Cost-Effective Modified Water Flood Strategy for High-Temperature Offshore Carbonates: Examining Surfactant-Brine Synergy Through Innovative Displacement Experiments

Hydrocarbon exploitation from offshore carbonate reservoir is always challenging. India's large offshore carbonate fields have reached mature waterflood stage. To enhance waterflood recovery, significant research has been done by modifying injection water chemistry. Low salinity waterflood pilot is ongoing. Low salinity water works well for high temperature, high salinity reservoirs, although mechanism of salinity shock is still debatable (Morrow N. et.al, 2011). The wettability altering surfactant co-injection with low salinity water can also offer additional recovery depending on its chemical characteristics. This study assesses the synergy of these two methods in two typical offshore carbonate reservoirs, also introducing a single-well wettability test for field evaluation. Typical screening of compatibility and thermal stability was performed. Subsequent to detailed compatibility tests, surfactant brine solutions with non-ionic and cationic surfactant were first investigated using contact angle experiments. The change of wettability was verified using Amott wettability method. A sequence of surfactant-brine and low salinity brine was injected over conventional water flood in tertiary mode to find incremental gain and saturation reduction in coreflood. The formulation was also validated in static imbibition kinetics experiments. Typical cationic surfactant appeared as effective in low salinity brine for a particular reservoir. For another set, it is observed that the efficacy of non-ionic surfactant is not dependent on brine salinity. Hence, surfactant in sea water is evaluated as useful formulation for the second reservoir. Low salinity and surfactant flooding individually draw attention to enhance recovery from offshore carbonate. The synergy of these two methods is studied here as no such conclusive reports are available in published literature. Further, single well wettability test is planned to test the efficacy of this wettability altering system in field. Reduction of sea water salinity needs reverse osmosis facility and reject water management, whereas surfactant flooding requires supply and quality assurance. The evaluation of best technology in such cases will help in taking timely and cost effective decisions for full field implementation.