Waterflood performance of fractured chalk reservoirs are strongly influenced by wettability,and spontaneous imbibition of water from fractures to matrix is a key mechanism for oil production. To determine the enhanced oil recovery(EOR)potential for induced wettability change,for instance during a surfactant injection,stable and uniform wetting preferences representing reservoir conditions must be established during core analysis to avoid laboratory artifacts. A systematic investigation of wettability stability in laboratory aged core plugs was performed by repeated cycles of spontaneous and forced imbibitions. Altered wetting states,ranging from strongly water-wet to moderately water-wet conditions were established by aging outcrop Rordal chalk samples in crude oil at 80°C for 2 to 6 days at a range of initial water saturations during aging. The Amott-Harvey water index(Iw)was found stable over two cycles of spontaneous imbibition tests,with an average change of Iw±0.04 between the first and second cycle. The oil recovery rate in partly waterflooded regions and transition zones was evaluated by varying the initial water saturation.
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