BIOGEOCHEMICAL CHANGES ASSOCIATED WITH BIODEGRADATION OF SYNTHETIC DRILLING FLUIDS

The Antarctic Drilling Program (ANDRILL) Southern McMurdo Sound Project (SMS) utilized readily biodegradable drilling fluids to recover an 1142 m long sedimentary rock core. During the drilling operation a total of greater than 5.60×10⁵ liters of drilling fluid were lost down the borehole. The drilling fluids consist of McMurdo surface seawater and sixteen densifying additive compounds. Synthetic drilling fluids were constructed, in the biogeochemistry laboratory at Northern Illinois University, chemically identical to those used at the SMS drill site. Fluid components were purchased from vendors and mixed individually in sterilized seawater (Catalina Water Company, LLC). Isolation of each fluid additive allows for the measurement of chemical change caused by the degradation, both biologic and abiotic, of each compound. To investigate biodegradation, the synthetic fluids were inoculated with Colwellia (ATCC BAA-681) and Psychrobacter species (ATTC 700754); two of the most dominant microbial species in Antarctic surface seawaters. Degradation experiments were setup by incubating 50 mL aliquots of synthetic fluids at temperatures documented in the subsurface, between 4°C and 56°C, at depths which the SMS drilling fluids were lost. Individual aliquots were removed from incubation and tested for geochemical changes associated with biodegradation. Geochemical parameters tested include: pH, dissolved oxygen, oxidation/reduction potential, cation and anion concentrations, and organic compound composition. The organic chemistry was measured using a High Performance Liquid Chromatograph coupled with a Mass Spectrometer (HPLC/MS). Results from this study are an essential step towards understanding the impact of current drilling methodologies on subsurface geochemistry and microbiology.