Abstract
Surfactant flooding in high salinity reservoirs (total dissolved solid (TDS) levels ≥ 100,000 mg/L) has been challenging, because many currently available surfactant candidates will fall out of the solution due to the solubility limit. Stewart Fee and SE Hewitt are two reservoirs of interested with high salinity formation brine of 165,000 mg/L and 102,300 mg/L TDS, respectively. Two ternary surfactant systems composed of conventional and extended surfactants were formulated for each reservoir. Single well chemical EOR pilots were performed in both reservoirs and single well chemical tracer tests (SWCTT) were conducted to estimate the near wellbore residual oil saturation (Sor) before and after chemical EOR injection.
Ternary surfactant systems were optimized using phase behavior tests and IFT measurements for each reservoir. Laboratory sand pack and coreflood tests were conducted to evaluate the displacement efficiency of optimized surfactant systems prior to field injection. The optimized systems reached ultra-low IFT in the order of 10−3 mN/m, recovered more than 60% of the Sor in sand packs and coreflood tests with actual formation brine salinity from each reservoir.
As part of the SWCTT, the partitioning coefficient of the reactive tracer (ethyl formate) is measured in the lab under reservoir temperature and formation brine salinity. Numerical simulation was used to model and interpret the SWCTTs. The number of layers, Sor, dispersivity coefficient, ester reaction rate and flow fraction in each layer were used as the matching parameters. Total injection rate was adjusted according to the material balance tracer recovery.
The partitioning coefficient decreased with presence of surfactant in experimental tests. Simulation results indicated that the reaction rate of reaction tracer was decreased in post-SWCTTs relative to pre-SWCTTs in both cases. This is because the coupled effects of lower partitioning coefficient and the drop of reservoir temperature due to water injection. The field test tracer profiles of two reservoirs were all best fitted by a two layers model. For Stewart Fee, the Sor in pre-chemical EOR SWCTT were 0.49 and 0.22 in each layer, and were decreased to 0.15 in both layers after the chemical EOR.For SE Hewitt, the Sor in pre-chemical EOR SWCTT were 0.20 and 0.12, and were decreased to 0.03 in both layers after the chemical EOR.
From the simulation results, the post EOR oil recovery in the field was between 60-90%, showing that the developed ternary systems are all very promising and effective for EOR in high salinity reservoirs.
The combined efforts of conducting experiments and modeling and numerical interpretation of SWCTTs for two high salinity reservoirs showed that the formulated two ternary surfactant systems composing of extended and conventional surfactants mobilized more than 60% of residual oil at low surfactant concentration of 0.5 wt%, which are all very promising for chemical EOR in high salinity reservoirs.
