Effect of oil chemistry on the performance of low salinity waterflooding: An integrated experimental approach

Chemical enhanced oil recovery (cEOR) relies on the interactions of the injected chemicals with the surface-active components (SACs) of the oil-in-place and the rock surface to induce favorable physico-chemical changes. In this study, we investigate the effect of oil composition on the performance of low salinity waterflooding (LSWF) 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 LSWF application in carbonate reservoirs by using model oils that have different SACs, but the same TAN. A variety of characterization techniques including thermal gravimetric analysis (TGA), 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 LSWF. 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 LSWF in carbonate rocks in terms of Darcy-scale oil recovery. Results of this study show that oil chemistry significantly influences the performance of LSWF 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. Carboxylic acid chain length also affects the strength of adsorption, which in turn impacts the magnitude of wettability alteration during LSWF. Further, partitioning of SACs in brine is observed to influence the type of interactions taking place in the rock-oil-brine system, where carboxylate salts (soaps) generated in-situ are detected 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 low salinity brine at similar experimental conditions. Finally, the value of TAN is found to be insensitive to the type of acidic SACs present in the oil phase as long as they are monoprotic and their molar concentration in the oil is the same. Even within the same class of SACs, namely carboxylic acids, the usefulness of TAN is limited in differentiating between the type and molecular structure of the acid (i.e.: straight chain versus aromatic).