Optimized Smart Water Composition at Ekofisk Conditions

Abstract

Seawater has been performing as a wettability modifier in chalk reservoirs and has been used for a long time. This wettability alteration enhances microscopic then overall displacement efficiencies. Having wettability altering capability makes seawater possible to spontaneously imbibe into the chalk matrix and produce extra oil. Studies show that Ca+2, SO4-2, and Mg+2, potential determining ions, can be chemically active and desorbs the organic oil acids from the chalk mineral surface. This release provides a more water-wet wetting condition to the reservoir. Smart Water is a designed injection brine that can modify reservoir wettability towards water wet. Knowing that seawater already functioning as a Smart Water in chalk reservoirs, to design ideal Smart water, seawater is considered as a base injection brine. Smart Water for chalk needs to be abundant in Ca+2, SO4-2, and Mg+2 concentrations. It is also shown that low Na+ and Cl concentrations lead to better displacement performance. The wettability alteration capability of a Smart Water is directly related to reservoir temperature. Although slight Smart Water effect may be observed in low temperatures 90 ºC, as temperature increases to 130 ºC significant wettability alteration by Smart Water is observed. In this thesis, finding a Smart Water composition and concentration, and stabile equilibrium condition of the Smart Water is aimed. Stevns Klint Chalk cores used for its analogy importance to Ekofisk. Experiment temperature is also chosen as reservoir temperature of Ekofisk, 130 ºC to provide a better resemblance. Totally four cores cleaned with de-ionized water, restored 10% Swi with sulfate-free formation water, and 90%Soi with an oil withAN:0.53 mgKOH/g. All cores aged for 14 days at 130 ºC. After a batch test was done Smart Water is decided to be prepared as a 10 M CaSO4 solution. All cores spontaneously imbibed, by FW in secondary mode by SW and CaSO4 solution-Smart Water in secondary and tertiary mode. Spontaneous imbibition test with FW recovered 30% while SW 58% and CaSO4 30% of OOIP in secondary mode. In tertiary mode, CaSO4 showed a slight wettability alteration and recovered an extra 8% while SW performed very well wettability modification and recovered 55% of OOIP after 30% of FW.SW also experimented in tertiary mode after CaSO4 solution-Smart Water and recovered 54% of OOIP confirming its wettability alteration capabilities.