Chemical enhanced oil recovery projects occasionally introduce an alkali agent to meet design requirements. The alkali agent reacts with reservoir rock components upon injection in reservoirs. It has been reported that the interaction of the injected alkali with some minerals in the rock assemblage, particularly anhydrite, is responsible for the very large alkali consumption, formation of secondary minerals, and regulates water chemistry. These effects, when unanticipated, can jeopardize the success of a chemical flooding project. In this study, single and two-phase flow flooding tests were carried out using rock samples from a sandstone reservoir in Wyoming to investigate the impact of multiphase flow on anhydrite dissolution at high-pH conditions. Effluent water chemistry was analyzed to investigate rock-fluid interactions taking place during an alkaline flood. Rock samples were CT-scanned to find out anhydrite distribution. Mitigation of harmful effects of rock-fluid interactions under alkaline flooding has been proposed through the addition of ethylenediaminetetraacetic acid (EDTA) to act as a calcium chelating agent. The effectiveness of EDTA was tested in single- and two-phase flow experiments. An alternative approach to mitigate damaging effects of alkali injection in anhydrite-containing rock, based on conditioning of injection water, was tested in this work. Results show that anhydrite dissolution diminishes when crude oil is present, but the effect depends on rock exposure time to oil (aging). In spite of the apparent decreased reactivity, anhydrite dissolution is still very pronounced in two-phase flow experiments. Results also show conclusively that water conditioning intended to diminish anhydrite dissolution chemical driving force is a more effective strategy to attain sustainable flooding conditions. (c) 2012 Elsevier Ltd. All rights reserved.