Abstract
Abstract Extended surfactants (ESs), obtained by inserting an oligomeric oxypropylene segment often together with an oligomeric oxyethylene segment between the hydrophobic tail and an anionic polar headgroup, have attracted considerable interest for enhanced oil recovery (EOR) due to their high salinity tolerance and favorable synergistic interactions with conventional anionic surfactants used for EOR. Sulfated ESs, in particular, have been extensively studied and applied in combination with alkylbenzene sulfonates (ABS) and olefin sulfonates. However, the limited thermal stability of sulfate head groups restricts their application in high-temperature reservoirs. In this study, three ESs with identical hydrophobic tail and the same oxypropylene-oxyethylene chain but different hydrophilic head groups – sulfate, sulfonate, and carboxylate – were synthesized and characterized. The three ESs showed comparable CMC values and excellent salinity tolerance. The sulfonate- and carboxylate-based ESs remained stable for several weeks at 120 °C, whereas the ES with a sulfate end group was rapidly hydrolyzed. All ABS/ES formulations formed Winsor Type III microemulsions with similar solubilization ratios, although the ABS/carboxylate system exhibited lower optimum salinity than the other two systems. Coreflooding using Berea sandstone outcrops achieved a yield of 50–56 % of the original oil in place (OOIP) after waterflooding.