Abundance and diversity of microorganisms, including H2S-producing and corrosive bacteria, in the produced water collected from different locations at a Central Arabian Gas-Oil Separation Plant (GOSP) were examined by culture-based and culture-independent methods. Conventional MPN techniques showed that abundance of general heterotrophic bacteria exceeded acceptable levels more frequently in water samples from High Pressure-Production Trap (HPPT) and Water-Oil Separator (WOSEP) than in water from Production Header (PH). MPN assay of corrosive sulfidogenic sulfate-reducing bacteria (SRB) and thiosulfate-reducing bacteria (TRB) exceeded acceptable levels in 11% of samples. Cellular ATP and qPCR tests confirmed that bioburden and SRB exceeded acceptable levels more frequently in samples from HPPT and WOSEP than in the PH water, for instance, SRB exceeded acceptable levels in 67% of PH samples and 89% of samples from HPPT and WOSEP. Profiling microbial population using 16S rRNA gene amplicon sequencing revealed that microorganisms in the PH water differed significantly from microbial community in the HPPT and WOSEP samples. Bacterial amplicons predominated over archaeal in all examined samples. Microorganisms in the PH water were chiefly represented by aerobic heterotrophic bacteria and fermenters. In contrast, the HPPT and WOSEP samples contained a diverse bacterial population specialized primarily on the sulfur redox reactions. In conclusion, this study demonstrated that processing of produced fluids at the GOSP prompted the increase in H2S-producing and corrosive bacteria in the water samples from HPPT and WOSEP. An effective microbial monitoring and control programs for oil-producing facilities have to integrate multiple lines of evidence on the bacterial abundance, activity and diversity in order to prevent progressing of microbial souring and corrosion through the water handling facilities and reservoir.
Water-handling oil producing facilities often become target for microbial contamination because treated waters are not sterile – they are inhabited by various microorganisms and contain sufficient inorganic and organic nutrients to support microbial growth1. The bacterial contamination and bioburden are to extravagate easily if environmental conditions in the facilities, for instance, moderate temperature (<45°C) and salinity (<50 g/l TDS), favor microorganisms. Growing bacterial population distributes along the system and forms biofilms on the surfaces of pipelines, valves, vessels, tanks, etc. Such spreading of free-floating (planktonic) and sessile (biofilm) bacteria in industrial systems is referred to as biofouling2. Developed biofilms, particularly if complexed with sediments, scale and sludge, negatively impact flow properties posing operational challenges3. Bacteria oxidize residual oil organics to multiple metabolites (i.e., volatile organic acids and CO2) that trigger corrosion. Furthermore, the presence of sulfates in the water promotes a growth of sulfate-reducing bacteria and archaea that reduce sulfates to sulfides via oxidation of available organic and inorganic electron donors4. This phenomenon of emerging biogenic sulfide, known as bacterial souring, is a marker of elevated corrosion risks due to high corrosivity of sulfides. Souring in the crude processing systems presents other concerns to petroleum producers, i.e., high health and environmental threats due to the sulfide's toxicity, as well as economic loss, since sulfide partitioned into the hydrocarbon phase reduces its commercial value1. Modern engineering practices include regular monitoring of deleterious microorganisms in the systems of interest in order to avoid bacteria-related implications. This monitoring commonly relies on the quantitative assessment of bioburden using standardized techniques: ATP (adenosine tri-phosphate) bioluminescence assay, most probable number (MPN), quantitative polymerase chain reaction (qPCR) and others. Each of the methods has its limitations that can influence the data interpretation. Therefore, a combination of methods is to be used in parallel, along with sophisticated evaluation of the analytical results (tailored to a particular case), in order to avoid incorrect conclusions. Here we report key findings from microbiological assessment of water samples from Central Arabian oil-water separation facility, and discuss the challenges and solutions of data interpretation.
