Dr Ken Wunch is Energy Technology Fellow at Lanxess Materials Protection Products, which provides chemical solutions for the energy industry: these deal with reservoir souring, microbial-induced corrosion (MIC) and biofilms and biofouling which can contaminate and obstruct equipment, pipelines and the reservoirs themselves.

‘Once primary pressure has stopped the production of oil, we have to inject sea water to keep the pressure up… But in doing so, you introduce literally trillions of bacteria into the reservoir.’ And monitoring these organisms is not easy: ‘The only snapshot you get is the fluid that’s going into the reservoir, and you don’t get the fluid till it comes out the reservoir. It could be a year later.’

Dr Heike Hoffmann is a consultant microbiologist with the Aberdeen-based Exploration & Production Services division of testing and quality assurance firm Intertek: ‘We’re dealing with microbes offshore, in platform installations, looking at bacteria which are dragged up with the seawater which is used in secondary water injection to increase the pressure to get more oil out.’ Real-time monitoring of bacteria in a reservoir is not yet possible, and Hoffmann confirms, ‘it’s really difficult to get samples downhole – we have to wait until they come back up.’

Dr Gabrielle Scheffer has two roles, as a director at eDNA Explorer – a platform for environmental DNA (eDNA) data used to identify and classify organisms – and as CEO of Alberta-based bioinformatics startup Decodomics. She looks at the genetic signature of the organisms found in reservoirs to see what approaches can be taken to optimise the environment.

Wunch says that in the past, the Forum dealt with traditional oil and gas reservoirs: ‘Mostly offshore, large fields that have problems with microorganisms getting into the facilities and causing infrastructure damage via corrosion or causing Reservoir Souring, where the organisms start producing H₂S [hydrogen sulfide].’ These sulfate-reducing bacteria (SRB) – also more generically known as sulfate-reducing microorganisms (SRM) – are the main issue in reservoir microbiology, because H₂S is flammable, highly poisonous and corrosive. As Wunch puts it, ‘H₂S corrodes infrastructure, kills people and sours the crude’. ‘Sour’ crude has excess sulfur in it, which must be refined out at considerable cost and emissions.

Heike Hoffmann points out that these sulfate-reducing bacteria ‘are the main culprit’ for biological souring, although some souring is caused by the geological environment itself. ‘And then we also look for bacteria which can cause corrosion in the pipelines and the systems; that’s again SRBs, but also methanogens and other types of bacteria.’

‘The bread and butter of our business is analysing samples from offshore… looking at different systems, and giving our assessment. We’re looking at the seawater injection system, at the production system, and utilities as well: the cooling medium, fire water systems, offshore diesel and crude oil storage tanks and the pipelines connecting platforms and the mainland.’

But the definition of a reservoir has changed, says Wunch: ‘There’s carbon capture, geothermal reservoirs are becoming more important, and also hydrogen storage [in] salt caverns, aquifers, and depleted oil and gas fields… these are all reservoir microbiology issues.’ With hydrogen storage, for instance, ‘the problem still exists – there are lots of microorganisms in there that can still use the hydrogen and they produce H₂S and basically sour or foul the hydrogen going downhole’.

Microbial carbon capture is a relatively new technique which involves organisms such as cyanobacteria and microalgae to turn CO₂ into biomass and, ultimately, into useful products. But this is vulnerable to interference from other organisms: Ken Wunch says: ‘It’s very difficult to introduce a product that would be able to kill one organism and completely leave another organism intact.’

So what is the best approach to mitigating the problems caused by microbes? Predictably, the answer is not straightforward. Anyone who has worked with large fuel storage tanks is aware of fuel contamination, which may be a combination of bacterial and fungal issues. This is relatively straightforward to deal with, using biocides to neutralise the organisms and separation to remove excess water. But ‘fuel contamination is more controlled’ than reservoir microbiology, says Dr Wunch: ‘We work in a wild environment.’

Biocides (sometimes referred to as souring remediation chemistry) can be applied to a reservoir in a low concentration through a continuous injection, or at a higher concentration via a batch dose application.

As Heike Hoffmann puts it: ‘It's always easier to keep a clean system clean. So, are your filtrations working? Are you using enough of the biocide? Are you injecting the biocide at the right place? We usually take a whole screen of samples and then track the bacteria through the system to see where are the hotspots and where something might be falling down.’

‘If you put the biocide in once a week, at a certain dose at a certain area, could you maybe do it twice a week but at a lower dose [so] you don't give the bacteria a week to grow up again? Can you hit them a lot more often? Obviously that's a question of budget.’

One approach is to remove the sulfate from the seawater which is to be injected into the reservoir. Ken Wunch explains: ‘Because the microorganisms basically breathe sulfate the way we breathe oxygen, and release H₂S the way we release carbon dioxide. So if you can take out the sulfate, the organisms won't be able to make H₂S. But that’s a half billion-dollar retrofit of a platform, [which] shows you the scale of the problem in reservoir souring.’

Another technique is to inject nitrate into the reservoir. ‘Nitrate stimulates nitrate-reducing bacteria [NRB], and they compete with the sulfate-reducing bacteria for food. The problem is, nitrate is a fertiliser, and when you do so, you just increase the biomass downhole, and you have to keep adding nitrate’. There has even been evidence, Wunch adds, ‘that the biomass can adapt its metabolism from nitrate-reducing to sulfate-reducing: if you don't give it nitrate any more, it just takes all the sulfate and it exacerbates the problem.’

Yet another approach could be bacteriophages – viruses which can infect and destroy bacteria. But Ken Wunch says that this has not been successful so far: ‘A phage, unfortunately, is very specific. It will shut down one strain of a microorganism – and the microorganisms adapt very quickly.’

In fact, the bacteria have adapted to some pretty nasty environments: ‘It was assumed that if a reservoir was at around 100 degrees that there would be no bacteria surviving,’ says Heike Hoffmann, ‘but nowadays we know that’s not true’. Organisms known as ‘extremophiles’ are now known to thrive in very high temperatures and pressures. ‘So there was the assumption that the bacteria causing the problems must have been introduced. But there might be some indigenous bacteria which cause problems as well.’

Ken Wunch asks: ‘Are we adding organisms from hydrothermal vents that are going downhole? We are adding extremophiles, because… some of these reservoirs can be 80–100°C , 5,000 psi [>300bar] and anaerobic and exposed to a variety of different harsh chemistries. And they are surviving and doing quite well in these environments.’

So what subjects will be discussed at the next Forum? ‘Carbon capture and hydrogen storage’, says Heike Hoffmann, ‘but it’s mainly analytical advances. Traditionally the culture-based method was used, but not everything can be cultured and especially from extreme environments. So more molecular methods are coming in, looking more for DNA.’ A recent development is that ‘we can look for functional genes of bacteria. Just because this bacteria is there doesn't mean that they're actually causing any harm.’

The latest techniques can see if a particularly harmful gene is active. Scheffer’s firm Decodomics is ‘looking at specific genes in bacteria for oil and gas operation. So we're looking at souring potential, corrosion potential and instead of looking at just the type of bacteria – do I have a Pseudomonas there – we're looking at that specific potential for the activity.’

‘Right now, people infer what the activity might be based on literature of what Pseudomonas has been doing… we go a step forward with metagenomics: some Pseudomonas might have these genes for hydrocarbon degrading, but not all of them. I’m going to look for that specific gene which we know produces that sulfide.’

Scheffer points out that this sort of precise understanding will become even more important as organisms become resistant to chemical or biological treatments: ‘Bacteria evolve so fast, right? Because they can replicate a lot faster than anything else.’ She believes that other sectors could learn from reservoir microbiology: ‘I think it's very transferable to many different industries, because bacteria are everywhere.’

• Further reading: Cracking the carbon storage code, from well test to success. What is CCS, and why is well testing the foundation for successful projects? The answer lies deep underground, in the geology of the subsurface, writes James Yard, Senior CCUS Development Manager at international well services company Expro.
• Getting to know CO₂. The UK is developing three carbon capture and storage (CCS) hubs, HyNet in Liverpool Bay, the Peak Cluster and the East Coast Cluster around the River Humber. They all envisage pumping quantities of CO₂ gas through pipelines. While those facilities are being designed and built, more searching questions about CO₂ streams – in terms of the physical plant, CO₂’s large-scale behaviour, and what are the maximum limits of impurities in the stream – are being tackled through fundamental research.