Laboratory Assessment of Rock Wettability Effect on Polymer Retention in Carbonates

Abstract

Polymer retention is one of the controlling aspects of an effective polymer flooding process. Very few studies discussed the effect of rock wettability on polymer retention, with no consensus on the outcome. While some studies described that oil-wet rocks have low polymer retention, others reported the opposite. This work investigates the effect of rock wettability on the retention of an ATBS-based polymer onto carbonates at high salinity and moderate temperature conditions. In this study, static and dynamic retention tests of an ATBS-based polymer onto high permeable Indiana limestone outcrops were conducted in both absence and presence of oil. These tests were conducted at 50 °C using representative crude oil and formation water (167,114 ppm) of Middle East carbonate reservoir conditions. For the two-phase dynamic tests, the cores were aged at 90 °C for different times (8 hours, 3 and 14 days) to create different wettability conditions, which were verified by Amott index to water. Then, polymer retention and in-situ rheology, including RF and RRF, were determined. Similar procedure was followed for dynamic single-phase tests, but without core aging. Furthermore, single- and two-phase static tests were conducted under identical experimental conditions to compare the retention values. The results of Amott index to water showed that the selected aging times were suitable for creating different wettability conditions, where cores with longer aging times had a wettability more towards oil-wetting state. It was observed that three-days period of aging was enough to restore the wettability of Indiana limestone outcrops used in this study. Also, polymer dynamic retention was found lower in the presence of oil by about 35 to 47% as opposed to its absence. A further decrease in polymer retention by 14% was obtained for cores with a more oil-wetting condition resulting in a retention level of about 25 µg/g-rock. This is because oil-wet cores have a larger and effective surface area covered by the oil film, leading to a lower surface area left for polymer adsorption as opposed to cores with a wettability towards a more water-wetting state. On the other hand, single- and two-phase static adsorption tests showed non-comparable and very high retention values in the range of 305-337 µg/g-rock. This finding indicates that aging of the rock in such tests does not play a decisive role in obtaining representative polymer retention levels comparable to the dynamic tests. This study is one of the very few works that discuss the effect of rock wettability on polymer retention in carbonates. The study provides an essential insight into the inconclusive results in the literature by highlighting the role of wettability effect on polymer retention based on both static and dynamic retention tests.