A Study of Wetting Behavior and Surfactant EOR in Carbonates With Model Compounds

Abstract

Abstract 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 wetting of the carbonate surfaces from oil-wet to 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 oil-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. Experiments with different surfactants and NA-treated calcite powder provide information about mechanisms responsible for sought after reversal to a water-wet state. Results indicate this flotation and a calcite chip cleaning test are rapid screening tools to identify better EOR surfactants for carbonates. The study considers the application of surfactants for enhanced oil recovery (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 for this process. Introduction Typically only about 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. About half of world's discovered oil reserves are in carbonate reservoirs and many of these reservoirs are naturally fractured.[1] According to a recent review of 100 fractured reservoirs,[2] carbonate fractured reservoirs with high matrix porosity and low matrix permeability especially could use enhanced oil recovery (EOR) processes. The oil recovery from these reservoirs is typically very low via conventional technology, due in part to fractured carbonate reservoirs (about 80%) being originally oil-wet, or at least, mixed wettability. Injected water will not penetrate easily into the oil-wet porous matrix and so can not displace the oil in place. Wettability of carbonate reservoirs has been widely recognized an important parameter in oil recovery by flooding technology.[3–6] Because altering the wettability of rock surface to preferentially 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[7]. Vijapurapu and Rao at Louisiana University studied the capability of certain ethoxy alcohol surfactants to alter wettability of the Yates reservoir rock from strongly oil-wet to water-wet. They reported that the advancing contact angle of water can be reduced from 158° to 39° by addition of the surfactant at a concentration of 3500 ppm.[8] Seethepali and co-workers at University of Houston reported that several anionic surfactants (SS-6656, Alfoterra 35, 38, 63 65 and 68) 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.[9] Zhang and co-workers at Rice University 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 about intermediate oil-wet to preferentially water-wet condition with alkaline/anionic surfactant systems. Adsorption of anionic surfactants on a dolomite surface can be significantly reduced in the presence of sodium carbonate.[10] Xie and co-workers at University of Wyoming reported that after imbibition of reservoir brine had ceased, immersion of a core in surfactant solution can produce an additional recovery of 5 to 10% OOIP, and they ascribed this additional oil recovery to increased water wetness of the core.[11] Enrique and co-workers examined wettability conditions of solid/brine/n-dodecane systems at various surfactant concentrations and different ionic strength. They concluded that the wettability in solid/oil/brine systems could be changed by diffusion, through the aqueous phase, of surfactant species that were originally present in the oil phase while the gradual adsorption of these molecules on the solid walls modifies the surface energy. [12]