Improving oil recovery in mature carbonate reservoirs is challenging due to inherent harsh conditions, i.e. high temperature, salinity and divalent ion concentration. This paper reports the development of a surfactant formulation that injected in sea water resulted in an incremental oil recovery higher than 20% at 120°C in carbonate field rocks.

A systematic study comprising blend and salinity scans of a lineal alkyl benzene sulfonate (ABS) and a co-surfactant was done in order to select a suitable formulation optimized to be injected in sea water. The formulation performance was evaluated by measuring the solubility, interfacial tension (IFT) and phase behavior. The robustness of the formulation was assessed by changing the surfactant concentration and the brine salinity.

Sandpack experiments with the addition of tracers were performed to determine the dynamic adsorption and coreflood experiments in low permeability reservoir cores were conducted to determine the injectivity, incremental oil recovery and surfactant adsorption/retention.

The formulation was designed by blending at different ratios lipophilic surfactants that reduce the oil/water IFT with more hydrophilic co-surfactants that improve the solubility. The obtained formulation, comprising an ABS and a co-surfactant, was stable at ambient temperature and 120°C. It was optimized to be injected at sea water salinity providing ultra-low IFT (<0.005 mN/m) and creating a microemulsion. The designed formulation was tested by injection in a previously water flooded low permeability core. Promising results were obtained in terms of incremental oil recovery (+20%) and injectivity, but too high adsorption/retention produced a delay on oil production.

In order to reduce surfactant losses, an optimization step consisting on the addition of a mineral oil and/or a low MW polymer was faced. A substantial reduction on adsorption/retention, maintaining incremental oil recovery values, was achieved by adding a specific amount of a specific mineral oil and/or a low MW polymer.

The results presented show a robust formulation able to produce high incremental oil recovery under harsh carbonate reservoir conditions.

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