Laboratory Evaluation of Alkali and Alkali-Surfactant-Polymer Flooding Combined with Intermittent Flow in Carbonatic Rocks

Conventional alkali flooding in numerous carbonatic reservoirs is often not effective due to insufficient contact time to reverse rock wettability from oil-wet into a more favorable condition to get the highest ultimate oil recovery, i.e., water-wet or partially water-wet conditions. Therefore, conventional flooding might be not completely efficient and/or adequate. Applying intermittent flow helps rock surface to complete its wettability reversal and to improve the final oil recovery. In the present study, laboratory evaluations of alkali and Alkali-Surfactant-Polymer (ASP) flooding combined with intermittent flow are presented and comparatively discussed. An alkaline solution is injected into oil-wet homogeneous carbonatic core samples and aged for a week. This aging period represents an “intermittent flow” which allows rock surface to reach a more favorable condition for oil recovery; oil is repulsed from the rock surface and the core is then flooded by displacing brine. Core flood test is performed vertically in order to avoid the early breakthrough of injected water phase. In the solely alkali flooding, the results demonstrates that the concentration of alkali strongly affects the flooding efficiency. Alkali concentration is intentionally kept low.In fact, high alkali concentration (of about 0.5 molar) is not recommended because the insolubility of in-situ soap (soap produced by the reaction of the alkaline substance with the acid present in the oil phase) causes pore space plugging and therefore permeability reduction. The application of intermittent flow is therefore less significant in the case where high initial water saturation is present. The second part of this study emphasizes on alkali-surfactant-polymer flooding. In particular, laboratory tests highlighted that the highest final oil recovery with the lowest Water-to-Oli Ratio (WOR) is obtained by aging the alkaline solution after the injection of a surfactant, and then flooding the whole system by a polymer solution. Fractured oil-wet carbonatic reservoirs seem the best candidates for the application of this technique since aging time would allow alkali to diffuse and reverse rock wettability also in otherwise unaccessible zones. The proposed technique seems very effective to increase the ultimate oil recovery of these reservoirs. Traditionally, alkali flooding in oil-wet carbonatic reservoir is believed to require the consumption of a large mass of alkaline substances. However, the mass of this substance is responsible for the wettability reversal into a more favorable condition for oil recovery, and can be acceptably managed by a proper design of the flooding parameters.