Abstract

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, water flooding 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 EOR techniques like surfactant flooding1–5 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.6–9 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 200310–12, 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 50–90% of oil. However, the cost involved is still high due to 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.13 CT scans revealed that surfactant imbibition was due to counter current flow in the beginning and due to 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 Zhang14 have studied different ethoxy and propoxy sulfates to achieve very low interfacial tension and alter wettability from oil-wet to intermediate-wet. The presence of Na2CO3 reduces the adsorption of anionic surfactant by lowering the zeta potential of calcite surfaces, 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 to make 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 interface15 and adsorb onto the mineral surface thus rendering the surface oil-wet.

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