Foam Stability Influenced by Displaced Fluids and by Pore

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Thermodynamics, Transport, and Fluid Mechanics

Foam stability influenced by displaced fluids and by pore size of porous media Mohammad Javad Shojaei, Kofi Osei-Bonsu, Simon Richman, Paul Grassia, and Nima Shokri Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05265 • Publication Date (Web): 20 Dec 2018 Downloaded from http://pubs.acs.org on December 23, 2018

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Industrial & Engineering Chemistry Research

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Foam stability influenced by displaced fluids and by pore size of porous

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media

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Mohammad Javad Shojaei1, Kofi Osei-Bonsu2, Simon Richman1, Paul Grassia3

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and Nima Shokri*1

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1School

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Manchester, Manchester, UK

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2Department

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The USA

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3Department

of Chemical Engineering and Analytical Science, University of

of Petroleum Engineering, University of Wyoming, Laramie, WY

of Chemical and Process Engineering, University of Strathclyde,

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Glasgow G1 1XJ, United Kingdom

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*Corresponding Author

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Dr. Nima Shokri

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School of Chemical Engineering and Analytical Science

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Room C26, The Mill

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The University of Manchester

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Sackville Street, Manchester, M13 9PL, UK

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Tel: 0441613063980

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Email: [email protected] 1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Group website: http://personalpages.manchester.ac.uk/staff/nima.shokri/

2 ACS Paragon Plus Environment

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Abstract

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Stability of foam in the presence of hydrocarbons is a crucial factor in the

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success of its use in various applications in porous media, such as soil

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remediation and enhanced oil recovery (EOR). It is generally believed that

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shorter chain hydrocarbons with lower density and viscosity have more

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detrimental effect on foam stability than longer chain hydrocarbons. However, it

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is still unclear how pore size of porous media could influence this behaviour.

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The main objective of the present study was to investigate the combined effect

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of the hydrocarbon chain length and hence the hydrocarbon’s viscosity and pore

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size of porous media on the foam stability and its displacement efficiency. To

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this end, a systematic series of experiments was conducted using an empty

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Hele-Shaw cell and glass bead packs with different pore size. The results in the

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Hele-Shaw cell and with coarse and medium beads revealed that the lighter, less

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viscous oil (Isopar G) was more destructive to foam. However, the results in the

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fine glass bead pack experiments did not correlate well with this finding. In the

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fine bead pack, foam appeared to have higher displacement efficiency in the

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presence of the lighter, less viscous oil. Generally, our results suggest that the

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pore size of the porous medium plays a more important role on the foam

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displacement efficiency compared to the type of oil.

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Keywords: Foam stability, Foam–oil interaction, Hele-Shaw cell, Pore size of

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porous media 3 ACS Paragon Plus Environment


1.

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INTRODUCTION

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Foam is a dispersion of a large volume of gas in a small volume of liquid in

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which gas bubbles are made discontinuous by thin liquid films called lamellae.1-

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3

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surfactant solution) making it a desirable mobility control agent for fluid

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displacements.4-8 The stability of foam is one of the key determining parameters

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that influences the displacement efficiency of foam flooding projects.9-12 Many

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studies have been conducted to investigate and describe the factors controlling

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foam stability. Derjaguin and Landau 13 introduced the theory of film disjoining

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pressure (Π), dependent upon film thickness (h), to explain the stability of a

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foam film. The disjoining pressure (Π) is defined as the sum of the repulsive

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positive electrostatic force per unit area (ΠEL) and attractive negative van der

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Waals force per unit area (ΠVW) according to classical DLVO theory.14 At stable

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equilibrium, the disjoining pressure of a lamella film is equal to the capillary

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pressure, i.e., Π = ΠEL + ΠVW = PC which is the pressure difference across the

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interface of gas and surfactant (solution). Adsorption of surfactant to the gas

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liquid interface results in an increase of the repulsive positive electrostatic force

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(ΠEL) which stabilizes the lamellae. In contrast, attractive van der Waals forces

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destabilize the thin film. If ΠEL > │ΠVW│+PC foam will be stable, conversely, if

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the negative component is stronger (ΠEL < │ΠVW│+Pc), the foam will be

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destabilized.15, 16

Foam has a higher apparent viscosity compared to its constituents (gas and


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The effect of oil on foam stability has been explained by three mechanisms, i.e.

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entry of oil droplet into gas-liquid interface17, 18, spreading of oil on the gas-

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liquid interface19, or formation of an unstable bridge across the lamella which

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destroys it.20 So called, entry, spreading and bridging coefficients can be

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determined using interfacial tension and the signs of these coefficients are

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believed to govern stability. However, several studies showed that this theory

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may not be able to offer an accurate prediction of foam stability.21-25 For

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example, Hadjiiski et al. 25 concluded that, no direct relation exists between the

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entering and spreading coefficients and the detrimental effect of oil on foam

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stability. Instead, they demonstrated that there is a strong correlation between

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the destabilising effects of oil and the magnitude of an “entry barrier” that is

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formed on the pseudoemulsion films between the oil droplet and the gas-liquid

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interface. The suppression of this oil drop entry is dictated by various surface

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forces (e.g. electrostatic, van der Waals) which are influenced by the

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physiochemical properties of the oil.

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The effect of oil on foam stability within porous media is still not very well-

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understood as the presence of the medium adds another layer of complexity

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with only limited data available on foam-oil interactions in porous media.

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Indeed, although foam stability has been generally studied using bulk foam

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tests, several studies have demonstrated that bulk foam stability does not

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necessarily correlate with the stability of that same foam during flow in porous



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media. DalIand et al.

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foam in bulk systems and porous media. Jones et al. 27 found apparent viscosity

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of foam in porous media is correlated to stability of foam in the bulk system in

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the absence of oil, but this correlation was not confirmed in the presence of oil.

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Osei-Bonsu et al. 28 showed stability of foam in bulk does not correlate with its

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effectiveness in oil displacement in porous media. Although the vast majority of

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research suggest that shorter chain hydrocarbons have more destabilising effect

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on foam than longer chain hydrocarbons with the shorter chain systems being

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typically less viscous

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size and hydrocarbon chain length and viscosity of the oil influence foam

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stability in porous media. The primary objective of this study was therefore to

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investigate the influence of the bead size (i.e. pore size) on foam stability during

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oil displacement in porous media composed of packed bead. In addition, we

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investigate the influence of the viscosity of oil on foam stability during the

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displacement of oil by foam in a glass bead system and in a Hele-Shaw cell.

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reported there is no correlation between the stability of

24, 29-32,

it is still unclear how the combined effect of pore

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2.

MATERIAL AND METHODS

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2.1.

Experimental Setup and Procedures

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In order to investigate the effects of the various parameters on foam stability in

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bulk and porous media, two types of experiment were conducted. In both

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experiments a transparent cell (either empty or packed with glass beads) was

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initially saturated fully with oil/water. Pre-generated foam was then injected at a 6 ACS Paragon Plus Environment

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constant rate to displace the oil/water from the cell. The resulting displacement

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dynamics and foam-oil interactions were recorded using a camera. In the

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following more details about the experiments are described.

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2.1.1. Hele-Shaw Cell Experiments

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A customised Hele-Shaw cell (32 × 20 cm dimensions) was fabricated to

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investigate the dynamics of oil displacement by foam in bulk scale.

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The Hele Shaw cell was similar to the one described in Osei-Bonsu et al.33 It

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consisted of two transparent glass plates fixed to a Plexiglas frame. During the

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experiment these glass plates were clamped together, with a gasket sandwiched

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in between to create a small gap (1 mm) in which the two fluids (oil and foam)

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could flow. This gasket dictated the size of the gap (assuming the gasket was

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incompressible) and prevented any leakage of fluid around the edges of the cell.

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The top plate/frame contained a hole with a screw fitting at either end to allow

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fluid to move in and out of the cell creating a flow path. A pressure transducer

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(Thurlby 30V-2A, with 0·3% accuracy and working range starting from 1 mbar)

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was installed at the inlet of the Hele-Shaw cell. The outlet of the experiments

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was connected to the atmosphere.

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Prior to each round of experiments, the surfactant solution was mixed again for

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10 minutes using the magnetic stirring device, ensuring consistency between

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experiments. Before assembling the Hele-Shaw cell, each glass plate was



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cleaned thoroughly on both sides to remove any residual oil or deposits stuck to

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the surfaces. Initially the plates were washed using a combination of water and

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laboratory detergent. A cloth was then doused in Isopropanol and used to wipe

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the surfaces in order to dissolve and remove any residual oil, ensuring the

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results could not be influenced by these residues. It was vital that care was taken

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when handling and cleaning the glass plates to ensure the surfaces did not

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become scratched, maintaining a smooth and consistent flow path for oil and

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foam. This was also important for obtaining clearer images of the flow. The

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Hele-Shaw cell was then placed over a light box and the camera positioned

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above it using a clamp stand. The light box was used to ensure there was

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sufficient light to capture clear images showing a highly defined foam network,

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with individual lamellae easily distinguishable. The cell was then filled with oil

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using a syringe connected to a tube which was temporarily attached to the

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entrance of the cell. A metal tap was attached to the outlet to allow fluid to flow

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out of the cell and into a collection beaker. During this oil injection, the Hele-

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Shaw cell was tilted (at the outlet end) to ensure any gas bubbles trapped in the

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oil escaped at the exit. Once the oil reached the collection beaker (i.e. the cell

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was full) the outlet tap was closed. Two identical syringes (diameter of 19.3

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mm) were filled with equal measures of surfactant solution (initially filled to 20

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ml mark) and placed onto the syringe pump, where they were clamped in place.

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The corresponding syringe diameter was programmed into the pump and the

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rate was set according to the desired foam quality (see later calculations). A 8 ACS Paragon Plus Environment

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tube was attached to each syringe and then connected to the foam generator,

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with the tubes arranged either side of the gas inlet. The gas cylinder was

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connected to the foam generator via the mass flow controller. Initially, the gas

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cylinder outlet pressure was turned from 0 to 1 bar to introduce a gas supply.

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The FlowDDE program was opened on the lab computer and communication

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with the mass flow controller was initiated. FlowView was then used to control

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the gas flow rate. Both the gas and liquid flows were switched on

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simultaneously to begin generating foam. In this experiment the gas flow rate

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was kept constant at 2 ml/min. The flow rate of the surfactant solution was

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adjusted to 0.22 ml/min and 0.35 ml/min to produce the two different foam

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qualities chosen for this experiment – 90 % and 85 % respectively before the

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injection of foam into the system. Foam quality was calculated based on the

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equation fg = q q where qg and ql are gas and surfactant flow rates g+ l

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respectively. Each experiment was repeated at least three times to ensure the

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repeatability.

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qg

2.1.2. Experiments with Glass Beads

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The cell packed with glass beads had a similar design to the Hele-Shaw cell

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described above. The gap size between the plates was 2 mm. This time,

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however, the two plates were fixed permanently in place using a number of

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screws fitted around the outside of the cell. This was necessary to allow glass

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beads to be packed into the cell sufficiently without damaging the glass plates. 9 ACS Paragon Plus Environment


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The inlet to this cell was very narrow to ensure that no glass beads could escape

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and potentially disrupt the packing. A fine metal mesh was fixed at the inlet of

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the model, again this was to prevent the glass beads from escaping and

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potentially disrupting the packing. Three different bead diameter ranges were

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used in this investigation: 1.25-1.55 mm, 0.50-1.00 mm and 0.15-0.21 mm. This

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enabled us to analyse the effects of grain/pore size of porous media on foam-oil

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interactions in porous media. The porosity of these three glass bead packings

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was approximately the same and equal to 35 %, but the permeability was

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different and equal to 1100, 450, and 24 D, for coarse, medium, and fine

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systems respectively. The permeability of the glass beads were calculated by

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injection of water at different flow rate using the obtained pressure data and

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employing Darcy equation. The glass bead was preferentially wet by surfactant.

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The cell contained a pressure transducer port located above the entrance of the

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cell to allow pressure drop to be measured. Like the Hele-Shaw cell, this bead

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pack design allowed us to obtain a 2D visual description of foam-oil

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displacement dynamics.

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At the beginning of each experiment the clean, empty cell had to be packed with

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glass beads. Initially, the cell was fixed vertically in place on the edge of the

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worktop using clamps, with the inlet in a downward-facing position. The inlet

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and pressure transducer port were temporarily blocked to prevent any fluid

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exiting the cell. The plate containing the outlet holes was removed to expose the


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outlet end of the cell. The cell was filled with oil (or water) and glass beads

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were then poured in, sinking to the bottom under gravity. The glass beads were

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inserted in stages, with compaction carried out using a metal ruler between each

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stage to ensure packing was as consistent as possible. Any excess oil that

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overflowed at the outlet was removed using a syringe. It was important that the

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same packing procedure (i.e. number of bead insertion/compaction stages) was

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used for each experiment to ensure that the porosities/total pore volumes were

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roughly the same (for experiments using the same bead size distribution). This

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ensured that packing inconsistencies did not distort any of the results and

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subsequent conclusions. Once the cell was packed, the detachable plate

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containing the outlet tubes was clamped to the outlet end of the cell. The glass

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bead pack was restored to its horizontal position and placed on top of the light

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box. The camera, which was connected to the lab computer, was then placed

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above the cell using a clamp stand. The precise positioning of the camera was

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adjusted by looking at the live field of view displayed by its supporting

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software, Capture, on the lab computer. A pressure transducer was attached

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above the entrance of the cell. In this experiment the outlet was assumed to be at

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atmospheric pressure so the pressure drop was taken as the gauge pressure at the

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cell entrance. A collection beaker was placed beneath the outlet tubes to collect

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any fluid exiting the cell during the experiment. In this experiment the

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displacements of oils Isopar V and Isopar G were investigated in glass bead

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packs at three different bead sizes. Besides the detrimental effect of oil on foam 11 ACS Paragon Plus Environment


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stability, capillary suction influences the stability of foam in porous media

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Water displacement experiments with foam were performed at different pore

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size to see how capillary pressure influences foam stability. The experimental

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procedure of these experiments were exactly the same as for oil displacement

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and just the type of fluid was changed.

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In all the glass-bead experiments, the gas flow rate was the same as that used in

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the Hele-Shaw cell experiment (2 ml/min) and so the surfactant solution flow

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rate was set accordingly to achieve 85% foam quality before injection to the

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system. Each experiment was repeated at least three times to check the

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repeatability.

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34.

2.1.3. Foam Generation and Fluid Properties

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To generate foam, air and aqueous surfactant solution were pumped

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simultaneously, at specified and well-controlled flow rates, through a foam

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generator. Surfactant solution was injected by the syringe pump (Harvard

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Apparatus, USA with ±0.35% accuracy and working range from 0.0001 µl/hr to

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100 ml/sec) while a mass flow controller (Bronkhorst, UK, with ±0,5%

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accuracy and working range from 1 ml/min to 100 ml/min) was used to deliver

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accurate and precise gas flow rates from the gas supply cylinder. 33

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The same surfactant solution (which forms the aqueous phase of the foam) was

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used in all experiments conducted in this investigation. This solution contained


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a 1:1 ratio of two different surfactants; Cocamidopropyl betaine (Cocobetaine)

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(The Soap Kitchen, UK, with 61789-40-0 CAS Number) and Sodium dodecyl

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sulphate (SDS) (Sigma, UK), each at 1% concentration (mass concentration) in

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a 0.1M NaCl aqueous solution. Osei-Bonsu et al.24 showed that this surfactant

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provides more foam stability than the surfactants taken individually under our

235

experimental conditions with the viscosity and surface tension being equal to

236

0.35 Pa.s and 0.13 mN/m respectively.24 Two different oils, both belonging to

237

the same hydrocarbon family (Isoparaffins), were used in these experiments to

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investigate the influence of oil properties on foam stability. It was important

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that these oils were of a similar type so that specific oil properties could be

240

compared. The oils used were Isopar V and Isopar G (Sil-Mid Limited, UK);

241

the former being the heavier and more viscous oil. Table 1 summarises the

242

properties of these isoparaffin oils.

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Table 1. Properties of the Isoparaffin Oils Used in these Experiments. Isopar

Carbon chain

Dynamic viscosity 25ºC (Pa.s)

Surface tension (mN/m) 24

V

C14-C19

1.08

25.44

G

C10-C12

0.08

22.57

244

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2.1.4. Image Processing

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An automated monochromic camera (Dalsa Genie TS-2500) with a resolution of

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2560 x 2048 pixels was used to capture high quality images of the foam 13 ACS Paragon Plus Environment


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evolution and oil displacement process over time. The captured images were

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processed and analysed using Image J software. The images were segmented

250

and analysed each image sequence at a time (one experiment provided one

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image sequence). Initially, an image sequence was imported into ImageJ and the

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scale was set, converting the image pixel dimensions to a corresponding length

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in mm measured during the experiments. The horizontal dimension of 2560

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pixels corresponded to a length of 228 mm. This enabled the software to

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calculate areas in mm2 which therefore allowed us to calculate volumes of oil

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and gas (gap depth = 1 mm). The images were then segmented using the

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“Process>Find Edges” function which converted the images from greyscale to

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black and white. This allowed the software to detect lines, curves and

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boundaries within the images which represented individual lamellae (foam

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films), oil/gas and oil/liquid interfaces and the perimeter around the cell (edge

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of the gasket). The contrast and brightness were adjusted using the

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“Adjust>Brightness/Contrast” function to allow these lines to become more

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distinguishable to ensure the software was able to detect them during the

264

analysis.

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The foam stability during oil displacement in the Hele-Shaw cell was quantified

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by calculating the volume of the released gas. It was expected that the higher

267

the foam stability, the smaller volume of gas released from the foam network

268

during the course of the displacement. Typical examples of the segmented


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images are presented in Figure 1 showing the displacement of oil by foam in the

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Hele-Shaw cell (in the absence of glass beads). Figure 1 qualitatively shows that

271

as the foam progresses through the cell, the volume of the released gas from the

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foam increases as a result of the detrimental effect of oil on foam stability.

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Once gas was released from the foam network, it either accumulated ahead of

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the foam front or along the walls of the Hele Shaw cell. This gas had a tendency

275

to accumulate and form larger gas bubbles that were easily distinguishable from

276

foam bubbles in terms of shape and size. Certain criteria were used to

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distinguish between foam bubbles and escaped gas as a result of destabilization.

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The first criterion was that the released gas bubble had to be located ahead of

279

the foam front or along the walls of the cell. The second criterion was that the

280

gas bubble area had to be larger than 10 mm2. This value was chosen after

281

meticulously studying numerous images from a range of image sequences. It

282

represented a bubble size larger than that of the usual generated foam bubbles

283

with 0.4 mm typical diameter.



284 285

Figure 1. Two typical images taken at 100s time intervals showing the

286

displacement of oil (Isopar G in this case) by foam in the Hele-Shaw cell at 85

287

% foam quality and 2 ml/min flow rate. The white colour in the image

288

represents the gas and oil while the grey colour represents the lamellae.

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3.

RESULT AND DISCUSSION

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3.1.

Effect of Oil Type on Foam Stability in Hele Shaw Cell

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Figure 2 shows cumulative gas volume released (mm3) from the foam network

292

over time during oil displacement in the Hele-Shaw cell (in the absence of glass

293

beads) at two different foam qualities (85 and 90 %). The pressure drop of the

294

system during the Hele-Shaw cell experiments was modest (less than 10 mbar)

295

so that the compressibility effect of foam can be ignored.


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Figure 2. The cumulative volume of gas released from the foam network over

298

time during oil displacement in the Hele-Shaw cell. V and G in the legend

299

represent Isopar V and Isopar G respectively and the numbers represent the

300

foam qualities in percentage. The error bars (half the length) indicate the

301

standard deviation over 3 repeat measurements.

302

Figure 2 demonstrates the influence of oil chain length (hence viscosity) and

303

foam quality on the stability of foam in the presence of oil during Hele-Shaw

304

cell experiments. It can be inferred from Figure 2 that more gas volume is

305

released from foam during oil displacement at the higher foam quality (90%).

306

This resulted in a final cumulative gas volume (at foam breakthrough point) that

307

is approximately 66% higher for the higher foam quality for the same oil; a

308

substantial difference. The higher the volume of gas released, the more the

309

destabilisation of the foam by the oil. This implies that lamella rupture (i.e.

310

coalescence) is much more prevalent at higher foam qualities.33 The comparison

311

between foam qualities in the presence of Isopar V (the more viscous oil) also



312

follows the same trend as for the less viscous Isopar G and hence supports this

313

interpretation. These results are in agreement with previous research on the

314

influence of foam quality on foam stability in the presence of oil.33, 35-37

315

There are several reasons to explain why lower quality foams exhibit a higher

316

tolerance to the defoaming activity of oil. One of the main reasons is that both

317

foam films and the Plateau borders between those films are thicker, relative to

318

bubble size. At lower foam qualities, these thicker films are more capable of

319

suppressing the penetration of oil into gas-water interfaces, a mechanism which

320

often leads to film rupture.33 Thicker borders also ensure that there is a lower

321

capillary suction pressure which means less liquid drainage within the films.

322

Another possible reason for the increased stability at lower foam qualities is that

323

the length of the foam films separating bubbles, in relation to total bubble

324

perimeter, is lower meaning it is less probable that oil will enter the foam films

325

in the first place and cause film rupture.

326

Figure 2 indicates that the lighter oil shorter carbon chain Isopar G tends to have

327

a more detrimental effect on foam stability. In fact, the rate of foam coalescence

328

(i.e. gas release) initially appeared to be very similar in the presence of both

329

oils. However, as the experiments progressed and overall contact times between

330

oil and foam increased, the rate of gas release from the foam in contact with

331

Isopar G (lighter, less viscous) increased relative to Isopar V. A possible

332

explanation for this is that as time progressed more of the lighter, less viscous 18 ACS Paragon Plus Environment

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333

oil was able to emulsify and form multiple smaller droplets that could be drawn

334

up into foam films and potentially destabilise them. This can be attributed to the

335

fact that less viscous oils emulsify more quickly which would consequently

336

accelerate the rate of lamellas collapse in relation to the more viscous oil.38

337

The final cumulative volume of gas released in both experiments (85% and 90%

338

quality) was approximately 30% higher in the presence of Isopar G than Isopar

339

V. Comparing this to the effect of foam quality discussed in the previous

340

section, a 5% increase in foam quality appeared to have a much stronger

341

influence on foam stability in the presence of oil than the differences between

342

the two oils themselves (66% difference compared with 30%).

343 344

3.2.

Effect of Type of Oil on Foam Stability in Porous Media with Different Pore Size

345

In contrast to the Hele-Shaw cell, it was challenging to measure the amount of

346

gas released from the foam in the experiments with porous media. It is

347

hypothesised that the gas released as a result of foam destruction was able to

348

penetrate the glass bead-pack rapidly, forming gas channels connected to the

349

outlet rather than accumulating ahead of the foam front in gas pockets.

350

Accordingly, the area within the model occupied by foam (foam saturation) and

351

the pressure drop across the glass bead pack were measured as an indication of

352

foam stability and its efficiency for fluid displacement.



353

Figure 3 shows foam saturation, defined as the volume occupied by foam

354

relative to total pore volume, and the pressure drop during the displacement of

355

oil/water by foam in the porous media with different pore size.

356 357

Figure 3. (a), (b), (c) Foam saturation as a function time during the displacement

358

of water, Isopar V and Isopar G respectively (d), (e) and (f) Pressure drop vs

359

time during the displacement of oil/water by foam at coarse, medium and fine

360

sand pack beads respectively. The gas flow rate and foam quality were 2 ml/min 20 ACS Paragon Plus Environment

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361

and 85 % respectively at the inlet of the model. The error bars (half the length)

362

indicate the standard deviation over 3 repeat measurements.

363

Figure 3 clearly demonstrates the destructive effect of oil on foam stability and

364

its displacement efficiency in porous media. Saturation of foam and the pressure

365

of the system are higher in the presence of water compared to oil when the other

366

experimental conditions are the same. Moreover, Figure 3 (a) in the presence of

367

water appear to show a significant difference in the behaviour of foam at

368

different pore sizes as more time is needed to ensure that the foam saturates all

369

the system as the pore size of the porous media decreases. This suggests that a

370

significantly larger amount of foam was destroyed during water displacement in

371

the finer glass bead pack due to capillary suction. In porous media, capillary

372

pressure can be considered as the force per area required for squeezing a

373

hydrocarbon droplet through a pore throat, working against the interfacial

374

tension between oil and water. This capillary pressure is higher in smaller pores.

375

Hence this increased capillary pressure have destabilised the foam to a greater

376

extent, making foams films more susceptible to rupture as they attempt to force

377

oil through tighter pore constrictions.34

378

Figure 3 shows foam provided higher pressure drop as the pore size of the sand

379

pack decreases. This is due to the lower permeability of finer sand pack. As the

380

foam is in general a compressible fluid, it seems compressibility could influence

381

the texture of the foam and its saturation. The texture of foam is generally 21 ACS Paragon Plus Environment


Page 22 of 28

382

determined by the porous medium in which it resides and the surrounding

383

pressure. It follows that pre-generated foam (used in this investigation) is re-

384

shaped and re-textured by the porous medium through which it is injected

385

Accordingly, we can expect foam would have higher foam quality in the

386

upstream of the system compare to downstream as the outlet of the system is

387

connected to atmosphere and the inlet of the system is influenced by the

388

pressure upstream inside the porous media. Lower foam quality in the upstream

389

has more liquid content that made it more tolerant to detrimental effect of oil

390

and capillary suction. This help the system to increase its foam saturation as the

391

time passed. The quality of pre-generated foam before injection into the model

392

was 85 %. If we assume the gas phase of foam behaved as ideal gas, it can be

393

expected that the rise in pressure could lead to the decrease in volume of gas

394

based on the equation P = V . For example 500 and 1000 mbar increase in 1 2

395

pressure, could change to 33 and 50 % change in gas volume that can decrease

396

the foam quality to 79 and 74 % respectively.

397

Comparing Figure 3 (b) with (c) shows similar to the Hele-Shaw cell case,

398

heavier oil (Isopar V) provide more foam stability in the medium and coarse

399

bead system compared to fine beads. However, Figure 3 (b) shows the stability

400

of foam in the presence of Isopar V is actually greater in the medium bead

401

system compared to the coarse bead system whereas for Isopar G stability is

402

greater in coarse bead systems compared to medium bead ones as Figure 3 (c)

P2

40.

V1


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403

shows. Visual observations of displacement of oil by foam showed two fronts.

404

The first front (further back) was the front of foam and second front (further

405

forward) was the front of the escaped gas that resulted from foam coalescence.

406

The second front was not effective to displace all the oil and there were some

407

unswept oil areas. Visual inspections showed this unswept oil did not have a

408

high detrimental effect on the foam for heavier oil in medium bead systems and

409

the first front propagated effectively and achieved higher foam saturation.

410

However, these unswept areas had a detrimental effect on foam for lighter oil

411

(Isopar G) and delayed the formation of a strong foam bank in medium bead

412

systems.

413

34Figure

414

type of oil changes. This difference is also manifested in the pressure drop

415

curves in Figure 3 (f). This implies that for fine beads the foam was more

416

effective to displace fluid in the presence of the lighter, less viscous Isopar G,

417

which contradicts our original finding made during the foam displacement in

418

coarse and medium beads and Hele-Shaw cell. It was observed that the released

419

gas resulted from the detrimental effect of oil and capillary suction, fingered

420

more in the low permeability medium. Accordingly, the subsequent injection of

421

foam followed these preferential flow paths and produced from the outlet

422

without propagation to the other parts of the system. As Figure 3 (b) shows

423

foam in the fine glass bead packs was not able to propagate across the whole of

3 shows foam behaves significant differently in fine bead packs as the



424

the cell during heavier oil displacement (Isopar V) due to the high level of

425

viscous fingering.

426

SUMMARY AND CONCLUSION

427

Oil displacement experiments were carried out in a Hele-Shaw cell and glass

428

bead packs to investigate the combined effect of type of oil and pore size of

429

porous media on foam stability and its displacement efficiency. The effects of

430

foam quality studied in the Hele-Shaw cell showed conclusively that lower

431

foam qualities result in formation of foam with an increased tolerance to the

432

destructive effects of oil.

433

The type of oil appeared to have a strong influence on foam stability during oil

434

displacement in the Hele-Shaw cell and porous media. Foam showed lower

435

foam stability and displacement efficiency in the presence of the lighter, less

436

viscous oil (Isopar G) in the Hele-Shaw cell and also in the coarse and medium

437

porous media.

438

The effect of oil type in the fine glass bead experiment did not appear to

439

correlate well with the trends displayed in the Hele-Shaw cell and coarser

440

porous media. The fine bead pack actually demonstrated the opposite effect as

441

the foam appeared to have higher displacement efficiency in the presence of

442

lighter oil. Overall these results suggest that the geometry of the porous medium


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443

(i.e. average pore size) has a much more important influence on the foam

444

displacement efficiency in the presence of oil than the type of oil itself.

445

ACKNOWLEDGMENTS

446

We would like to acknowledge the UK Engineering and Physical Sciences

447

Research Council (EPSRC) for providing the PhD studentship for Mohammad

448

Javad Shojaei.

449

AUTHOR INFORMATION

450

Corresponding Author

451

E-mail: [email protected].

452

Notes

453

The authors declare no competing financial interest.



454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506

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Table of Contents Graphic

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Foam Stability Influenced by Displaced Fluids and by Pore

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