Brimstone and Treacle: Understanding Oil-Driven Microbial Souring In Petroleum Reservoirs

Reservoir souring is a widespread problem within the petroleum industry. It is characterised by an increase in sulfide concentrations within a reservoir, and it's produced fluids, over time. This reduces crude oil quality and consequently its market value, drives corrosion of infrastructure, and presents safety concerns for workers. Accommodating for these issues significantly inflates the cost of drilling and refining operations. Sulfate reducing microorganisms (SRM) are thought to be responsible for the majority of souring. SRM "breathe" sulfate, using it during respiration as an acceptor of electrons obtained from food (electron donors) and reducing it to sulfide. As souring is intrinsically linked to the activity of SRM, it is heavily influenced by conditions within an oil reservoir, such as temperature, salinity, pH, availability of nutrients and electron donors/acceptors. Due to the myriad of problems souring poses to the petroleum industry, there has been a concerted effort to understand its underlying mechanisms and produce accurate predictive models.
This project will focus on the role of crude oil composition in determining the likelihood/extent of souring observed within a petroleum reservoir. Crude oil is the main provider of electron donors for SRM, and some oils contain hydrocarbon fractions that have displayed toxicity to microbes. As crude oils show massive variations in their composition, they may also display significant variation in their propensity to support souring. The key aim of this project is to describe the relationship between oil composition and souring, in the hopes of improving the ability to predict how souring may develop within an oil reservoir. This will be done by sourcing a wide array of crude oil samples, and analysing them using cutting edge chromatography techniques to gain a detailed picture of their composition. Using laboratory microcosms as analogues to oil reservoirs, the souring potentials of these oils will be assessed. Using these highly controlled laboratory systems will allow for a definitive assessment of composition/souring relationships, unachievable simply by in situ observations of oil reservoirs. In addition to testing the souring potential of each oil, detailed analysis of the microbial communities they support will be undertaken using metagenomics. This will hopefully allow for a deeper understanding of the interactions between oil composition and souring, as a detailed view of the microbial communities supported by an oil can be established. Initial areas of investigation will involve establishing general trends between the differences in souring potential of non-biodegraded (light) oils compared to more biodegraded (heavier) oils. Less biodegraded oils are more abundant in smaller hydrocarbon fractions that are more easily used as a source of electron donors by reservoir microbes, however they also contain the higher concentrations of toxic hydrocarbon fractions. Once a general trend is established there will be further probing into the nuances of this relationship, as in the real world two oil reservoirs containing oils with similar levels of bio-degradation can display vastly different levels of souring. Subject to this research, further study into individual oil components, or groups of oils components and their culpability in supporting/inhibiting souring may be undertaken. Microcosms present a useful tool to investigate souring under highly controlled conditions and will be used extensively during this project, however there are certainly criticisms as to how well they truly represent a complex reservoir environment. Later stages of the project will therefore involve the use of advection based systems to study souring, in the form of packed bioreactors which are more representative of the turbulent conditions within an oil reservoir during drilling activities.