Low Salinity Flooding - A Critical Review

Abstract

Low Salinity flooding is an emerging technology to improve waterflood oil recovery for both sandstone and carbonate reservoirs. Extensive laboratory experiments investigating the effect of low salinity are available in the literature. To quantify the low salinity effect, either spontaneous imbibition and/or tertiary waterflooding experiments have been performed. In few cases, the experimental flooding data was interpreted using numerical simulation to derive relative permeability curves for both low and high salinity water to be used in field simulation. The field experience using change in brine salinity to improve oil recovery is yet very limited. The mechanisms suggested explaining low salinity effect (LSE) is very diverse. This paper will evaluate the different proposed mechanisms with focus on wettability alteration as the main mechanism. The paper will discuss 1- whether spontaneous imbibition experiments, which are often performed to demonstrate the change of wettability, are sufficient to demonstrate the potential of low salinity effect and 2- whether changing the rock to more water-wet will lead to reduction in residual oil saturation (Sorw) in light of the well-established fact that Sorw is higher for water-wet rock. Core flood results as a tertiary low salinity process is mostly reported in the literature, and these experiments often give a low increase in oil recovery smeared over a long production period. The question arise that possible a lot of the literature data is influenced by capillary end-effects. Some solid low salinity response is documented with stepping up the flow rate at the end of the primary waterflood, and still shows added recovery after change in injected brine salinity. An alternative approach to control capillary end-effects would be to use monitoring of in-situ saturation. Unfortunately, the low salinity literature lacks information about local saturation and thereby also on how additional oil is mobilized. The paper conclude on what we in our opinion find is the proven main low salinity mechanisms, best approach for core flooding studies of low salinity, and modeling approach for matching core floods and also estimate field response.