Abstract
Summary Reservoir wettability is an important parameter that can affect the fluid flow characteristics in porous media. The change in wettability toward a more water-wet state underlies several enhanced oil recovery (EOR) methods. One of these methods, Smart Water flooding, is based on the injection of ion-modified water to induce chemical interactions in the formation and improve oil recovery by increasing microscopic sweep efficiency. Smart Water EOR effects have been studied for several years worldwide, however the experimental results from different researchers are not consistent. One reason for this is the difference in experimental conditions, that is, the combination of a certain crude oil chemistry, formation brine composition and core mineralogy. The interactions between crude oil, brine and rock (COBR) will create an initial wetting, which is the main characteristic determining the Smart Water EOR potential. In this work, the effect of the formation water (FW) composition on the wetting in sandstone reservoirs will be considered. Outcrop sandstone cores with complex mineralogy were used in oil recovery tests and surface reactivity experiments. Previously published work performed on the same core system has shown potential for an average increase in oil recovery of 10% OOIP by injecting Smart Water in tertiary mode. In those experiments a FW with a high content of calcium ions (Ca2+), was used. In the present study, a high salinity NaCl brine (100 000 ppm) was used as FW in order to study the effect of the absence of divalent cations on the initial wetting and Smart Water EOR potential. An oil recovery test with FW injection was performed to evaluate the initial wetting in the core, and a low salinity (LS) NaCl brine (1000 ppm) was used to determine the Smart Water EOR potential. A pH-screening test was thereafter performed to confirm the reactivity of the core minerals, and the COBR-interactions. The results from this work confirmed that the core was slightly less water-wet using a FW free of Ca2+-ions, resulting in a tripled Smart Water EOR effect of +30% OOIP, in tertiary mode. The wettability of the core is determined by the adsorption of reactive polar organic components onto mineral surfaces. Ca2+ ions present in the FW compete with polar organic components for adsorption sites on the mineral surfaces, making the rock surface initially more water-wet. Thus, the reduction in Ca2+ ions in the FW made the core less water-wet, increasing the Smart Water EOR effect.
Published Version
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