Ionic Liquids Application in Surfactant Foam Stabilization for Gas

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Ionic Liquids Application in Surfactant Foam Stabilization for Gas Mobility Control Alvinda Hanamertani, Rashidah Pilus, Ninie Suhana A Manan, Shehzad Ahmed, and Mariyamni Awang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00584 • Publication Date (Web): 30 May 2018 Downloaded from http://pubs.acs.org on May 31, 2018

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Energy & Fuels

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Ionic Liquids Application in Surfactant Foam Stabilization for Gas

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Mobility Control

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Alvinda Sri Hanamertania, Rashidah M. Pilusa,*, Ninie A. Mananb, Shehzad Ahmeda, Maryamni

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Awangc

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a

Department of Petroleum Engineering, Universiti Teknologi PETRONAS, 32610, Perak, Malaysia

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b

Department of Chemistry, University of Malaya, 50603, Kuala Lumpur, Malaysia

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c

Elsa-Energy Sdn Bhd, Malaysia

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*Corresponding Author:

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Rashidah M. Pilus

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Department of Petroleum Engineering, Universiti Teknologi PETRONAS, 32610, Perak, Malaysia

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E-mail address: [email protected]

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Tel: +60 5-368 7036

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ABSTRACT

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Foam application in enhanced oil recovery (EOR) processes has been promoted primarily to

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address mobility control issue during gas flooding which leads to poor sweep efficiency. Some

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critical factors such as foam stability and strength in porous media need to be addressed to ensure

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the effectiveness of foam flooding. In this research, a relatively new additive for foam

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stabilization composed of ionic liquid (IL) has been introduced. A systematic foam experiments

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in static condition and porous medium were performed to investigate the potential of IL-based

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additives to enhance surfactant foam stability for gas mobility control. Screening on the mixtures

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of surfactant and different types of IL were initially conducted based on bulk foam stability

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measurement at high temperature. Core flooding experiment was then executed to evaluate the 1 ACS Paragon Plus Environment

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foam characteristic using the best formulation in the absence of oil under reservoir conditions.

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Results from bulk foam experiment indicated that the presence of IL as additive was able to

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increase foam stability up to certain extent depending on the type of IL used and its formulation

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with surfactant. In comparison with the base case, the foam stability increment achieved by the

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best formulation was 136%. The ability of selected IL to lower the surface tension of surfactant

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solution was found to be in a good accordance with its improvement on foam stability. In core

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flooding experiments, the acceleration of foam generation was noticed in the presence of IL

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indicated by immediate increase in mobility reduction factor (MRF) upon early nitrogen

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injection with 30% increment at maximum point. Small slug SAG (surfactant alternating gas)

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injection was able to optimize the performance of surfactant/IL mixture used in reducing gas

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mobility by effectively producing stronger foam. This research has provided a strong indication

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of the capability of IL to be employed as additive for foam stabilization, hence improved foam

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performance in reducing gas mobility during EOR processes.

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Keywords: foam additive, foam stability, surfactant, ionic liquid, mobility control, SAG

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1.

Introduction

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Over last two decades, gas injection projects have been reported to have an increasing

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trend and this method has been the most widely applied to recover the light, condensate and

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volatile oil 1. However, during gas injection, some challenges are still encountered such as

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gravity override and gas fingering through high permeability streaks which allows gas to bypass

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the displaced fluid leading to low volumetric sweep efficiency 2, 3. Water alternating gas (WAG)

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method has been attempted to reduce gas fingering issue, however, this method still faces a

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major problem which is gravity segregation due to density difference between water and gas 4, 5.

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In this regard, the formation of gas phase in a foam in situ by the presence of foaming agent has 2 ACS Paragon Plus Environment

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been promoted as a possible mechanism to minimize problems during gas injection

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presence of foaming agent which is typically a surfactant solution can increase the stability of

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two-phase interfaces by decreasing the gas/liquid surface tension leading to the formation of

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foam. The concentration of surfactant commonly used for foam EOR application is in the range

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of 0.1 – 1 wt% 13. Foam generation in porous media results in a resistance to flow, reducing the

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mobility of both liquid and gas phases by increasing gas saturation and decreasing water

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saturation

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less permeable regions as it blocks the high permeability regions which will eventually improve

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the sweep efficiency leading to an increment in trapped oil recovery. The types of gas commonly

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used for foam EOR applications are N2, CO2, and hydrocarbon gases. Application of N2 foam

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and CO2 foam for EOR purpose has been continuously studied to observe and compare their

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performance at high pressure and temperature conditions. Previous studies reported that N2 foam

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was able to exhibit a higher foam strength compared to CO2 foam, indicated by a greater

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pressure drop across the core sample during foam flooding experiment which ultimately

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improved either the mobility reduction factor or foam apparent viscosity 15-19. The differences in

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foam behavior produced using different gases are affected by the gas solubility in water,

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interfacial properties, the wettability effects, surfactant types and concentrations, and effect of

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pH 16.

14

. The

. As foams act as a mobility controller, foam causes more gas to flow through the

In porous media, there are specific conditions required to generate foam which are only

65

20, 21

66

above a minimum flow rate and pressure gradient or below a critical capillary pressure

67

Chou

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with surfactant solution. Otherwise, foam generation could be initiated by creating region having

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high saturation of surfactant. This can be done by injecting small slug of gas and surfactant

17

.

also ascertained that foam is readily formed whenever the core has been pre-saturated

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solution alternately or co-injection of surfactant and gas using pulse or gas shut off mechanism.

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This study also reported that high surfactant saturation can be preferably created in situ during

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surfactant alternating gas (SAG) which is favorable for foam generation in the subsequent

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drainage process. Furthermore, due to the existence of gas channels in a large scale in reservoir,

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alternate injection is preferred over co-injection of gas and surfactant, so that the foam can

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propagate deeper into the reservoir. It was suggested that SAG can be considered as beneficial

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foam treatments since the injection is easier to perform below fracturing pressure which has been

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operated in Snorre field as the world’s largest application to control gas mobility in-depth of

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reservoir

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12, 22

.

In practical applications, the ability of foam to block and divert gas flow is directed to 12

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change the injection profile, delay gas breakthrough, and to store more gas in the reservoir

.

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During these processes, the gas mobility should be controlled and it depends on the number of

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lamellae generated which is affected by the surfactant concentration, the presence of additives

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and the reactions rates toward foam itself 23. On the laboratory scale, mobility control ability and

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foam strength in porous media can be described in terms of mobility reduction factor (MRF) or

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apparent viscosity (μ ). A high increase in MRF or apparent viscosity indicates the stronger

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foam with better mobility control. It has been suggested that a strong foam is easier to be formed

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experimentally when the low flow rate is applied, employing sufficient concentration of

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surfactant and by using longer core size. Moreover, foam can be generated during drainage

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process instead of imbibition, whereby the gas is injected whenever the surfactant solution has

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already resided inside the core 17.

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To perform effectively, foam has to be sufficiently stable while working as blocking

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agent either in the absence or presence of oil. Some studies reported that the individual surfactant


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still have inadequate capability to produce long-lasting foam especially in harsh conditions of the

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reservoir

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additives has been proposed to be applied in a proper composition. There are several categories

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of additives which can be used to stabilize foam, such as organic compounds, electrolytes, finely

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divided particles, polymers, biopolymers, and liquid crystals

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been employed, such as alcohol, polymer, salt, and combination of different surfactants have

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shown their potential to enhance the performance and stability of conventional foams produced

24, 25

. In order to enhance the performance of surfactant, the application of chemical

27-34

26

. Several additives which have

100

from single surfactant system

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applications, for instance, the use of high molecular weight polymer is not favorable for some

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reservoirs due to its possibility to plug the low permeable rocks. Moreover, significant amount of

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polymer is needed to encounter high salinity reservoir brine which will make the process become

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uneconomical

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and stability by inhibiting the bubble coalescence through its effect on improving disjoining

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pressure across the foam lamellae and influencing the electrostatic stabilization at the interface

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was depending on the type of salt and surfactant as well as the concentration used

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presence of salt was also reported to be able to reduce gas solubility in solution and also lower

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the rate of bubble coalescence hence promoting the stability of foam

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investigation conducted by Yekeen, et al.

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concentration (CMC), the presence of salt containing monovalent ions could not produce a

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higher foam stability as it increases the rate of bubble coalescence and coarsening. The use of

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salt containing trivalent ions on the other hand, are more detrimental to foam stability, while

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other findings indicate that an increase in salt concentration negatively affect the foam stability.

25

. However, some limitations were still found during their

. The presence of salts is expected to influence the surfactant foam generation

. Some studies reported the positive effect of salts addition on foam stability in which its extent

34

36

36-38

. The

. However, recent

shows that above surfactant critical micelle


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Similar to this study, the destabilizing effect by the addition of salts has been previously

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ascertained

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green chemical with high solubility as well as high thermal and chemical tolerance, also having

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the ability to alter the surface behavior of surfactant at the gas/liquid interface are required to be

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investigated for enhancement of foam stability.

29, 39

. With regard to this, alternative chemical additives which are considered as a

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Some researchers have recently tried to investigate the capability of ionic liquids (ILs) as

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alternative chemicals in enhanced oil recovery processes. ILs have also been promoted as novel

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surfactant due to its ability to reduce oil/water interfacial tension at various salinity and

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temperatures 40-42. ILs are known as ionic compounds which have melting point less than 100°C.

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ILs are typically composed of both organic cation and organic or inorganic anion

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beneficial properties such as negligible vapor pressure, wide liquid range and high chemical and

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thermal stability

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possess high thermal stability which is up to 400°C

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ability of ILs in affecting surfactant behavior at the surface. The presence of ILs having surface

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activity such as phosphonium- and imidazolium-based ILs, was able to stabilize the electrical

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repulsion between ionic head groups of surfactant, forming closed arrangement of molecules at

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the interface. This is indicated by the reduction of critical micelle concentration (CMC) value,

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surface tension, and/or water/oil IFT 41, 49-51. An alternation in surface activity of surfactant in the

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presence of ionic liquid (IL) is entirely influenced by some interactions that occur between ionic

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moiety of surfactant and IL, depending on their charge types. The mechanism of ILs in affecting

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surface behavior of surfactant corresponds to the way by which additives could help to stabilize

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foams. Therefore, the application of ionic liquid as alternative additive to stabilize foam has

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brought an intense interest for this chemical to be further investigated. Besides common types of

44, 45

43

. ILs have

. Certain ILs which contain the imidazolium ring have been reported to 45, 46

. Javadian, et al.


47, 48

also reported the

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ILs, eutectic-based ILs have also attracted interests from many researchers. They have been

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promoted as novel solvents which can be applied in many applications due to their high solute

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solubility, economical use, wide potential window, and good environmental compatibility.

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Eutectic term is defined as a mixture of two compounds which have strong interaction at

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equilibrium composition resulting in the alternation of each property e.g. melting point. Ionic

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liquid such as choline chloride (the quaternary ammonium salts) can be associated with various

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complexing agents to form eutectic mixture called Deep Eutectic Solvent (DES) 52, 53. As a novel

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material, DES has many advantages over the common ILs types, which are water-tolerant, non-

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toxic, biodegradable, non-flammable, versatile, less expensive, and easier to make. Similar to

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ILs, DES can be custom-made according to the desired targets and also expected to have surface

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activity. Therefore, DESs have been developed and used in several fields, such as extraction,

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separation, catalytic processes, etc. Recently, the application of DESs have been introduced in

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petroleum field focusing on enhance heavy oil recovery processes

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choline chloride-glycerol (1:2) and choline chloride - urea (1:2) to alter several mechanism

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contributed to heavy oil recovery such as emulsification, wettability alteration, surface properties

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alteration, and residual oil displacement at reservoir conditions have been investigated

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results indicated that DESs used were able to alter the wettability of the rock surface and also

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improve solution/oil viscosity ratio resulting in an improvement of residual heavy oil recovery

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after water flooding.

54

. The effectiveness of

54

. The

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The favorable ability of certain types of ILs to improve the surface activity of surfactant

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in addition to other beneficial characteristics of ILs and DESs as ionic liquid-based novel

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material has encouraged further investigation into their possibility to be applied as additives on

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foam applications. ILs presence are expected to potentially improve the surface properties of


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foam, hence assisting surfactant to generate stable foam with longer life-time 55. This research is

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aimed to address their favorable properties to improve foam stability by conducting a series of

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experiment employing specific types of ILs and DESs as IL-based additives to surfactant. Bulk

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foam stability experiments at reservoir temperature i.e. 90°C were performed to investigate the

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effect of ILs presence on foaming properties of surfactant. Bulk foam stability test was also used

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as a screening tool to select the best surfactant/IL formulation to be further evaluated in core

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flooding experiment at high temperature high pressure (HTHP). It has been previously reported

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that results obtained from core flooding experiments in the absence of oil are positively

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correlated with bulk foam stability test, therefore, experiments in bulk foam can be considered as

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a good screening tool to indicate the performance of tested solution further in porous media 31. In

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core flooding experiments, the mobility reduction factor (MRF) was used as indicator to

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investigate the ability of selected IL in assisting surfactant foam to reduce gas mobility in porous

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media. Bulk foam and core flooding experiments were conducted in the absence of oil to

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eliminate the external factor that can affect the results for IL performance investigation.

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2.

Materials and methods

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2.1.

Materials

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The surfactant used in this research was in-house surfactant, MFOMAX with active

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content of 20%, supplied by PETRONAS Research Sdn. Bhd. (PRSB). MFOMAX surfactant is a

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mixture of anionic and amphoteric surfactants which is completely soluble in water. Different

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types of ionic liquid-based material including imidazolium-based IL and deep eutectic solvent

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(DES) were employed as additives to surfactant. The details of ILs used are listed in Table 1.

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The structure of each IL molecule is depicted in Figure 1. All solutions either containing


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individual surfactant or mixture of surfactant and ILs were prepared in brine with 2 wt% salinity.

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The synthetic brine was composed of 95% sodium chloride (NaCl, Merck) and 5% calcium

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chloride (CaCl2, R&M) dissolved in deionized water (pH 6.9, density 1.011 g/cm3 at 25°C). To

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prepare different mixture of surfactant and ionic liquid, the concentration of surfactant was fixed

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based on the results obtained from concentration screening which was well above the surfactant

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CMC, and the concentration of ionic liquid was varied and calculated based on surfactant/IL

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mass ratio ranging from 90:10 to 60:40. N2 with a purity of 99.98% was utilized for generating

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foam both in bulk and porous media. The outcrop of Berea sandstones were used for core

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flooding experiments with dimension of 1.5 inches in diameter and 6 inches in length. The

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details of the core properties are given in Table 2.

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Table 1

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Details of ionic liquid-based additives Reference No.

Name

Synonym

MW (g/mol) Code

1. 2.

1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

[C4mim][NTf2]

419.37

IL-A

Glyceline

107.94

IL-B

Choline Chloride - glycerol

196 197

Figure 1. Chemical structure of IL-based additives


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Table 2

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Physical properties of Berea core samples

Core

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Length

Diameter

Pore Volume

Porosity

Kbrine

(cm)

(cm)

(ml)

(%)

(mD)

Experiment

A

IL-free case

14.46

3.82

33.32

20.08

175

B

IL case

14.99

3.83

38.04

22.03

215

200 201

2.2.

Experimental procedures

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2.2.1. Bulk Foam Experiment

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A systematic foam stability tests on surfactant in the absence and presence of IL-based

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additives were performed using Foamscan instrument, manufactured by Teclis Company, France

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(Figure 2a). In order to conduct the experiment, 60 ml of tested solutions was initially prepared

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and placed in the foam column having inner diameter of 32 mm. The foam column (depicted in

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Figure 2b) was covered by two jackets where the first jacket was connected to the oil bath for

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temperature control and the second jacket was containing Squalene liquid to maintain the

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clearness of images captured by the camera. The testing temperature was fixed at 90˚C for all

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tests. The gas flow rate was set at 50 ml/min and the foam volume was adjusted to be 150 ml.

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The Foamscan instrument was connected to gas pressure regulator of 10 bar, the maximum

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allowable pressure set. Foam was generated by sparging nitrogen gas at constant flow rate

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through porous glass frit (≈40µm) into the surfactant solution. The gas sparging was

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automatically stopped right after reaching the preset foam volume. All operations were

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controlled and monitored in the connected Foamscan software. Foam volume in foam was

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determined using Charged-Coupled Device (CCD) camera based on the black and white contrast 10 ACS Paragon Plus Environment

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produced by quantifying the gray level. The gray level was corresponding to the light source

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directed to the foam column which could differentiate the light and dark part due to the presence

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of foam. This device measured the foam volume generated during gas sparging and recorded

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foam decay profile over time after foam generation. The pairs of electrodes attached to the foam

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column were used to quantify the amount of liquid retained in foam as a function of time. Foam

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measurements were stopped few minutes after reaching foam half-life (time taken to reach a half

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of initial foam volume).

224 225

Figure 2. (a) Set up of Foamscan instrument 56, (b) Screenshot of the foam column

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In order to identify the contribution of surface tension lowering ability to an improvement

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in foam stability, surface tension of selected formulations obtained from bulk foam experiments

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against N2 was investigated at 90°C using IFT 700 equipment manufactured by Vinci

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Technologies. The pendant drop method was chosen for each experiment and surface tension

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values were determined from the Laplace-Young equation based on the complete shape of the

231

droplet 57.


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2.2.2. Core flooding experiment

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Core flooding experiment was carried out using HPHT Core Flooding System from

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Sanchez Technologies (France) which was connected to a computer system for data acquisition.

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The confining pressure was controlled at 1800 psig and temperature was at 90˚C, the conditions

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of Malaysian oilfield (Baronia). The equipment is composed of core holder, climatic air bath,

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stainless steel liquid accumulators, injection pumps (Stigma 300, Sanchez technologies),

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confining pump, back pressure regulator (BPR), wet rotary gas meter, and measurement system.

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Stigma pumps have an accuracy of volume measurement of ±0.001 cc and flow rate accuracy of

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0.1%. The measurement system attached comprises of the electronic temperature and pressure

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regulators, temperature and pressure sensors, sensors for differential pressure measurement, and

242

automatic valves. The HTHP system consisting of core holder, accumulators, and back pressure

243

regulator were located inside the oven. Different injection lines for liquids and gas phases are

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directed to two injection ports attached to the inlet part of the core holder system, where one port

245

is allocated for liquids and another port is for gas. High pressure N2 accumulator was connected

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to the core flooding system. The core sample was initially loaded in the core holder which was

247

then placed horizontally inside the oven. Brine and foaming agent were then placed in the

248

accumulators. The foaming agent tested for the first experiment was surfactant solution without

249

IL (IL-free case) and the second was surfactant/IL mixture at certain ratio (IL case). System

250

cleaning and leakage checking were carried out prior to pressure build-up. The differential

251

pressure or pressure drop (∆) during foam generation in porous media was monitored and

252

recorded in which the reading was obtained from the measurement by pressure sensors

253

connected to the inlet and outlet of the core. The set-up of core flooding equipment is illustrated

254

in Figure 3. 12 ACS Paragon Plus Environment

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Figure 3. Schematic diagram of experimental set-up for core flooding experiment

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Before being used for core flooding experiment, the core sample was initially dried in

258

oven for 2 days and its dry weight was measured. Core saturation was carried out by immersing

259

the core in brine inside the desiccator under vacuum condition for more than 24 hours. The wet

260

weight of the core was measured afterwards. The pore volume was calculated based on the

261

difference between wet weight after saturation and dry weight in gram divided by the density of

262

brine. At initial stage of core flooding experiment, the core sample was flooded with brine at

263

different flow rates, until reaching steady state condition and the pressure drop was recorded to

264

determine the absolute brine permeability. N2 gas was then injected at flow rate of 0.2 ml/min

265

until no more brine produced and the pressure drop measured was used as a base case providing

266

the gas mobility profile (without foam generated). Foam was generated in situ by alternate 13 ACS Paragon Plus Environment

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injection of the foaming solution and gas into the core sample. In the case of foam generation,

268

the flow rate of all injected fluids was fixed at 0.2 ml/min. Prior to foam generation in each run,

269

8 PV of foaming solution was injected in order to saturate the core and also to satisfy surfactant

270

adsorption, hence creating the conducive condition for the foam to be readily formed. It was then

271

followed by N2 injection until the steady state condition was achieved. On the other run, the

272

same set of experiment was repeated for the use of foaming solution containing selected IL. This

273

one cycle surfactant alternating gas (SAG) was performed to indicate the ability of IL in

274

improving surfactant performance in reducing gas mobility by generating stronger foam. Small

275

slugs SAG injection was then conducted by alternately injecting 0.4 PV of the selected

276

formulation and gas in three cycles at constant flow rate (0.2 ml/min) controlled by the injection

277

pump with accuracy of 0.1%. Each measurement was terminated after reaching a steady state

278

regime at which the pressure drop values remained stable. The effectiveness of foam generated

279

in porous medium with and without using additive was evaluated and characterized by

280

comparing the pressure drop (∆) profiles and calculating the mobility reduction factor (MRF)

281

value using the following equation 30, 58,

=

∆ ∆ = ∆ ∆

(1)

282

where ∆ and ∆ are the steady-state pressure drop (psi) across the core sample

283

measured in the presence and absence of foaming agent in the liquid phase, respectively. The

284

presence of strong foam was indicated by a high pressure drop and MRF value 28, 59.


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3.

Results and discussion

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3.1.

Bulk foam experiments

287

Bulk foam experiments at reservoir temperature of 90°C were performed to investigate

288

the effect of IL presence on the stability of foam generated by the surfactant. Screening processes

289

to select the surfactant concentration and also the best surfactant/IL formulation were carried out

290

in this stage. The analysis of foam stability was based on the foam decay profile as well as the

291

foam half-life recorded.

292

3.2.1. Effect of surfactant concentration

293

Surfactant concentration has an important role in affecting the effectiveness of foam

294

application. Previous studies ascertained that determination of surfactant concentration is

295

required to balance the chemical costs, taking into account several factors such as surfactant

296

CMC, surfactant foaming properties in bulk and porous medium, and surfactant adsorption 13, 60,

297

61

298

its foam stability at different temperatures to confirm the adequate concentration for further

299

investigation. Figure 4 presents the foam half-life measured as a function of MFOMAX

300

concentration at two different temperatures. The concentrations tested in this screening were well

301

above the CMC which is around 0.02-0.03 wt%

302

increase in MFOMAX concentration decreases the stability of foam generated. At low

303

temperature, the foam half-life at concentration of 0.1 wt% was found to be two times higher

304

than that at 0.5 wt% and 1 wt% concentration. The similar result was obtained at high

305

temperature i.e. 90°C where the low concentration of surfactant exhibited a higher foam stability.

. In this study, concentration screening was performed for MFOMAX surfactant by evaluating

62

. From the result, it can be observed that an


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306 307

Figure 4. Foam half-life for different surfactant concentrations at temperatures of 25⁰C and 90⁰C

308

The decline in foam stability with increasing surfactant concentration indicates that a

309

high amount of surfactant molecules in the liquid phase by which the rapid micelles formation

310

favorably take place, destructs the necessary surfactant distribution at the interface after certain

311

period, thus the lamellae strength cannot be maintained. This phenomena is also affected by the

312

bulky surfactant molecules which could produce steric constraints among their molecules. In this

313

regard, some literatures also reported the reduction of foam stability as concentration increases

314

above the CMC, suggesting that a significant micelle concentration in surfactant solution could

315

affect surface elasticity as well as disjoining pressure over the foam thin film which leads to

316

foam rupture

317

investigations as it was found to be sufficient to possess relatively high foam stability compared

318

to other much higher concentrations.

63-65

. Therefore, concentration of 0.1 wt% was chosen to be used on the next


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319

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3.2.2. Effect of Ionic liquid (IL) as additive on surfactant foam stability

320

Bulk foam experiments for investigating the effect of IL as additive in the surfactant

321

system on foam stability were carried out by employing two different types of IL and varying the

322

ratio between MFOMAX and IL whereby the concentration of MFOMAX was fixed at 0.1 wt%.

323

The foam half-life for each mixture is presented in Figure 5. The base case refers to the foam

324

stability produced by single surfactant system (ratio of 100:0) for comparison with the foam

325

stability in the presence of IL. In general, the presence of IL-A and IL-B was able to improve the

326

stability of MFOMAX foam. Result shows that surfactant foam stability gradually increases with

327

increasing IL concentration which is within the range of tested MFOMAX/IL ratios. This

328

improvement indicates the ability of each IL to influence surfactant behaviour at gas/liquid

329

interface which results in more stable foam lamellae. The extent of foam stability enhancement

330

was found to be dependent on the mixture ratio and also the type of IL used.

331 332

Figure 5. Foam half-life measured for MFOMAX/IL mixtures with different ratios at 90°C 17 ACS Paragon Plus Environment

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Page 18 of 42

333

The addition of IL-A at ratio from 90:10 to 70:30 was able to increase the half-life of

334

MFOMAX foam within the relatively same increment which was around 35% higher than the

335

half-life of MFOMAX without IL. The highest improvement was achieved at ratio of 60:40

336

where the half-life was prolonged from 417 seconds to 676 seconds, increased by a factor of 1.6.

337

The foam stability enhancement in the presence of IL-A was possibly induced by favorable

338

interactions between IL-A and MFOMAX at the gas/liquid interface consisting of electrostatic

339

attraction between the head groups and hydrophobic interactions between the tail groups. A

340

higher content of IL-A in the mixture solution could promote a packed arrangement with

341

surfactant at the interface, leading to stabilized foam lamellae. On the other hand, a comparable

342

result was shown for IL-B application where an increase in IL-B concentration could prolong the

343

half-life of MFOMAX foam dramatically. The addition of lower concentration of IL-B

344

noticeably increased the foam half-life, hence even longer than that in the presence of IL-A at

345

any ratio. It indicates that the interactions occurred between MFOMAX and IL-B could provide

346

a better alteration on the surface properties which favorably control the elasticity of the lamellae

347

after gravity drainage and bubble coarsening phenomena. The lower molecular weight of IL-B

348

might have less steric hindrance which could also promote slower breakage of thin foam film

349

generated. It has been reported that the lamellae breakage in dry foam system is mainly

350

controlled by the surface viscosity and elasticity which are highly affected by the ability of

351

surfactant to sufficiently distribute itself across the lamellae

352

lamellae due to the presence of IL-B could also be influenced by its ability to induce surfactant

353

molecules redistribution at the interface which causes strong cohesive bonding through some

354

favorable interactions, for instance, via hydrogen bonding network. This phenomena is also able

355

to provide a better disjoining pressure where a metastable films are formed after further thinning.


66-68

. In this regard, the stabilized

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356

Similar to IL-A case, the highest improvement in foam stability for MFOMAX/IL-B was noticed

357

at 60:40 ratio. In this case, the foam stability was found to be more than two times higher than

358

the base case by which the increment was from 417 to 985 seconds. Of all ratios tested, the

359

highest performance was noticeably observed at 60:40 ratio in the use of both ILs. For further

360

analysis, the performance of IL-A and IL-B was compared at MFOMAX/IL ratio of 60:40 as the

361

effective ratio obtained.

362 363

Figure 6. Foam volume produced by MFOMAX in the absence and presence of IL-A and IL-B at

364

mixture ratio of 60:40 as a function of time

365

Figure 6 shows the comparison of MFOMAX foam decay profile with and without IL-A

366

and IL-B at ratio of 60:40. Foam stability enhancement was noticed from their ability to reduce

367

the rate of foam coarsening and foam coalescence. It was indicated by a change in slope of the

368

foam decay trend resulting in a prolonged half-life. As a result, the time taken to reach half of

369

initial foam volume (75 ml) exhibited by MFOMAX/IL-A and MFOMAX/IL-B at ratio of 60:40


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370

were extended 62% and 136%, respectively, meaning that MFOMAX/IL-B foam provided the

371

most prolonged foam decay.

372 373

Figure 7. Liquid drainage profile as a function of time for MFOMAX in the absence and

374

presence of IL-A and IL-B at ratio of 60:40

375

Besides the foam volume, the liquid volume in foam generated with selected

376

formulations was recorded over time in order to observe the liquid drainage profile in the

377

absence and presence of ILs, presented in Figure 7. The plot shows that the liquid in the foam

378

continuously drained within certain period once the foam generation was stopped. As a result,

379

the amount of liquid decreased significantly until reaching a stable volume at which the foam

380

became dry. At this point, foam contained small amount of liquid residing in the lamellae.

381

Compared to the base case, the presence of ILs in the surfactant solution shows a slight effect on

382

the liquid drainage. In the presence of IL-A, the time consumed for the liquid phase to drain was


Page 20 of 42

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383

approximately 280 seconds similar to the base case and a little longer drainage was noticed on

384

MFOMAX/IL-B foam whereby the liquid volume in the foam become very less and liquid

385

volume in column was almost back to the initial state after around 350 seconds. This results

386

show that the addition of IL-A and IL-B at the specified ratio of 60:40 did not provide significant

387

effect in reducing the rate of liquid drainage as the liquid was completely drained out of the

388

lamellae after about 300 seconds, almost similar to the base case. However, even though the

389

liquid in foam has been mostly drained, it is possible for the thin lamellae to remain stable for

390

certain period. At this stage, surface elasticity factor plays an important role in preventing foam

391

rupture giving rise to stability 66, 69. This can be noticed from Figure 6 and Figure 7, that foam

392

half-life was achieved after the liquid volume in foam reached the minimum volume. A complete

393

liquid drainage for IL cases happened within 280-350 seconds while the foam half-life was

394

achieved after 600 seconds. This indicates that the main role of ILs in stabilizing the foam is not

395

by affecting the liquid phase viscosity, hence delayed the liquid drainage. Nevertheless, ILs

396

possess the role of maintaining the stability of dry lamellae. Another finding from the results is

397

that foam destruction was not necessarily affected by fast liquid drainage, but foam coarsening

398

which is thermodynamically driven as well as Ostwald ripening also play the essential role in

399

foam rupturing process.

400

With regard to foam stability indication, a visual observation in foam column after 800

401

seconds for the three cases i.e. individual MFOMAX, MFOMAX/IL-A, and MFOMAX/IL-B

402

with mixture ratio of 60:40 is presented in Figure 8. A pronounced improvement in maintaining

403

foam stability after certain period was noticed in the use of IL but depending on the type of IL

404

used. A higher contrast in color inside the foam column of MFOMAX/IL-B indicates that the

405

foam generated was still containing stable dispersed coarse bubbles after 800 seconds as


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Page 22 of 42

406

compared to MFOMAX/IL-A. The foam produced by MFOMAX/IL-A was drier and more

407

susceptible to high temperature indicated by a faster foam collapse due to the continuous foam

408

coarsening and coalescence. The foam volume recorded after 800 seconds for MFOMAX/IL-B

409

was found to be two times higher than that of MFOMAX/IL-A, confirming that IL-B presence

410

imparts positive effect in reducing the rate of foam rupture at high temperature.

411 412

Figure 8. Images of foam column in the use of MFOMAX, MFOMAX/IL-A, and MFOMAX/IL-

413

B after 800 seconds

414

As mentioned earlier, a noticeable improvement of foam stability in the presence of IL-B 70

415

could be induced by some possible interactions. Considered as electrolytes

416

tendency to stabilize the electrostatic repulsion between similarly charged surfactant head groups

417

which results in an increase in the rate of surfactant adsorption at the interface. This behavior has

418

been suggested by Javadian, et al.

419

through electrostatic stabilization leading to the alteration of surface behavior. In aqueous

420

system, the chloride ions from IL-B molecules are released and the choline cations become free

48

, IL-B have high

, in which IL presence can facilitate surfactant association


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421

to move 71. Therefore, these ions favorably interact with the oppositely charged component from

422

surfactant molecules. IL-B also could interact with surfactant molecules via hydrogen bonding

423

due to the presence of hydrogen bond donors (HBD) moieties in IL-B molecules which is

424

glycerol. The hydroxyl based moieties attached to IL-B molecules provides bulk hydrogen bond

425

donor with a high capacity which produces a large hydrogen bond networks with surfactant. This

426

additional interaction was contributed to lamellae stabilization. All interactions exhibited by

427

MFOMAX/IL-B resulted in strong cohesive interactions enhancing the molecules packing

428

density at the interface. This effect can be attributed to an improvement in lamellae stability

429

against Ostwald ripening and bubbles coalescence. Consequently, MFOMAX/IL-B exhibited

430

higher foam stability than MFOMAX/IL-A.

431

3.2.3. Effect of surface tension on foam stability

432

The relationship between bulk foam stability and the surface tension of the tested solution

433

against N2 is presented in Figure 9. It can be seen from the figure that a clear link was found

434

between foam stability and surface tension. This result also reveal that IL has been able to reduce

435

surface tension of MFOMAX solution at given temperature. As the best formulation, based on

436

foam stability results, MFOMAX/IL-B at 60:40 ratio exhibited a higher reduction of surface

437

tension as compared to MFOMAX/IL-A which was from 23.99 mN/m to 20.14 mN/m. The

438

ability of ILs to reduce surface tension was linked to their effect on surface activity of

439

MFOMAX by having some interactions as explained earlier. When ILs could have high

440

contribution in surface tension lowering, it means the number of surfactant molecules increases

441

per unit area at N2/solution interface or at lamellae layers. This behavior is favorable for thin

442

lamellae stability against mechanical shock due to its tendency to control the equilibrium surface

443

tension under film deformation, providing Gibbs-Marangoni effect to counteract lamellae rupture 23 ACS Paragon Plus Environment

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444

14,

72

445

MFOMAX/IL-B as the best formulation.

. Result from this experiment corresponds to foam stability results ascertaining

446 447

Figure 9. Foam half-life plotted as a function of surface tension against N2 at 90°C for

448

MFOMAX in the absence and presence of IL-A and IL-B at mixture ratio of 60:40

449

3.2.

Core flooding experiment

450

3.3.1. N2 foam generation in the absence and presence of IL

451

The effect of IL as additive on MFOMAX foam performance in reducing gas mobility in

452

porous media was evaluated utilizing the best formulation which was MFOMAX/IL-B with

453

60:40 ratio selected based on bulk foam experiment results. In the first investigation on IL effect,

454

one cycle formulated surfactant alternating gas (SAG) was carried out where the core sample

455

was initially saturated with the tested formulation, then foam was generated in situ by injecting

456

N2 thereafter. The evaluation of IL effect was based on the pressure drop profile during N2

457

injection and then followed by comparison of the calculated MRF in the use of surfactant with 24 ACS Paragon Plus Environment

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Energy & Fuels

458

and without IL additive. Test on N2 injection after core saturation with brine without foaming

459

agent was conducted to serve as a baseline case for comparison in evaluating the pressure drop

460

and to provide the steady-state pressure drop in the absence of foam for MRF calculation. Figure

461

10 shows the measured pressure drop across the core sample during N2 injection after brine

462

injection (no foam), surfactant injection (IL-free case), and surfactant/IL mixture injection (IL

463

case). During N2 injection into brine saturated core, an initial small increase in pressure drop

464

indicates the flow resistance of N2 due to the formation of gas front encountering the brine. Then,

465

the pressure drop decreased and remained stable at approximately 0.22 psi for core A and 0.27

466

for core B. Early gas breakthrough, taking place when the gas undergoes channeling and by

467

passing the brine quickly, was observed after about 0.3 PV injected. In the use of MFOMAX

468

without IL (IL-free case), pressure drop increases steeply at early N2 injection whereby the

469

maximum pressure drop reached after 0.5 PV injected. From the graph, it was found that during

470

foam generation for IL-free case, strong foam region was formed in the range of 0.3-0.8 PV

471

injected and the gas breakthrough occurred after 1 PV injected. The fluctuating pressure drops

472

during the gas flow where the wetting phase is surfactant solution are typically affected by the

473

foam generation and coalescence phenomena in the porous media

474

was recorded as data acquisition from the high sensitivity sensor for pressure drop reading. In

475

this regard, MFOMAX exhibited an improvement in reducing gas mobility based on the gas

476

breakthrough observed as compared to the base case where there was no foam generated.

477 478


73-75

by which the fluctuation

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479 480

Figure 10. Pressure drop profile during N2 injection after brine, MFOMAX, and MFOMAX/IL-B

481

injection

482

In comparison with IL-free case, the presence of IL-B as additive has successfully

483

increased MFOMAX performance by generating stronger foam which was noticed from the early

484

injection of N2. The maximum pressure drop reached was found to be 13 psi, 1.6 times higher

485

than maximum pressure drop for MFOMAX case. After reaching maximum point, the pressure

486

drop stepwise decreased and ultimately following the pressure drop profile of the base case. A

487

higher increase in pressure drop indicates the formation of strong foam offering a high resistance

488

for gas to flow. In this regard, the generated foam was able to control gas mobility effectively

489

compared to IL-free case suggesting a higher stability of lamellae was formed. This result

490

corresponds to foam stability result showing the synergetic effect of MFOMAX and IL-B,

491

therefore selected as the best formulation. Gas breakthrough was found to occur after about 0.9


Page 26 of 42

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Energy & Fuels

492

PV injected where the pressure drop became stabilized at rather higher values compared to the

493

baseline pressure drop. The stronger foam generated at early stage of N2 injection indicates the

494

potential of this formulation to have a higher pressure drop providing the optimized reduction of

495

gas mobility if some SAG cycles using small slug size are performed.

496 497

Figure 11. Comparison of MRF profile between IL-free and IL-stabilized foam case

498

In order to determine the foam strength generated in core flooding experiment with and

499

without IL as additive, the MRF was calculated based on the comparison of pressure drop

500

obtained from both cases with the baseline pressure drop and thereafter plotted as a function of

501

PV injected. The MRF has been commonly used to describe the efficiency of foam to control gas

502

mobility in porous media. Generally, a higher MRF indicates a stronger foam which is able to

503

stabilize the gas front, delaying the gas breakthrough15. Figure 11 presents the MRF provided by

504

MFOMAX foam with and without IL as a function of PV injected during one cycle SAG 27 ACS Paragon Plus Environment

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505

injection. It was noticed that at early stage of N2 injection (less than 1 PV injected), the MRF was

506

considerably increased due to the presence of IL in surfactant solution (IL-stabilized foam case)

507

as compared to the MRF in the IL-free case. MRF in the use of MFOMAX/IL-B reached a

508

maximum value of about 48 before 0.5 PV indicating that N2 mobility was greatly lowered

509

immediately upon its contact with formulated surfactant solution, while the maximum MRF

510

reached for IL-free case was about 36 after 0.5 PV injected. In this regard, the formulated

511

surfactant, MFOMAX/IL-B, could accelerate foam generation which offered higher foam

512

strength by showing greater ability to reduce N2 mobility (30% higher at maximum point) as

513

compared to the MFOMAX case. This result was in accordance with bulk foam stability results

514

shown in Subsection 3.2.2., where foam half-life exhibited by MFOMAX/IL-B was 2.2 times

515

higher than half-life of individual MFOMAX foam. Since the foam generation became faster in

516

the presence of IL as additive, whereby the strong foam was generated as soon as N2 injection,

517

this formulation was further examined by conducting small slug SAG injection. Small slugs SAG

518

injection is able to provide regions of high foaming agent saturation in-situ which is necessary

519

for immediate foam generation 17.

520

3.3.2. IL-stabilized surfactant foam during small slug SAG injection

521

MRF profile during small slugs SAG injection for MFOMAX/IL-B is presented in Figure

522

12. The injection volume for MFOMAX/IL-B solution and N2 was 0.4 PV and each fluid was

523

alternately injected in three cycles at fixed flow rate i.e. 0.2 ml/min. Results obtained indicate

524

that small slugs injections enhanced the foam generation and foam stability in the porous media.

525

The ascending trend represented the formulated surfactant injection stage and the descending

526

trend represented the gas injection stage. High MRF obtained during small slugs SAG injection

527

indicates the formation of a substantial amount of foam along the core sample when gas was in 28 ACS Paragon Plus Environment

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Energy & Fuels

528

contact with formulated surfactant solution. This can be observed by a significant increase in

529

pressure drop. Based on the results, an increase in MRF was noticeably observed from the first

530

injection of 0.4 PV of MFOMAX/IL-B. This high initial MRF showed the formulated surfactant-

531

foam could perform as a good gas mobility controller. The injection of liquid slug created the

532

wetter system and produced lower foam quality which provided a greater resistance

533

shown by relatively high MRF compared to MRF during gas slug injection. The gradual decline

534

in foam resistance during the injection of subsequent gas slug was due to the formation of drier

535

foam system with less saturation of liquid phase which susceptible to coalescence leading to

536

foam destruction. The injection of formulated surfactant in the second cycle boosted up the foam

537

generation by providing the required foaming agent so that the foam formation can be

538

maintained. This behavior continued up to the third cycle producing higher MRF. In this case,

539

foam was continuously generated in-situ and it also increased the viscosity of the fluid

540

throughout the core sample, thus the mobility of gas was greatly reduced. The trend of small

541

slugs SAG injection shown in Figure 12 is in agreement with the previous studies which used the

542

similar injection mode 23, 61, 76.


76

. It was

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543 544

Figure 12. MRF as a function of PV injected for small slug SAG injection in the use of

545

MFOMAX/IL-B

546

Of all cycles, the highest MRF achieved was 116 during the third injection of 0.4 PV

547

MFOMAX/IL-B solution. This increment was almost 2.5 times higher than the maximum MRF

548

achieved during one cycle injection. In average, a high MRF was reached with values within the

549

range of 50-100 during second and third cycle injections which effectively provided stronger

550

foam possessing higher stability. A higher degree of mobility reduction obtained from this

551

experiment was attributed to the fact that stronger foam can be generated in situ when a

552

relatively higher amount of foaming agent is present in the porous media. According to this

553

result, a role of IL as additive to improve surfactant performance in reducing N2 mobility can be

554

optimized using this mode of injection by providing a continuous foam generation inside the

555

porous media.


Page 30 of 42

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556

Energy & Fuels

4.

Conclusion

557

The utilization of IL-based additives for foam stabilization has been introduced for the

558

first time in this study. Their potential in increasing foam stability was addressed to improved

559

mobility control during gas flooding. Bulk foam stability experiment at reservoir temperature

560

was performed to investigate the effect of IL on foaming properties of surfactant and was also

561

used as a screening tool to select the best surfactant/IL formulation with certain ratio. Results

562

obtained from bulk foam experiments at 90°C clearly show the potential of ILs tested to stabilize

563

MFOMAX foam. Of all mixture ratios tested, the highest performance was observed at 60:40

564

ratio of surfactant to additive. At formulation ratio of 60:40, the MFOMAX foam stability was

565

noticeably increased by 62% and 136% in the presence of IL-A (imidazolium-based IL) and IL-

566

B (eutectic-based IL), respectively. Thus, the best formulation for the highest bulk foam stability

567

at reservoir temperature was found to be MFOMAX/IL-B at ratio of 60:40.

568

Core flooding experiment was conducted to evaluate the foam characteristic using the

569

best formulation in the absence of oil under reservoir conditions. The results show that the

570

presence of IL-B has improved MFOMAX performance in the porous media by generating the

571

stronger foam during one SAG cycle experiment, indicated by a higher pressure drop from the

572

early gas injection stage. In the use of IL-B as additive, MRF was immediately increased upon

573

gas injection with a maximum MRF value in MFOMAX/IL-B formulation reaching 30% higher

574

than MRF for the base case. This indicates the presence of IL was able to accelerate foam

575

generation which offers higher foam strength. Small slug SAG injection, on the other hand, was

576

able to optimize the performance of MFOMAX/IL-B formulation in reducing gas mobility by

577

effectively producing stronger foam. The highest MRF achieved during the third injection of 0.4

578

PV MFOMAX/IL-B solution in small slug SAG experiment was almost 2.5 times higher than the 31 ACS Paragon Plus Environment

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579

maximum MRF achieved during one SAG cycle experiment. To recapitulate, the results in bulk

580

and core flooding experiments have indicated the capability of IL to be employed as additive for

581

foam stabilization and to improve the performance of surfactant foam in reducing gas mobility in

582

porous media. The performance and efficiency of the formulated foam are recommended to be

583

further studied in the presence of oil, in order to evaluate the oil recovery as an ultimate effect

584

from controlled gas mobility.

585

Acknowledgments

586

The authors acknowledge research funding from SHELL-UTP-TU-Delft collaboration

587

project under Grant No. 0153 AB-DA3 and the materials from PETRONAS Research Sdn. Bhd.

588

(PRSB). Authors would also like to thank Centre of Research in Enhanced Oil Recovery

589

(COEOR), Universiti Teknologi PETRONAS for the laboratory facilities and technical

590

assistance.

591

References

592

1. Alvarado, V.; Manrique, E., Enhanced Oil Recovery: An Update Review. Energies 2010, 3

593

(9), 1529-1575.

594

2. Belgrave, J.; Win, T. In Vertical sweep efficiency and oil recovery from gas injection in

595

heterogeneous horizontal reservoirs, SPE Western Regional Meeting, Anchorage, Alaska,

596

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Ionic Liquids Application in Surfactant Foam Stabilization for Gas

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