Comparing Inexpensive, customized brine compositions for enhanced oil recovery in high temperature outcrop and reservoir chalk

The effectiveness of seawater as a wettability modifier in chalk and limestone can be significantly improved by depleting NaCl from the seawater, and/or adding extra sulfate (SO 4 2- ) that together with the divalent calcium and magnesium cations (Ca 2+ , Mg 2+ ) will desorb negatively charged oil molecules from positively charged rock surfaces. Customized brine compositions were designed, aiming to be inexpensive to manufacture, stable with respect to solid precipitation and acting as Smart Water by enhancing oil mobilization by wettability alteration in chalk compared to that by seawater. Different brine compositions were tested in which the Ca 2+ , Mg 2+ , and SO 4 2- content was varied while keeping the salinity low. The compositions were chosen such that the brines could be made from adding cheap and readily available salts to fresh water. Few enhanced oil recovery studies using Smart Water have been performed on reservoir chalk samples due to reservoir rock sample scarcity. Therefore, parametric studies were first performed on Stevns Klint outcrop chalk cores prepared in the laboratory to slightly water-wet conditions. The most promising brine composition from the parametric study was tested and compared against seawater injection in restored reservoir chalk cores originating from the Norwegian continental shelf. The chemical and physical properties of the outcrop and reservoir chalk material were compared, and it was found that the Stevns Klint outcrop chalk was a very good analogue to the reservoir chalk used in this study. Oil recovery tests by spontaneous imbibition and viscous flooding were performed in both outcrop and reservoir chalk cores. The SmW2 brine, containing 20 mM CaSO 4 and 20 mM MgCl 2 , was the most effective oil recovery brine in the outcrop chalk at 130 °C, surpassing the spontaneous imbibition performance of seawater in secondary mode. The same brine was also found to be the best performing brine in the reservoir chalk cores. These oil recovery experiments suggest that it is possible to prepare optimized Smart Water brines that do not precipitate at high temperature but are able to mobilize extra oil through wettability alteration by creating positive capillary forces and improving reservoir sweep.