CO2-Soluble Ionic Surfactants and CO2 Foams for High-Temperature

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CO2‑Soluble Ionic Surfactants and CO2 Foams for High-Temperature and High-Salinity Sandstone Reservoirs Zheng Xue,† Krishna Panthi,‡ Yunping Fei,† Keith P. Johnston,† and Kishore K. Mohanty*,‡ †

McKetta Department of Chemical Engineering and ‡Department of Petroleum and Geosystems Engineering, The University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *

ABSTRACT: The sweep efficiency of CO2 enhanced oil recovery can be improved by forming viscous CO2-in-water (C/W) foams that increase the viscosity of CO2. The goal of this study is to identify CO2-soluble ionic surfactants that stabilize C/W foams at elevated temperatures up to 120 °C in the presence of a high salinity brine using aqueous phase stability, static and dynamic adsorption, CO2 solubility, interfacial tension, foam bubble size, and foam viscosity measurements. An anionic sulfonate surfactant and an amphoteric acetate surfactant were selected to achieve good thermal and chemical stability, and to minimize adsorption to sandstone reservoirs in the harsh high-salinity high-temperature brine. The strong solvation of the surfactant head by the brine phase and surfactant tail by CO2 allows efficient reduction of the C/W interfacial tension, and the formation of viscous C/W foams at high salinity and high temperature. Furthermore, the effect of temperature and methane dilution of CO2 on foam viscosity was evaluated systematically in both bulk and porous media. High temperature reduces the stability of foam lamella, which leads to lower lamella density and, therefore, lower foam viscosity. Methane dilution of CO2 reduces the solvation of surfactant tails and makes the surfactant less CO2-philic at the interface. The consequent increase of the interfacial tension decreases the stability of foam lamella, as seen by the increase in foam bubble size, thereby reducing foam viscosity.

1. INTRODUCTION Carbon dioxide (CO2) flooding in continuous or water-alteringgas (WAG) mode has been used commercially for enhanced oil recovery (EOR) for over 40 years.1 CO2 is miscible with crude oil at pressures higher than the minimum miscibility pressure (MMP) and displaces oil efficiently at the pore scale. Despite its commercial success, the recovery efficiency of miscible CO2 EOR floods is often in the range of 10%−15% of original oil in place (OOIP). The displacement efficiency is very high, but the sweep efficiency is not, because of the low density and low viscosity of CO2, as well as the heterogeneity of oil reservoirs.1−3 In particular, the low viscosity of CO2 (∼0.01−0.1 cP), relative to crude oils, which are typically in the range of tens to hundreds of centipoise (cP), easily leads to viscous fingering, and the consequent early breakthrough of CO 2 reduces sweep efficiency.4 The sweep efficiency can be improved by increasing the viscosity of CO2, using CO2-in-water (C/W) foams, instead of direct CO2 flooding or WAG injection.5 CO2 is typically in the supercritical state under reservoir conditions. The dispersion of supercritical CO2 bubbles in liquids, or C/W foams (macroemulsions), can reduce the mobility of CO2 in the porous reservoir and stabilize the displacement front of CO2 flooded zones against viscous fingering, thereby increasing the recovery efficiency of oil reservoirs.1,3 CO2 EOR is now being considered for an increasing number of onshore sandstone reservoirs where CO2 supply is abundant. However, many reservoirs contain high-temperature and highsalinity fluids, and they often have high concentrations of divalent salts. For CO2 foam flooding to be successful, the surfactant that stabilizes C/W foams must maintain good thermal and chemical stability, as well as good foamability under the harsh reservoir conditions.6,7 Also, another major challenge is that the © XXXX American Chemical Society

adsorption/retention of surfactants onto mineral surfaces should be minimized to avoid formation damage and surfactant loss.7 Anionic or zwitterionic surfactants are typically used in sandstone reservoirs to minimize the adsorption to negatively charged mineral surfaces by electrostatic repulsion. The presence of high concentrations of salts, especially the divalent salts that significantly contributes to the total ionic strength of the brine solution, inevitably leads to screening of electrostatic interactions between surfactant head groups and the mineral surfaces, and therefore poses significant challenge to the adsorption loss of surfactant. Also, the divalent cations can bridge surfactant head groups and surfactant-mineral surfaces, which raise additional challenges to the stability and adsorption of surfactants.8 Generally speaking, relatively few studies have examined the formation of C/W foams at high temperatures of >80 °C, given the limitations of surfactant solubility and chemical stability in high-salinity high-temperature brine. Nonionic surfactants such as alcohol ethoxylates or ethoxylated polymeric surfactants are widely studied as good C/W foaming agents at low to moderate temperatures,6,9,10 but they inevitably suffer from precipitation at high temperatures, where the hydrogen bonding between EO groups and water are weakened and eventually leads to a loss of solvation.6,11,12 While ionic surfactants have a tendency to have good solubility at elevated temperatures,13 quaternary ammonium salts are often subjected to thermodegradation at elevated temperatures,14,15 and various anionic surfactants that contain sulfates may readily undergo hydrolysis at high temperature and acidic conditions of pH 1, the surfactant is more CO2-philic and has a tendency to forms a W/C emulsion. The interfacial tension goes through a minimum as HCB varies when the surfactant equally favors the CO2 and aqueous phase. CO2 is a far weaker solvent than water, because of the lack of permanent polar moment; thus, the surfactant is always more hydrophilic and has a tendency to stabilize C/W foams. Upon increasing the temperature, the surfactant head groups are more strongly solvated by brine45,46 and the tails are less solvated by CO2, because of the decrease in the density of CO2, as discussed earlier, leading to a higher HCB of the surfactant at the C/W interface, and thus an increase of IFT. The effect of methane dilution on C/W IFT is shown in Figure 5. For AMPHOAM surfactant, even with 20% methane in the CO2 phase, the IFT was still reduced significantly, to 8 mN/m, indicating good solvation of the surfactant tail in CO2−methane mixtures. Similarly, with the LDMAA surfactant, the IFT was

Figure 4. Interfacial tension isotherm between CO2 and 14.6% TDS brine solution with the presence of 0.5% w/v AMPHOAM surfactant at temperatures of 60, 90, and 120 °C.

decreases with the addition of the surfactant and plateaus above the critical micelle concentration (CMC). As the temperature increases, IFT increases. Interfacial tensions between CO2 and water are rarely reported at temperatures above 100 °C, because of the limitations of solubility and chemical stability of most common surfactants.6,34 The IFT between CO2 and 14.6% TDS brine without surfactants is ∼29 mN/m at 60 °C and increases to 37 mN/m at 120 °C, in agreement with previous reports.42 The interfacial tension with the addition of AMPHOAM above the CMC increases as the temperature increases at a constant

Figure 5. Interfacial tension between CO2−methane and brine containing 0.5% w/v (a) AMPHOAM and (b) LDMAA surfactant with methane mole fractions up to 20% at 120 °C. F

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much smaller than the 762 μm diameter of the capillary tube; therefore, the foam flows as a bulk foam in the capillary tube viscometer.49 The apparent viscosities of foams in the capillary tube are shown in Table 3. Clearly, the viscosities of pure CO2 foam are very similar for both surfactants. As the methane molar fraction increased to 20%, a significant decrease of the foam viscosity was observed, correlating with an increase of the Sauter mean bubble size. Generally speaking, foam viscosity is dependent on the foam bubble size, the interfacial tension, and the continuous phase viscosity. The viscosity of bulk foams or concentrated emulsions was given by Princen et al.50 as

reduced to 10 mN/m. It was observed that, for both surfactants, the IFT increases upon increasing molar fractions of methane in CO2. As CO2 was diluted by methane, the solvation of the surfactant tail further decreases, because of the even-lower cohesive energy density of methane, relative to CO2. As a result, the HCB of surfactant is further shifted to higher values, leading to an increase of IFT. It is also instructive to examine the CO2−brine partition coefficient of the surfactant. The partition coefficient of 0.25% w/ w AMPHOAM between CO2 and the 14.6% TDS brine at room temperature and 5000 psia were measured. A low partition coefficient below 0.002 (weight ratio of surfactant in CO2 over water) was found, in agreement with the high hydrophilicity of the ionic head groups, as indicated by the thermal stability tests in Table 2. The ionic headgroup in water is strongly solvated via the ion−dipole interactions, and the solvation of the hydrocarbon tail in CO2 is relatively weak, favoring a low partition coefficient. In parallel, the HCB value is low and favors curving of the C/W interface around the CO2 phase to form CO2-in-water foam. 3.6. Morphology and Viscosity of Foam in the Bulk State. C/W foam was generated at a superficial velocity of 208 ft/day with 0.5% surfactant in 14.6% TDS brine at room temperature and 5000 psia. Here, the porous glass bead pack was used as the foam generator. The micrographs of the as-generated foams in the high-pressure microscopy cell are shown in Figure 6.

μfoam =

As can be seen, the foam bubbles appear to be spherical, even at a quality of ΦC = 0.9, because of the relatively large polydispersity, as shown in Table 3.47,48 For the case of pure CO2 foams, the Sauter mean diameter (Dsm) values of the CO2 bubbles for both surfactants are very similar (Dsm ≈ 40−50 μm). When CO2 was diluted with 20% methane, an increase of the bubble size to ∼70−80 μm was observed. For all cases, the bubble size was

W* =

Table 3. Viscosity (μfoam) and Sauter Mean Bubble Size (Dsm) of C/W Foams

AMPHOAM LDMAA

μfoam (cP)

Dsm (μm)

Upoly

μfoam (cP)

43 41

0.32 0.29

36 33

81 70

0.35 0.33

20 18

(5)

πh f 2 σ h 2 γ

(6)

where hf is the thickness of the film, σh the interfacial tension of the curved border of the hole, and γ the interfacial tension of a planar interface. Clearly, thinner lamella are more prone to breakage. 3.7. Effect of Temperature on Foam Viscosity in Porous Media. The foam flow in porous media is quite different from foam flow in the bulk state, where the bubble size is orders of magnitude smaller than the capillary radius. In the porous glass

80% CO2 + 20% CH4

Upoly

⎛ h2 ⎞ dh f = ⎜⎜ f 2 ⎟⎟ΔPfilm dt ⎝ 3μe R f ⎠

where ΔPfilm is the difference of the capillary pressure (Pc), which drives the thinning of film due to curvature difference between the plateau border and thin film region, and the disjoining pressure (∏d), which stabilizes the lamella, because of the repulsive forces between head groups of surfactants adsorbed at the interface (ΔPfilm = 2(Pc − ∏d); μe is the viscosity of the thin film continuous phase; and Rf and hf are, respectively, the radius and thickness of the thin film.54 Clearly, the higher capillary pressure due to the higher IFT for the methane diluted CO2 foams could lead to quick drainage of the lamella, and eventually a thinner lamella is formed, which is more susceptible to rupture due to capillary wave instabilities and the nucleation of holes in the film. The thermal activation energy for coalescence can be expressed as55

Figure 6. Micrographs of ΦC = 0.9 C/W foams with either 100% CO2 or 80% CO2 and 20% CH4 as the internal phase. Foams are stabilized with 0.5% AMPHOAM or 0.5% LDMAA surfactant in 14.6% TDS brine at 5000 psia and room temperature. A scale bar of 100 μm is located inside each of the micrographs.

Dsm (μm)

(4)

where τ0 is the yield stress, γ̇ the shear rate, ϕi the internal phase volume fraction (referenced as the foam quality for C/W foams), μe the viscosity of the continuous phase, R the bubble size, and γ the interfacial tension. The similar viscosities obtained for both surfactants in the capillary tube agree with the similar bubble size (see Figure 6), interfacial tension (Figure 4), and continuous phase viscosity. The significant decrease of the foam viscosity upon methane dilution of CO2 can be attributed to the larger bubble size (Figure 6). The bubble size is controlled by the lamella creation and lamella destruction.51,52 The increased interfacial tension upon methane dilution of CO2 increases the energy penalty to create new bubbles. The newly generated foam lamella can be further destabilized by drainage of the continuous phase, followed by coalescence.53 Here, the increased interfacial tension contributes to higher capillary pressure and therefore higher lamella drainage rate of foam lamella. The drainage rate (V), when assuming noslip boundary conditions, can be described by V=−

100% CO2

−1/2 ⎛ μ γR ̇ ⎞ τ0 + 32(ϕi − 0.73)μe ⎜ e ⎟ γ̇ ⎝ γ ⎠

G

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Figure 7. Effect of temperature on the apparent viscosity of C/W foams stabilized with 0.5% w/v (a) AMPHOAM and (b) LDMAA surfactant in 14.6% TDS brine in a 23 Darcy glass bead pack at ΦC = 0.9 foam quality, 5000 psia, and two different superficial velocities (208 and 833 ft/day).

bead pack, the pore throat size is ∼26 μm and is comparable to or smaller than the CO2 bubble size. Therefore, the foam adopts the “train of bubbles” configurations, where single aqueous lamellae separate adjacent CO2 bubbles.49 The foam behavior in porous media was studied by co-injecting CO2 and 0.5% w/v surfactant in high-salinity brine of 14.6% TDS. The foam quality (CO2 volume fraction) was controlled to be 0.90. Injection was conducted at total superficial velocities of 208 and 833 ft/day. The permeability of the glass bead pack (180 μm diameter beads) was 23 Darcy. All of the experiments were conducted at a constant pressure of 5000 psia. As shown in Figure 7, even in the presence of high-salinity 14.6% TDS brine, strong C/W foam with an apparent viscosity of 11 cP was formed at 120 °C and superficial velocity of 208 ft/day with 0.5% AMPHOAM surfactant. At lower temperatures, the apparent viscosity increases, for example, to 21 cP at 60 °C and 38 cP at room temperature. Similar trends were also observed at the higher superficial velocity (833 ft/day). Clearly, the surfactant efficiently adsorbs to the C/W interface to reduce the interfacial tension and stabilize the lamella, despite the harsh salinity and temperature conditions, because of the aforementioned good solubility in both 14.6% TDS high-salinity brine and CO2. Similarly, the LDMAA surfactant also generates strong foams, even up to a high temperature of 120 °C, as shown in Figure 6b. The decrease of the foam apparent viscosity with the temperature increase for both surfactants can be mainly ascribed to the reduced continuous phase viscosity and lamella density at higher temperatures. As described in Hirasaki and Lawson’s smooth capillary tube model,49 the apparent viscosity of foams (μapp) in small capillary tubes where the foam adopts a “train-ofbubbles” configuration is the sum of three different mechanisms, as listed sequentially on the right-hand side (RHS) of eq 7: (a) viscous dissipation due to the liquid slugs between bubbles, (b) dissipation due to the deformation of bubble interfaces when passing through capillary tubes, and (c) interfacial flow induced by the surface tension gradient due to the surfactants being depleted from the front of the bubble and accumulates at the back as the bubble moves forward. That is,

where Ls is the length of the liquid lamellae, nL the number density of lamellae, rc the radius of curvature of the liquid/gas interface, Rc the capillary tube radius, U the velocity of the bubble, Ns a nondimensional number for interfacial tension gradient, and NL the nondimensional bubble length. Clearly, at the same superficial velocity and foam quality (CO2 volume fraction), the apparent viscosity of foam in a certain porous medium is dependent on the continuous phase viscosity and the lamella density. The viscosity of the continuous phase decreases at elevated temperatures, thus leading to decreased foam apparent viscosity. Furthermore, lamella can be destabilized by drainage (thinning of the aqueous film), followed by rupture of the thin film when the lamella is drained to a critical thickness, as discussed earlier.54,56 The increase of temperature at constant pressure leads to a lower density of CO2, and, consequently, weaker solvation of the surfactant tail and less CO2-philic surfactant. The consequent increase of IFT hinders the generation of lamella and increases the capillary pressure that drives thinning and coalescence of lamella, thereby leading to foams with coarse texture (i.e., lower lamellae density) and lower apparent viscosity. The lower viscosity of the continuous phase at elevated temperatures also contributes to higher drainage rates, which eventually leads to lamella rupture and decrease of the lamella density. 3.8. Effect of Superficial Velocity on Foam Viscosity in a Porous Medium. The effect of total superficial velocity on the apparent viscosity of foams stabilized with 0.5% w/v AMPHOAM in 14.6% TDS brine in a 23 Darcy glass bead pack at a foam quality of Φ = 0.90 is demonstrated in Figure 8. The apparent viscosity of C/W foams shows reduced viscosity at higher superficial velocities at different temperatures, namely, shear-thinning behavior was observed. For example, at 90 °C, the apparent viscosity of C/W foam decreased from 16.9 cP at 208 ft/day to 8.8 cP at 1250 ft/day. For field application of CO2 foam enhanced oil recovery, such shear-thinning behavior is favored. The foams will have low viscosity at the near-well-bore region, where flow rates are high. The low viscosity at injection allows good injectivity. When foams flow far into the formation, where the flow rates decrease, the viscosity will increase, allowing efficient mobility control. The shear-thinning behavior of foams flow in porous media at both temperatures of 60 and 90 °C can be explained by the model of Hirasaki and Lawson,37 as demonstrated in eq 7, where both the second and third terms on the RHS of the equation show the dependence of the foam apparent viscosity on the velocity of the gas phase. When a single bubble moves into a capillary tube at a

⎤ ⎛ n R ⎞⎛ 3μ U ⎞−1/3⎡⎛ rc ⎞2 ⎢⎜ ⎟ + 1⎥ = LsnL + 0.85⎜ L c ⎟⎜ e ⎟ ⎥⎦ ⎢⎣⎝ R c ⎠ μe ⎝ rc/R c ⎠⎝ γ ⎠

μapp

⎛ 1 − e−NL ⎞ ⎛ 3μ U ⎞−1/3 + (nLR c)⎜ e ⎟ Ns ⎜ ⎟ ⎝ γ ⎠ ⎝ 1 + e−NL ⎠

(7) H

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The increase of the apparent viscosity from a foam quality of ΦC = 0.6 to a foam quality of ΦC = 0.9 can be ascribed to the increase of the population of CO2 bubbles, i.e., higher lamellae density as the bubble size in the porous medium can be assumed to be constant. The increase of lamella density and also the reduction of the thickness of the thin water film between CO2 bubbles and capillary tube walls will increase the flow resistance of bubbles and, hence, the apparent viscosity of foams in porous media.49 The drastic reduction in the apparent viscosity of foam quality from ΦC = 0.9 to ΦC = 0.95 can be ascribed to the reduction of the population of CO2 bubbles, because of the limiting capillary pressure effect in porous media.57 As the foam quality increases, the wetting phase (water) saturation decreases, which leads to an increase of the capillary pressure between the wetting phase and the nonwetting phase. Eventually, the capillary pressure reaches the limiting capillary pressure, whereby the disjoining pressure exerted by the surfactants is insufficient to overcome the capillary pressure; thus, the bubble coalesces and the foam texture coarsens. Such a coarsening of foam texture leads to the drastic reduction of the apparent viscosity of the foam beyond a foam quality of ΦC = 0.9 and eventually leads to the transition from a strong foam to a weak foam, hence, the low viscosity at the extreme foam quality of ΦC = 0.95.58 Similarly, as shown in Figure 9b, the apparent viscosity of foam stabilized with LDMAA surfactant also increases as the foam quality increases to ΦC = 0.9 and then decreases as the foam quality further increases to ΦC = 0.95, because of the capillary limiting pressure effect. 3.10. Effect of Methane Dilution on Foam Viscosity in a Porous Medium. The CO2 was diluted with methane (at mole fractions up to 20%) and the formation of foam was tested. Figure 10 shows the apparent viscosity of CO2−methane/W foams in a porous glass bead pack at a temperature of 120 °C and a superficial velocity of 208 ft/day. As seen in Figure 10a, the foam viscosity decreases as the methane molar fraction increases. For example, the viscosity decreased from 11 cP to 5.2 cP as the methane molar fraction increased to 20% for foams stabilized with 0.5% AMPHOAM. Similarly, the viscosity of the foam decreased to 3.5 cP for the case of LDMAA surfactant with 20% methane. Dilution of the methane decreased the solvation of the surfactant tail, and the surfactant became less CO2-philic. The consequent increase in IFT, as shown in Figure 4, increases the energy penalty for generation of interfaces, and also increases the capillary pressure that drives the thinning of foam lamella and eventually leads to lamella rupture. Therefore, the lamella density is expected to be decreased by methane dilution of CO2, and thus the lower foam viscosity in the glass bead pack. The effect of temperature on the apparent viscosity of foam with 20% methane

Figure 8. Effect of total superficial velocity on the apparent viscosity of C/W foams stabilized with 0.5% w/v AMPHOAM surfactant in 14.6% TDS brine in a 23 Darcy glass bead pack at a foam quality of ΦC = 0.9, 5000 psia, and two different temperatures of 60 and 90 °C.

higher velocity, the thickness of the water thin film between the CO2 bubble and the capillary tube wall increases and gives greater bubble deformation. As a result, the curvature of the bubble and the pressure drop across the bubble increase. The pressure drop becomes proportional to the 2/3 power of the velocity of the gas phase and, consequently, gives a −1/3 power dependence of the apparent viscosity of foams on the velocity of the gas phase, as shown in the second term on the RHS of eq 2. In addition, the surfactants at the C/W interface are dragged toward the rear part of the bubble when bubbles flow forward and give rise to an interfacial tension gradient. Such a Gibbs−Marangoni effect retards the flow of bubbles and also contributes to the −1/3 power dependence of C/W foam apparent viscosity on the velocity of the gas phase, as shown in the third term on the RHS of eq 7. 3.9. Effect of Quality on Foam Viscosity in a Porous Medium. The effect of foam quality on the apparent viscosity of foams stabilized with 0.5% AMPHOAM surfactant in 14.6% TDS brine in a 23 Darcy glass bead pack at total superficial velocities of 208 and 833 ft/day at 90 °C, and 5000 psia, is shown in Figure 9a. At a superficial velocity of 833 ft/day, as the foam quality increased from ΦC = 0.6 to ΦC = 0.9, the foam apparent viscosity gradually increases and eventually reaches a maximum of 16.9 cP at a foam quality of ΦC = 0.9. Further increases in foam quality from ΦC = 0.9 to ΦC = 0.95 decreased the foam apparent viscosity dramatically, and eventually no foam was formed at the extreme quality of ΦC = 0.95, as indicated by the very low apparent viscosity. A similar maximum viscosity at a foam quality of ΦC = 0.9 was observed at a superficial velocity of 208 ft/day.

Figure 9. Effect of foam quality on the apparent viscosity of C/W foams stabilized with 0.5% w/v (a) AMPHOAM and (b) LDMAA surfactant in 14.6% TDS brine in a 23 Darcy glass bead pack at 5000 psia and superficial velocities of 833 and 208 ft/day, respectively. I

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Figure 10. (a) Effect of methane molar fraction on the apparent viscosity of C/W foams at 120 °C. (b) Effect of temperature on the apparent viscosity of C/W foams where CO2 phase was diluted with a methane molar fraction of 20%. Foams are stabilized with 0.5% w/v AMPHOAM or LDMAA surfactant in 14.6% TDS brine in a 23 Darcy glass bead pack at 5000 psia and a superficial velocity of 208 ft/day.

dilution of CO2, which could potentially allow C/W foam flooding using the produced gas and reduce the cost of gas.

is shown in Figure 10b. Similar to the C/W foam without methane dilution, the apparent viscosity decreases as the temperature increases.

■

ASSOCIATED CONTENT

* Supporting Information

4. CONCLUSIONS In this study, an anionic sulfonate surfactant (AMPHOAM) and an amphoteric surfactant ((lauryldimethylammonio)acetate, LDMAA) were shown to stabilize viscous C/W foams at a high salinity of 14.6% TDS brine at elevated temperatures up to 120 °C. The effect of temperature and methane dilution of CO2 on foam viscosity was evaluated systematically in both bulk and porous media. The following conclusions can be drawn from this work: • Very few surfactants are aqueous stable in high-salinity brines at high temperatures and soluble in CO2. AMPHOAM and LDMAA are two of these surfactants. CO2 solubility tests show that the surfactants are CO2-soluble, because of the strong solvation of the surfactant tails by the CO2 phase. • Consequently, the IFT between CO2 and aqueous surfactant solutions was reduced to very low values (