Stability and performance of poly γ-(glutamic acid) in the presence of sulfate ion for enhanced heavy oil recovery

First, the extracellular poly gamma-(glutamic acid) (PGA) was synthesized by bacillus licheniformis LMG 7559. Then, the applicability of this biopolymer in the microbial enhanced oil recovery (MEOR) was evaluated. The impact of sulfate ions, as strong potential determining ions (PDI' s) toward carbonate surfaces, on the stability and performance of PGA flooding, was investigated. The stability of the PGA solution was reduced by increasing the biopolymer content and adding salts. The contact angle measurement showed that sulfate ions played an important role in altering of wetting state to a water-wet condition (less oil-wet). After the treatment of glass and carbonate rock plates in the PGA solution (0.25% w/v), the contact angles were changed about 18 and 44 units, respectively. Also, the PGA solution with a two-fold concentration of typical sulfate ions content (SBP#2 S) changed the contact angle about 47-fold and 92-fold at the glass and rock surfaces, respectively. It was concluded that the SBP#2 S decreased the surface charge of carbonate rock to the value of -6.5 mV compared to the sulfate-free sea brine-PGA solution (SBP#0 S) with the value of -2.3 mV. Furthermore, the adsorption of PGA onto the carbonate particles and the fluids viscosity as well as the interfacial tension (IFT) between PGA solution and crude heavy oil were examined. We observed that as the PGA content increased to 0.5% w/v, PGA adsorption on the carbonate rock surface increased to 21.1 mg/g. The Freundlich isotherm model was more acceptable for the experimental results than the Langmuir model. Also, the results of viscosity measurement showed that the viscosity of PGA solutions decreased in the presence of the salts. Stable fluids were injected in a quarter five-spot heterogeneous glass micro-model. The PGA solution with the content of 0.5% (w/v) recovered 31.45% of oil in place and the SBP#2 S produced 27.82% of oil in place compared to the SBP#0 S solution (19.1%).