Secondary Versus Tertiary Polymer Flooding: A Laboratory Evaluation in Low Permeability HT/HS Carbonate Reservoir Conditions

Abstract

Polymer flooding is a well-established EOR technique widely implemented in sandstone reservoirs. Sulfonated acrylamide-based copolymers recently proved their potential in harsh HT/HS carbonate reservoirs with pilot tests upcoming in the Middle East. While polymer flooding is often classified as a tertiary recovery method, the present study aims to evaluate EOR efficiencies in secondary and tertiary recovery scenarios. The polymer selection process and characterization in porous media are described in detail. The study was conducted on a lab-scale at a temperature of 120°C. The salinity of seawater used for injection was 42 g/L TDS. The polymer selection was based on a thorough rheology and filterability study. The coreflooding tests were performed on limestone outcrops with permeability in the range of 40 mD. Single-phase experiments were performed to evaluate the impact of a pre-shearing step and study the in-situ rheology. Secondary and tertiary oil recovery experiments were conducted using reservoir dead oil. In all experiments, the tracer method was used to determine dynamic adsorption. The rheology and filterability study identified the best candidate (SAV 10) among three different molecular weight polymers containing the same high-sulfonation level. The target viscosity for the desired mobility ratio was 3 cP at 120°C, achieved with a polymer concentration of 3500 ppm. The single-phase experiments suggested that pre-shearing the polymer reduced the apparent shear thickening at high velocities due to a reduction in viscoelastic properties. The best oil recovery performances were obtained in the secondary mode (polymer flood applied at Swi) with a 15% higher recovery when compared to the tertiary mode (after reaching Sorw). The results correlate with a later breakthrough for the earlier polymer flood case. In both cases the mobility ratio was comparable (0.18-0.20) and much lower than waterflooding (1.5). The polymer dynamic adsorption estimated with the tracer method was about 140 μg/g rock in brine saturated cores, decreasing to ∼100 μg/g rock in the presence of residual oil saturation. The in-situ rheology evaluation after stable recovery was reached following polymer flooding (Sorp), evidenced less apparent shear thickening, which could be related to a change in the conformation of the pores in the presence of oil. Most studies reported on secondary and tertiary polymer flooding focused on the case of heavy-oil sandstone reservoirs. The present work introduces new insights on early implementation of polymer flooding to optimize oil production by maximizing the performance of the method in HT/HS carbonate reservoirs. Furthermore, our study provides new insights about sulfonated acrylamide-base copolymer rheology in porous media for low permeability carbonate cores.