Safe wells for Europe’s CCS highway

When thousands of CO2 wells are to be drilled – which is needed to comply with the Paris Agreement – situations will arise with inflow of CO2 into the well during drilling. How should this be handled to avoid danger to people and to the environment? What do we know about how CO2 and drilling fluids behave in relation to each other, and what do we need to find out more about? The Norwegian Continental Shelf has already been approved for storage of CO2 from Europe, and a “highway” for CO2 in the form of a pipeline network from the continent to storage formations under the seabed in the North Sea is being planned. With the help of research and technology development, we can ensure safe operations when the storage wells for CO2 are to be drilled.

Capture of CO2 from large industrial emissions with subsequent storage in geological formations has been pointed out as part of the solution to the climate crisis. Here, the North Sea stands out as a very suitable area for storage, with capacity large enough to handle emissions from all of Europe. To pump CO2 into the formations, wells are needed – many wells – if it is to make sense in the big picture. To fully realize such CO2 storage, several thousand wells are required globally. In Norway, we have decades of experience from drilling oil wells – but can we do everything the same when we drill wells for CO2 storage and what are the risks associated with this?

Experience from the oil industry

One of several points that stand out as a possible challenge is well control – how to carry out the drilling process, especially to ensure that there is no inflow of gas or liquid from the reservoir and into the well, and further up towards the drill floor. Since oil exploration began in the North Sea in the 1960s, thousands of wells have been drilled on the Norwegian shelf, mostly into areas where petroleum is believed or known to exist, i.e. oil and/or natural gas. When drilling into a formation with oil or gas, care must be taken to ensure that this does not flow into the well. By having a high enough pressure in the well in relation to the pressure in the reservoir, this is avoided, but at the same time the pressure cannot be too high either, because then the wall of the well can fracture. In other words, you have to find the right pressure level. It has happened that one has failed at this, and then the consequence can be that you get an inflow of oil or gas, also called a well control event. Gas can be particularly scary to get into the well, as this will expand dramatically as it moves upwards towards lower pressure. If you do not manage to regain control, the result can in the worst case be catastrophic. The most famous example in recent times is the Macondo accident in 2010. Fortunately, a lot of knowledge and experience has been gained in the industry, both through learning from accidents and near misses, and through targeted research.

CO2 vs. natural gas

Fortunately, much of the knowledge and experience from oil and gas wells is transferable to CO2 wells and will be useful here. However, new risk situations can arise when drilling wells for CO2 storage. This is especially true when drilling wells into formations where CO2 has already been stored. Then a whole new risk comes into the picture: What if CO2 that has already been pumped into the formation starts to flow into the well that is being drilled? Fortunately, this is not expected to happen very often, as there are good routines with solid safety margins built in to prevent this. But one has to be prepared for it to happen, and one thing that can be said right away is that CO2 will not behave in the same way as natural gas.

Doesn’t burn, but…

Natural gas, with methane as its main component, is highly flammable, while CO2, on the contrary, can be used to extinguish fires. One of the reasons why CO2 is well suited for this is that the gas is heavier than air and thus settles like a blanket over the fire and suffocates it. This property is a concern when drilling storage wells for CO2.  Although it is good that CO2 does not catch fire, CO2 that comes up on the drill floor during the drilling process will be a major safety risk. Since it is heavier than air, it will not be easily ventilated to the surroundings. If the concentration of CO2 in air exceeds 8%, the air mixture is fatal to humans.  CO2 that is depressurised as it flows towards the drill floor can be cooled down (Joule-Thomson effect). Locally, CO2 can thus rapidly cool safety valves and put them out of action. This presents completely different challenges for equipment and personnel than natural gas. Another possible side effect of this is that cold CO2 together with aqueous drilling fluid can form hydrates, a solid similar to snow or ice crystals. Such crystals can cause pipes and valves to plug, with the ripple effects this can cause.

Large volumes – important to control the critical drilling operation

Otherwise, there are many differences in how these two gases behave under the pressures and temperatures typically found in a well. CO2 will typically be in the supercritical or liquid phase when it enters the well. It has significantly higher solubility in the drilling fluid than natural gas. Compared to natural gas dissolved in drilling fluid, outgassing from dissolved CO2 in drilling fluid will occur abruptly and much higher up in the well. In such a situation, there will be a potential for large, difficult-to-handle volume streams of drilling fluid and CO2. CO2 that mixes with the drilling fluid may also change the properties of the drilling fluid, such as density and viscosity, and reduce the ability to maintain sufficiently high pressure in the well.

The video link below can give an idea of the process of outgassing CO2. The cylinders both contain base oil, the main component of an oil-based drilling fluid, respectively without CO2 and with CO2 loading. With the help of ultrasound, a process is triggered that allows CO2 to be released from the liquid. This represents what can happen in a liquid with dissolved CO2 flowing up a well towards the drill floor, where pressure in the liquid drops and causes CO2 to exit the liquid as a gas. We see that when CO2 is released, the volume increases dramatically.

As with petroleum wells, it is important to control the pressure in the well so that the inflow of CO2 does not occur. However, should such a situation arise, it is crucial to know how the incoming CO2 will behave together with drilling fluid that fills the well. This knowledge must be incorporated into the companies’ procedures so that the inflow can be detected early and the right measures taken.

What is SINTEF doing to secure drilling operations for CO2 storage?

SINTEF has been a research partner in the Climit Demo project “Well control for CO2 wells”, led by eDrilling and supported by Gassnova, operators and supplier companies. SINTEF has carried out laboratory experiments with drilling fluids and CO2, and based on this developed calculation models for how CO2 affects the drilling fluid and the condition of the well. This in turn has been integrated into existing models for flow, pressure and temperature in the well as a tool for simulating well control scenarios, i.e. to map the severity of inflow of CO2 during drilling.

The project is now moving forward as a Joint Industry Project to further improve these models. Among other things, the effect of CO2 contaminated with SOx or NOx or mixed with natural gas is investigated. Furthermore, the risk of hydrate formation and mechanisms for how quickly CO2 is released from the drilling fluid as it flows upwards in the well will be investigated.

SINTEF’s contribution to the project will be important for securing the reputation for CO2 storage in the future, especially when CO2 storage is used on a large scale with the extensive drilling activity that this entails.