Oil reservoirs become storage areas for greenhouse gases

The same oil reservoirs that have made Norwegians wealthy can also be used as storage areas for CO2. But then we must be certain that the CO2 gas will remain in place.

“The oil and gas industry has not needed to think about the risk of leakage. When oil is discovered, they know that it has been in the reservoir for millions of years, without having leaked out. They have the benefit of historical hindsight. However, as researchers into CO2 storage, we have to grapple with the drawback of future uncertainty. So we have interesting challenges ahead of us,” says Sarah Gasda at Uni CIPR.

Fossil fuels require CO2 management
CO2 storage is a relatively new activity at CIPR, where work has principally been concentrated on research into increasing oil extraction. But these areas are nonetheless closely related, says Gasda, since the same methods are used in both cases.

”Clearly our objectives differ, however we can re-apply the petroleum researchers’ expertise on how fluids move in the ground. We merely have to adjust the method a little, to adapt it to our purposes.”

The idea of storing CO2 in the ground is a result of the fact that humankind cannot continue to emit CO2 at steadily increasing rates if we wish to avoid catastrophic climate change. At the same time, fossil fuels are such a well established source of energy that it would be unrealistic to refrain from their use for the foreseeable future.

Thus, we are compelled to find a way of handling the CO2 gas that results from production and combustion, and it is therefore, as Gasda says, ”logical to put it back where we found it.” There is sufficient room to store huge quantities of CO2 in the North Sea, but there are some practical limitations.

CO2 is lighter than water, and will therefore form a layer on top of the water that is already in the reservoir. Gasda and her colleagues are looking into ways to displace the water to enable more CO2 to be added to the space. This will determine how much CO2 can actually be stored in a given reservoir.

The research chiefly involves modelling work. To investigate whether the output of the model tallies with the actual situation, they use data from for example the Utsira formation, where for many years Statoil has been storing excess CO2 from the natural gas produced at the Sleipner field.

Problem-free storage was a problem for the model
”Utsira has worked very well for Statoil, it has not resulted in any leakages or other problems. But for us, it has been difficult to simulate how the CO2 is behaving,” she says.

This is because it took an unexpectedly long time for the CO2 to gather at the top of the reservoir. What nobody knew, was that in reality the reservoir was divided into several layers, with barriers in between.

During its ascent, the CO2 was held back by these barriers, and it could only be detected at the top of the formation after several years had passed.

”If we don’t know that these boundary layers exist then we can’t build a model that will produce the correct results. Then you can ask: As no-one knew that this was going to happen, what else could there be, that we don’t know about? So what was a perfect and problem-free project for Statoil, turned into an interesting problem for us. On the other hand, the model has demonstrated its ability to simulate what was occurring in each of the layers. But we still need to figure out how the different layers are communicating with each other and exchanging CO2.”

The laboratory in the North Sea
”Norway is in a fortunate position regarding research into these mechanisms, since there are large areas in the North Sea that are of interest for CO2 storage, and the infrastructure has already been built because of the oil extraction activities. And since the storage area is situated off the coast, and not beneath our homes, it is not difficult to sell to the public. But, of course, we must take into account the fisheries, which Norway is dependent on. We need to find the right balance here,” she says.

But when you inject CO2 into a geological formation that is riddled with oil wells, won’t the CO2 simply come up again?

”That’s a good question. In the U.S., where I’m from, there are many old oil wells that are no longer in use, and that are now derelict while fluids are leaking out. But as long as we have an overview of the oil wells and look after them, we don’t think there is a risk of leakage,” she says.

The risk of leakage
The oil wells in the North Sea are normally filled with cement. Unfortunately when CO2 is mixed with water it becomes an acid that corrodes the cement. As long as the corrosion is occurring on the underside, it will take many thousands of years for the CO2 to be released, but if the cap is not completely sealing the area, it can be quicker.

”We don’t conduct research into this issue, but there are many other people that do, since there is an awareness of the problem.”

Gasda thinks that CIPR’s research on CO2 storage has the potential to grow, and efforts are underway to assemble a research consortium composed of international partners. The North Sea is used as a research laboratory to investigate how certain we can be that stored CO2 will not leak out again.

”We need to develop knowledge and tools in order to model a potential leakage. It is not so much a matter of answering the question ’will there be a leakage?’, but more about answering ’are we able to discover a leakage, if it were to occur?’. We must know that the surveillance technology is giving us meaningful information. In a sense, it is not popular to talk very much about leakages, because it is not very comforting. I think that the storage is safe and secure. But we must be able to say how safe and secure,” concludes Gasda.

June 5, 2013, 9:38 a.m.

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