Effects of non-active salt and sulfate ion on smart waterflooding in a U.A.E. carbonate reservoir

Recent studies have shown that decreasing the salinity and varying the ionic composition of the water that is injected into the reservoir alters crude oil/brine/rock interaction and improves recovery. Several laboratory studies have been conducted and are underway, while a few pilot tests have also been initialized. The majority of the studies were performed on sandstone reservoirs; however, few tests have also been performed on carbonate reservoirs. With few of the UAE carbonate reservoirs reaching their maturity stage and with still considerable oil left behind; there is a need for a more effective approach. Waterflooding was previously viewed as a physical process. However, smart waterflood that alters the wettability of the crude oil/brine/rock (CBR) interaction, in addition to conventional viscous forces, has been seen to have promising results. Smart waterflood is the alteration of the concentration of the active and/or non-active ions or the salinity of the injected water that has shown to alter the initial equilibrium established between the CBR in recent studies. This approach is economically more feasible than other EOR methods and also conforms to the Kyoto protocol as the U.A.E is a signatory to this protocol.;In this study, laboratory tests were conducted on U.A.E reservoir carbonate core samples. The following scenarios were considered---(a) the brine salinity was lowered within a range of 13960--228240ppm (b) the concentration of the SO42- ions was varied and (c) the concentration of the non-active ions was decreased in order to assess the effect of salinity and ionic composition variation on wettability and recovery. The cores were initially flooded with formation water as the base brine and subsequent incremental recovery was assessed by serial injection of various smart waters. The pressure differential and the wettability alteration leading to incremental oil recovery were also analyzed.;The main observations were improved oil recovery and wettability alteration with decreasing non-active ions concentration and increasing SO4 2- ion concentration. Incremental oil recovery of about 2--8% OOIP in tertiary recovery was observed when switching to brine of lower ionic strength. The improved recovery was not solely due to the reduction of salinity but also a combined effect of the variation in ionic composition. The results were analyzed and discussed within the boundaries of the mechanisms previously mentioned in literature and provide a better understanding of the several mechanisms previously mentioned that contribute to better recovery through smart waterflooding.;The study concluded that modifying the chemistry of the injection brine can lead to incremental recovery. However, when this approach is extended to field application, the formation of scaling and souring could pose as a challenge due to the increase in SO42- ion concentration beyond a certain limit. Therefore, a thorough study on the limit of SO 42- ion concentration that can be used without any precipitation or scaling or harm from souring has to be performed.