Decarbonisation Technology - February 2025 Issue

Reservoir injection As the flow of CO₂ reaches its endpoint at the storage site in the subsurface, flow assurance remains equally important due to the change in pressure. The high pressures required to ensure CO₂ remains in its supercritical state could be met by the lower pressures at the injection point, depending on the site. For example, injection into a depleted hydrocarbon field with low pressures can result in adiabatic cooling, with the reduced temperatures possibly leading to hydrate formation and freezing of pore water (Loeve et al., 2014) . CO₂ injection into an aquifer is less likely to have the risk of pressure change and freezing, as no fluids have been extracted, therefore pressure remains stable. However, the risk of lower pressure within aquifers remains, depending on factors such as the depth of the aquifer, the height of the water column, and nearby hydrocarbon production, potentially changing the regional pressure. The advanced capabilities of software like bMark include accessing historical production data from reservoirs, as well as temperature and pressure data (see Figure 4 ), which can help further understand the characteristics of a potential CO₂ storage site. CCS projects The selection of appropriate locations for permanent CO₂ storage is also fundamental to

of certain impurities, to reduce the risk of corrosion.

Single and two-phase flow The transport of CO₂ is currently limited to single-phase flow, either in its dense or supercritical form. Existing tools for the flow assurance of hydrocarbons can be used for CO₂ if it remains in a single phase. If it is kept at high pressure and temperature, CO₂ will stay in its single, supercritical phase. However, as discussed previously, any leaks or depressurisation will risk CO₂ going into two phases. The two-phase flow of CO₂ is when the gas and liquid forms coexist. The issue with two phases in flow assurance is the potential for severe slugging within the pipeline, which involves the intermittent flow of gas and liquid. Slugging can disrupt the flow, causing significant pressure and temperature variations that may cool pipelines, making them brittle and prone to rupturing (Yang et al., 2021) . Again, these issues have been learned from the oil and gas industry, where slugging is a major challenge during production. Due to these disruptions in flow, various methods have been developed to identify and handle changes in phase, including dynamic models based on mass, momentum, and energy conservation. To manage the potential of phase changes within CO₂ pipelines, it is crucial to control the temperature and pressure within the pipeline to ensure stable conditions.

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Initial pressure (psi)

Figure 4 Depth vs pressure plot of fields in the North Sea

Source: Belltree bMark