The objective of this study is to develop strategies to improve oil recovery in highly fractured carbonate reservoirs at high temperatures (100°C and above). Such reservoirs usually contain high salinity and high hardness formation brines. The use of nonionic surfactants or anionic surfactants (Carboxylates and Sulfonates) to alter oil-wet reservoirs towards more water-wet was investigated under harsh reservoir conditions. A nonionic surfactant Ethomeen® T/25 has shown aqueous stability at high salinity and temperature with high effectiveness in wettability alteration and imbibition oil recovery. Anionic surfactant formulations were developed to recover oil mainly by gravity drainage since they did not show wettability alteration effect in hard brine. Both experimental and simulation studies have shown that sulfonate/carboxylate surfactants achieve high performance in wettability alteration and spontaneous imbibition only when they mix with chelating agents in formulations. Chelating agents, especially EDTA.4Na and sodium polyacrylate (NaPA) have been tested for their compatibility and effectiveness in the surfactant formulations. By sequestering divalent ions in hard brine, chelating agents free anionic surfactants to react at the solid-fluid interface to alter wettability of carbonates from oil-wet towards more water-wet. The chelating agents trigger mineral dissolution, but that does not lead to wettability alteration or contribute to the imbibition oil recovery directly.

Keywords:
waterflooding, concentration, surfactant, wettability alteration, chemical flooding methods, surfactant formulation, experiment, hard brine, enhanced recovery, Brine
Subjects:
Improved and Enhanced Recovery, Unconventional and Complex Reservoirs, Waterflooding, Chemical flooding methods, Carbonate reservoirs
Copyright 2014, Society of Petroleum Engineers
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