Alkali/Polymer Flooding of High-TAN Oil: Live Oil and Dead Oil Evaluation

Abstract

Summary Alkali/polymer (AP) flooding of high-TAN oil is a promising enhanced oil recovery (EOR) method. Phase tests reveal that the generated emulsions are thermodynamically unstable macroemulsions rather than Winsor-type emulsions as observed in alkali/surfactant (AS) systems. We investigated the effect of gas on the phase behavior and displacement efficiency of AS systems. The reason is that the impact of gas in solution on the displacement efficiency in alkali projects is significant, neglecting the gas effects underestimates the incremental recovery factor by >15%. Experiments and analysis were performed to investigate the effects of alkali injection into a live and dead high-TAN oil. Viscosity measurements using a capillary rheometer and oscillating u-tube were done to ensure the same viscosity of the dead oil (adding cyclohexane) to live oil. Alkali phase behavior scans were used to determine the amount of emulsions formed initially and over time. The structure and characteristics of the emulsions were investigated using a high-resolution microscope. Micromodel experiments (dead oil only) were performed to elucidate the displacement efficiency effects on pore scale, while flooding experiments showed the displacement efficiency on core scale. Phase experiments showed that initially, a substantial amount of emulsions is formed. The volume of the emulsion is changing over time reaching zero for the live and dead oil. The microscope pictures show that in the initial stage, a “middle phase” macroemulsion is present. With time, the middle phase disappears supporting the results of thermodynamically unstable emulsions seen in the phase experiments. Micromodels show that oil is mobilized by AP injection on a local scale by elongating ganglia and reducing the size of trapped oil and only a limited amount of macroemulsions is formed at the oil/alkali/water interface. The increased oil recovery is thus an effect of the local capillary number and mobilization of ganglia. Here, no stable three-phase system consisting of oil/microemulsion/water as in AS system is generated. Live oil AP corefloods lead to recovery factors of 95% compared with 74% for dead oil. The gas in solution improves the local pore scale sweep efficiency and needs to be included in the evaluation of AP flooding to ensure that incremental oil production is not underestimated for high TAN number oils. The main findings are as follows: Phase experiments of alkali with dead and live high TAN oil show that initially a large amount of emulsions is generated. However, these emulsions are thermodynamically unstable macroemulsions. Micromodel investigations show that the local pore scale displacement efficiency is improved by injecting AP solutions. Gas in solution is substantially improving the local displacement efficiency and needs to be included to correctly determine incremental oil production from AP flooding.