Mechanism Of Foam Generation Research Articles

Foam EOR as an Optimization Technique for Gas EOR: A Comprehensive Review of Laboratory and Field Implementations

Foam-enhanced oil recovery (EOR) is poised to become one of the most promising tertiary recovery techniques to keep up with the continuously increasing global energy demands. Due to their low sensitivity to gravity and permeability heterogeneities that improve sweep efficiency, foams are the preferred injection fluids over water or gas. Although foam injection has been used in the field to improve oil recovery and control gas mobility, its success is still hindered by several conceptual and operational challenges with regard to its stability and foamability under reservoir conditions. This can be attributed to the insufficient attention given to the mechanisms underlying foam generation and stability at the microscopic level in many studies. For a deeper understanding, this study reviews the most pertinent published works on foam-EOR. The major objective is to provide a broad basis for subsequent laboratory and field applications of foam-EOR. In this work, we highlighted foam classification and characterization, as well as the crucial factors impacting foam formation, stability, and oil recovery. Additionally, the principal mechanisms of foam generation are thoroughly explained. Finally, the most recent developments in foam generation and stability improvement are discussed. Foam-EOR is comprehensively reviewed in this work, with an emphasis on both theoretical and practical applications.

Dynamic Behaviors and Mechanisms of Air-Foam Flooding at High Pressure and Reservoir Temperature via Microfluidic Experiments.

Air injection has been proven to be an effective improved oil recovery technique for deep and light oil reservoirs with low permeability and poor water injectivity. But the efficiency of air injection in highly heterogeneous reservoirs is low due to poor gas sweeping that may lead to early oxygen breakthrough caused by gas channeling and viscous fingering. Foam can be used to assist air injection to overcome the obstacles of early gas breakthrough and to increase the displacement and sweeping efficiency. In this paper, laser-etched visual microscopic pore models were used as microfluidic devices to study the air-foam flooding process in porous media at reservoir temperature and high pressure. The dynamic behaviors and relevant mechanisms of air-foam flooding were investigated. Typical mechanisms of foam generation in porous media are achieved in different parts of the micromodel, which can be listed as follows: lamella leave-behind, lamella division, and snap off. Analysis on flow states of air foam showed that foams migrate in porous media by bursting and regenerating during the flooding process. It can be observed that the flow mode of foam in porous media is the separate flow of gas and liquid through microscopic displacement experiments, suggesting that foam should not be treated as a homogeneous phase in heterogeneous porous media. The pressing, occupying, and selective blocking effects of foam in porous media exhibited different oil displacement performances with the presence of various pore geometries and networks. Tiny foams also showed stripping and carrying effects on larger oil droplets benefiting from the lipophilicity of foam. Through comprehensive analysis on overall and local oil displacement mechanisms, air-foam injection could enhance the microscopic sweep volume and improve the oil displacement efficiency.

Modeling foam propagation in pore network with designated pressure constraints

Foam has been used for decades in petroleum industry for enhanced oil recovery (EOR) due to its mobility adjustment ability. A variety of mechanistic models have been proposed to study foam flow in porous media from multiple scales. In these models, such as population balance models, some important parameters (e.g., foam generation/coalescence rate, flowing foam fraction) are treated empirically since they cannot be obtained from conventional lab experiments. To improve the accuracy and reliability of foam description in these foam-assisted processes, pore network model interwoven with invasion percolation with memory (IPM) method has become a powerful tool in studying foam flow characteristics in porous media, which can predict relative permeability and flowing gas fraction with fully consideration of pore-scale mechanisms of foam generation, destruction, and propagation. However, the assumption of exactly sufficient incremental pressure difference used in conventional IPM-based pore-scale foam modeling is rigorous, not only making it difficult to conduct relevant experimental validations, but also creating potential algorithmic conflicts when simulates displacements in weak foam regime or displacements incorporated with foam coalescence mechanisms. In this study, a novel mixed-sorting rule, which proceeds the displacement by describing the transition between immobile foam bank and mobilized foam flow quantitively, is incorporated into IPM-based foam propagation model. In this way, the immobile foam bank is identified based on effective lamellae generation rate, whereas the mobilized foam flow is distinguished by frontal displacing velocity, respectively. Results estimated with proposed method successfully capture key properties that define pore-scale foam behavior at excessive pressure constraints from formation of immobile foam bank, foam mobilizing pressure thresholds, and fractal dimension of displacement pattern, etc.

WITHDRAWN: A comprehensive review on critical affecting parameters on foam stability and recent advancements for foam-based EOR scenario

Foam injection process is a promising new technique of enhanced oil recovery (EOR), which tackles the issues induced by gas-based EOR scenarios such as viscous fingering and gravity segregation phenomena. The potential of foam-based EOR technique has attracted the great attention of the upstream oil industry over the past decades. Besides the successful oilfield applications of the foam injection for improving oil recovery efficiency, there are many challenges regarding to the stability, quality and foamability of the foam solution at elevated pressures and temperatures. In addition, the responsible mechanisms of foam injection at micro-scale has not been well attended in the available literature. This paper reviews the most relevant published researches on the foam-based EOR for a better understanding and delineation. In this work, the critical affecting parameters on foam stability and quality are discussed in details. In addition, the predominant mechanisms of foam generation are comprehensively clarified. In the following, the practical methods for investigation of the foam stability are explained and compared. Finally, the recent advancements for development of foam solution and enhancement of the foam stability are presented and the flaws of studies in this subject are illustrated.

Pore- and Core-Scale Insights of Nanoparticle-Stabilized Foam for CO2-Enhanced Oil Recovery.

Nanoparticles have gained attention for increasing the stability of surfactant-based foams during CO2 foam-enhanced oil recovery (EOR) and CO2 storage. However, the behavior and displacement mechanisms of hybrid nanoparticle–surfactant foam formulations at reservoir conditions are not well understood. This work presents a pore- to core-scale characterization of hybrid nanoparticle–surfactant foaming solutions for CO2 EOR and the associated CO2 storage. The primary objective was to identify the dominant foam generation mechanisms and determine the role of nanoparticles for stabilizing CO2 foam and reducing CO2 mobility. In addition, we shed light on the influence of oil on foam generation and stability. We present pore- and core-scale experimental results, in the absence and presence of oil, comparing the hybrid foaming solution to foam stabilized by only surfactants or nanoparticles. Snap-off was identified as the primary foam generation mechanism in high-pressure micromodels with secondary foam generation by leave behind. During continuous CO2 injection, gas channels developed through the foam and the texture coarsened. In the absence of oil, including nanoparticles in the surfactant-laden foaming solutions did not result in a more stable foam or clearly affect the apparent viscosity of the foam. Foaming solutions containing only nanoparticles generated little to no foam, highlighting the dominance of surfactant as the main foam generator. In addition, foam generation and strength were not sensitive to nanoparticle concentration when used together with the selected surfactant. In experiments with oil at miscible conditions, foam was readily generated using all the tested foaming solutions. Core-scale foam-apparent viscosities with oil were nearly three times as high as experiments without oil present due to the development of stable oil/water emulsions and their combined effect with foam for reducing CO2 mobility

Visualizing pore-scale foam flow in micromodels with different permeabilities

The mechanisms of pore-scale foam transport in porous media help further understanding how foam flows in porous media with different permeabilities. In this work, the pore-scale behaviors of N2 foam flows in porous media with different permeabilities have been investigated using transparent polydimethylsiloxane (PDMS) micromodels with foam qualities in the range of 50 %–90 %. The foam transport in homogeneous and heterogeneous micromodels, and the effects of permeability on foam texture and gas trapping, were evaluated through direct visual observations. Bubble generation based on the pre-existing foam bubbles occurred for trapped and flowing bubbles, which could change the distribution of flowing and trapped gases in the micromodel. Two foam generation mechanisms, namely snap-off and lamella division, were observed in this study. The mean size of bubbles in the low-permeability micromodel was smaller than that in the high-permeability micromodel especially at high foam qualities. The trapped gas saturation in the low-permeability micromodel was larger than that in the high-permeability micromodel. For foam flow in the heterogeneous micromodel, the process of foam diversion from the high-permeability region to the low-permeability region was directly displayed at the pore scale. Regeneration of large trapped bubbles in the low-permeability region under the pressure difference across the micromodel played an important role in this process.

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.

A pore-scale study on improving CTAB foam stability in heavy crude oil−water system using TiO2 nanoparticles

The main challenge of foam injection for enhanced oil recovery (EOR) is foam stability at harsh reservoir conditions. With regard to this matter, this research aims to investigate the effect of TiO2 nanoparticles (NPs) in improving the stability of foam bubbles in porous media. The conductivity of hexadecyltrimethylammonium bromide (CTAB) solutions was measured to determine critical micelle concentration (CMC). Thereafter, TiO2 NPs were added into the foam solution and pH, contact angle, and interfacial tension (IFT) in the presence and absence of CTAB were measured. The stability of foam bubbles was evaluated through bulk foam tests and absorbance measurements. Finally, different nanofluids (at the concentrations of 0.00, 0.01, 0.03, 0.06, and 0.10 wt%), nano-CTAB solutions (at the concentrations of 0.00, 0.01, 0.03, 0.06, and 0.1 wt% NPs at the CMC of CTAB) and foams (at the concentrations of 0.00, 0.01, 0.03, 0.06, and 0.1 wt% NPs and at the CMC of CTAB) were injected into a heavy oil saturated heterogeneous porous medium. The results of the conductivity measurements showed that the highest adsorption of CTAB molecules onto the NPs surfaces occurs at 0.03 wt% NPs and 0.03 wt% CTAB. The results of micro- and macroscopic images from bulk foam tests revealed improved foam stability for this nano-CTAB foam. Moreover, contact angle and IFT results showed further improvement in interfacial properties for the mixture of NPs and CTAB solutions. Ultimately, in terms of oil recovery, the highest oil recovery factor (54%) was observed upon injection of foam with 0.03 wt% TiO2 NPs and 0.03 wt% CTAB, compared to the 22% recovery factor of DW injection. The visualization experiments showed that 32% enhancement in the recovery factor was mainly due to oil production from dead−end pores. This was because of different functions of NPs including stabilization of foam bubbles, wettability alteration, and improved foam generation mechanisms.

Pore Network Investigation of Trapped Gas and Foam Generation Mechanisms

The mobility of gas is greatly reduced when the injected gas is foamed. The reduction in gas mobility is attributed to the reduction in gas relative permeability and the increase in gas effective viscosity. The reduction in the gas relative permeability is a consequence of the larger amount of gas trapped when foam is present while the increase in gas effective viscosity is explicitly a function of foam texture. Therefore, understanding how foam is generated and subsequent trapped foam behavior is of paramount importance to modeling of gas mobility. In this paper, we push the envelope to enlighten our decisions of which descriptions are most physical to foam flow in porous media regarding both the flowing foam fraction and the rate of generation. We use a statistical pore network interwoven with the invasion percolation with memory algorithm to model foam flow as a drainage process and investigate the dependence of the flowing foam fraction on the pressure gradient and to shed light on foam generation mechanisms. A critical snap-off probability is required for strong foam to emerge in our network. The pressure gradient and, hence, the gas mobility reduction are very low below this critical snap-off probability. Above this snap-off probability threshold, we find that the steady-state flowing lamellae fraction scales as $$(\nabla \tilde{p})^{0.19}$$ in 2D lattices and as $$(\nabla \tilde{p})^{0.32}$$ in 3D lattices. Results obtained from our network were convolved with percolation network scaling ideas to compare the probabilities of snap-off and lamella division mechanisms in the network during the initial gas displacement at the leading edge of the gas front. At this front, during strong foam flow, lamella division is practically nonexistent in 2D lattices. In 3D lattices, lamella division occurs, but the probability of snap-off is always greater than the probability of lamella division.

Foam Generation and Rheology in a Variety of Model Fractures

Gas is used in petroleum reservoirs to displace oil for enhanced oil recovery. The microscopic displacement efficiency of gas is very good, but at the reservoir scale the process suffers from poor sweep efficiency, especially in naturally fractured reservoirs. Foam can improve the sweep. There have been considerable scientific contributions toward understanding foam flow in nonfractured porous media, with relatively little work on foam flow in fractured porous media. We investigate foam-generation mechanisms in five fully characterized glass models of fractures with different apertures and correlation lengths of the aperture distribution. We also study the rheology of the in situ-generated foam by varying the superficial velocities of the gas and surfactant solution. We compare the measured pressure gradient against the fracture attributes, aperture, and the correlation length of the aperture. We also compare foam texture as a function of position within the fracture as the generated foam propagates through the fracture. Gas mobility was greatly reduced as a result of in situ foam generation in our model fractures. Foam was generated predominantly by capillary snap-off and lamella division. The measured mobility reduction depends on fracture attributes. Fracture-wall roughness, represented by both the hydraulic aperture and the correlation length of the aperture, plays an important role in foam generation and mobility. The average bubble size increases as the aperture increases, which results in a significant decrease in pressure gradient. Two model fractures show the same two foam-flow regimes central to the understanding of foam in nonfractured porous media: a low-quality regime where pressure gradient is independent of liquid velocity and a high-quality regime where pressure gradient is independent of gas velocity. The mechanisms thought to be behind these two regimes in nonfractured porous media do not apply to these experiments, however.

Study of foam generation and propagation in fully characterized physical-model fracture

Foam greatly reduces gas mobility for gas enhanced-oil-recovery (EOR) projects. It substantially increases both the effective viscosity of gas and gas trapping. Numerous studies have been conducted to understand foam rheology in rock matrix both theoretically and experimentally. The knowledge of foam flow in fractured porous media is incomplete, however. This study aims to contribute to the understating of foam generation and propagation in a fully characterized physical-model fracture. We investigate foam-generation mechanisms and the propagation of pre-generated foam. Gas mobility was greatly reduced as a result of in-situ foam generation. Foam-generation mechanisms similar to those seen in 3D porous media were observed on this model fracture. Foam was generated predominantly by capillary snap-off and lamella division. Lamella division was observed at high gas fractional flow at two different superficial velocities. Fracture wall roughness played an important role in foam generation. In the case of pre-generated foam, two very distinct bubble sizes were injected: fine-textured bubbles much smaller than the roughness scale and coarse-textured foam with bubbles much larger than the roughness scale. The first case did not show any significant change in bubble size as foam propagated through the model fracture, while in the second case, the fracture played a role in reducing bubble size. Inter-bubble diffusion did not regulate bubble size in our apparatus because residence time in the fracture is relatively short. We cannot confirm that foam reached local equilibrium in our experiments, but we believe that local equilibrium lies between the cases of in-situ- and pre-generated foams.

Investigation on stabilization of CO2 foam by ionic and nonionic surfactants in presence of different additives for application in enhanced oil recovery

Application of foam in upstream petroleum industry specifically in enhanced oil recovery (EOR) has gained significant interest in recent years. In view of this, an attempt has been paid to design the suitable foaming agents (foamer) by evaluating the influence of three surfactants, five nanoparticles and several additives. Experimental investigations have been carried out in order to examine the mechanism of foam generation in presence of sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB) and polysorbate 80 (Tween 80) as anionic, cationic and nonionic surfactants by using the CO2 as gaseous component. It has been found that ionic surfactants show the higher foam life compared to nonionic surfactant. Out of different nano particles used, namely alumina (Al2O3) zirconium oxide (ZrO2), calcium carbonate (CaCO3), boron nitride (BN) and silica (SiO2), boron nitride shows the maximum improvement of foam stability. The foam stability of surfactant-nanoparticles foam is further increased by addition of different additives viz. polymer, alcohol and alkali. The results show that, the designed foaming solution have nearly 2.5 times higher half-decay time (t1/2) compared to the simple surfactant system. Finally, it has been found that gas injection rate plays an important role in obtaining a uniform and stabilized foam.

Modeling Techniques for Foam Flow in Porous Media

Summary Foam, a dispersion of gas in liquid, has been investigated as a tool for gas-mobility and conformance control in porous media for a variety of applications since the late 1950s. These applications include enhanced oil recovery, matrix-acidization treatments, gas-leakage prevention, as well as contaminated-aquifer remediation. To understand the complex physics of foam in porous media and to implement foam processes in a more-controllable way, various foam-modeling techniques were developed in the past 3 decades. This paper reviews modeling approaches obtained from different publications for describing foam flow through porous media. Specifically, we tabulate models on the basis of their respective characteristics, including implicit-texture as well as mechanistic population-balance foam models. In various population-balance models, how foam texture is obtained and how gas mobility is altered as a function of foam texture, among other variables, are presented and compared. It is generally understood that both the gas relative permeability and viscosity vary in the reduction of gas mobility through foam generation in porous media. However, because the two parameters appear together in the Darcy equation, different approaches were taken to alter the mobility in the various models: only reduction of gas relative permeability, increasing of effective gas viscosity, or a combination of both. The applicability and limitations of each approach are discussed. How various foam-generation mechanisms play a role in the foam-generation function in mechanistic models is also discussed in this review, which is indispensable to reconcile the findings from different publications. In addition, other foam-modeling methods, such as the approaches that use fractional-flow theory and those that use percolation theory, are also reviewed in this work. Several challenges for foam modeling, including model selection and enhancement, fitting parameters to data, modeling oil effect on foam behavior, and scaling up of foam models, are also discussed at the end of this paper.

Pore-network study of the mechanisms of foam generation in porous media

Understanding the role of pore-level mechanisms is essential to the mechanistic modeling and simulation of foam processes in porous media. Three different pore-level events can lead to foam formation: snapoff, leave behind, and lamella division. The initial state of the porous medium (fully saturated with liquid or already partially drained), as surfactant is introduced, also affects the different foam-generation mechanisms. Bubbles created by any of these mechanisms cause the formation of new bubbles by snapoff and leave behind as gas drains liquid-saturated pores. Lamellae are stranded unless the pressure gradient is sufficient to mobilize those that have been created. To appreciate the roles of these mechanisms, their interaction at the pore-network level was studied. We report an extensive pore-network study that incorporates the above pore-level mechanisms, as foam is created by drainage or by the continuous injection of gas and liquid in porous media. Pore networks with up to 10 000 pores are considered. The study explores the roles of the pore-level events, and by implication, the appropriate form of the foam-generation function for mechanistic foam simulation. Results are compared with previous studies. In particular, the network simulations reconcile an apparent contradiction in the foam-generation model of Rossen and Gauglitz [AIChE J. 36, 1176 (1990)], and identify how foam is created near the inlet of the porous medium when lamella division controls foam generation. In the process, we also identify a new mechanism of snap-off and foam generation near the inlet of the medium.

A mechanistic population balance model for transient and steady-state foam flow in Boise sandstone

Foam in porous media is discontinuous on a length scale that overlaps with pore dimensions. This foam-bubble microstructure determines the flow behavior of foam in porous media and, in turn, the flow of gas and liquid. Modeling of foam displacement has been frustrated because empirical extensions of the conventional continuum and Newtonian description of fluids in porous media do not reflect the coupling of foam-bubble microstructure and foam rheology. We report a mechanistic model for foam displacement in porous media that incorporates pore-level mechanisms of foam generation, coalescence, and transport in the transient flow of aqueous foams. A mean-size foam-bubble conservation equation, along with the traditional reservoir/groundwater simulation equations, provides the foundation for our mechanistic foam-displacement simulations. Since foam mobility depends heavily upon its texture, the bubble population balance is both useful and necessary, as the role of foam texture must be incorporated into any model which seeks to predict foam flow accurately. Our model employs capillary-pressure-dependent kinetic expressions for lamellae generation and coalescence, and incorporates trapping of lamellae. Additionally, the effects of surfactant chemical transport are included. All model parameters have clear physical meaning and, consequently, are independent of flow conditions. Thus, for the first time, scale up of foam-flow behavior from laboratory to field dimensions appears possible. The simulation model is verified by comparison with experiment. In situ, transient, and steady aqueous-phase liquid contents are garnered in a 1.3 μm 2 Boise sandstone using scanning gamma-ray densitometry. Backpressures exceed 5 MPa, and foam quality ranges from 0.80 to 0.99. Total superficial velocities range from as little as 0.42 to 2.20 m/d. Sequential pressure taps measure flow resistance. Excellent agreement is found between experiment and theory. Further, we find that the bubble population balance is the only current means of describing all flow modes of foam self-consistently.

Numerical Simulation of Foam Flow In Porous Media

Abstract This paper presents a new mathematical formulation that provides one possible representation of foam behaviour in a porous medium. Foams are gaining increasing importance in the petroleum industry in a number of roles, notably as mobility control and/or blocking agents in enhanced oil recovery operations. It has been shown in the past that appropriate foams can indeed improve the recovery efficiency of a given process, and foams can serve as temporary blocking agents. Even though many laboratory studies have been conducted to understand the rheology of foams and the mechanics of foam flow in porous media, relatively view efforts have been made toward the mathematical simulation of foam rheology and flow in a porous medium. Recently a few researchers have addressed the problem but often no experimental data were available to validate the numerical simulation results. Besides no attempt was made to incorporate effects of variable absolute permeability, the presence of oil, or optimum surfactant concentration. Experimental evidence has shown that oil acts as a defoamer for most surfactants. The formulation developed in the present work explains the blocking mechanism of foam in the presence of oil and water. The mathematical model is tested against experimental results, showing good agreement. Introduction There have been several attempts to use foam as blocking agents. However, most of these tests were applied in enhanced oil recovery by gas or steam injection(1). The initial work with foam indicated that it could improve the conformance of gasdrive oil recovery processes because it selectively reduces the relative permeability to gas in a reservoir rock(2). Bernard et al. (3) showed that foam flooding recovered more oil mainly because it created a higher trapped-gas saturation which indirectly yielded a lower relative permeability to water. Marsden and Khan(4) observed a higher mobility reduction in higher permeability cores. A microscopic model study was carried out by Owette et al(5). They used visual models saturated with surfactant solutions. They observed that when gas alone was injected only a few interfaces were formed behind the gas liquid front. However, when foam was injected the bubbles were larger. They also observed that larger channels could not be blocked by the foam and most of the gas flow took place through those larger channels. This flow mechanism was independent of surfactant concentration. Even though at low concentrations flow was similar to that in a homogeneous medium, at higher concentrations foam bubbles were actually generated and the process of "breaking and reforming" as described by Holm(6) was observed. Radke and Ransohof(7) also used visual observation to identify foam generation in glass bead packs. They showed the importance of pore geometry on the mechanisms of foam generation. Snap-off and lamella divisions were observed in flow across a boundary from low to high permeability but neither mechanism was seen below a critical capillary number when flowing from a high to low permeability zone. Based on experimental studies, five major mechanisms of foam flow have been recognized. These are:

Mechanisms of Foam Generation in Glass-Bead Packs

SummaryThe fundamental, pore-level mechanisms of foam generation are investigated in monodisperse bead packs. First, direct visual observations identify the following generation mechanisms: lamella leave-behind, gas-bubble snap-off, and lamella division. Then, to ascertain the relative importance of these mechanisms, quantitative experiments are pursued on the role of bead- pack permeability (bead sizes from 0.25 to 1 mm [0.01 to 0.04 in.]), gas-phase velocity (0.001 to 0.8 cm/s [0.0004 to 0.3 in./sec]), gas-phase fractional flow (0.60 to 1.0), permeability variations, and surfactant type [sodium dodecyl benzene sulfonate (SDBS), sodium dodecyl sulfate (SDS), Chevron Chaser SD1000™, and Suntech IV 1035™]. We discover a critical velocity, above which a "strong" foam is generated and below which only "weak" foam is formed. The snap-off mechanism is the primary mechanism responsible for the formation of the strong foam. A simple model, based on the concept of a "germination site," is developed to predict the onset of snap-off at higher gas velocities. New experimental data obtained in the homogeneous glass-bead packs for the critical capillary number necessary to form a strong foam are in excellent agreement with the proposed germination- site model.

An Experimental Investigation of Gas-Bubble Breakup in Constricted Square Capillaries

Summary. Recent advances in EOR involve generating foam within underground porous media to displace the oil. We investigate the important snap-off mechanism of gas-bubble generation in constricted square capillaries experimentally. The snap-off of smaller bubbles from a larger bubble as it moves through the constriction is recorded on 16-mm movies. The time required for bubbles to snap off once they move past the constriction and the length of the generated bubbles are obtained from viewing the movie frames. The bubble capillary number, uvT/, is varied from 10–5 to 5 × 10 -3 by adjusting the wetting-fluid viscosity, A, and the surface tension, a, by adding aqueous surfactants to mixtures of glycerol and water and by altering the bubble velocity, VT. Results show that a dimensionless time to snap-off depends weakly on the capillary number and that the generated bubble size increases almost linearly with increasing- capillary number. Surfactants create dynamically inmobile interfaces for surfactant solutions of I wt% sodium dodecyl benzene surfactants (SDBS) and Chevron Chaser SD1000. Compared with the surfactant-free solutions, the time to breakup with surfactants increases by a factor of about 3; generated bubble length increases by a factor of at most 3. Introduction Use of steam foams as displacing fluids for EOR has been successful in recent field applications. Foam improves oil recovery by increasing the resistance to flow of the steam through the underground oil-bearing porous medium, thus improving mobility control and decreasing gravity override. Although foam has proved successful in field applications, modeling of foam displacement processes is still at an early stage. To understand foam flow in porous media better, we investigate the mechanisms of foam generation in porous media. Foam ‘veneration processes are important because they control the texture (i.e., the bubble size) of a foam as it flows through the medium. In turn, texture strongly affects the pressure-drop/flow-rate relationship for foam in porous media. Our objective is to quantity in a realistic single-pore model let the time required to snap off a bubble and the size of the generated bubble. The pore space between three touching spheres (an ideal porous medium) is quite angular 4 (see Fig. 1a of Ref. 4). It looks like a constricted triangular capillary. Square capillaries maintain the angularity and also are readily available. Hence, constricted square capillaries are studied in this work. Snap-off occurs when a bubble is disconnected from a larger gas bubble by a growing liquid collar that blocks a pore neck, as shown in Fig. 1. As the gas bubble moves through a constriction, it leaves a channel of liquid in the corners of the capillary. as shown in Steps 1 and 2 of Fig. 1. Liquid then fills in at the pore neck, as depicted in Step 3. Eventually, sufficient liquid collects at the pore neck, and the liquid rearranges to form a lens that blocks the capillary, thus snapping off a bubble in Step 4. Previous Work Roof originally studied snap-off as a mechanism that traps oil drops in porous media. Fried, and in a more careful study, Mast Showed that snap-off is an important mechanism for bubble generation in porous media. Most investigations of snap-off are in cylindrical glass capillaries. Often, a groove is cut along the wall of the capillary to form a channel along which liquid can flow. The advantage of square capillaries is that they naturally have the channels. Arriola et al. studied the trapping of oil drops in constricted square capillaries and determined how small oil drops snap off from a trapped drop. We also study square constricted capillaries. In our case, however, the gas bubbles are always driven through the constriction at a constant volumetric flow' and are not allowed to trap. Accordingly, we observe a mechanism of snap-off different from the one Arriola et al. reported. Roof argued that if the hemispheric front of a nonwetting phase protrudes seven throat radii past the neck of the constriction, the front of the nonwetting phase will snap off. This is a static analysis. It cannot predict dynamic behavior. Strand et al., however, observed that the flow rate of an oil drop through a constriction is important with larger oil drops being snapped off when the oil drop is driven through a capillary at a higher velocity. JPT P. 1137^