Coinjection of Seawater and Produced Water to Improve Oil Recovery from Fractured North Sea Chalk Oil Reservoirs

Abstract

Carbonate oil reservoirs are often fractured with moderate water-wet conditions, which prevent spontaneous imbibition of water into the matrix blocks. Enhanced oil recovery by water flooding is therefore seldom successful, and the average oil recovery from carbonates is usually much less than 30%. Hence, the improved oil recovery potential is very high in these types of reservoirs. Recent studies on chalk cores from the North Sea formations have shown that seawater is able to change the wettability toward a more water-wet condition at high temperatures, >100 °C. The successful injection of seawater into the Ekofisk formation, where the oil recovery is now estimated to reach 50%, is a good example. Seawater contains favorable concentrations of the potential determining ions Ca2+, Mg2+, and SO42− that are active in the displacement of strongly adsorbed carboxylic material from the chalk surface. The initial formation water will partly mix with seawater. Therefore, the amount and composition of the produced ...