Low Salinity Oil Recovery On Clayey Sandstone: Experimental Study

Abstract

It's now largely agreed from laboratory and field tests results that brine composition and ionic strength can influence oil recovery (Jadhunandan and Morrow, 1991; Jadhunandan and Morrow, 1995). Morrow et al. in the 90's first carried out a large study on the ability of low salinity waterflooding to become an EOR method (Yildiz and Morrow, 1996; Tang and Morrow, 1999). More recently BP, carried out a large program on low salinity injection and proposed additional recovery mechanisms (Mc Guire and Chatham, 2005; Lager et al. 2006). Although there is substantial evidence of this phenomenon, the physical causes are still uncertain and several possible recovery mechanisms have been put forward in the literature: clay release, wettability alteration, IFT modification, key role of kaolinite etc. In this paper, we present experiments dealing with tertiary and secondary recovery tests performed on an outcrop sandstone containing 9.2% of clays without kaolinite. Each experiment is performed following a three step procedure during which pressure drop, pH, brine composition and oil recovery are monitored. First, the core is saturated with a high salinity brine at ambient temperature until equilibrium is reached (pressure drop and pH are constant and ion exchange is achieved). Second, the initial water saturation is established using a crude oil. Finally, after ageing, high salinity brine waterflooding followed by low salinity brines in the case of tertiary experiment or direct low salinity brines flooding in the case of secondary recovery are performed. Additional oil recovery of about 10% of original oil in place is observed in tertiary experiments when going from 50g/l to 1g/l brine. This oil production occurs after 2 or 3 pore volumes in the form of an oil bank displacement, suggesting a more dispersive behavior of the matrix in the presence of oil. It is worth noting that this additional oil recovery has been obtained in a clayey sandstone that does not contain kaolinite. When low salinity brine was injected, both pH and pressure drop increased. Successive dilutions of injection brine from high salinity did not lead to an oil recovery increase until using 1g/l, showing a salinity concentration threshold, whereas pressure drop and pH were increasing significantly at each salinity dilution. Waterflooding at different temperatures shows that oil recovery in secondary mode increases with increasing displacement temperature. But additional oil recovery in tertiary mode is more positive for moderate temperatures.