Evaluation of the critical parameters in oil recovery from fractured chalks by surfactant injection

Parameters affecting the oil recovery by surfactant injection such as interfacial tension (IFT), surfactant type, concentration and critical micelle concentration (CMC), solubility characteristics of surfactants and initial water in naturally fractured chalky reservoirs were examined. Both viscous (forced imbibition) and capillary displacement (spontaneous imbibition) experiments were performed as they could be possible recovery mechanisms. To identify the importance of existing water phase, some surfactant injection experiments were performed after a total waterflood.In primary surfactant recovery (forced imbibition into a virgin core), the IFT plays a major role. For the secondary surfactant injection (forced imbibition after a total waterflood), surfactant type, possibly adsorption and IFT are critical. It is worthwhile noting that the assessment of the adsorption effect is speculative as no measurement and analysis were done in this regard. The effect of the CMC on oil recovery can be pronounced for cationic surfactants especially for the forced imbibition cases. In case of capillary imbibition recovery, the IFT change was not significant for non-ionic surfactants if there is no initial water in the rock. When there is initial water, however, no significant imbibition recovery was observed. This could be attributed to the high solubility of non-ionics in oleic phase.The analysis and observations presented in this study provide a surfactant selection criteria for highly or partly fractured chalky oil reservoirs.