Decommissioned oil and gas fields can play climate-friendly role as carbon storages

Important link to industry

According to Charlotte Nørgaard Larsen, the centre’s detailed understanding of the North Sea subsurface is a huge advantage, as a large part of the data collection needed elsewhere can be skipped. This makes it both easier and cheaper. In this way, the centre’s historical role as a research platform for the oil and gas industry gives DTU Offshore a head start in solving some of the challenges inherent in carbon storage.

“The combination of geological knowledge and engineering does not exist anywhere else in the Danish research environment,” explains Charlotte Nørgaard Larsen, before pointing out another important element:

“We have two handfuls of employees with backgrounds in the offshore industry, so we have this link to the industry. Over the years, we’ve built a role where we ensure that research in oil and gas extraction is brought together and used in the industry. This is a role and a connection that we can now use to accelerate carbon storage in Denmark.”

Denmark’s chalk reservoirs

The majority of the oil and gas extraction from the Danish part of the North Sea has come from chalk reservoirs a few kilometres below the seabed. Here, oil and gas has been stored in microscopic pores in underground chalk, which can now be used to deposit CO2. However, using the reservoirs for carbon storage creates a unique, because when CO2 comes into contact with water, carbon dioxide is formed. Since acid dissolves chalk, unintentional dissolution of chalk in the reservoirs might be a risk, which could possibly affect the reservoir’s strength properties.

DTU Offshore has therefore conducted a number of laboratory experiments where they have pumped CO2 into chalk samples from Danish oil and gas fields under the same temperature and pressure conditions as would be present in the underground. The experiments have shown that no issue will arise to prevent carbon storage.

Sealed storage

One thing is to fill a reservoir with CO2, but the carbon storage must also be properly sealed to prevent the greenhouse gas from fizzling back out. There are two essential elements to this: the wellbores and the sealant rock which is located right on top of the reservoir.

The plan is to reuse some of the wellbores used for oil and gas extraction. But since a completely different gas will now run through the installations, it must be ensured that no unfortunate reactions will occur. Especially corrosion, i.e. undesired decomposition of metals, is a worry. In collaboration with DTU’s academic environment within construction and mechanical engineering and with Aarhus University, model calculations to predict corrosion are carried out, and a coating that can prevent corrosion of the drilling pipes is being developed.

In abandoned oil and gas fields, you will also typically find older wellbores that have been sealed with a certain type of cement. Here, it is important that the cement retains its seal integrity so that the stored CO2 does not leak through the micropores in the chalk and reenters the atmosphere via the wellbore. This is one of the research areas that DTU Offshore is also working on.
Oil and gas fields always have clay rock above the reservoir itself which functions as sealant. It is this geological seal that has kept the hydrocarbons in the ground for millions of years before we started extracting them.

The seal will have the same role when the reservoir serves as carbon storage in the future. The research conducted by DTU Offshore aims, among other things, to assess whether it is possible to influence the clay rock so that it can enhance the sealing effect of the cement seal surrounding the abandoned wellbores. In addition, DTU Offshore performs model calculations to predict whether small fractures will occur in the sealing clay rock when pressure conditions change in connection with the injection of CO2.

But it is not just decommissioned oil and gas fields that are of interest. DTU Offshore is also investigating whether some of the so-called saline aquifers - i.e. porous geological layers with salty groundwater that occur in many places in the North Sea - can be used for carbon storage. This may significantly expand storage potential.

Storage monitoring

According to the EU directive on carbon storage, there is a requirement to monitor the storage for any leaks.

“This monitoring is clearly part of the basis for public acceptance that is needed when you start storing carbon in the underground,” explains Simon I. Andersen, Professor at DTU Offshore.

There are no methods for permanent monitoring of CO2 leakage from carbon storages to the marine environment across large areas. This creates a problem, as it can be difficult to assess whether you are observing actual leakage or natural seasonal variations when you only carry out scattered measurements at large time intervals. DTU Offshore has therefore focused on developing sensors for continuous measurements of CO2 at the seabed.

The choice fell on a fibre optic technology where the sensors themselves consist of a micrometre-thin coating of a chemical substance composed of both metal ions and organic molecules. The amount of carbon in the coating varies according to the variations in carbon content in the water. This also affects the light signal sent through the optical fibre, and this change can then be recorded and converted into the precise carbon content in the seawater, explains the professor.

The plan is to place sensor elements at intervals on the long optical fibres, which will then be laid out in a network on the seabed above the carbon storage. In this way, they can accurately record where a leakage occurs - if it occurs. This type of monitoring equipment is brand new and a patent is currently pending for parts of the equipment.

Important for climate strategies

Birgitte Dalsgaard Larsen, technical advisor at DTU Offshore, sees a great need to utilize DTU Offshore’s competences in connection with carbon storage projects over the coming years.

“Carbon storage will be an important part of Denmark’s and the EU’s climate strategies. The centre’s knowledge and expertise are therefore needed to solve the climate challenges.”

The centre is able to contribute in two areas in particular, she points out:

“One area is tailoring work programmes to address the most uncertain parameters of carbon storage. The second is to develop new technologies to ensure efficient and safe carbon storage.”