New research being presented at the Letters in Applied Microbiology ECS Research Symposium shines a fresh light on the persistence of microbial communities that are linked to the contamination of oilfield reservoirs - a problem that costs the energy sector billions of dollars a year.

The research work, entitled ‘Evaluating VFA and Nitrate Utilisation in Complex Microbial Communities from High-Pressure Bioreactor Systems’ will be presented at the symposium on May 15 by Matt Streets, a senior research scientist at Rawwater Engineering Company Ltd.

thumbnail_Rawwater 10--3-220390 1

The research demonstrates that microbial oilfield communities, grown at elevated pressure and exposed to residual hydrocarbon as the sole carbon source for the last decade, can still utilise volatile fatty acids for both nitrate reduction and sulfate reduction. Meanwhile, sour gas generation from diverse microbial communities under high-pressure bioreactor incubations changed under different periods of shut-in, the study shows.

Oilfield reservoir souring

Matt explains: “Oilfield reservoir souring - the contamination of hydrogen sulfide, H2S, gas in production fluids - has been problematic for operators globally for decades. However, it wasn’t considered until the late 1980s that the cause could be related to microbial activity. Fast forward 30+ years, deleterious microbiology continues to cost the energy sector billions of dollars a year.

“During waterflooding operations (termed ’secondary recovery’), large volumes of water are introduced into the deep subsurface for pressure maintenance, sweeping hydrocarbon (HC) towards a production well. The injection of large volumes of cold water (often seawater) into the deep subsurface results in cooling of the formation rock around the water injection well.

“As the rock cools, microorganisms begin to attach and proliferate in this cooled region. Problematic organisms such as sulfate-reducing microorganisms (SRM) can readily convert the available sulfate from the injected seawater to H2S, a toxic and corrosive byproduct. Although seawater is relatively low in carbon, the souring communities can often access hydrocarbon components from the residual crude oil.

“After many swept volume turnovers, the residual crude oil becomes heavily washed, leaving the relatively low soluble HC components behind. The components can still provide energy for the sulfate reducers, but typically result in reduced rates of sulfide production, relative to some of the more soluble HC components prior to the ‘heavy washing’.”

Produced water reinjection

Over the last 10 years, more operators have been re-injecting produced water back into the formation, a process known as produced water re-injection or PWRI. This production water is often rich in dissolved HC components and other carbon sources, such as volatile fatty acids (VFAs). Due to injection water volume requirements, many operators commingle seawater and produced water prior to injection, and this SW/PWRI mix is therefore rich in both carbon and sulfate.

“Our research was to determine whether (i) how long could sulfide production be supported from HC-degrading conditions, (ii) how would souring rates change during different periods of shut-in, and (iii) would a mixed community be capable of quickly switching and accessing VFAs and/or nitrate to support sulfide production and/or nitrate reduction,” Matt says.

Investigating microbial souring

In 2014, Rawwater was commissioned to set up a number of sand-packed, oil-saturated bioreactors as part of the Seriatim Oilfield Reservoir Souring JIP in order to investigate microbial souring from crude oil-derived electron donors. The study operated for 21 months under daily batch flow injection in columns operating at 1,000psig and 30oC.

The bioreactors were only ever injected with biologically-active anoxic synthetic seawater and at the start of the study, the columns were all seeded with effluent from an archived, oil-saturated bioreactor from the Rawwater laboratory.

During this study, the team observed some interesting souring profiles from two of the high-pressure bioreactor columns: 13Do and 15Do. At the start of summer in 2016 (around project day 600), the columns ceased operating in the Seriatim Oilfield Reservoir Souring JIP and were adopted as in-house bioreactors at Rawwater. Rawwater was keen to explore the further development of the souring trends at elevated pressure and temperature PT conditions and continued to operate the columns over the next 2,000 days.

Shut-in periods

“We then began to explore the impact of sour gas generation under different periods of bioreactor shut-in (i.e. no injection / flow). Over a period of 18 months, we increased and then decreased the shut-in periods of the high-pressure bioreactors to determine the impact on produced sulfide concentrations (mg/l), and also the rate of souring (mg/l/day),” Matt says.

“In general, we found that longer periods of shut-in resulted in high sulfide concentrations, though the rates of souring under extended shut-in periods appeared to decrease.

“During this shut-in study, we extracted produced water samples from both bioreactors to investigate whether these planktonic communities were still capable of utilising VFAs and nitrate in atmospheric-pressure bottle tests.

“During this 7-week incubation, it was found that VFA addition could readily support sulfate reduction, and combined nitrate and VFA addition could support microbial nitrate reduction. Microbial community analysis from the bottle test study indicated a proliferation of Marinobacter, with many species of this genus known to be VFA-utilisers and HC-degraders.”

Surprising finds

The research yield two surprising finds - firstly that microbial sour gas generation could be supported by heavily-washed crude oil in high-pressure bioreactors after around 800 swept volume turnovers, and secondly the ease with which the microbial communities could still readily utilise VFAs and nitrate despite having not been directly exposed to these chemical compounds for around 10 years.

“We have demonstrated that microbial communities are capable of low-level (but significant) sulfide production from oil biodegradation, despite ‘many hundreds’ of ‘bioreactor swept volume turnovers’. This information is key for souring forecasting and modelling,” Matt says.

“We also demonstrated that different periods of shut-in under oil degradation conditions resulted in different sulfide concentrations and different rates of sour gas production.

“The injection of PWRI back into a low (but significant) souring formation would introduce sulfate and VFAs, with the latter potentially exacerbating the sour gas generation in the downhole formation. This information should be taken into account as part of the decision process to move forward with PWRI.”

Next decade

Matt is now keen to continue to operate the two long-term bioreactors to determine whether sour gas generation continues for the next 10 years.

“In addition, it would be really interesting to repeat the bottle test study and use a sample of the bioreactor core (sessile community) to inoculate the bottles. There are still a lot of unknowns regarding the correlation between planktonic and sessile (biofilm) communities, and it is always important to use a representative inoculum source to simulate any downhole geomicrobial activity,” he says.

The project was led by Matt Streets at the Rawwater laboratory and various research scientists and laboratory technicians have been involved in operating the high-pressure bioreactors since 2014.

“The genomic profiling of the bottle test communities was conducted at the University of Manchester. And a huge thank you must go to the Seriatim Oilfield Reservoir Souring JIP sponsors, for initiating the constructing and initial operation of the two high-pressure bioreactors back in 2014, and for engaging in and supporting the work over the last 10 years,” he says.

The Letters in Applied Microbiology Early Career Scientist Research Symposium will be held at University of the West of England (UWE) in Bristol, UK, on 15 May 2024.  To find out more, visit the event page.