Feb-2025

Key elements of flow assurance in carbon capture and storage

The challenges and strategies involved in optimising CO2 flow for safe and effective storage.

Abbey Grant-Belltree
Belltree Group

Article Summary

Emerging technologies such as carbon capture and storage (CCS) are crucial for reducing and removing anthropogenic carbon dioxide (CO₂) emissions from the atmosphere. Since the Paris Agreement in 2015, countries worldwide have set targets to limit global average temperatures from reaching 1.5°C above pre-industrial levels. The urgency to meet these targets has driven the development of CCS projects from previous concepts of CCS, such as enhanced oil recovery (EOR), which began in the 1970s. There are now 50 commercial-scale CCS projects operational and 534 in development worldwide (Global CCS Institute, 2024).

Flow assurance plays a critical role in the CCS process, ensuring that CO₂ can be transported from its point of capture to its permanent storage site without disruptions. This involves maintaining CO₂ in its supercritical state during pipeline transport, managing pressure and temperature to prevent phase changes, and addressing potential risks like corrosion and blockages. New and developing CCS projects can anticipate and mitigate these challenges by making flow assurance a key factor in the successful deployment of CCS at scale, drawing on decades of experience with CCS in the oil and gas industry.

Selecting the appropriate subsurface storage location is also a crucial component of any CCS project. Using data-driven tools such as bMark can allow screening of potential storage prospects to help identify the optimum storage site. This article explores the intricacies of flow assurance in CCS, highlighting the challenges and strategies involved in optimising CO₂ flow for safe and effective storage.

Supercritical CO₂
CO₂ can be transported in any form, but it is often compressed into a liquid because it occupies significantly less volume compared to its gaseous form. When transported via pipeline, it is most efficient for CO₂ to be in its supercritical state with pressures higher than 74 bar and temperatures higher than 31°C. This represents highest point in which it can exist as a vapour and liquid in equilibrium (TWI, 2010). In this state, CO₂ has the density of a liquid but the viscosity of a gas (Drax, 2022). The viscosity is up to 100 times lower than that of the liquid phase (TWI, 2010), significantly reducing drag during pipeline flow. This improvement increases CO₂ throughput, leading to lower operational costs.

Figure 1 shows the various transport methods for CO₂ associated with the phase during operation. The difficulty with transporting CO₂ in this state is that it must remain supercritical, requiring the temperature and pressure to be maintained to avoid phase changes.

Pipeline transport of supercritical CO₂
Various methods for transporting CO₂ are available, including pipelines, ships, road, and rail. However, pipelines remain the most cost-effective and widely used option, particularly for long distances and larger volumes of CO₂. As shown in Figure 2, pipelines generally have a lower cost compared to ships for distances up to 800 km. Offshore pipelines, especially in regions like the North Sea, will be crucial for the development of commercial-scale CCS projects.

Currently, the US leads the world in the number of operational and developing CCS projects, supported by its extensive 5,000+ mile onshore CO₂ pipeline network. Many existing pipelines previously used for oil and gas can be repurposed for CO₂ usage, which helps to reduce the cost significantly. Reusing pipelines typically costs around 1-10% of constructing new ones (Drax, 2022) while also minimising the need for new infrastructure. However, to meet climate targets in the next few decades, approximately 100 times more pipeline infrastructure than currently available will be required (Global CCS Institute, 2018).

To support the expansion of CCS pipeline networks, tools like bMark provide access to up-to-date data sources, including information on existing oil, gas, and CO₂ pipelines worldwide. This data is essential for planning, designing, and scaling up pipeline infrastructure effectively. Figure 3 highlights the pipeline data for the US, available on bMark.

Safety of CO₂ transport
One of the main public concerns about CCS is the safety of CO₂ transport in pipelines and its secure storage underground. However, in EOR projects, CO₂ has been transported over decades, so it is not an unfamiliar technology. It poses no greater risk than oil and gas transport, which has a long history of safe management. Additionally, transporting CO₂ is much safer than other substances as it is not explosive or flammable when mixed with air (Drax, 2022). A combination of governmental legislation and international standards will ensure maximum safety in CO₂ transport. Ultimately, one of the most important steps in safeguarding CO₂ transport via pipeline is flow assurance. This critical process involves understanding and mitigating the factors that could disrupt the flow of CO₂ from the capture point to its storage location.

Flow assurance
Flow assurance was initially based on ensuring the successful and economical flow of hydrocarbons from the reservoir to their destination. However, this now directly applies to the transport and injection of CO₂ during CCS projects. Flow assurance studies are an essential part of the design process for oil and gas operations, such as the front-end engineering and design (FEED) process. Factors affecting flow assurance include the formation of solids/blockages, pressure drop, temperature variation, and purity of the CO₂.

Among the factors involved in flow assurance, the primary concern is the potential for blockages within pipelines, whether it is hydrocarbons or CO₂. Studies focus on preventing and controlling the formation of solids which may block the flow. Many things can affect this, especially the pressure, temperature, and chemistry of the product flowing through the pipeline.

Pressure drop is one of the key concerns in flow assurance, with many contributing factors affecting wellbore pressure, fluid properties, and temperature. Here again, CO₂ injection projects can use the experience gained from the oil and gas industry. In hydrocarbon production, wellbore pressure is a significant factor in pressure drop, with up to 80% of the lost pressure occurring during the flow from the subsurface to the surface.