An Experimental Study of Wetting Behavior and Surfactant EOR in Carbonates With Model Compounds
Authors: Wu Yongfu , Shuler Patrick J. , Blanco Mario , Tang Yongchun , Goddard William A. 
Published: March 2008

Summary
This study focuses on the mechanisms responsible for enhanced oil recovery (EOR) from fractured carbonate reservoirs by surfactant solutions, and methods to screen for effective chemical formulations quickly. One key to this EOR process is the surfactant solution reversing the wettability of the carbonate surfaces from less water-wet to more water-wet conditions. This effect allows the aqueous phase to imbibe into the matrix spontaneously and expel oil bypassed by a waterflood.
This study used different naphthenic acids (NA) dissolved in decane as a model oil to render calcite surfaces less water-wet. Because pure compounds are used, trends in wetting behavior can be related to NA molecular structure as measured by solid adsorption; contact angle; and a novel, simple flotation test with calcite powder. Experiments with different surfactants and NA-treated calcite powder provide information about mechanisms responsible for sought-after reversal to a more water-wet state. Results indicate this flotation test is a useful rapid screening tool to identify better EOR surfactants for carbonates.
The study considers the application of surfactants for EOR from carbonate reservoirs. This technology provides a new opportunity for EOR, especially for fractured carbonate, where waterflood response typically is poor and the matrix is a high oil-saturation target.

Introduction
Typically only approximately a third of the original oil in place (OOIP) is recovered by primary and secondary recovery processes, leaving two-thirds trapped in reservoirs as residual oil. Approximately half of world's discovered oil reserves are in carbonate reservoirs and many of these reservoirs are naturally fractured (Roehl and Choquette 1985). According to a recent review of 100 fractured reservoirs (Allan and Sun 2003), carbonate fractured reservoirs with high matrix porosity and low matrix permeability especially could use EOR processes. The oil recovery from these reservoirs is typically very low by conventional waterflooding, due in part to fractured carbonate reservoirs (about 80%) being originally less water-wet. Injected water will not penetrate easily into a less water-wetting porous matrix and so cannot displace that oil in place.
Wettability of carbonate reservoirs has been widely recognized an important parameter in oil recovery by flooding technology (Tong et al. 2002; Morrow and Mason 2001; Zhou et al. 2000; Hirasaki and Zhang 2004). Because altering the wettability of a rock surface to preferentially more water-wet conditions is critical to oil recovery, alteration of reservoir wettability by surfactants has been intensively studied, and many research papers have been published (Spinler and Baldwin 2000). Vijapurapu and Rao (2004) studied the capability of certain ethoxy alcohol surfactants to alter wettability of the Yates reservoir rock to water-wet conditions. Seethepali et al. (2004) reported that several anionic surfactants in the presence of Na2CO3 can change a calcite surface wetted by a West Texas crude oil to intermediate/water-wet conditions as well as, or even better than, an efficient cationic surfactant. Zhang et al. (2004) investigated also the effect of electrolyte concentration, surfactant concentration, and water/oil ratio on wettability alteration. They reported that wettability of calcite surface can be altered to approximately intermediate water-wet to preferentially water-wet conditions with alkaline/anionic surfactant systems. Adsorption of anionic surfactants on a dolomite surface can be significantly reduced in the presence of sodium carbonate.