CO2 Storage in Depleted Oil and Gas Fields

The centre’s CO2 storage programme is tailored to address the risks and uncertainties associated with storage in Denmark. The vision is to unlock storage potential in Denmark by reducing uncertainties and developing cost effective and low risk solutions.

DTU Offshore finalized a CO2 storage state-of-the-art study in early 2021, identifying knowledge and technology gaps. This gap analysis is the starting point for defining the scope for the research programme.

Re-using existing oil and gas reservoirs

The existing oil and gas fields provide an opportunity to accelerate the permanent storage of CO2 thanks to:

• A well-described reservoir storage capacity
• Decades of accumulated knowledge of the subsurface
• A reservoir seal which has been proven to be gas tight over geological time
• Long distance to shore and inhabited areas

The majority of the oil and gas fields in Denmark are chalk reservoirs, and the CO2 storage state-of-the-art study revealed that there are concerns about using chalk reservoirs for CO2 storage with regard to rock dissolution and mechanical integrity.

To address these concerns and to understand the suitability of the chalk reservoirs for CO2 storage, a group of research projects have worked to identify any showstoppers for CO2 storage in chalk reservoirs. The research was focusing on a narrow porosity range and the conclusion was that there are no showstoppers.

Based on these conclusions, a second phase was initiated looking at a wider porosity range, including operational aspects such as the impacts of CO2 impurities and remaining hydrocarbons to understand the operating envelope.

Well barriers

One of the main challenges when re-using existing oil and gas fields for CO2 storage is the integrity of existing well penetrations, which might result in potential leak paths if the well barriers are breached after abandonment. This is a concern as the wells and some of the abandonment barriers were designed and installed before CO2 storage was considered.

One of the concerns when injecting CO2 is the corrosion of the tubulars. Based on experimental data, researchers are developing a CO2 corrosion prediction model which includes the effect of the CO2 impurities. This model is looking at the corrosion inside the tubulars, but it is also creating an understanding of what happens in the interphase between the casing and the cement with CO2 present in the reservoir.

Another research group is looking into developing a coating to protect against CO2 corrosion, by repelling water from the surface of the steel. This coating has been validated in lab conditions and pilot testing is the next step.

Cement is the main barrier element in the abandoned wells and this barrier may be affected if exposed to CO2. Based on both experiments and modelling, research is creating an understanding of the impact of CO2 on barrier integrity.

Using shale as a barrier material for the wells will ensure integrity over geological time. Researchers are investigating where naturally occurring shales can form part of a well barrier and thereby ensure long term integrity. The swelling capabilities will also be tested within a CO2 environment.

Containment

When evaluating a potential storage site, not only abandoned wells must be evaluated for leakage potential, but the cap rock integrity must also be verified. Researchers have developed a modelling tool which can evaluate the potential for fracture propagation in the caprock during the injection period.

Monitoring

Monitoring of CO2 injection is key for public acceptance of a CO2 storage site. When injecting CO2 into the subsurface, two things must be monitored: How does the CO2 plume move in the subsurface reservoir and how do a leak from the storage reservoir develop towards the seabed.

The monitoring research within the CO2 Storage Programme covers both aspects, with the focus on developing subsea leakage monitoring systems, as this has been identified as a gap.

Various types of sensors have been developed and verified at lab conditions, ranging from single point sensors which can be deployed stationary or on a moving remotely operated vehicle (ROV), and sensor networks to cover a larger area. Work is ongoing to get the sensors pilot tested at offshore conditions.

Environmental impact

Storing CO2 in the subsurface can have an impact on the marine environment in case of a leakage. It is therefore important to establish a pre-injection environmental baseline to be able to understand the impact of a potential leak. Research is ongoing to develop a toolbox for environmental baselining of CO2 storage sites.

If a leak is developing, it will most likely not be pure CO2 which will be seen at the seabed. As CO2 is injected with impurities into the storage reservoir, the CO2 will react both chemically and biologically with the surrounding fluids. When moving up through the formations towards the seabed the CO2 will again react with the surroundings.

Research is therefore ongoing to understand the composition of a potential CO2 leak and how it will impact the microbial environment at the seabed.

Energy transition

Research is ongoing to develop a software which estimates the energy consumption of various potential CCS (Carbon Capture and Storage) value chains, linking emitters throughout Europe with storage sites. The software includes various capture methods, depending on the flue gas origin, and various transport methods.

The offshore area will in the future be part of the energy transition in various ways, both when it comes to energy generation and energy storage. Research is ongoing to understand if the remaining hydrocarbons in the depleted oil and gas reservoirs can be converted to hydrogen by using microbial processes.

If successful this would mean that the existing installations might produce hydrogen instead of hydrocarbons, which e.g. could be used to decarbonize offshore CO2 storage facilities. Another research project is looking into whether CO2 hydrates can be used for temporary storage of excess energy.