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.