Biodegradation Mitigation and Protection Strategies for the Biopolymer Schizophyllan

Abstract

Abstract Polymer flooding is a widely applied enhanced oil recovery (EOR) technique using soluble polymers to increase the injection water viscosity and therefore enhance the sweep efficiency in the field. Biopolymers are an environmentally friendly alternative to synthetic polymers and can have good salt- and temperature tolerance like the polymer Schizophyllan. Because biopolymers are often biodegradable, it is important to protect them against potential microbial degradation at subsurface conditions, especially in the near wellbore region consisting of the first few meters after injection. Three individual sandpack experiments were performed to assess biodegradation of Schizophyllan at original reservoir conditions. An enriched, biodegrading microbial community was injected into all sand packs and two treatment options were tested: a) adding the biocide Bacillat after a mature degrading biofilm was developed; b) adding biocide prior to mature biofilm formation. Different biocide concentrations were tested. Rest viscosity (i.e. level of biodegradation) was determined by measuring viscosity injected and produced from the sandpack columns. Various microbial (cell numbers, metabolite production and identity) and petrophysical (differential pressure, permeability) parameters were assessed during the experiments. The results show that the relative loss in the effluent viscosity was lower than 10 % when 375 ppm biocide was added to the injection fluid 24 hours after the inoculation period (prior to mature biofilm development). Microbial cell counts were low and byproducts because of degradation could not be measured even after 80 days of injection. The same concentration proved to be ineffective to improve the effluent viscosity (90 % viscosity loss) measured in the column with a mature biofilm. Successive concentration increase (375, 750 and 1900 ppm) did not have a significant effect on viscosity maintenance and were not able to inactivate biodegradation. Initial high biocide concentrations (750 ppm and 1900 ppm) could not protect the polymer in the presence of an active biofilm Furthermore, degradation of the biopolymer could not be prevented by using 200 ppm biocide at the start of injection before biofilm buildup. This shows a strong resistance and adaptability of the biofilm towards the used biocide. Permeability of the sand packs containing a growing biofilm decreased drastically, indicated by a continuous increase in differential pressure. Our study shows that Schizophyllan could be protected from bio-degradation if the right biocide concentration is used at the beginning of the injection period. The sandpack study shows the importance of a well-designed biopolymer protection strategy prior to field implementation and the need for an early mitigation of biodegradation and/or biofilm formation. Such experiments enable the possibility to test the different biocidal treatments under reservoir-like conditions and predict biopolymer stability in the field.