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«Paper 4 Comparing the static and dynamic foam properties of a fluorinated and an alpha olefin sulfonate surfactant Comparing the static and dynamic ...»

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Paper 4

Comparing the static and dynamic foam properties of

a fluorinated and an alpha olefin sulfonate surfactant

Comparing the static and dynamic foam properties of

a fluorinated and an alpha olefin sulfonate surfactant

Anne Kari Vikingstad and Morten Gunnar Aarra

Centre for Integrated Petroleum Research, CIPR, University of Bergen, Norway

Morten Gunnar Aarra Anne Kari Vikingstad

Centre for Integrated Petroleum Research Centre for Integrated Petroleum Research Realfagbygget, Allègaten 41 Realfagbygget, Allègaten 41 N-5007 Bergen N-5007 Bergen Norway Norway E-mail address: Morten.Aarra@cipr.uib.no E-mail address: Anne.Vikingstad@cipr.uib.no Telephone: +47 55583563 Telephone: +47 55583668 Fax: +47 55588265 Fax: +47 55588265 Abstract To improve the understanding of the influence of oil type and of oil saturation on foam stability and foam generation, core flooding experiments and static foam tests have been conducted using three different North Sea oils. The experiments were performed using two surfactants - an alpha olefin sulfonate (AOS) and a fluorinated surfactant (FS-500) - both with and without oil.

Foam was generated by both surfactants in all core flooding experiments both with and without oil.

The experiments showed that the surfactants generated foam with similar strengths and stability in both dynamic foam experiments and static foam tests without oil. In the core flooding experiments with residual oil saturation, the AOS surfactant generated a weaker foam than that of the FS-500 surfactant, although foam propagated more rapidly with the AOS.

The static experiments showed that foam generation with FS-500 seemed to be independent of the presence of oil. Furthermore, the FS-500 foam generation in core flood experiments seemed independent of the presence of residual oil saturation with respect to foam strength. Foam propagation was significantly delayed, however, in the presence of oil.

For the AOS surfactant, the foam generated using 2 of 3 North Sea oils was destabilized in static foam tests. AOS generated foam with differing foam strengths for the different crude oils in the core flooding experiments. The correlation between the static and dynamic foam experiments was poor for the AOS surfactant. AOS showed more rapid foam propagation with, than without oil.

Keywords: Foam, foam-oil interactions, core flooding experiments, alpha olefin sulfonate, fluorinated surfactant Introduction Gas is the discontinuous phase in a foam system, separated by thin liquid films (Schramm, 1994, Holm, 1968). Foam formation in a porous media reduces gas mobility. In Enhanced Oil Recovery (EOR) foam has been used both to improve gas sweep efficiency and to shut off gas production in production wells. Foam can improve results in situations of, for example, poor area sweep, gas channeling and gravity override (Rossen, 1996, Schramm, 2005). Several field applications are discussed in: Hanssen et al., 1994, Aarra et al., 1996 and 2002, Blaker et al., 2002, and Skauge et al., 2002. In many of these North Sea field tests an alpha olefin sulfonate surfactant was used to generate foam.

A great number of core flooding experiments have been conducted to evaluate the properties of foam generation in the presence of oil (Nikolov et al., 1986, Jensen and Friedmann, 1987, Chou, 1991, Dalland et al., 1992, Kristiansen and Holt, 1992, Aarra et al., 1994 and 1997, Holt et al., 1996, Vassenden et al., 1998, Mannhardt and Svorstøl, 1999, Mannhardt et al., 2000). Usually, foam is used to reduce gas mobility in zones already flooded by gas. Thus, it is important to perform the core flooding experiments at residual oil saturation (Aarra et al., 1997 and 2002, Mannhardt and Svorstøl, 1999, Mannhardt et al., 2000).

From the literature, most data suggest that oil may limit the efficiency of foams in reducing gas mobility. Some define a critical oil saturation above which foam does not form (see discussion by Schramm, 1994), but several studies show that it is possible to generate strong foams at relatively high oil saturations (Aarra et al., 1997 and 2002, Mannhardt and Svorstøl, 1999, Mannhardt et al., 2000). Another suggests that a high concentration of light hydrocarbons in the oil appears to be the main reason for reduced foam stability (Kuhlman, 1990). In our earlier work (Vikingstad et al., 2005), we include the results for alkanes in static bulk foam tests. Lower molecular weight alkanes provided a less favorable environment for foam than alkanes with a higher molecular weight, as indicated by others (Suffridge et al., 1989, Meling and Hanssen, 1990).

Oil in the core during foam experiments has been reported to reduce the propagation velocity of foam through the core (Jensen and Friedmann, 1987, Aarra et al., 1997, Vassenden et al., 1998, Mannhardt and Svorstøl, 1999). Chou (1991) reports that foam propagation without oil depends on the initial condition of the core material.

Pre-saturating the core with surfactant solution prior to foam generation seemed beneficial for both foam generation and foam propagation.

To characterize the strength of the generated foam, the mobility reduction factor (MRF) is often defined (Schramm, 1994, Mannhardt et al., 2000):

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∆Pfoam and ∆Pno-foam are the measured differential pressure across the porous medium with and without foam respectively. A high MRF corresponds to a strong foam.

Other methods used to describe foam strength in porous media include reporting the differential pressure of the full core and in parts of the core (Chou, 1991, Mannhardt et al., 1996, Svorstøl et al., 1996, Aarra et al., 1997, Mannhardt and Svorstøl, 1999 and 2001, Siddiqui et al., 2002), or to observe the time needed for foam to propagate throughout the core.

In this paper we have examined foam generation capability for an alpha olefin sulfonate and a fluorinated surfactant in cores with residual crude oil saturation. The results from core experiments are compared to results from static foam tests (Vikingstad et al., 2005 and 2006, Aarra et al., 2006). The spreading coefficient (S), entering coefficient (E), lamella number (L), and bridging coefficient (B) have been calculated using the measured surface and interfacial tension values for the oil and surfactant solution. The aim has been to try to find correlations that can elucidate and improve the understanding of foam generation and foam stability in porous media.

Dynamic core experiments were conducted at high pressure and temperature. Three different North Sea crude oils were used in separate experiments, as well as trials without oil, for each surfactant.

Dalland et al. (1992) and Mannhardt et al. (2000) found that fluorinated surfactants formed foams that were very stable in the presence of oil. Mannhardt et al. (2000) added a fluorinated surfactant to different types of surfactants. The addition of a fluorinated surfactant enhanced the oil tolerance of some, but not all, foams. Further, Dalland et al. (1992) categorized four of eight fluorinated surfactants as creating oiltolerant foams. In their experiments, they observed foams with properties ranging from effective gas blocking foams to oil sensitive foams. Chukwueke et al. (1998) studied the AOS surfactant and two fluorinated surfactants for foam generation for use in gas shut-off. Core flooding experiments showed that gas-blocking performance under reservoir conditions was poor for one of the fluorinated surfactants. AOS combined with a polymer showed good gas blocking, as also reported by Aarra et al. (1997).

Methods and Materials The core material used in the experiments was outcrop Berea sandstone. Each core was one piece, about 30 cm in length, and around 3,5 cm in diameter. The permeability of the cores varied between 260 mD and 310 mD. The porosity was approximately 20%, pore volume ~ 65 ml.

Two different surfactants were used: an anionic C14-C16 alpha-olefin sulfonate surfactant, AOS, with a molecular weight of 324 g/mole; and a fluorinated surfactant.

The fluorinated surfactant was a Perfluoroalkyl betaine, FS-500, supplied by DuPont.

The surfactant was zwitterionic, and, according to the vendor, the molecular weight was comparable to the AOS surfactant. The surfactant concentration was 0,5wt% in all experiments. First reference experiments without oil were conducted for the two surfactants. Experiments with three different crude oils, oil 1-3, from the North Sea for both of the surfactants followed. Two parallel experiments were done with oil 1 using FS-500. The Gas Oil Ratio (GOR), viscosity and density for the three crude oils are presented in Table 1. The oil formation volume factor, Bo, was close to 1 for each of the oils.

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The experiments were conducted at 50°C (45°C for oil 2 using AOS) and an outlet pressure of 120 bar on a horizontally oriented core. In the core flood experiments the foam quality was 80%; that is, the gas volume fraction was 0,8 and constant at the inlet throughout each experiment. The N2-gas and the surfactant solution were injected simultaneously at a total flow rate of 40 ml/h; injection rates were controlled directly by two high pressure Quizix pumps. A visual cell was placed at the outlet of the core to observe the texture of the foam and to try to find the approximate time for foam propagation through the core. The pressure was measured at the inlet, the outlet and at the pressure tab located 17,8 cm from the inlet, that is, about 3/5 of the core length from the inlet (Figure 1). This allowed comparing pressure development through the core during foam generation and evaluation of foam propagation.

Figure 1: Illustration of pressure measurements configuration on the core.

The water-filled core was drained by Marcol (~11 cP) to irreducible water saturation.

The Marcol was then exchanged by one of the nitrogen-saturated North Sea oils. A gravity stable water flooding was conducted to bring the cores to residual oil saturation after water flooding (Sorw). Prior to foam generation, two pore volumes of

surfactant were injected. The synthetic seawater had the following composition:

2,489wt% NaCl, 0,173wt% CaCl2, 1,112wt% MgCl2, 0,019wt% NaHCO3, 0,406wt% Na2SO4, and 0,067wt% KCl. The procedure for static foam experiments is described in Vikingstad et al., 2005 and 2006, and Aarra et al., 2006.

Results and Discussion We discuss here the results of foam core flooding experiments and static experiments. Results are compared against our earlier static bulk foam experiments (Vikingstad et al., 2005 and 2006, Aarra et al., 2006). The role of similarity or lack thereof between static and dynamic foam tests is a subject of ongoing debate in the literature. Mannhardt et al. (2000) reports a large number of experiments and finds it difficult to correlate foam performance in core floods with bulk foam stability, etchedglass micro model observations or interfacial parameters. The same result is reported by Dalland et al. (1992).

Overview of static foam experiments One of the main findings of the static bulk experiments was that, in the presence of oil, the fluorinated surfactant (FS-500) generated more stable foam over time than the AOS. FS-500 seemed nearly unaffected by oil as foam tests with and without oil showed equal stability for this surfactant (Vikingstad et al., 2005 and 2006, Aarra et al., 2006). The FS-500 generated 16-18 cm foam for all the foam tests.

For the AOS surfactant, results for the three North Sea oils used in this study are shown in Figure 2. Two of three oils destabilized foam. Foam stability was good for several other of the crude oils investigated (Vikingstad et al., 2005 and 2006, Aarra et al., 2006). Further, foam tests with decane and alkanes with lower molecular weights destabilized the foam. The ionic strength of the brine also influenced the stability of AOS foams in the presence of oil.

Foam height (cm)

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In general the spreading coefficient (S), entering coefficient (E), and lamella number (L) indicated stable foam for the FS-500 (Table 2). The bridging coefficients (B) were negative in most cases. This is consistent with the results of the static foam experiments. No such correlation between S, E, L, and B, and static foam stability was evident for the AOS surfactant (Table 3 and Figure 2).

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In these static foam tests the stability of the pseudo-emulsion film was not investigated but may be important for foam stability.

Another important results of these earlier studies was that foam generated below cmc for both surfactants, and that both reached a constant maximum foam height at 0,1-0,5wt% surfactant. In the presence of oil FS-500 generated stable foam at lower surfactant concentrations than the AOS surfactant.

Core flooding experiments In Figure 3 the differential pressure (dP) during foam generation in the core experiments without oil is shown for the two surfactants, with a surfactant concentration of 0,5wt%. Based on dP measurements, both surfactants generated strong foams without oil in the core. Without oil the AOS surfactant generated even stronger foam than the fluorinated surfactant (compared after 2,5 PV fluid injected).

Differential pressure (bar)

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Figure 3: Differential pressure (dP) as a function of pore volumes injected fluid for core flooding experiments without oil with AOS and FS-500 surfactants. The thicker line represents the full core dP: the thinner line represents dP over the last part of the core.

In the FS-500 experiment 15 PV of surfactant solution and N2-gas were injected.

Considering the position of the pressure tabs, the dP/cm in the last part of the core was about 1,5 times higher than in the first part of the core, indicating generation of even stronger foam. This is consistent with core experiments performed by Mannhardt et al., 2000, Mannhardt and Svorstøl, 1999 and 2001. They report a lower pressure drop in the first part of the core than over other sections both with (Mannhardt et al., 2000, Mannhardt and Svorstøl, 1999) and without oil (Mannhardt and Svorstøl, 2001). Foam propagation velocity was similar for the two surfactants in these experiments.

Core flooding experiments with residual oil saturation Three core flooding experiments in 30 cm Berea core material were conducted for both surfactants with North Sea crude oils (1, 2, 3) at residual oil saturation after water flooding, see Table 4.

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