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Transport in Porous Media

, Volume 131, Issue 1, pp 175–192 | Cite as

A Pore Scale Study of Non-Newtonian Effect on Foam Propagation in Porous Media

  • Galang B. Ramadhan
  • Quoc P. NguyenEmail author
Article
  • 161 Downloads

Abstract

CO2 injection is one of the most promising techniques to enhance oil recovery. However, an unfavorable mobility ratio, reservoir heterogeneity and gravity segregation can reduce the macroscopic sweep efficiency. In situ foaming of injected CO2 is the method that has the most potential for improving sweep efficiency based on controlling CO2 mobility. This study investigates the foaming behavior of N,N,N′-trimethyl-N′-tallow-1,3-diaminopropane (DTTM) surfactant with CO2 in a transparent porous microflow model with natural rock pore structures. It focuses on the effect of the salinity induced non-Newtonian behavior of DTTM solution on foam propagation. The performance of foams stabilized by 0.5 wt% DTTM solution over the viscosity range from 0.71 (at 5 wt% NaCl) to 41 cp (at 20 wt% NaCl) was compared with conventional polymer-enhanced foams whose liquid phase contained a commonly used foaming surfactant, C15–18 Internal Olefin Sulfonate (C15–18 IOS) and a hydrolyzed polyacrylamide. Such comparisons have also provided insight into the respective impacts of liquid phase viscosification by worm-like surfactant micelles and polymer on foam texture associated with its rheological characteristics. It was found that at low aqueous phase viscosity (injection liquid viscosity of 0.71 cp) the maximum achievable viscosity of DDTM foam was around 1000 cp, which was 80 times IOS stabilized foam. The interfacial tension of DTTM was higher than that of IOS, resulting coarser foam texture and higher individual lamella resistance. An increase in DTTM solution viscosity by a factor of 33 decreased foam generation and viscosity for gas injection. This was not observed for the simultaneous injection of gas and DTTM solution. Overall, the effect of liquid phase viscosity on transient foam behavior during gas injection is similar for both DTTM and IOS regardless of the difference in the nature of viscosifying agents (WLM vs 3330 s polymer). An increase in gas injection pressure without liquid injection delayed foam propagation and reduced the magnitude of foam viscosity. The results from this study indicated that DTTM surfactant is an important alternative to commercially available polymers that have been used to enhance foam performance in porous media. This particular surfactant type also overcomes several disadvantages of polymers such as limited temperature and salinity tolerance, shear degradation, and filtering in low permeability formations.

Keywords

Alkyl diamine surfactant CO2 foam Viscoelasticity Microfluidics Polymer 

1 Introduction

One of the major challenges in gas-enhanced oil recovery (EOR) is the poor mobility control of the injection gas. The principal factors effecting this adverse mobility include the low viscosity of the injection gas (causing viscous fingering), the high-density contrast between the injection gas and resident liquids (causing gravity override), and the reservoir heterogeneity (causing gas channeling). Aqueous foam is one of the most extensively studied solutions for mobility control in porous media. Over the years, foam has been proven to alleviate gas conformance issues both in the laboratory setting (Li et al. 2010; Talebian et al. 2014) and field trials (Sanders et al. 2012; Mukherjee et al. 2016).

The addition of polymer to a foaming surfactant solution has been suggested to improve foam propagation in porous media (Sydansk 1994; Kutay and Schramm 2004). It results in an increase in aqueous phase viscosity due to the polymer chain entanglement (Graessley 1967), leading to a more stable foam with a higher apparent viscosity (Kutay and Schramm 2004). Despite the benefits, the use of a polymer in a foaming solution does have several drawbacks and limitations. Some of the drawbacks include reduced injectivity due to the high viscosity of the liquid slug and polymer filtering in low permeability formations. The application of polymer-enhanced foam is also confined by several limitations, which include low-temperature tolerance, significant viscosity reduction in high salinity conditions, and irreversible shear degradation. Therefore, it is necessary to develop an alternative additive for enhancing foam performance.

This study investigates the foaming behavior of N,N,N′-trimethyl-N′-tallow-1,3-diaminopropane (DTTM). Above the critical micelle concentration (CMC), DTTM monomers form worm-like micelle (WLM) at an elevated salinity. WLM possesses characteristics that improve foam viscosity and stability (Liebum et al. 2018). Similar to polymer macromolecules, WLM entanglement leads to the viscosification of the liquid phase (Magid 1998). Also, WLM forms a mesh-like structure in thin foam films that contributes to steric disjoining pressure and thus resisting film thinning (Klitzing et al. 2001). The time required for a WLM structure to recover from a shear-induced degradation and return to its original conformation is defined as micellar relaxation time. Micellar relaxation time typically spans over a short period (e.g., milliseconds to fractions of a second) (Patist et al. 1999), which makes the shear-induced degradation of WLM reversible. WLM filtering in low permeability formations is minimal due to the lower molecular weights of WLMs relative to polymers (Kumar et al. 2014).

In addition to replicating the role of polymer in the liquid phase, DTTM surfactant offers several unique features. DTTM possesses a high solubility in supercritical CO2 (Liebum et al. 2017). This property is advantageous as it allows DTTM delivery in the CO2 phase (Le et al. 2008). The delivery of surfactant in the foam gas phase could alleviate the injectivity issue typically associated with high viscosity polymer-surfactant solution. This injection strategy has also been proven to reduce the surfactant adsorption to the rock matrix; thus, reducing the amount of surfactant loss and shortening the delay for foam formation and propagation (Ren et al. 2013, 2014; Ren and Nguyen 2017). Unlike polymer macromolecules, WLM comprised of DTTM can withstand high temperatures (up to 120 °C) and a high salinity (up to 25 wt% NaCl) environment (Liebum 2016; Liebum et al. 2017, 2018).

The main objective of this study is to investigate how the non-Newtonian behavior of the DTTM solution influences the dynamics of foam behavior in a transparent two-dimensional microflow model with natural rock pore structures. The model is optically clear for flow visualization (so-called Hele Shaw Cell). The definition of rock pore structures is controlled through lithographic production techniques. For the purpose of this work, two different salinities were used: (1) a “low” salinity solution (5 wt% NaCl, 0.5 wt% DTTM) associated with the low liquid phase viscosity; and (2) a “high” salinity solution (20 wt% NaCl, 0.5 wt% DTTM) associated with the high liquid phase viscosity. The performance of these DTTM stabilized foams was compared with conventional polymer-enhanced foams whose liquid phase contained a commonly used foaming surfactant, C15–18 Internal Olefin Sulfonate (C15–18 IOS) and a hydrolyzed polyacrylamide. Such comparisons would also provide insight into the respective impacts of liquid phase viscosification by surfactant WLM and polymer on foam rheology. The importance of liquid phase viscosification over surface tension reduction in enhancing in situ foam propagation for all surfactant formulations was evaluated based on the measurements of interfacial tension (IFT), rheology and the direct observation of foam texture, mobile and trapped gas saturation during transient foam flow.

2 Background

Direct observation of foam flow in micromodels has revealed several lamella creation mechanisms of which snap-off, lamella division, and leave-behind have been well documented (Schramm 1994). The lamella division is the dominant lamella creation mechanism when snap-off becomes thermodynamically impossible or unrepeated (Rossen 2003). For gas injection into a medium initially saturated with a surfactant solution, all the lamella creation mechanisms may play a role during the transient displacement. When the depleted liquid content does not allow for the continuation of snap-off, the mobilization and division of stationary (leave-behind) lamellae are the primary mechanisms used to maintain foam texture. The addition of polymer to the surfactant solution may reduce the rate of lamella creation. For snap-off, the time for the invading gas to expand downstream a pore-throat (determined by interstitial gas velocity) is necessarily longer than that for liquid flowback for snap-off. This is particularly true for one-dimensional flow channels. With higher pore connectivity, the liquid coming from the surrounding region of a snap-off site might be less sensitive to the gas rate in this site. This implies that polymer-induced liquid mobility reduction can reduce the frequency of or even inhibit snap-off. However, it is intuitive that viscosifying foam films with polymer would not inhibit their ability to multiply by division.

The addition of polymer can delay foam lamellae rupturing and bubble coalescence. Polymer improves the stability of thick foam film by increasing its viscosity and reducing the rate of film drainage. The film drainage rate is inversely proportional to the liquid viscosity as described by the following film thinning model (Huang et al. 1986).
$$ - \frac{{\text{d}h}}{{\text{d}t}} = \frac{{2h^{3}\Delta P}}{{3\eta R^{2} }} $$
(1)
where h is the instantaneous thickness of the film; ΔP is the pressure difference between the film and plateau border; η is the viscosity of the liquid; and R is the radius of the foam bubble.

In addition to bulk viscosity, surface elasticity and viscosity, which are commonly evaluated as the Marangoni effect (Marangoni 1869; Stubenrauch and von Klitzing 2003), reduce the rate of film drainage by the opposing liquid flow out of high surface tension region of the film. Previous study shows that drainage time is proportional to increased Marangoni effect (Bergeron et al. 1996). Marangoni effect can be influenced by the rate of surfactant exchange between the surface and the aqueous solution, which is a function of solution viscosity. For thin films (< 100 nm), the rate of drainage deviates from that predicted by Eq. 1. The deviation is mainly due to the increasing influence of disjoining pressure. Disjoining pressure resists film thinning forces through an interplay between van der Waals forces, electrostatic double-layer force, solvation and steric forces (Petkova et al. 2012). The steric forces arise from a mechanical interaction between the lamella interfaces and a structure in the lamella film that resists film thinning. The structures providing resistance could be in the form of entangled polymers, stratified oil droplets, stratified spherical micelles (Schramm and Wassmuth 1994), or entangled cylindrical micelles (Klitzing et al. 2001; Nguyen et al. 2007).

Foam reduces gas mobility in a porous medium by two mechanisms: (1) resisting the motion of gas bubbles by liquid lamellae and (2) trapping gas bubbles by local capillary pressures. The first mechanism can be described by the following theoretical model of foam apparent viscosity (\( \mu_{\text{f}} \)) in a periodically diverging–converging channel (Nguyen et al. 2004).
$$ \mu_{\text{f}} \approx \mu_{\text{g}} + \frac{{(n_{\text{l}} \bar{R})}}{4}\left( {\frac{{2\mu_{\text{l}} }}{a}\left( {\frac{{3\mu_{\text{l}} v_{\text{f}} }}{{\sigma_{\text{gl}} }}} \right)^{{ - \frac{2}{3}}} + \frac{{\lambda E_{\text{g}} \ln \bar{A}_{s} }}{{v_{\text{f}} }}} \right) $$
(2)
where \( \mu_{\text{g}} \) is the viscosity of gaseous phase; \( \mu_{\text{l}} \) is the viscosity of liquid phase; \( \sigma_{\text{gl}} \) is the gas liquid interfacial tension; \( n_{\text{l}} \) is the lamella density per unit length; \( \bar{R} \) is the average pore diameter; \( v_{\text{f}} \) is the mean velocity of lamellae train; \( \lambda \) is the porous media geometric correction factor; \( E_{\text{g}} \) is the Gibbs surface elasticity; \( \bar{A}_{s} \) is the average foam bubble aspect ratio. Equation 2 indicates that the primary factors contributing to the flow resistance of an individual lamella include (1) the viscosity of liquid phase, (2) Gibbs elasticity, and (3) interfacial tension. The impact of these factors is multiplied by lamella density as a measure of foam texture. Previous studies performed on the equilibrium surface tension of anionic polyelectrolyte–anionic surfactant system (Wreath 1989) and dynamic surface tensions of nonionic polyelectrolyte–anionic surfactant system (Israelachvili et al. 1976) indicated the insignificant effect of added polyelectrolytes on the surface tension.

The second mechanism of gas mobility reduction in a porous medium is bubble trapping, which reduces the effective permeability of gas by reducing the number of flow channels (Schramm and Wassmuth 1994). The ability of foam bubbles to flow from one pore to another requires the local pressure gradient to exceed the Laplace pressures imposed by the liquid lenses occupying the pore throats (Schramm and Wassmuth 1994). It has been reported that the fraction of trapped foam in porous media could be over 65% pore volume; depending on foam texture, flow rate, and pore network morphology (Chen et al. 2006). The effect of polymer addition on trapped foam saturation has not been investigated. However, the increased apparent viscosity and stability of a polymer-enhanced foam can lead to an increase in trapped foam fraction.

3 Experimental Description

3.1 Materials

The following surfactants, N,N,N′-trimethyl-N′-tallow-1,3-diaminopropane (DTTM) and C15–18 Internal Olefin Sulfonate (IOS), were donated from AkzoNobel and Shell, respectively. DTTM exhibits an interconversion between nonionic and cationic forms. The molecular structures of the two forms of DTTM surfactant are illustrated in Fig. 1. The interconversion between the nonionic and cationic forms of DTTM forms is triggered by the concentration of surrounding hydrogen ions (H+). At a low H+ concentration (high pH environment), the two amine head groups of DTTM are not protonated, and DTTM assumes its nonionic form. At a higher concentration of H+ (low pH environment), one or both amine head groups of DTTM are protonated, and DTTM assumes its cationic form. The nonionic form of DTTM possesses a high solubility in supercritical CO2 (Liebum et al. 2017).
Fig. 1

Chemical structure of DTTM surfactant head in the nonionic form (left) and the cationic form (right)

The IOS was synthesized by olefin metathesis (i.e., ethylene oligomerization). C15–18 internal olefins form the hydrophobe, which was subsequently sulfonated, resulting in a proportion of alkene and hydroxyl alkane sulfonates (~ 60:40 mixture), each with several isomers depending on the double-bond positioning in the olefin. All the surfactants were used without further purification.

A hydrolyzed polyacrylamide, Flopaam 3330S polymer, was donated from SNF. A concentrated polymer stock solution was prepared with 5000 ppm polymer and 1000 ppm NaCl, mixed for more than 24 h, and filtered through a 1.2 µm. Polymer solution hydrated sufficiently generally has a filtration ratio less than 1.2.

3.2 Interfacial Tension Measurement

The series of IFT measurements were performed on surfactant, polymer, and surfactant–polymer solutions with CO2 at 22 °C and atmospheric pressure. Our IFT measurement system consisted of a ram¬é-hart F4 Series camera focused on a flat Teflon surface and placed opposite a 150w fiber optic illuminator. Measurements were taken in the sessile drop mode via ram¬é-hart DropImage Advanced software, which captured the curvature of the drop and calculated surface tension via Young–Laplace methods. Table 1 shows the composition and IFT of seven aqueous solutions used in this work.
Table 1

Composition and CO2 IFT of 7 aqueous solutions used in microflow experiments

Solution name

Surfactant concentration (wt%)

Polymer concentration (ppm)

Polymer MW (MDa)

Salinity NaCl (wt%)

Interfacial tensiona (dyne/cm)

DTTM High

0.5

0

20

35.15

DTTM Low

0.5

0

5

35.30

IOS

0.5

0

5

27.61

Flopaam 3330 s/IOS

0.5

3000

8

5

26.82

a22 °C and atmospheric pressure

3.3 Rheology Measurement

A TA instrument AR-G2 rheometer with smart swap was used to analyze all samples. The Couette concentric cylinder was selected based on the solution’s fluid behavior ranging from Newtonian to nonlinear viscoelasticity. About 19 mL of the solution was dispensed into the cylinder and pre-conditioned for 1 min before starting the steady-state shear rate analysis from 1 to 100 s−1 with four points per decade.

Figure 2 shows the viscosity of 0.5 wt% DTTM solution at 22 °C as a function of NaCl concentration. At a concentration above CMC, DTTM solution exhibits a sharp increase in viscosity for NaCl concentration above 15 wt%. This sharp increase in viscosity is attributed to the salinity-triggered transformation of spherical micelle to WLM (Hirasaki and Pope 1974). An increase in the electrolyte concentration screens the charges of ionic surfactant head groups. This screening effect reduces the effective geometrical volume of the surfactant head; thereby allowing closer interaction between surfactant monomers. As a result, surfactant monomers can agglomerate more densely; and eventually, evolve from a spherical to worm-like conformation. Additionally, an increase in salinity has been attributed to the shortening of micellar persistence length (Bird et al. 2007). Persistence length is the extent of the rigid portion of a WLM (Dreiss 2007). A reduction in the micellar persistence length leads to a more flexible WLM structure. The increased flexibility could lead to a larger micellar entanglement network; thus, resulting in a higher viscosity of the liquid solution. To study the effect of DTTM viscosity on foam performance, two different salinities (5 and 20 wt% NaCl) were used to make DTTM Low and DTTM High solutions (Table 1), respectively, based on the rheology of 0.5 wt% DTTM solution at 22 °C (Fig. 2).
Fig. 2

Viscosity of 0.5 wt% DTTM surfactant solution as a function of NaCl concentration at 22 °C and three different shear rates

Figure 3 shows the respective viscosities of two high viscosity solutions, DTTM High and Flopaam 3330 s/IOS (Table 1), as a function of shear rate. Note that the presence of 0.5 wt% IOS did not modify the viscosity of Flopaam 3330 s and that the viscosities of the DTTM Low and IOS were about 0.71 cp and nearly constant within the range of shear rate from 1 to 100 s−1.
Fig. 3

Viscosity of DTTM High and Flopaam 3330 s/IOS solutions at 22 °C

To compare the foaming performance of DTTM High with Fopaam 3330 s/IOS, it is desired to obtain the same effective viscosity for these solutions in single phase flow in microflow model at the selected constant liquid injection rate of 0.6 µL/min. The apparent shear rate (\( \dot{\gamma }_{\text{a}} \)) in the microflow model for this injection rate can be estimated from Eq. 3 (Hirasaki and Pope 1974).
$$ \dot{\gamma }_{\text{a}} = \left( {\frac{3n + 1}{4n}} \right)^{{\frac{n}{n - 1}}} \frac{12u}{{\sqrt {150k\phi} }} $$
(3)
where n is the bulk solution power law index; u is the Darcy velocity, calculated based on the liquid injection rate and the microflow model dimensions (Table 2); k and ϕ are the micromodel permeability and porosity, respectively (Table 2). Equation 3 results in an apparent shear rate of 45.7 s−1. Note that the microflow model is a two-dimensional porous medium. The apparent shear rate can also be estimated from the following theoretical model for incompressible flow in a porous narrow slit.
$$ \dot{\gamma }_{\text{a}} = 12\frac{u}{\phi h} $$
(4)
where h is the height of the microflow model, respectively. Equation 4 results in an apparent shear rate of 38.8 s−1. The apparent shear rates given by the two theoretical models are quite consistent. The viscosities of DTTM High and Flopaam 3330 s/IOS are also very similar within the shear rate range from 38.8 to 45.7 s−1 as indicated from Fig. 3. Therefore, an average viscosity of 41 cp was used to determine the concentration of FLOPAAM 3330 s (3000 ppm).
Table 2

Microfluidic porous media chip specification

Property

Value

Unit

Permeability

2.75

Darcy

Pore volume

2.30

µL

Porosity

0.57

Porous medium length

2

cm

Slit height

22.4

µm

Slit width

9

mm

3.4 Foam Flow in Microflow Model

A Micronit porous microflow model with natural rock pore structures was used for all foam flow experiments. The properties of the microflow model are presented in Table 2.

A diagram of the setup is shown in Fig. 4. CO2 is routed from a tank and joined to a chemical solution-bearing Hamilton Gas Tight Syringe in a Chemyx Fusion Syringe Pump via a tee connection. CO2 was injected at a constant pressure in all experiments, while liquid was injected at a constant rate of 0.6 µL/min. The gas injection rate was quantified through volumetric water displacement method, where the mass of the collected water was recorded throughout the injection period. Two absolute pressure transducers were placed at the respective inlets of the chip and the backpressure regulator (BPR). Backpressure of 55 psig was used to reduce the effect of gas compressibility. Gas–water flow was observed and recorded via an AmScope MU300 Microscope Digital Camera connected to a laptop computer.
Fig. 4

Diagram of the microflow experiment setup

Two different injection strategies were studied: (1) pressure-controlled CO2 injection (gas injection) and (2) simultaneous injection of CO2 and an aqueous solution (co-injection). For the gas injection, the porous medium was saturated with the aqueous solution before CO2 injection. Gas injection experiments were performed at three different applied pressure drops (between the inlet and the outlet of the model): 1, 2, and 3 psi. The same procedure was used for the co-injection experiments, but at higher applied pressure drops (3, 4, and 5 psi). The gas and liquid rate were used with the aid of Darcy law to calculate the foam apparent viscosity μf
$$ \mu_{\text{f}} = \frac{k\Delta P}{uL} $$
(5)
where \( \Delta P \) is the applied pressure drop; L is the length of the porous medium. All experiments were performed at 22 °C. After each experiment, the microflow model was cleaned with 10 mL of DI water over 30 min. A permeability test was performed before each experiment to ensure that the medium permeability was not modified.

4 Results and Discussion

4.1 Effect of Gas–Liquid Interfacial Tension

This section examines the foaming behaviors of the DTTM Low and IOS solutions. These solutions had the same surfactant concentration (0.5 wt%) and viscosity (0.71 cp). The notable difference between the two solutions is their IFTs (\( \sigma_{\text{gl}} ) \) with CO2 (35.30 and 27.61 dyne/cm for DTTM Low and IOS, respectively). The overall pressure drop applied to the gas flow was 1 psi for the gas injection and 3 psi for the co-injection for both surfactants. The resulting foam apparent viscosities (\( \mu_{\text{f}} ) \) versus time are presented in Fig. 5. The associated foam flow images are shown in Fig. 6.
Fig. 5

Foam apparent viscosity for DTTM Low and IOS solutions during a gas injection and b co-injection

Fig. 6

Foam flow images for DTTM Low and IOS during a gas injection and b co-injection

4.1.1 Gas injection

Figure 5A shows that \( \mu_{\text{f}} \) for both DTTM Low and IOS reached high values (over 14 times the viscosity of surfactant solution) almost immediately after the onset of gas invasion. From Fig. 6a, the images taken before five injected pore volumes (PVs) show robust foam propagation for both DTTM Low and IOS. Gas saturation was quite high at 2.5 PVs in both cases, indicating good liquid displacement by foam. However, a remarkable difference in foam texture was observed. DTTM Low foam was considerably coarser than IOS foam, which can be attributed to the IFT difference between the surfactants. The lower IFT with IOS surfactant promoted the generation of gas–liquid surface area by the lamella creation mechanisms and reduced the minimum local pressure gradients required to mobilize foam films, leading to increasing lamella multiplication by division (Rossen 1990). In addition, the number of snap-off events probably increases as it is easier for the gas to invade more smaller liquid occupied pores at reduced IFT (i.e., reduced entry capillary pressure). Despite having a coarser texture, DTTM Low foam exhibited higher apparent viscosity during liquid desaturation (before 10 PVs). From Eq. 2, this could only be possible with a higher individual lamella resistance to gas flow for DTTM than IOS as lamella resistance increases with the interfacial tension (\( \sigma_{\text{gl}} ) \).

Lamella creation was significantly reduced for both DTTM and IOS during late time gas injection due to very low water saturation. In such a “dry” environment, foam film drainage accelerates due to the higher capillary pressure. The drainage continues until the lamellae either rupture or reach a metastable state. Figure 6a shows a clear reduction in lamellae density for IOS after five PVs. However, such reduction was not observed for DTTM Low.

As the gas injection continued, the number of stable lamellae displaced out of the medium increased, leading to the formation of continuous gas channels and further reduction of foam apparent viscosity after 10 PVs for both surfactants. It is important to observe that the immobilization of lamellae during this late time injection reduced the effective permeability of gas throughout the medium, which explains why the foam apparent viscosity was significantly higher than the pure CO2 viscosity (0.0148 cp at 22 °C and 55 psi) after 15 PVs (Fig. 5a).

4.1.2 Co-injection

DTTM Low and IOS foams exhibited a similar \( \mu_{\text{f}} \) buildup rate through eight PVs (Fig. 5b) for this injection strategy. The early time \( \mu_{\text{f}} \) for the co-injection (Fig. 5b) was lower than that for the gas injection (Fig. 5b) for both surfactants. Examining the images of foam flow over this period (Fig. 6) reveals that bubble density was consistently higher for the gas injection than the co-injection. This could be attributed to the fact that the in situ foam quality was gradually reduced as foam propagated throughout the medium because of the pressure-controlled gas injection and the rate-controlled liquid injection. The increasing liquid flow in the medium reduced the rate of foam creation but did not prevent foam texture from building up to its maximum at around 10 PVs. For both gas injection and co-injection, the magnitude of \( \mu_{\text{f}} \) for DTTM was higher than that for IOS even though the IOS foam texture was finer. This observation could be explained based on the IFT difference between the two surfactants. The higher \( \mu_{\text{f}} \) for DTTM resulted in further reduction of gas injection rate, giving rise to the transient displacement of foam by the injected liquid. This phenomenon can be clearly seen from the images of foam flow for DTTM at 10 and 14 PVs in Fig. 6b. Such cyclic occurrence of lamella creation and displacement by liquid caused \( \mu_{\text{f}} \) to fluctuate strongly after 10 PVs as observed in Fig. 5b.

4.2 Effect of Liquid Phase Viscosity

This section examines the effect of liquid phase viscosity on the foaming behaviors of DTTM and IOS surfactants. Foam flow experiments were performed using two DTTM solutions (DTTM High and DTTM Low shown in Table 1). The IFTs of these two solutions were nearly identical. The bulk viscosity of DTTM Low and DTTM High was 23.45 and 0.71 cp at 40 s−1, respectively. Figure 7 shows the results of foam apparent viscosities for both the gas injection and the co-injection.
Fig. 7

Foam apparent viscosity for DTTM High and DTTM Low solutions during a gas injection and b co-injection

For the gas injection, DTTM High foam exhibited lower \( \mu_{\text{f}} \) than DTTM Low over 13 PVs. The texture of DTTM High foam did not develop as rapidly as that of DTTM Low foam (at 2.5 and 5 TPV, Fig. 8a). The increase in liquid viscosity would reduce the mobility of liquid and thus lamella creation rate. On the other hand, it increases individual lamella resistance to gas flow based on Eq. 2. In our experiments with DTTM High and DTTM Low, it appears that the increase in lamella resistance with liquid viscosity was not strong enough to compensate for the loss of foam apparent viscosity caused by the reduced lamella creation.
Fig. 8

Foam flow images for DTTM High and DTTM during a gas injection and b co-injection

However, lamella creation was dramatically reduced after DTTM High and DTTM Low foams reached their finest texture associated with the highest gas saturation at around 7.5 TPV. After this point, a gradual decline was observed in the \( \mu_{\text{f}} \) of DTTM Low foam, while DTTM High foam was able to maintain its \( \mu_{\text{f}} \). This difference in \( \mu_{\text{f}} \) behavior could be indicative of foam stability. After five PVs, DTTM Low foam experienced a continuous gas breakthrough, which was not observed in DTTM High foam. The lamella density of DTTM High remained relatively constant after five PVs, which suggests an increase in foam stability with liquid viscosity (i.e., decrease in film thinning rate). Moreover, the WLM structures in the DTTM High liquid phase could enhance the steric disjoining pressure for better foam stability.

For the co-injection, the \( \mu_{\text{f}} \) buildup rate was similar between DTTM High and DTTM Low foams through five PVs (Fig. 7b). However, the development of foam texture for DTTM High was slower than that for DTTM Low through 10 PVs (Fig. 8b), which is consistent with the observation from the gas injection experiment. DTTM Low foam reached its finest texture and highest gas saturation at around 10 PVs before entering its strong fluctuation phase. During this phase, DTTM Low foam was repeatedly displaced by liquid and then regenerated to reach the same finest texture. DTTM High, on the other hand, went through more cycles of foam displacement and regeneration before reaching almost the same finest texture and highest gas saturation at around 27 PVs. With such an insignificant difference in texture, it is expected to observe that DTTM high exhibited a higher \( \mu_{\text{f}} \) due to its higher lamella resistance (Fig. 7b after 25 PVs).

The transient behaviors of the DTTM foams above indicate that liquid phase viscosification by WLM could influence lamella creation, stability, and lamella resistance to flow. Therefore, it can be hypothesized that polymer-induced viscosification would result in similar dynamics of foam behavior and rheology. This hypothesis was tested on two IOS solutions (IOS and Flopaam 3330 s/IOS) whose properties are given in Table 1. Note that the presence of Flopaam 3330 s did not significantly modify the IFT between IOS solution and CO2 and that the viscosity difference between these IOS solutions was the same as that between DTTM High and DTTM Low. The results of foam apparent viscosity for IOS solutions are shown in Fig. 9 for both the gas injection and the co-injection.
Fig. 9

Foam apparent viscosity for IOS and Flopaam 3330 s/IOS solutions during a gas injection and b co-injection

For the gas injection, the \( \mu_{\text{f}} \) of IOS foam with 3330 s polymer was higher than that without the polymer during the first 2 PVs and after 15 PVs (Fig. 9a). The images of foam flow (e.g., at 2.5 and 15 PVs shown in Fig. 10a) indicate that the bubble density and gas saturation were lower in the presence of polymer, which explains the lower apparent viscosity for the 3330 s/IOS solution observed between 3 and 15 PVs (Fig. 9a). However, the stability of foam was apparently improved by the addition of 3330 s, which helped reduce the foam coalesce rate during late time injection (after 15 PVs). Such improvement is important at high local capillary pressure for maintaining foam flow resistance, as evidenced by the higher apparent viscosity of 3330 s polymer-enhanced foam at very low liquid saturation (after 15 PVs).
Fig. 10

Foam flow images for IOS and 3330 + IOS during a gas injection and b co-injection

Overall, the effect of liquid phase viscosity on transient foam behavior during gas injection is similar for both DTTM and IOS regardless of the difference in the nature of viscosifying agents. However, a clear discrepancy was observed for the co-injection. The \( \mu_{\text{f}} \) of the IOS foam was consistently lower than the 3330/IOS foam (Fig. 9b), which was not observed for DTTM High and DTTM Low before 25 PVs (Fig. 7b). In addition, the foam texture development was slower without the viscosifying polymer, which was quite opposite to the dynamics of foam texture by the viscosifying WLMs of DTTM. The observed difference in viscosity effect by DTTM and IOS for the co-injection strongly reflects the important role of IFT. A combination of IFT reduction and viscosity enhancement appears to give the highest foam propagation rate. However, the significance of the IFT effect is diminished once strong foam has developed, which is evidenced by the higher foam apparent viscosity for both 3330 s/IOS (Fig. 9b after 15 PVs) and DTTM High (Fig. 7b after 25 PVs). Moreover, the DTTM High foam exhibited greater foam stability over the duration of the injection, while the 3330/IOS foam still experienced substantial texture coarsening, particularly at high gas saturation (i.e., high capillary pressure). This suggests that DTTM High could outperform the polymer-enhanced foam in the reservoirs where foam stability is a critical factor.

4.3 Effect of Applied Pressure Gradient on DTTM Foam Behavior

In the previous gas injection experiments, the applied overall pressure drop was fixed at 1 psi. This section examines the effect of a higher pressure drop (2 psi) on the transient foam behavior for DTTM Low and DTTM High. The results of foam apparent viscosity are shown in Fig. 11a and b. It was observed that an increase in the applied pressure gradient delayed the peak value of \( \mu_{\text{f}} \) and reduced its magnitude for both DTTM solutions. The foam texture was coarser associated with lower gas saturation as the pressure gradient increased. The delay of foam propagation increased with DTTM viscosity. This impact of applied pressure gradient on the transient DDTM foam behavior can be attributed to the unstable displacement of the resident liquid. The mobility ratio between the displacing gas and the liquid would be higher due to weaker foam or higher viscosity of the liquid that results in an earlier gas breakthrough by viscous fingering. The severity of this effect increases with gas velocity (or applied pressure gradient in these experiments), particularly when the gravity does not influence the displacement by segregating the two fluids. As a consequence, lamella creation is delayed or even completely inhibited when the liquid phase reaches prematurely its residual saturation during gas injection. However, the co-injection of gas and surfactant solution is expected to enhance lamella creation even at very low liquid saturation. This is well evidenced by the foam apparent viscosity of the DTTM solutions during the gas–liquid co-injection shown in Fig. 11c and d. Despite the significant difference in the bulk viscosities of DTTM solutions, the increase in the applied pressure gradient by a factor of two did not significantly impact the rate of foam viscosity buildup and the displacement efficiency.
Fig. 11

Foam apparent viscosity for DTTM Low and High at different applied pressure gradients

5 Summary

This study presents the transient CO2 foam behaviors of DTTM and IOS surfactants in a physical microflow model with natural rock pore structures. It was found that without enhancing the aqueous phase viscosity, the foam apparent viscosity for DTTM surfactant could reach 1000 cp during gas injection and nearly 800 cp with co-injection, which were over 80 times the maximum viscosity of IOS foam under the same injection conditions. The higher interfacial tension of DTTM solution resulted in coarser foam texture due to reduced lamella creation, but higher individual lamella resistance to flow as compared to the IOS surfactant.

The enhancement of DTTM solution viscosity at elevated salinity (from 0.71 at 5 wt% NaCl to 23.45 cp at 20 wt% NaCl) enhanced lamella resistance, but reduced lamella creation during gas injection, resulting in a decrease in foam apparent viscosity. However, foam stability was improved at a very low liquid saturation (high capillary pressure) for the higher DTTM solution viscosity, which helped sustain foam viscosity during late time gas injection. The effect of reduced lamella creation at high liquid viscosity could be reduced by continuous liquid injection (i.e., co-injection with gas). This increased the foam apparent viscosity with its liquid phase viscosity.

The effect of liquid phase viscosity on transient foam behavior during gas injection is overall similar for both DTTM and IOS regardless of the difference in the nature of viscosifying agents (WLM versus 3330 s polymer). At high liquid phase viscosity, the significance of the IFT effect on foam texture is diminished once strong foam has developed. Moreover, the DTTM High foam exhibited greater foam stability than the 3330/IOS foam, which is important for foam application at a field scale.

An increase in gas injection pressure without liquid injection delayed foam propagation and reduced the magnitude of foam apparent viscosity. This effect was attributed to earlier gas breakthrough as gas injection rate increased. Maintaining lamella creation with liquid injection minimized the effect of applied pressure gradient on foam viscosity buildup and the displacement efficiency.

Notes

Acknowledgement

This study was supported by Gas EOR Consortium the University of Texas at Austin and the University of Texas at Austin.

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Copyright information

© Springer Nature B.V. 2018

Authors and Affiliations

  1. 1.The University of Texas at AustinAustinUSA

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