InnovOil looks at how data is helping to combat oilfield souring
Oilfield souring has plagued the oil and gas industry for decades. Oilfields that once produced sweet oil gradually start producing hydrogen sulphide-rich sour oil instead.
In order to understand the causes of this better, oilfield services specialist Rawwater has run a suite of more than 30 bioreactors for 12 years, collecting 400 years’ worth of oilfield souring data.
InnovOil spoke with Rawwater’s founder and managing director, Dr Robert Eden, and senior project officer, Matthew Streets, about what their data reveal about oilfield souring.
In the 1980s, the mechanisms behind oilfield souring were unknown. “Back then, there were a number of candidates’ mechanisms, and most of these mechanisms were purely chemical,” Dr Eden said. “One of the mechanisms was biological, and it didn’t have much credibility. What my team at the University of Manchester Institute of Science and Technology did was look at all the different candidates for souring, not expecting it to be biological, but at the end of the two-year study, we discovered that souring was a biological problem.”
The culprit was revealed to be bacteria, which enter a well when seawater is pumped into it in order to maintain its pressure. “The introduction of this seawater introduces microbiology, which are able to generate hydrogen sulphide,” Streets explained. “They chemically reduce sulphates, which naturally occur in seawater, and there’s plenty of it in seawater for them to use, in a similar way we use oxygen in the atmosphere. They chemically reduce the sulphates and they convert it to sulphides.”
Eden added: “They’re happy in these reservoirs, in the warm temperatures and the high pressures, and they will convert parts of the crude into their own biomass and in the same way that we eat food, they eat the crude oil. It’s the destruction of the crude oil that generates the hydrogen sulphide.”
Working under pressure
Oilfield souring causes a number of problems for oilfield operators. Not only is sulphurous oil less desirable than low-sulphur oil, the corrosive effects on oilfield infrastructure can be disastrous. This is especially true for assets designed for use with sweet oil and not treated for exposure to sulphur compounds.
“When these oil reservoirs were going sour, there was bit of a panic as to why the reservoirs were going sour. How can we save ourselves the consequence of a cracked pipeline, environmental damages and reputational damage?” Eden noted.
In order to gather the data, Rawwater built a large collection of bioreactors, steel tubes filled with sand and a variable amount of oil, and then injected them with bacteria-containing filled seawater. Rawwater then measured hydrogen sulphide production over time.
“The most bioreactors we’ve been running is 85 bioreactors all at once,” Eden said. “We’re pretty sure we have the largest suite of these bioreactors in the world.”
Rawwater’s bioreactors are an improvement on conventional bioreactors used in similar studies. Conventional bioreactors are generally unpressured, and thus fail to simulate downhole conditions accurately.
Rawwater’s bioreactors are pressurised to reflect life in an oil reservoir better. Operating conditions range from atmospheric pressure to 12,000 pounds per square inch, at temperatures from five degrees centigrade to just below the boiling point of water.
“You can have three bioreactors running at atmospheric pressure, under all of the same conditions as a pressurised set of bioreactors, and you get quite different groups of bacteria growing in the different pressure conditions, and these different bacterial communities act quite differently for under different treatment scenarios,” Streets explained.
Bad eggs and good eggs
Oilfield souring has been rumoured to consume around a third of typical production chemistry budgets. As such, oil majors and chemical service companies have been keen to research the phenomenon with an eye to finding a permanent solution.
“One of the big service offerings and one of the large-scale studies that we’re running is a joint industry project called the Seriatim series of work,” Streets said. “We’ve been running this with a number of the big blue chip oil companies.”
Seriatim has had over US$10 million of funding provided to it since 2006 by its investors. “The data we collect from our pressurised bioreactor suite within the Seriatim project imparts data into theirs and our souring simulators, which allows them to conduct field assessments on an asset-by-asset basis to determine the likelihood of sour gas production in one reservoir compared to another, and if it is going to support significant sulphide production, to what extent in order for them to set up for adequate metallurgy and or chemical basing programmes,” Streets said.
The Seriatim project has helped identify the factors that can cause a reservoir to go sour. “From our database, we found that about 20% of oilfield reservoirs don’t sour,” Eden noted.
The goal of the funding is to solve the problem of oilfield souring once and for all. Rawwater’s data have helped them develop a number of solutions so far.
“What we have observed is that there’s no one size fits all in terms of managing a sour reservoir” said Eden. “Some reservoirs may not be producing high hydrogen sulphide concentrations, so it’s possible to scavenge it at the producer, and some reservoirs are forecast to go very sour, so the way of managing that is the water injection chemistry.
“Some reservoirs might see intermediate souring, so it could be a combination of the two. It’s all down to understanding how sour a particular oil is going to make a reservoir and whether or not the physics of the reservoir will support hydrogen sulphide production.”
The growth of oil-eating bacteria is dependent on three factors – biology, chemistry and physics.
“What we tend to say is that the biology will get down there regardless,” Streets said. “From a chemistry point of view, if you don’t have either the sulphate there or the organic carbon source from the crude oil, it won’t support biological growth.
“For physics, if the pressure downhole is far too high for life to be supported then the biology won’t grow. In a similar way, if you don’t put enough cold water down the reservoir to cool the formation you won’t generate a thermal viability shell to facilitate bacterial growth,” Streets added.
Eden added: “With the Seriatim programme, we’re still doing research. But the research is becoming very focused now, and that’s primarily focused on particular assets’ chemistries.”