Foam Generation Research Articles

Role of viscous cross-flow and emulsification in recovery of bypassed oil during foam injection in a microfluidic matrix-fracture system

Role of viscous cross-flow and emulsification on foam displacement process, has been experimentally studied in a microfluidic device (consists of homogeneous matrix pattern and a fracture in one side) by changing the injectant type (gas/foam/aqueous solution of surfactant/water) and foam quality and presence of connate water.Two distinct regions have been identified during foam displacement: 1) A dynamic region where gas and foam saturation is low and it mostly consists of aqueous phase. Emulsions of oil phase is generated at the front of dynamic region. 2) A static region where foam saturation is high and fluids’ mobility is low.The discussion has been also supported by analysis of calculated oil recovery variation versus foam quality with a bell-shaped trend, matched with foam rheology reported in standard quality scan tests. Foam injection tests and gas injection ones as the case of no viscous cross-flow have been contrasted and the results confirmed the proposed role of viscous drives. Proper condition of emulsification has been discussed in foam injection tests and it has been shown that lack of such conditions in surfactant injection leads to weak emulsification. Moreover, foam generation mechanisms which have been observed during the experiments are snap-off and pinch-off.

ВПЛИВ РОБОЧИХ ПАРАМЕТРІВ НА ТЕХНІЧНІ ХАРАКТЕРИСТИКИ ФУНКЦІОНАЛЬНИХ ЗРАЗКІВ СТВОЛІВ-ГЕНЕРАТОРІВ ПІНИ ВИСОКОЇ КРАТНОСТІ

Проаналізовано сучасний стан застосування піни високої кратності як засобу пожежогасіння об’ємним способом та обґрунтовано необхідність розроблення в Україні пожежних стволів-генераторів піни високої кратності. Проведено патентний пошук пристроїв для генерування піни високої кратності з робочих розчинів піноутворювачів, де вказано на особливості складників під час утворення піни високої кратності. Наведено результати досліджень робочих параметрів розроблених експериментальних зразків генераторів піни високої кратності вентиляторного та ежекційного типів. Установлено залежність величини кратності піни, продуктивності за піною та кількості витраченого робочого розчину від співвідношень геометричних розмірів складників генераторів і витрати (тиску) робочого розчину піноутворювача та витрати повітряного потоку. Розроблено два функціональні макети генераторів піни високої кратності ежекційного та вентиляторного типу.

Monodisperse liquid foams via membrane foaming

HypothesisIt is possible to generate fairly monodisperse liquid foams by a dispersion cell, which was originally designed for the generation of fairly monodisperse emulsions. If this is the case, scaling-up the production of monodisperse liquid and solid foams will be no longer a problem. ExperimentsWe used the dispersion cell - a batch process - and examined the influence of stirrer speed, membrane pore diameter and injection rate on the structure of the resulting liquid foams. We used an aqueous surfactant solution as scouting system. Once the experimental conditions were known we generated gelatin-based liquid foams and methacrylate-based foamed emulsions. FindingsWe found that (a) the bubble size of the generated liquid foams can be adjusted by varying the membrane pore diameter, (b) no stirrer should be used to obtain monodisperse foams, and (c) the bubble size is not influenced by the air injection rate. Since (i) the output for all investigated systems is up to two orders of magnitude larger compared to microfluidics and (ii) the membrane technology can very easily be scaled-up if run in a continuous process, the use of membrane foaming is expected to be heavily used for the generation of monodisperse liquid and solid foams, respectively.

Optimized foam-assisted CO2 enhanced oil recovery technology in tight oil reservoirs

In this paper, foam-assisted CO2 EOR and anti-gas channelling technology are investigated and optimized to enhance tight oil recovery. A series of laboratory experiments, including pressure−volume−temperature, foam generation and evaluation, and coreflood tests, and phase behaviour theoretical mathematical models are performed to evaluate foam agent, analyze anti-gas channelling mechanisms and influential factors, and optimize foam-assisted CO2 EOR technique. The specific CO2 bulk fluid behaviour and phase behaviour in wellbore are determined through experimental and theoretical models. Two distinct stages are found to be divided prior to and after the CO2 gas breakthrough. Most oil productions, which is in the range of 28–40%, occur prior to the gas breakthrough, whereas only additional 5–8% oil is produced after the gas breakthrough. A higher injection rate and/or permeability ratio result in an earlier gas breakthrough and causes less oil to be produced before gas breakthrough while the oil recovery factor slightly increases after the breakthrough by increasing injection rate. Gas diffusion in water-saturated core reach equilibrium faster than that in the oil-saturated core. An overall evaluation parameter is developed to select foam agent. The optimized static condition for the selected foam agent here is approximately 9 MPa at low temperatures while dynamic performance is improved at a higher gas but lower liquid injection rate. The simultaneous water-alternating-gas injection scheme in subsequent of an initial gas injection with liquid−gas ratio of 1:1 performs better than the water-alternating-gas scheme, which is proven to be effective for the core samples with fracture width of 82.67 μm. Finally, the oilfield surface foaming operational system is designed to upscale laboratory research to practical applications with specific operating setup and procedures, which has been applied in the target oil reservoir and performs well as expected.

Development of sustainable and cost efficient textile foam-finishing and its comparison with conventional padding

Conventional padding is a non-sustainable textile processing technique, which consumes excessive water, chemicals and energy. To support the survival of the textile processing industry, researchers have identified the foam technology for application of dyes and finishes. Foam technology is more ecofriendly than the conventional padding. However, the successful foam generation for different finishes is a challenging task. In addition, it is more difficult and complicated task to effectively apply the foam on the fabrics and get the results which should be comparable with the conventional padding. This paper compares the pad-finishing with foam-finishing on the basis of sustainability, cost, productivity, and performance using 11 different non-toxic and sustainable finishes including cross-linkers, oil and water repellents, softeners, and fire retardant on the cotton fabric samples. Cost, productivity, performance and sustainability were estimated through the specific methods. The paper organizes the problem as analytic hierarchy process model and solves the model using super decisions software. The results reveal that the foam-finishing technique is more preferable in terms of cost, productivity and sustainability, if optimized properly. In addition, the successful foam-finishing recipes have been generated and the performance of foam-finishing has been comparable with pad-finishing. For instance, if all the criteria were given the same priority, the foam-finishing recipe with less quantity of chemical was 84.61% better than the pad-finishing. The optimized foam-finishing recipe was 84.55% better than the pad-finishing. In addition, the optimized foam-finishing recipe indicated better finishing performance in term of some important tests as compared to padding.

The effect of rock microheterogeneity on steam foam rheology

The injection of steam into a heavy oil reservoir as an enhanced oil recovery method is a mature technology. However, due to steam's high mobility, this process can suffer from gravity override, viscous fingering, and loss to thief zones. Steam foam has been used successfully in the 1970's and 1980's to improve the sweep efficiency of steam injection processes, however more work needs to be done to better understand and optimize the process for successful commercial field implementation. Recent development of better surfactants for high temperature foams above 200 °C enabled application in more reservoirs, but the physics of steam foam in varying reservoir properties has been neglected. The purpose of this work is to study the change in the rheological behavior of steam foam in rocks with varying degrees of microheterogeneity. Core floods were conducted using 1-D sand packs to determine the apparent viscosity of steam foam in varying conditions of permeability, foam quality, and shear rate. Homogenous and heterogeneous sand packs with low and high permeability were used to study the effect of rock microheterogeneity on steam foam behavior. Furthermore, the quality and velocity of the foam were varied to better understand how to optimize a steam foam process in reservoirs with varying rock quality. Based on the results, it is concluded that permeability and foam quality govern the trend of apparent viscosity growth and decay for a given sand permeability. It is determined that there is an offset of optimum foam quality between high and low permeability sands. Steam foam performs better at higher foam qualities in high permeability systems. The heterogeneity of a sand does not affect this. It was also determined that at high velocities, the shear thinning effect on steam foam is offset by new foam generation in heterogeneous sands. This phenomenon keeps the apparent viscosity of the foam stable, both in low and high permeability environments. While the permeability of the sand more significantly influences foam viscosity, it was concluded that the microheterogeneity of a sand pack also affects foam dynamics. The complexity of the pore network in a heterogeneous environment aids in the generation of foam. Overall, this work has added knowledge to the understanding of steam foam rheology in porous media. With advances in both foaming surfactant formulations and the understanding of steam foam physics, the field application of steam foam can be better optimized when taking into consideration the reservoir environment.

Homogeneous gelation leads to nanowire forests in the transition between electrospray and electrospinning

We demonstrate that homogeneous gelation of droplets in electrospray leads to the generation of nanowire forests and foams.

Influence of aqueous foam syneresis on the shock wave propagation velocity

Numerical simulation of the spherical shock pulse propagation in aqueous foam with volumetric liquid fraction of 0.0083 has been carried out in accordance with the published experimental data on the explosion of HE in aqueous foam. The assumption is used that the foam structure is destroyed by the shock wave, which leads to the transformation of the foam into a monodisperse gas-droplet mixture. The system of equations for the two-phase gas-droplet model of aqueous foam includes the laws of conservation of mass, momentum, energy for each phase and the equation for the dynamics of the volumetric liquid fraction in a single-pressure, two-velocity, two-temperature approximations in a three-dimensional formulation and takes into account the forces of the Schiller-Naumann interfacial drag, the Ranz-Marshall interphase contact heat exchange and the effect of foam syneresis on the initial distribution of its volumetric liquid fraction. Realistic equations of state in the form of Peng-Robinson and Mie-Gruneisen are used to describe the thermodynamic properties of air and water that make up a gas-droplet mixture. Numerical modeling of the processes under consideration was carried out in the open software of computational fluid dynamics OpenFOAM using the finite volume method based on the iterative two-step PIMPLE algorithm. The analysis of the effect of foam syneresis on the dynamics of shock pulse in aqueous foam is given. It was found that the uneven distribution of the liquid fraction in the foam, caused by its sedimentation under the gravity, leads to the increase in the shock pulse velocity in upper layers of the foam. In comparative analysis of numerical solutions and experimental data at sensor locations, the importance of taking into account syneresis phenomena in modeling the dynamics of shock wave in aqueous foam is shown. The reliability of calculations obtained by the proposed model is confirmed by their agreement with experimental data.

Core-scale sensitivity study of CO2 foam injection strategies for mobility control, enhanced oil recovery, and CO2 storage

This paper presents experimental and numerical sensitivity studies to assist injection strategy design for an ongoing CO2 foam field pilot. The aim is to increase the success of in-situ CO2 foam generation and propagation into the reservoir for CO2 mobility control, enhanced oil recovery (EOR) and CO2 storage. Un-steady state in-situ CO2 foam behavior, representative of the near wellbore region, and steady-state foam behavior was evaluated. Multi-cycle surfactant-alternating gas (SAG) provided the highest apparent viscosity foam of 120.2 cP, compared to co-injection (56.0 cP) and single-cycle SAG (18.2 cP) in 100% brine saturated porous media. CO2 foam EOR corefloods at first-contact miscible (FCM) conditions showed that multi-cycle SAG generated the highest apparent foam viscosity in the presence of refined oil (n-Decane). Multi-cycle SAG demonstrated high viscous displacement forces critical in field implementation where gravity effects and reservoir heterogeneities dominate. At multiple-contact miscible (MCM) conditions, no foam was generated with either injection strategy as a result of wettability alteration and foam destabilization in presence of crude oil. In both FCM and MCM corefloods, incremental oil recoveries were on average 30.6% OOIP regardless of injection strategy for CO2 foam and base cases (i.e. no surfactant). CO2 diffusion and miscibility dominated oil recovery at the core-scale resulting in high microscopic CO2 displacement. CO2 storage potential was 9.0% greater for multi-cycle SAGs compared to co-injections at MCM. A validated core-scale simulation model was used for a sensitivity analysis of grid resolution and foam quality. The model was robust in representing the observed foam behavior and will be extended to use in field scale simulations.

Environmental effect of lubricity additives through dielectric molecular parameters

Lubricant additives are chemicals, nearly always-organic or organometallic, that are added to oils in quantities of a few weight percent to improve the lubricating capacity and durability of the oil. Specific purposes of lubricant additives are improving the wear and friction characteristics by provision for adsorption and extreme pressure lubrication, improving the oxidation resistance, control of corrosion, control of contamination by reaction products, wear a and other debris, reducing excessive decrease of lubricant viscosity at high temperatures, enhancing lubricant characteristics by reducing the pour point and inhibiting the generation of foam. These properties depend upon the molecular interactions between the chemical additives. So that, this study target on investigating the molecular interactions through the dielectric parameters such as Kirkwood correlation factor, Excess permittivity, Bruggemann factor and Dipolar excess free energy in the lubricative binary mixtures,1) caprylic acid + 2-ethyl-1-hexanol and 2) valeric acid + 2-ethyl-1-hexanol. The above dielectric parameters have been calculated from the experimentally measured values of capacitance and refractive index of mixtures, at temperatures 303 K, 313 K and 323 K. In the above studied systems, existence of short and long range, homo and hetero interactions are identified. These interactions are found to be concentration and temperature dependent. The hetero interaction results in the formation linear as well as closed multimers in the mixture. Due to the existing interactions, structural changes are found to take place.

Oxide-mediated mechanisms of gallium foam generation and stabilization during shear mixing in air

Foaming of gallium-based liquid metals improves their processability and-seemingly in contrast to processing of other metal foams-can be achieved through shear-mixing in air without addition of solid microparticles. Resolving this discrepancy, systematic processing-structure-property characterization demonstrates that many crumpled oxide particles are generated prior to air bubble accumulation.

Foamstability: The interplay between salt-, surfactant- and critical micelle concentration

It is central to understand the underlying factors that affect the generation and stabilization of foam. Nevertheless, reports on how foam stability is impacted by surfactant concentration, electrolyte concentration, and electrolyte nature are ambiguous. This paper summarizes the foaming properties below and above the critical micelle concentration (CMC) at various electrolyte concentrations. The commonly used surfactant, alpha olefin sulfonate (AOS), was used along with NaCl in a range of concentrations. The foam height, foam volume stability, bubble count, and foam structure were studied to determine the foam behavior at given conditions. The optimum surfactant concentration for the formation of a stable micellar film was found to be just above the CMC value and the foam stability on the addition of salts is related to the surfactant concentration. There exists another critical concentration (not CMC) that is important to determine the impact of salts on the stability of the foam. The addition of NaCl does not have the same impact below and above this critical concentration. If the concentration of surfactant is below the critical concentration, addition of NaCl reduces the half-life and foam stability. However, at higher surfactant concentrations, the presence of NaCl improves the half-life and foam stability. This study clarifies the ambiguities that are present in the literature pertaining to the effect of salt on foam stability and foaming properties.

Research-scale three-phase jet foam generator design and foaming condition optimization based on Box–Behnken design

Three-phase foam, namely surfactant-particle stabilized foam is a promising material in pool fire extinguishment. However, its effectiveness has not been validated. In this study, a research-scale three-phase jet foam generator, which allowed adjustments of foam slurry composition, nozzle type, air flow rate and screen aperture, was developed. In order to optimize the foaming condition, a three-factor three-level Box–Behnken design (BBD) was adopted. Fly ash (FA) particles were used as the solid phase to generate three-phase foams. As a result, the FA supported foam exhibited better stability over conventional fire-fighting foam, especially when the particle concentration exceeded a threshold value. In addition, the BBD results presented a good agreement between experimental data and fitted models. The optimal foaming condition was determined by numerical optimization. Small-scale fire extinguishing experiments were carried out and three-phase foam manifested better burnback performance compared to conventional fire-fighting foam. The design in this work can be used to study the firefighting efficiency of different three-phase foams and serve as a prototype to develop better generators for both lab research and practical application.

Генератори сценічних ефектів як постановочний засіб художньо-образної виразності на сцені

The purpose of the study is to study, analyze and systematize the generators of stage special effects as a means of presenting artistic and figurative information in the stage space, and to make an art assessment of their role in the works of the performing arts. The research methodology consists of the following approaches: system-historical is to study the context of the problem; art is to determine the generators role of stage special effects in contemporary stage practice, structural and analytical are to classify various special effects as the means of artistic expression in the scene. The scientific novelty of the proposed research is to artificially analyze the processes of stage space modernization by introducing a modern range of special stage effect generators (smoke, fog, snow, soap bubbles, foam, cryo, pyro and aroma generators, paper salutes, mobile systems, mobile systems), and also in the study of the significance of these technical artistic and production means of expression in contemporary works of the performing arts. Conclusions. Based on the above, it can be argued that special effects generators are becoming more widely used as artistic and production means of presenting figurative information in the works of the performing arts. However, while the modernization of the stage space is closely linked to scientific and technological progress, the demonstration of the possibilities of stage technique is not an end in itself and they use technical innovations to enrich artistic techniques, giving technological means, first of all, to perform aesthetic and visual functions on stage.

Studying the generation of foam in the presence of nanoparticles using a microfluidic system

In this paper, a study on the generation of foam in the presence of nanoparticles in porous media using microfluidics is reported. Drainage and co-injection tests were carried out and monitored with a high-speed camera. Convolutional neural network model was applied to quantify foam texture over time, efficiently. Microscopy images show a generation process characterized by early snap-off, followed by lamella-division, and finally leave-behind. In the presence of nanoparticles, the latter stage is delayed due to resistance to drainage imparted by capillary forces within the liquid films. Moreover, a general relationship between the generation rate and the pressure gradient, which resembles the classical constitutive equation for foam generation, i.e., rg∝∇Pα, could be formulated. This indicates that the generation of a foam in the presence of partially hydrophobic nanoparticles and anionic surfactant, used for strong foam formation, follows the same mechanism of a foam stabilized only with surfactant.

Screening of CO2-philic surfactants morphology for high temperature-pressure sandstone reservoir conditions

The conventional foaming surfactants have drawbacks such as premature lamellae rupture in contact with oil, copious water amount required for foam generation, adsorption issue or surfactant partitioning in water/oil interface, salinity tolerance problem and instability at temperature and high pressure. The addition of CO2-philic functionalities in surfactant structure and surfactant blending is suggested to alleviate these problems, improve mobility control, and accelerate foam propagation. In our previous publications, foaming performance of a newly developed surfactant blend with CO2-philic functionalities was described based on surfactant structure-property characterization in Malaysian reservoir conditions. In this study, we develop a fast-track screening procedure for sulfonate anionic based surfactant blend design accounting for the effect of surfactant tail groups (single, double and triple-tailed) and morphology on CO2 foam performance targeting high pressure/high temperature sandstone reservoirs. A visualized foam meter and HTHP coreflood was used to observe performance of the foam generated. IFT surfactant solutions with CO2were also used to examine the attraction of surfactant with CO2.It was observed that triple tailed surfactant perform better both in bulk and coreflood tests in terms of stability, propagation and oil recovery at a high temperature of 102 °C, pressure of 125 bar and salinity of 30,000 ppm. The addition of tail into surfactant structure also increased surfactant activity at C/W interface and reduced surface tension. However, the adsorption of surfactants onto Berea sandstone rock showed a direct relation with the number of tail groups that impose a limitation on it. The blend of selected superior surfactants with betaine foam booster were compared in terms of foam propagation in coreflood in the absence and presence of oil, where blend of triple-tail surfactant performed better than double-tail surfactants, as expected.

Improved foam application at the tunnel face with large ventilation volume and low pressure supplied water

Due to large energy loss, current foam technology for dust suppression requires high inlet water pressure, but performance is poor where ventilation volume is high. This paper proposes modifying key foam device components, including the agent mixing device, foam generator, and nozzle, to address these problems. A cavitating jet pump was simultaneously employed for accurate agent mixing and flow meter, guaranteeing accurate agent flow and low energy loss. A vertical foam generator was designed to increase foam volume and ensure even foam distribution among the foam nozzles. The two-layered foam nozzles were designed and fabricated using 3D printing to ensure even and thick foam impact area on the dust source, effectively blocking dust escape passages. Experiments were performed in the laboratory and on a real heading face. Ventilation volume was 560–570 m3/min for the field experiment with initial total dust concentration 589–679 mg/m3. The proposed system reduced required inlet water pressure to 0.87 MPa, while achieving 86.8% average total dust suppression efficiency and reducing long-term time average respirable dust concentration to 3.9–5.7 mg/m3. This study will help improve performance and expand application fields for foam dust suppression technology.

Innovative Approach To Enhance the Methane Hydrate Formation at Near-Ambient Temperature and Moderate Pressure for Gas Storage Applications

The solidified natural gas (SNG) technology via hydrates is a promising and potential method for storing natural gas. It offers a compact mode of natural gas storage with high degree of safety. In this work, we present the synergistic effects of tetrahydrofuran (THF), hollow silica (HS), and sodium dodecyl sulfate (SDS) in enhancing the kinetics of mixed methane–THF hydrate formation at 6 MPa and 293.2 K in an unstirred reactor configuration. HS at the low loading of 0.5% w HS/v in 5.56 mol % THF solution used in the current study is an innovative approach to improve the surface contact area resulting in kinetic enhancement at moderate conditions (low driving force). However, the mixture of 0.5% w HS/v in 5.56 mol % THF solution resulted in a long induction time (about 400 min), which is not favorable for the SNG technology. Addition of SDS decreased the induction time and increased the hydrate formation rate under similar experimental conditions. The induction time significantly decreased with the increase in the SDS concentration. Additionally, the presence of SDS played a key role in influencing the temperature profile and the hydrate morphology during mixed hydrate formation. The methane gas uptake was 0.0591 (±0.0007) and 0.0615 (±0.0023) mol of methane/mol of water without and with SDS, respectively. Thermal stimulation with ΔT = 15 K was employed to recover methane from hydrates. A recovery of 96–98% of the stored methane gas from the hydrates formed with and without SDS was demonstrated. Interestingly, the presence of HS was effective in preventing the foam generation during hydrate dissociation in the presence of SDS.

Impact of Crude Oil on Pre-Generated Foam in Porous Media

As foam is injected into an oil reservoir, the region near an injector can become oil-free due to the relatively high capillary number. Foam created in this region encounters oil further out in the reservoir. The impact of oil on foam in porous media is usually investigated by co-injecting surfactant, gas and oil, or by injecting pre-generated foam into an oil-saturated core. However, the former experiment does not give information on the impact of oil on pre-generated foam, and with the latter experiment one cannot easily obtain data at different oil fractional flows, necessary to model the impact of oil on pre-generated foam.Here we present a novel but relatively simple experimental technique for investigating the effect of oil on pre-generated foam in porous media. This allows one to compare the effects of oil on pre-generated foam and on foam generation without the experimental complications involved in using two separate porous media. We co-inject surfactant solution and gas into a relative narrow core (1 cm diameter), and inject oil into the core some distance downstream from the inlet through ports in the side of the core. The relatively narrow core allows rapid contact between the injected crude oil and pre-generated foam in a porous medium much larger than pore dimensions. By injecting the three phases into the core we investigate the flow behaviour of foam with oil at fixed fractional flows of all three phases.We illustrate the technique with a study of the effect of one crude oil with two surfactants. With this system, there is a progressive decrease in the apparent viscosity of the foam after encountering oil. Foams with a higher gas fraction experiences a more-significant weakening by oil over the length of the core than foams with a lower gas fraction. By the end of the core, the apparent viscosities of foam with a higher gas fraction approach values observed with three-phase co-injection. We present a novel, but relatively simple method to investigate the change of foam mobility as it encounters oil in a porous medium, at controlled fractional flows. We show that in our case the apparent viscosity of foam with oil can decrease by more than a factor of four over a distance of 15 cm, indicating that foam and oil reach steady-state (as observed with three-phase co-injection) almost instantaneously compared to the length of a reservoir-simulation grid-block.

Foam Generation in Flow Across a Sharp Permeability Transition: Effect of Velocity and Fractional Flow

Summary Foam reduces gas mobility and can help improve sweep efficiency in an enhanced-oil-recovery (EOR) process. For the latter, long-distance foam propagation is crucial. In porous media, strong foam generation requires that the local pressure gradient exceed a critical value (▿Pmin). Normally, this happens only in the near-well region. Away from wells, these requirements might not be met, and foam propagation is uncertain. It has been shown theoretically that foam can be generated, independent of pressure gradient, during flow across an abrupt increase in permeability (Rossen 1999). The objective of this study is to validate theoretical explanations through experimental evidence and to quantify the effect of fractional flow on this process. This article is an extension of a recent study (Shah et al. 2018) investigating the effect of permeability contrast on this process. In this study, the effects of fractional flow and total superficial velocity are described. Coreflood experiments were performed in a cylindrical sintered-glass porous medium with two homogeneous layers and a sharp permeability jump in between, representing a lamination or cross lamination. Unlike previous studies of this foam-generation mechanism, in this study, gas and surfactant solution were coinjected at field-like velocities into a medium that was first flooded to steady state with gas/brine coinjection. The pressure gradient is measured across several sections of the core. X-ray computed tomography (CT) is used to generate dynamic phase-saturation maps as foam generates and propagates through the core. We investigate the effects of velocity and injected-gas fractional flow on foam generation and mobilization by systematically changing these variables through multiple experiments. The core is thoroughly cleaned after each experiment to remove any trapped gas and to ensure no hysteresis. Local pressure measurements and CT-based saturation maps confirm that foam is generated at the permeability transition, and it then propagates downstream to the outlet of the core. A significant reduction in gas mobility is observed, even at low superficial velocities. Foam was generated in all cases, at all the injected conditions tested; however, at the lowest velocity tested, strong foam did not propagate all the way to the outlet of the core. Although foam generation was triggered across the permeability boundary at this velocity, it appeared that, for our system, the limit of foam propagation, in terms of a minimum-driving-force requirement, was reached at this low rate. CT images were used to quantify the accumulation of liquid near the permeability jump, causing local capillary pressure to fall below the critical capillary pressure required for snap-off. This leads to foam generation by snap-off. At the tested fractional flows, no clear trend was observed between foam strength and fg. For a given permeability contrast, foam generation was observed at higher gas fractions than predicted by previous work (Rossen 1999). Significant fluctuations in pressure gradient accompanied the process of foam generation, indicating a degree of intermittency in the generation rate—probably reflecting cycles of foam generation, dryout, imbibition, and then generation. The intermittency of foam generation was found to increase with decreasing injection velocities and increasing fractional flow. Within the range of conditions tested, the onset of foam generation (identified by the rise in ▿P and Sg) occurs after roughly the same amount of surfactant injection, independent of fractional flow or injection rate.