Displacing viscous oil by water leads to poor displacement efficiency owing to the high mobility ratio and viscous fingering. Polymer injection increases oil recovery by reducing viscous fingering and improving sweep efficiency. We are showing how Alkali-Polymer (AP) flooding is substantially improving production of reactive viscous oil from a Romanian oil field. IFT measurements, coreflood and micro-model experiments were used to understand and optimize the physico-chemical processes leading to incremental oil recovery.

Extensive IFT measurements were performed at different alkali and AP concentrations. In addition, phase behavior tests were done. Furthermore, micro-model experiments were used to elucidate effects at the pore-scale and as screening tool for which chemicals to use. Single and two-phase coreflood experiments helped defining the displacement efficiency on a core scale. Various sequences and concentrations of alkali and polymers were injected to reduce costs and maximize incremental recovery of the reactive viscous oil.

IFT measurements showed that saponification (110 μmol/g saponifiable acids) at the oil-alkali solution interface is very effectively reducing the IFT. With time, the IFT is increasing owing to diffusion of the generated soaps away from the interface. Phase experiments confirmed that emulsions are formed initially. Micro-models revealed that injection of polymers or alkali only leads to limited incremental oil recovery over waterflooding. For alkali injection, oil is emulsified due to in-situ saponification at the edges of viscous fingers. AP injection after waterflooding is very effective. The emulsified oil at the edges of the viscous fingers is effectively dragged by the viscous fluid substantially increasing recovery. Corefloods confirmed the findings of the micromodels. In addition, the effect of di-valent cations for the selection of the polymer concentration was investigated. Water softening leads to significantly higher viscosity of the AP slug than non-softened brine. Reducing the polymer concentration to obtain the same viscosity as the polymer solution containing divalent cations resulted in similar displacement efficiency. Hence, significant cost savings can be realized for the field conditions, for which AP injection is planned after polymer injection.

The results show that alkali solutions lead to initial low IFT of reactive viscous oil owing to soap generation at the oil-alkali solution interface increasing with time due to diffusion. Injecting alkali solutions into reactive viscous oil is not effective to reduce remaining oil saturation, a limited amount of oil is mobilized at the edges of viscous fingers. AP flooding of reactive viscous oil is substantially increasing incremental oil recovery. The reason is the effective dragging of the mobilized oil with the viscous fluid and associated exposure of additional oil to the alkali solutions. Furthermore, the economics of AP flooding projects can be substantially improved by adjusting the polymer concentration to the AP slug containing softened water.

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