We first set a threshold permeability (k_th) on the permeability density function (pdf) and use standard algorithms from percolation theory to check whether the high permeable grid blocks (i.e. those with permeability higher than k_th) with occupied fraction of “p” first forms a cluster connecting two opposite sides of the system in the direction of the flow (high permeability flow pathway). Then we estimate the effective permeability of the heterogeneous porous media in different ways: a power law (k_eff=k_th p^m), a weighted power average (k_eff=[p.k_th^m+(1-p).k_g^m ]^(1/m) with k_g the geometric average of the permeability distribution) and a characteristic shape factor multiplied by the permeability threshold value. We found that the characteristic parameters (i.e. the exponent “m”) can be inferred either from the statistics and properties of percolation sub-networks at the threshold point (i.e. high and low permeable regions corresponding to those permeabilities above and below the threshold permeability value) or by comparing the system properties with an uncorrelated random field having the same permeability distribution. These physically based approaches do not need fitting to the experimental data of effective permeability measurements to estimate the model parameter (i.e. exponent m) as is usually necessary in empirical methods. We examine the order of accuracy of these methods on different layers of 10th SPE model and found very good estimates as compared to the values determined from the commercial flow simulators.
Gago PA, King PR, Muggeridge AH, 2016, Fast estimation of effective permeability and sweep efficiency of waterflooding in geologically heterogeneous reservoirs
Geological heterogeneity can adversely affect the macroscopic sweep efficiency when waterflooding oil reservoirs, however the exact distribution of permeability and porosity is generally not known. Engineers try to estimate the range of impacts heterogeneity might have on waterflood efficiency by creating multiple geological models and then simulating a waterflood through each of those realizations. Unfortunately each simulation can be computationally intensive meaning that it is generally not possible to obtain a statistically valid estimate of the expected sweep and the associated standard deviation. In this paper we show how the volume of unswept oil can be estimated rapidly (without flow simulations) from a geometrical characterization of the spatial permeability distribution. A "constriction" factor is defined which quantifies the effective cross-section area of the zones perpendicular to the principal flow direction. This is combined with a 'net-To-gross ratio' (which quantifies the fractional reservoir volume occupied by the zones that contribute to flow) to estimate effective permeability and the expected recovery factor for that realization. The method is tested using a range of realistic geological models, including SPE10 model 2 and its predictions are shown to agree well with values obtained using a well established commercial flow simulator.
Sadeghnejad S, Masihi M, King PR, et al., 2016, Study the effect of connectivity between two wells on secondary recovery efficiency using percolation approach
Estimating available hydrocarbon to be produced during secondary oil recovery is an ongoing activity in field development. The primary plan is normally scheduled during early stage of field's life through master development plan studies. During this period, due to the lake of certain data, estimation of the field efficiency is usually based on rules of thumb and not detailed field characterization. Hence, there is a great motivation to produce simpler physically-based methodologies. The minimum necessity inputs of percolation approach make it a useful tool for foration performance prediction. This approach enables us to attain a better assessment of the efficiency of secondary recovery methods at early production time. The main contribution of this study is to establish a continuum percolation model based on Monte Carlo simulation that can estimate the connectivity of good sands between two wells. In the classical percolation, the connectivity is considered between two lines and two faces of the system in 2-And 3-D; whereas, hydrocarbon production is achieved through wells with the shape of lines (e.g., vertical, horizontal, or deviated wells). In addition, the results showed that not implementation of the correct geometry of wells can alter the estimated results from the percolation approach.
Masihi M, Gago P, King P, 2016, Percolation-based effective permeability estimation in real heterogeneous porous media
It has long been understood that flow behavior in heterogeneous porous media is largely controlled by the continuity of permeability contrasts. With this in mind, we are looking in new methods for a fast estimation of the effective permeability which concentrates on the properties of the percolating cluster. From percolation concepts we use a threshold permeability value (Kth) by which the gridblocks with the highest permeability values connect two opposite side of the system in the direction of the flow. Those methods can be applied to heterogeneous media of a range of permeabilities distribution and various underlying structures. We use power law relations and weighted power averages that can be inferred either from the statistics and the properties of percolation sub-networks at the threshold point. This approach does not need fitting to the experimental data of conductivity measurements to estimate the model parameter as is done in empirical methods. We examine the order of accuracy of these methods on some layers of 10th SPE model and found very good agreements with the values determined from the commercial flow simulators. The results of this work open insights on new methods in estimating the effective permeability using percolation concepts.
Gago PA, King PR, Muggeridge AH, 2016, Fast estimation of effective permeability and sweep efficiency of waterflooding in geologically heterogeneous reservoirs
Geological heterogeneity can adversely affect the macroscopic sweep efficiency when waterflooding oil reservoirs, however the exact distribution of permeability and porosity is generally not known. Engineers try to estimate the range of impacts heterogeneity might have on waterflood efficiency by creating multiple geological models and then simulating a waterflood through each of those realizations. Unfortunately each simulation can be computationally intensive meaning that it is generally not possible to obtain a statistically valid estimate of the expected sweep and the associated standard deviation. In this paper we show how the volume of unswept oil can be estimated rapidly (without flow simulations) from a geometrical characterization of the spatial permeability distribution. A "constriction" factor is defined which quantifies the effective cross-section area of the zones perpendicular to the principal flow direction. This is combined with a 'net-To-gross ratio' (which quantifies the fractional reservoir volume occupied by the zones that contribute to flow) to estimate effective permeability and the expected recovery factor for that realization. The method is tested using a range of realistic geological models, including SPE10 model 2 and its predictions are shown to agree well with values obtained using a well established commercial flow simulator.
Sadeghnejad S, Masihi M, King PR, et al., 2016, Study the effect of connectivity between two wells on secondary recovery efficiency using percolation approach
Estimating available hydrocarbon to be produced during secondary oil recovery is an ongoing activity in field development. The primary plan is normally scheduled during early stage of field's life through master development plan studies. During this period, due to the lake of certain data, estimation of the field efficiency is usually based on rules of thumb and not detailed field characterization. Hence, there is a great motivation to produce simpler physically-based methodologies. The minimum necessity inputs of percolation approach make it a useful tool for foration performance prediction. This approach enables us to attain a better assessment of the efficiency of secondary recovery methods at early production time. The main contribution of this study is to establish a continuum percolation model based on Monte Carlo simulation that can estimate the connectivity of good sands between two wells. In the classical percolation, the connectivity is considered between two lines and two faces of the system in 2-And 3-D; whereas, hydrocarbon production is achieved through wells with the shape of lines (e.g., vertical, horizontal, or deviated wells). In addition, the results showed that not implementation of the correct geometry of wells can alter the estimated results from the percolation approach.
Gago PA, Maza D, Pugnaloni LA, 2016, Ergodic-nonergodic transition in tapped granular systems: The role of persistent contacts, PAPERS IN PHYSICS, Vol: 8, ISSN: 1852-4249
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- Citations: 6
Pastor JM, Garcimartin A, Gago PA, et al., 2015, Experimental proof of faster-is-slower in systems of frictional particles flowing through constrictions, PHYSICAL REVIEW E, Vol: 92, ISSN: 1539-3755
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- Citations: 117
Gago PA, Maza D, Pugnaloni LA, 2015, Relevance of system size to the steady-state properties of tapped granular systems, PHYSICAL REVIEW E, Vol: 91, ISSN: 1539-3755
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- Citations: 8
Zuriguel I, Parisi DR, Hidalgo RC, et al., 2014, Clogging transition of many-particle systems flowing through bottlenecks, SCIENTIFIC REPORTS, Vol: 4, ISSN: 2045-2322
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- Citations: 203
Perge C, Alejandra Aguirre M, Alejandra Gago P, et al., 2012, Evolution of pressure profiles during the discharge of a silo, PHYSICAL REVIEW E, Vol: 85, ISSN: 1539-3755
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- Citations: 36
Pugnaloni LA, Sanchez I, Gago PA, et al., 2010, Towards a relevant set of state variables to describe static granular packings, PHYSICAL REVIEW E, Vol: 82, ISSN: 1539-3755
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- Citations: 39
Gago PA, Bueno NE, Pugnaloni LA, 2009, High intensity tapping regime in a frustrated lattice gas model of granular compaction, GRANULAR MATTER, Vol: 11, Pages: 365-369, ISSN: 1434-5021
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- Citations: 10
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