Polymer Augmented Low Salinity Flooding in Low Permeability Carbonate Reservoirs

Abstract

Low salinity waterflooding improves displacement efficiency in initially oil-wet reservoirs if it can alter wettability, but it is often a slow process. Polymer flooding usually does not improve displacement efficiency, but enhances sweep efficiency. In this work, the synergy between low salinity and polymer flooding is studied for low permeability carbonate rocks. Polymer solutions were consecutively filtered through a 1.2 µm mixed cellulose ester membrane and a 0.4 µm polycarbonate membrane. With the proper preparation method, two polymers (HPAM and AN132) with the molecular weight of 6 MDa were successfully injected into the oil-aged carbonate rocks with the absolute permeability of 10-20 mD. Low salinity polymer flood was carried out using HPAM prepared in diluted seawaters (with modified sulfate concentrations). After extensive water floods, HPAM prepared in the 10 times-diluted seawater produced the same incremental oil recovery (4-5% original oil in place) as the ATBS-polymer AN132 prepared in the seawater. Increasing the sulfate concentration by four- and eight-folds doubled the incremental oil from low salinity polymer floods.