Atomic Force Microscopy Study of Wettability Alteration by Surfactants

Summary Waterflooding recovers little oil from fractured carbonate reservoirs, if they are oil-wet or mixed-wet. Surfactant-aided gravity drainage has the potential to achieve significant oil recovery by wettability alteration and interfacial tension (IFT) reduction. The goal of this work is to investigate the mechanisms of wettability alteration by crude oil components and surfactants. Contact angles are measured on mineral plates treated with crude oils, crude oil components, and surfactants. Mineral surfaces are also studied by atomic force microscopy (AFM). Surfactant solution imbibition into parallel plates filled with a crude oil is investigated. Wettability of the plates is studied before and after imbibition. Results show that wettability is controlled by the adsorption of asphaltenes. Anionic surfactants can remove these adsorbed components from the mineral surface and induce preferential water wettability. Anionic surfactants studied can imbibe water into initially oil-wet parallel-plate assemblies faster than the cationic surfactant studied. Introduction Waterflooding is an effective method to improve oil recovery from reservoirs. For fractured reservoirs, waterflooding is effective only when water imbibes into the matrix spontaneously. If the matrix is oil-wet, the injected water displaces the oil only from the fractures. Water does not imbibe into the oil-wet matrix because of negative capillary pressure, resulting in very low oil recovery. Thus there is a need of tertiary or enhanced oil recovery techniques like surfactant flooding (Bragg et al. 1982; Kalpakci et al. 1990; Krumrine et al. 1982a; Krumrine et al. 1982b; Falls et al. 1992) to maximize production from such reservoirs. These techniques were developed in 1960s through 1980s for sandstone reservoirs, but were not widely applied because of low oil prices. Austad et al. (Austad and Milter 1997; Standnes and Austad 2000a; Standnes and Austad 2000b; Standnes and Austad 2003c) have recently demonstrated that surfactant flooding in chalk cores can change the wettability from oil-wet to water-wet conditions, thus leading to higher oil recovery (~70 % as compared to 5% when using pure brine). In 2003 (Standnes and Austad 2003a; Standnes and Austad 2003b; Strand et al. 2003), they identified cheap commercial cationic surfactants, C10NH2 and bioderivatives from the coconut palm termed Arquad and Dodigen (priced at US$ 3 per kg). These surfactants could recover 50 to 90% of oil in laboratory experiments. However, the cost involved is still high because of the required high concentration (~1 wt%) and thus there is a need to evaluate other surfactants. The advantage of using cationic surfactants for carbonates is that they have the same charge as the carbonate surfaces and thus have low adsorption. Nonionic surfactants and anionic surfactants have been tested by Chen et al. (2001) in both laboratory experiments and field pilots. Computed tomography scans revealed that surfactant imbibition was caused by countercurrent flow in the beginning and gravity-driven flow during the later stages. The basic idea behind these techniques is to alter wettability (from oil-wet to water-wet) and lower interfacial tension. Hirasaki and Zhang (2004) have studied different ethoxy and propoxy sulfates to achieve very low interfacial tension and alter wettability from oil-wet to intermediate-wet in laboratory experiments. The presence of Na2CO3 reduces the adsorption of anionic surfactant by lowering the zeta potential of calcite surfaces, and thus dilute anionic surfactant/alkali solution flooding seems to be very promising in recovering oil from oil-wet fractured carbonate reservoirs. It is very important to understand the mechanism of wettability alteration to design effective surfactant treatments and identify the components of oil responsible for making a surface oil-wet. It is postulated that oil is often produced in source rocks and then migrates into originally water-wet reservoirs. Some of the ionic/polar components of crude oil, mostly asphaltenes and resins, collect at the water/oil interface (Freer et al. 2003) and adsorb onto the mineral surface, thus rendering the surface oil-wet. In this work, we try to understand the nature of the adsorbed components by AFM. Recently, AFM has been used extensively to get the force-distance measurements between a tip and a surface. These force measurements can be used to calculate the surface energies using the Johnson-Kendall-Roberts (JKR), the Derjaguin-Muller-Toporov (DMT), and like theories (van der Vegte and Hadziioannou 1997; Schneider et al. 2003). AFM is also used extensively for imaging surfaces. It can be used in the contact mode for hard surfaces and in the tapping mode for soft surfaces. It can be used to image dry surfaces or wet surfaces; tapping mode in water is a relatively new technique. AFM images have been used to confirm the deposition of oil components on mineral surfaces (Buckley and Lord 2003; Toulhoat et al. 1994). In this work, crude-oil-treated mica surface is probed using atomic force microscopy before and after surfactant treatment to study the effects of surfactant. AFM measurements are correlated with contact-angle measurements. We also study surfactant solution imbibition into an initially oil-wet parallel plate assembly to relate wettability to oil recovery. Our experimental methodology is described in the next section, the results are discussed in the following section, and the conclusions are summarized in the last section.