Study on the application of surfactin for enhanced oil recovery

Surfactin is an anionic bio-surfactant produced by Bacillus subtilis. Although it has high capacity to decrease interfacial tension between oil and water and improve oil recovery, it has not been used for enhanced oil recovery because it forms precipitation by binding with divalent cations such as calcium ions which are contained in formation water. This study examined the countermeasure to prevent surfactin from forming the precipitation for applying it to enhanced oil recovery. Alcohols, chelating agents, a cationic surfactant and an ion capturing substance were selected as the candidates for inhibiting surfactin from forming the precipitation. Citric acid and trisodium citrate were selected as promising candidates through the measurements of turbidity of the mixture of the candidate, surfactin and calcium ions. Those chemicals also had a function as a co-surfactant for surfactin. The interfacial tension between crude oil and the mixture of citric acid or trisodium citrate (0.6 %) and surfactin (0.3 %) was decreased to 0.1 mN/m, whereas that between crude oil and 0.3 % of surfactin was 3.5 mN/m (see Figure 1). The permeability of a sandstone core into which the solution containing surfactin and trisodium citrate was injected was decreased significantly, whereas citric acid could be injected into the core without significant permeability reduction. Citric acid was therefore selected as the best inhibitor and subjected to the core flooding experiments. 25 % of oil remaining after the primary oil recovery was additionally recovered by the injection of the mixture of surfactin and citric acid after the primary oil recovery (see Figure 2). Moreover, the pressure drop was not increased during the injection of the mixture of surfactin and citric acid. Those results suggest that citric acid has a dual role as the binding inhibitor and co-surfactant for surfactin.