Influence of Crude Oil Components on Recovery by High and Low Salinity Waterflooding Nanji J. Hadia,* ,† Tone Hansen, † Medad T. Tweheyo, ‡ and Ole Torsæter † † Department of Petroleum Engineering and Applied Geophysics, Norwegian University of Science and Technology (NTNU), 7491 Trondheim, Norway, and ‡ Statoil ASA, Norway ABSTRACT: This study presents an investigation on the effects of various polar components of a crude oil on its recovery by high and low salinity waterflooding and correspondingly on some of the suggested mechanisms in the literature. Coreflooding experiments were performed on several Berea core plugs aged in four different variants of the same crude oil with relative amounts of acids, bases, and asphaltenes. The oils labeled as acid-free, base-free, asphaltenes-free (deasphalted), and original or normal crude oil were expected to initiate varying wetting conditions during aging. The results of high salinity waterflood showed that the plug aged with base-free oil provided the highest whereas that with acid-free oil provided the lowest final oil recoveries. A reduction in residual oil saturation (S orw ) by 1.4% to 2.9% PV for normal, base-free, and asphaltene-free crude oils after low salinity waterflooding (LSW) in tertiary recovery mode was observed. For the case of acid-free crude oil, the S orw was reduced marginally. A 2-3-fold increase in differential pressure was observed during injection of low salinity brines. The effluent brine pH was also increased by 1 pH unit during LSW. The observations from the present work indicate that different oil components initiate varying wetting conditions and that the initial wetting conditions influence the performance of a tertiary low salinity flood. In particular, a low salinity flood seems favorable when the initial wetting conditions are not water-wet. ■ INTRODUCTION Laboratory investigations and a few field observations conducted over the past few years reveal that injecting brine with low salinity improves oil recovery significantly. Literature so far has shown mixed results for LSW as a tertiary recovery process. The actual incremental recoveries measured indicated large variations on different reservoir cores and outcrops. Bernard 1 first provided an evidence of improved oil recovery along with increased pressure by injecting fresh water as compared to conventional high salinity waterflooding. Recent laboratory studies 2-5 also reported improved oil recovery with decreased brine salinity. Few field trials also reported improved recovery by LSW. 6-8 However, recently reported laboratory and field pilot results in a North Sea field showed poor response to the injection of low salinity brine. 9 Based on experimental observations, researchers have proposed various phenomena/mechanisms to explain the cause of increased oil recovery by low salinity water injection. Tang and Morrow 10 proposed a mechanism of fines migration during LSW. They proposed that mixed-wet clay particles get detached from the pore walls during injection of low salinity brines. This results into release of associated oil droplets and increases the oil recovery along with production of fines. Lager et al. 11 proposed a mechanism of cation exchange between mineral surfaces and invading brine as the primary mechanism for improved oil recovery during LSW. They also concluded that pH induced interfacial tension (IFT) reduction 7 or emulsification and fines migration may not always occur in LSW. The theory of electrical double layer (EDL) expansion during LSW has also been argued by Berg et al. 12 and Ligthelm et al. 13 and supported by Lee et al. 14 Recently, Austad et al. 15 suggested a chemical mechanism related to a local increase in pH by LSW, while Skrettingland et al. 9 suggested that initial wetting conditions are important and attributed the poor response of LSW in the Snorre field to the near optimal wetting conditions for waterflooding. There is, however, no consensus on a particular dominant mechanism although it has been shown that injection of low saline water may result in a wettability alteration toward a more water-wet behavior. 13 It is also widely accepted that the presence of clay minerals and crude oil containing polar components in the reservoir are important to observe benefits of LSW. 15,16 However, the effect of the presence of various crude oil components such as acids, bases, and asphaltenes on incremental oil recovery by LSW is not yet well understood. Recent work by Sandengen et al. 17 has shown core flooding results whereby the injection of low saline water was interpreted as yielding more oil-wetting conditions. They also reviewed the ion exchange mechanisms suggested by Lager et al. 11 to explain that injection of low salinity brines can alter wettability in both directions: either more oil-wetting or more water-wetting, depending on the oil and rock properties. The adsorption of polar components can alter the initial water-wetness. 18-23 The wettability of reservoir rocks plays a vital role in deciding the performance of waterflooding and other EOR processes. It has been reported that the presence of small amount of asphaltenes in the crude oil can change the wettability of originally water-wet surfaces. 24,25 However, Skauge et al. 26 showed that the acidic and basic components also play a major role for wettability alteration. Researchers Received: February 22, 2012 Revised: June 1, 2012 Published: June 4, 2012 Article pubs.acs.org/EF © 2012 American Chemical Society 4328 dx.doi.org/10.1021/ef3003119 | Energy Fuels 2012, 26, 4328-4335