Chemical Mechanism of Low Salinity Water Flooding in Sandstone Reservoirs

Both laboratory and single well field tests have documented that enhanced oil recovery can be obtained from sandstone reservoirs by performing a tertiary low saline waterflood. Due to the complexity of the crude oil-brinerock interactions, the mechanism behind the low saline EOR process has been debated in the literature for the last decade. Both physical and chemical mechanisms have been proposed, but it appears that none of the suggested processes has so far been generally accepted as the main contributor to the observed low salinity EOR effect. Based on published data and new experimental results on core flooding, effects of pH and salinity on adsorption of acidic and basic organic components onto different clay minerals, clay properties like ion exchange capacity and selectivity, and oil properties, a new chemical mechanism is suggested, which agrees with documented experimental facts. At reservoir conditions, the pH of formation water is about 4 due to dissolved acidic gases like CO2 and H2S. At this pH, the clay minerals, which act as cation exchange material, are adsorbed by acidic and protonated basic components from the crude oil, and cations, especially divalent cations, from the formation water, like Ca. Injection of a low saline fluid, which promotes desorption of Ca, will create a local increase in pH close to the brine-clay interface because Ca is substituted by H from the water. A fast reaction between OH and the adsorbed acidic and protonated basic material will cause desorption of organic material from the clay. The water wetness of the rock is improved, and increased oil recovery is observed. To observe low salinity EOR effects in sandstones, a balanced initial adsorption of organic components and Ca onto the clay is needed. Both the adsorption capacity and the pH-window for adsorption/desorption of organic material is somewhat different for various types of clay minerals. A detailed knowledge of the chemical mechanism behind the low saline EOR process together with information on formation brine composition, oil properties and type of clay material present, will make it possible to evaluate the potential for increase in oil recovery by a low salinity waterflood. Introduction A great number of laboratory tests by Morrow and co-workers (Tang and Morrow, 1999a; Tang and Morrow, 1999b; Zhang and Morrow, 2006; Zhang et al., 2007b) and also by researchers at BP (Lager et al., 2007; Webb et al., 2005a; Webb et al., 2005b) have confirmed that enhanced oil recovery can be obtained when performing a tertiary low salinity waterflood, with salinity in the range of 1000-2000 ppm. Based on 14 tests from different sandstone reservoirs, Lager et al. (Lager et al., 2007) have reported that the average increase in recovery was about 14%. The laboratory observations have even been confirmed by single well tests performed in an Alaskan reservoir (Lager et al., 2008b). As increasing amounts of laboratory experiment results have been published in the last decade, various suggestions of the mechanism behind the low salinity process have appeared. Unfortunately, none of the suggested mechanisms have so far been generally accepted as the “true” mechanism. The reason is that many parameters linked to the rock, to the reservoir fluids (oil and brine), and to the injection fluid are involved. In order to give the reader a good background to understand the proposed mechanism in this paper, a list of the accepted experimental conditions is given, followed by a short recap of the previously suggested mechanisms. Conditions for low salinity effects The listed conditions for low salinity effects are mostly related to the systematic experimental work by Tang and Morrow (Tang and Morrow, 1999a), but some points has also been taken from the work by BP (Lager et al., 2007; Lager et al., 2008a).