Effect of Oil Chemistry on the Performance of Low-Salinity Waterflooding in Carbonates : An Experimental Workflow to Characterize Rock-Oil-Brine Interactions

Chemical enhanced oil recovery (cEOR) relies on the interactions between surface-active components (SACs) of the oil-in-place and injected chemicals to induce favorable physico-chemical changes. In this study, we investigate the effect of oil composition on the performance of chemically-tuned waterflooding (CTWF) in carbonate rocks using an integrated experimental approach. In addition, we assess the extent of usefulness of total acid number (TAN) as an oil screening criterion for CTWF applications in carbonate reservoirs by using model oils that have different SACs, but the same TAN. A variety of characterization techniques including thermal gravimetric analysis, attenuated total reflectance (ATR-FTIR) and zeta potential are performed to investigate the molecular-scale effect of oil chemistry on rock-oil-brine interactions during CTWF. Sessile drop contact angle measurements are also performed to quantify the influence of different SACs on the wettability of carbonate rock samples. Concurrently, coreflood experiments are performed to evaluate the effect of oil composition on the performance of CTWF in carbonate rocks in terms of Darcy-scale oil recovery. Results of this study show that oil chemistry significantly influences the performance of CTWF at all scales. Different molecular-scale interactions are observed in the presence of different SACs owing to differences in their affinity to the rock surface, strength of adsorption, solubility in brine, as well as their distinct pore-scale wetting abilities. These differences translate into significant variation in Darcy-scale oil recovery. In addition, carboxylic acid chain length is found to affect the amount of SACs adsorbed onto carbonate rocks during aging. Chain length also affects the strength of adsorption, which in turn impacts the magnitude of wettability alteration during CTWF. Further, partitioning of SACs in brine is observed to influence the type of interactions taking place in the rock-oil-brine system, where soaps generated in-situ are detected in the oleic phase only in the presence of certain oil-brine pairs but not in the presence of others. Solubility of SACs in brine is also found to promote water-wetness. As a result, significant differences in the rate of oil recovery and ultimate recovery are observed when displacing four oils by the same chemically-tuned brine at similar experimental conditions