Niels J. Brussee

Demonstrating the Potential of Low-Salinity Waterflood to Improve Oil Recovery in Carbonate Reservoirs by Qualitative Coreflood

Low salinity waterflood (LSF) is a promising improved oil recovery (IOR) technology. Although, it has been demonstrated that LSF is an efficient IOR method for many sandstone reservoirs, the potential of LSF in carbonate reservoirs is still not well-established as only a limited number of successful coreflood experiments are available in the literature. Therefore, the aim of this study was to examine the oil recovery improvement by LSF in carbonate reservoirs by performing coreflood experiments. This paper proposes an experimental approach to qualitatively evaluate the potential of LSF to improve oil recovery and alter the rock wettability during coreflood experiments. The corefloods were conducted on core plugs from two Middle Eastern carbonate reservoirs with a wide variation of rock properties and reservoir conditions. Seawater and several dilutions of formation brine and seawater were flooded in the tertiary mode to evaluate their impacts on oil recovery compared to formation brine injection. In addition, a geochemical study was performed using PHREEQC software to assess the potential of calcite dissolution by LSF. The experimental results confirmed that lowering the water salinity can alter the rock wettability towards more water-wet, causing improvement of oil recovery in tertiary waterflood in plugs from the two reservoirs. Furthermore, seawater is more favorable for improved oil recovery than formation brine as injection of seawater after formation brine resulted in extra oil production. This demonstrates that the brine composition plays an important role during waterflooding in carbonate reservoirs, and not only the brine salinity. It was also observed that oil recovery can be improved by injection of brines that cannot dissolve calcite based on the geochemical modeling study. This implies that calcite dissolution is not the dominant mechanism of IOR by LSF. To conclude, this paper demonstrates that low-salinity waterflood has a good potential as an IOR technology in carbonate reservoirs. In addition, the proposed experimental approach ensures the verification of LSF effect, either it is positive or negative. However, more work is required to further explore the most influential parameters affecting LSF response and explain the dominant mechanisms. Introduction Low salinity waterflood (LSF) is a relatively mature improved oil recovery technique for sandstone reservoirs. The concept of LSF, for sandstones, is to lower the ionic strength of the injected brine, which leads to an alteration of the rock wettability towards more water-wet and hence an improvement of oil recovery. Numerous laboratory studies demonstrated the effect of LSF by spontaneous imbibition tests and coreflood experiments (Bernard 1967, Jadhunandan and Morrow 1991, Yildiz and Morrow 1996, Tang and Morrow 1997, Lager et al. 2006, Ligthelm et al. 2009, Masalmeh et. al. 2013). Furthermore, published data confirmed the positive response of LSF at the field scale (Webb et al. 2004, Lager et al. 2008, Vledder et al. 2010). However, the potential of LSF for carbonate reservoirs has not been well investigated. Several spontaneous imbibition tests were performed on Stevns Klint outcrop chalk (Austad et al. 2005, Zhang and Austad 2006, Strand et al. 2006). The results demonstrated the wettability alteration towards more water-wet by seawater or modified seawater. Increasing the sulfate concentration in seawater resulted in more change of wettability towards waterwetness. Ferno et al. (2011) performed spontaneous imbibition tests on different chalk outcrops (Stevns, Rordal, and Niobrara) using brines with and without sulfate. The effect of adding sulfate to the brines on wettability alteration was observed only in plugs from Stevns Klint chalk, but not from the other 2 chalk types. Webb et al. (2005) performed

The Critical Parameter for Low Salinity Flooding - The Relative Importance of Crude Oil, Brine and Rock

Improved oil recovery by low salinity flooding (LSF) in sandstone reservoirs is hypothesized to be the result of a wettability change of the crude oil, brine, rock (COBR) system to a more water-wet state. The exact mechanism behind the wettability change upon lowering the ionic strength of the brine is, however, not yet fully understood. It is generally accepted that a strong low salinity effect requires the presence of clay minerals in the reservoir rock and preferably a high salinity of the formation water containing divalent cations. Still, COBR systems that obey these minimum requirements may give a highly variable response to low salinity flooding. To create enhanced understanding of the critical parameter(s) controlling the low salinity effect, crude oil, rock and brine from three different reservoir systems were varied in all possible combinations in a series of spontaneous imbibition tests. These tests show that, for the COBR systems analyzed here, the rock is the most critical parameter for a strong low salinity effect. Cross-correlation of the change in water saturation upon exposure to low salinity, ΔSw LS, with various rock parameters indicated the strongest correlation with rock zeta potentials.