Contribution of make-up water to the microbial community in an oil field from which oil is produced by produced water re-injection

Much of the world's oil is produced by water injection. Produced water (PW) is often mixed with make-up or source water (SW) in a water plant and re-injected. The effluent from a sewage treatment plant is used as the SW in an oilfield in Southern Alberta. Untreated SW (SW-U) was subjected to continuous chlorination, as well as to quarterly treatment with the biocide acrolein, then filtered (SW-F) and further treated with the oxygen scavenger ammonium bisulphite. The treated water (SW-T) was then mixed with PW in the water plant. We aimed to determine whether the mixing of SW and PW created favorable conditions for microbial growth. Viable anaerobic heterotrophs, enumerated on tryptone-yeast extract plates, were 100–102 ml−1 for SW-U and SW-T, but higher for SW-F (105 ml−1), whereas those in PW were between 105 ml−1 and 106 ml−1. Mixing of the two types of water increased the CFU ml−1 by maximally 4-fold, indicating some additional growth. Analysis of the microbial community composition by pyrosequencing indicated that the PW samples were dominated by methanogens (Methanoculleus, Methanolinea, and Methanosarcinales), which were present in much lower fractions in SW samples. The latter had high fractions of Clostridium, Comamonadaceae, and Flavobacterium, which were not found in PW samples. The microbial communities in SW samples were more diverse than those in PW samples (Shannon index 2.36 to 4.05 and 1.49 to 2.16, respectively). The 44 most prominent taxa were all represented in SW samples, but only 25 of these were represented in PW samples. The results indicate that the microbial community, currently residing in the field, may have originated from the SW despite efforts (chlorination, biocide) to limit inoculation of the field with SW microbes.