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-brine-rock 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 5 due to dissolved acidic gases like CO 2 and H 2 S. 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 2+ . Injection of a low saline fluid, which promotes desorption of Ca 2+ , will create a local increase in pH close to the brine-clay interface because Ca 2+ is substituted by H + from the water. A fast reaction between OHand 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 2+ 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.

Waterflooding has for a long time been regarded as a secondary oil recovery method. In the recent years, extensive research on crude oil, brine, and rock systems has documented that the composition of the injected water can change wetting properties of the reservoir during a waterflood in a favorable way to improve oil recovery. Thus, injection of "smart water" with a correct composition and salinity can act as a tertiary recovery method. Economically, it is, however, important to perform a waterflood at an optimum condition in a secondary process. Examples of smart water injection in carbonates and sandstones are: (1) injection of seawater into high temperature chalk reservoirs and (2) low salinity floods in sandstone reservoirs. The chemical mechanism behind the wettability alteration promoted by the injected water has been a topic for discussion both in carbonates and especially in sandstones. In this paper, the suggested mechanisms for the wettability modification in the two types of reservoir rocks are shortly reviewed with a special focus on chemical similarities and differences. The different chemical bonding mechanisms of polar components from the crude oil onto the positively charged carbonate and the negatively charged quartz/ clay indicates a different chemical mechanism for wettability modification by smart water in the two cases.

Recently, a chemical mechanism for enhanced oil recovery by low salinity water flooding was suggested. The key step in the mechanism was a localized increase in the pH at the clay surface due to desorption of active cations, especially Ca2+, as the low saline injection water invaded the porous medium. An increase in pH will remove adsorbed acidic and basic material from the clay surface and increase the water wetness of the rock. In the present paper, parametric studies have been performed to verify the different steps in the suggested mechanism. Both dynamic core floods and static adsorption studies of model base and acid on three different clay minerals have been performed. The results from the dynamic experiments were supported by the static adsorption/desorption studies. The main observation regarding adsorption/desorption of basic organic material onto clays was that changes in pH appeared to play a greater role than the salinity of the brine. Thus, both the dynamic and the static studies supported the suggested chemical mechanism for the low salinity EOR process.

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Due to the lack of an exact chemical understanding of the mechanism for enhanced oil recovery by low salinity water flooding in sandstones, predictable EOR effects based on actual reservoir parameters have not been possible to obtain. Recently, we published a new chemical mechanism for the low salinity effect, which is in line with experimental documentation so far. In this paper, the results for low salinity lab experiments related to an offshore field in the North Sea are discussed in relation to the new chemical knowledge. The important reservoir parameters for the actual field are: total clay content 8-16 wt%, crude oil acid and base numbers 0.13 and 0.85 mgKOH/g, respectively, formation water salinity 201 000 ppm, and reservoir temperature 130 °C. The reservoir has been flooded with seawater, and the question is: "Is it possible to increase the oil recovery by injection of diluted seawater as the low saline fluid?" In most cases, the cores were first flooded with formation water in a secondary process followed by diluted seawater, 1600ppm, in a tertiary process. The extra oil recovery varied between 0-6% of OOIP. In light of the newly published mechanism, the results are discussed in terms of:• Clay content: type and amount • pH: initial and change in pH • Concentration gradient in the most active cation, Ca 2+ • Initial water saturation and composition.• Effects of aging and flooding temperature. For a given system, the low salinity EOR effect appeared to be strongly linked to the change in pH. Low initial pH, created by the presence of CO 2 , increased the adsorption of polar components onto clays, which normally will lead to enhanced oil recovery due to the low salinity water flooding effect.