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

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 water will vary with time. Due to environmental reasons, the produced water should be reinjected together with seawater into the chalk formation. The question that we ask in this paper is: “Will mixtures of seawater and produced water displace the oil in a similarly good manner as pure seawater?” This paper contains: (1) calculations of the compatibility of different mixtures of SSW (synthetic seawater) and artificial PW (artificial produced water) regarding precipitation of CaSO4, SrSO4, and BaSO4 at various temperatures. In most cases the fluids were compatible when the PW was diluted at least 4 times with SSW. (2) Experimental work on oil recovery from chalk cores of moderate water-wetness using various mixtures of SSW and PW. Both spontaneous imbibition and viscous flooding were performed. At T > 100 °C, the oil recovery by using PW:SSW mixtures in ratios ranging from 2:1 to 1:8 was significantly higher than by using pure PW in a spontaneous imbibition process. In a viscous flood, SSW appeared to be much more efficient than PW to displace the oil, and high oil recovery values were reached. This work is environmentally promising, because the conclusion drawn is that it should be possible to coinject produced water and seawater into a North Sea chalk oil reservoir, such as Valhall, without losing the good enhanced oil recovery properties of seawater.