RIASSUNTO
Abstract
Carbonate coreflood experiments have demonstrated the potential for Advanced Ion Management (AIMSM) to significantly increase oil recovery compared to waterflood using formation water. AIMSM involves adding and/or removing ions from the injection water to improve waterflood performance. AIMSM further improves current state-of-the-art processes through the addition of certain salts and/or the softening of water. Simulations have accurately matched the experiments, providing a tool to assess field-scale recovery and supporting the premise that the increased recovery is due to a change in wettability. This study
shows that a relatively inexpensive and straightforward modification of injection water composition can significantly increase oil recovery.
Introduction
Historically, there have been limited publications on the effect of changing ion composition for waterflooding of carbonate reservoirs, partially because the process often encountered problems due to adverse reactions between the injected water and anhydrite or gypsum which may exist in carbonate reservoir (Taber and Martin, 1983). This trend, however, has been changing recently due to an increased interest in enhanced waterflood, partially triggered by better-than-expected oil recovery from the fractured chalk reservoir at the Ekofisk field in the North Sea (Sylte et al., 1998; Hallenbeck et al., 1991). Much of that body of research has been conducted and published by Austad and co-workers at the University of Stavanger using outcrop chalk cores. Those studies focused on spontaneous imbibition tests for laboratory measurements with the goal of improving the imbibition processes suitable for fractured chalk reservoirs. Sulfate anions of various concentrations were added to seawater in those studies.
In addition, Ligthelm (2009) performed spontaneous imbibition experiments on Middle Eastern limestone with 5% increment in oil recovery (17% OOIP total recovery). However, Høgnesen et al. (2005) observed no effect of high sulfate brine during spontaneous imbibition on unfractured limestone treated with modified crude oil. Bortolotti et al. (2009) studied alkaline flooding with sodium hydroxide and sodium bicarbonate in a calcarenite outcrop and synthetic oil system. They concluded that a pH of 9 or above is required for effective alkaline flooding. To date, no laboratory study via coreflooding (as opposed to spontaneous imbibition tests) to enhance waterflood performance by adding inorganic salts (as apposed to significantly increasing pH as in alkaline floods or reducing IFT as in ASP floods) have been published for unfractured carbonates, including limestones and dolomites.