Understanding the microbial community dynamics in response to biocides for the improvement of reservoir souring prediction and mitigation
Abstract
Reservoir souring is the term used to depict the phenomenon that the level of hydrogen
sulfide (H2S) increases over time during the oil and gas production from a reservoir that
previously produced no H2S. Reservoir souring is usually caused by the activity of a group
of microorganisms capable of producing H2S in the reservoir, mainly sulfate-reducing
microorganisms (SRM). Souring control is critical for the safe and economical production
of oil and gas because H2S is both toxic and corrosive. Biocides have been increasingly
used for souring control, mainly to inhibit microbial growth. Biocides often fail, and the
poor understanding of the microbial community dynamics in response to biocides limits
the optimisation of souring prediction and mitigation. In this PhD project, the microbial
community dynamics in response to biocides was extensively explored using both static
microcosms and dynamic sand-packed flow-through bioreactors. Spectrophotometric
measurement and ion chromatography were used to assess the concentrations of the
products and substrates of microbial activity. DNA assays (i.e. qPCR and 16S rRNA gene
sequencing) were used to quantitatively assess a variety of microbial ecology metrics,
which represent the key microbial properties delineating the microbial community
responses (i.e. the abundance, alpha diversity and structure of the microbial community).
In two of the studies, PMA (propidium monoazide) technique was incorporated before
the DNA extraction so that dead cells were removed from the downstream DNA assays,
allowing for the assessment of the microbial community responses for the live-only
fraction of the microbial community. The studies revealed close and complex connections
between the development of souring under biocide treatments and the changes in the key
microbial ecological properties. In particular, the studies suggested that the change in
alpha diversity could be an early warning sign for the failure of souring control using the
biocides investigated in this project. The studies further revealed that biocides might
result in changes in the spatial pattern of the microbial abundance by introducing a high
abundance “shelter” zone, as well as leading to the long-term microbial community shift
towards the enrichment of spore-forming SRM (e.g. Desulfotomaculum and
Desulfurispora). Collectively, the studies demonstrated the importance and potential
practical benefits of understanding and monitoring the ecological properties of the biocide
treated microbial community. Finally, based on the findings of the studies, several
conceptual models were developed to describe the responses of the souring microbial
community to biocides. These conceptual models might provide novel theoretical
foundations for the development of next-generation souring modelling tools, which may
further contribute to the improvement of current souring control strategies.