Coinjection of Seawater and Produced Water to Improve Oil Recovery from Fractured North Sea Chalk Oil Reservoirs Tina Puntervold,* Skule Strand, and Tor Austad UniVersity of StaVanger, 4036 StaVanger, Norway ReceiVed NoVember 26, 2008. ReVised Manuscript ReceiVed February 20, 2009 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 Ca 2+ , Mg 2+ , and SO 4 2- 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 CaSO 4 , SrSO 4 , and BaSO 4 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. Introduction Chalk is a carbonate rock, and carbonate reservoirs are usually moderately water-wet to oil-wet and highly fractured, which means that the injected brine will follow the fractures from the injector to the producer and will only displace the oil contained in the fractures. The orientation of the fractures relative to the fluid flow direction also appears to have an impact on oil recovery. 1 Therefore, the oil recovery from carbonates is usually low, well below 30% on average. In that way, the naturally fractured Ekofisk chalk field in the North Sea appears to be an exception because the oil recovery is expected to exceed 50% by injection of seawater. The wetting condition of the Ekofisk field varies from preferentially water-wet in the Tor formation, to moderately water-wet in the Lower Ekofisk, and to neutral to preferentially oil-wet in the Upper Ekofisk formation. 2 Through a series of papers, we have recently documented that seawater acts as an EOR-fluid in chalk formations by increasing the water-wetness and improving spontaneous imbibition of water into the matrix blocks. 3-6 In that sense, injection of seawater into high temperature chalk fields can be regarded as a tertiary oil recovery technique. The chemical mechanism behind the wettability alteration caused by seawater has been studied in detail. 4,5 It was confirmed that seawater contained potential determining ions Ca 2+ , Mg 2+ , and SO 4 2- , which were able to displace strongly adsorbed carboxylic material from the chalk surface at high temperatures, T > 90-100 °C. This effect increased drastically as the temperature increased beyond 100 °C, due to increased adsorp- tion of SO 4 2- onto the chalk surface. 6 The presence of SO 4 2- is essential for the wettability alteration reactions to take place. In addition, the divalent cations Ca 2+ and/or Mg 2+ must be present. In seawater, all these ions are present in proper concentration ratios, that is, [Mg 2+ ] ∼ 2 [SO 4 2- ] ∼ 4 [Ca 2+ ]. The formation water, FW, and produced water, PW, contain significant amounts of Ca 2+ and Mg 2+ , but they only contain very small amounts of SO 4 2- , which is insufficient to promote a wettability modification toward a more water-wet condition. * Corresponding author. E-mail: tina.puntervold@uis.no; phone: +4751832213; fax: +4751831750. (1) Shedid, S. A. J. Pet. Sci. Eng. 2006, 50, 285–292. (2) Torsaeter, O. , An experimental study of water imbibition in chalk from the Ekofisk field, In Paper SPE12688 presented at the SPE/DOE Fourth Symposium on Enhanced Oil RecoVery, Tulsa, OK, USA, April 15- 18, 1984; 1984. (3) Zhang, P.; Austad, T. Colloids Surf., A 2006, 279, 179–187. (4) Zhang, P.; Tweheyo, M. T.; Austad, T. Colloids Surf., A 2007, 301, 199–208. (5) Zhang, P.; Tweheyo, M. T.; Austad, T. Energy Fuels 2006, 20, 2056– 2062. (6) Strand, S.; Høgnesen, E. J.; Austad, T. Colloids Surf., A 2006, 275, 1–10. Energy & Fuels 2009, 23, 2527–2536 2527 10.1021/ef801023u CCC: $40.75  2009 American Chemical Society Published on Web 03/20/2009 Downloaded by PORTLAND STATE UNIV on July 2, 2009 Published on March 20, 2009 on http://pubs.acs.org | doi: 10.1021/ef801023u