Measurement of CO2 throughout the carbon capture utilization and storage chain
Carbon capture utilization and storage (CCUS) is regarded as a key enabling technology for the large-scale displacement of natural gas by hydrogen in gas grids. While the production of hydrogen via electrolysis of water is the optimum solution and already widely used for small-scale production, large-scale production via electrolysis is not prevalent at present. Instead, large-scale production will be based on stream reforming of natural gas. Although this is a mature technology, the by-product is carbon dioxide (CO2). By incorporating CCUS into this process, the production would be significantly cleaner and help reduce anthropogenic CO2 emissions.
One of the many technical challenges to be overcome in establishing CCUS as a practical operational process is effective measurement and monitoring. Accurate measurement will be essential for environmental and safety needs and fundamental in reducing financial exposure in CO2 trading schemes. However, there are a large number of potential measurement challenges expected in CCUS, due to both the physical properties of carbon dioxide (CO2) and the processes involved in CCUS projects. CO2 is unusual because of the relationship and closeness of its triple point and critical point to the temperatures and pressures commonly found in industrial processes, Fig. 1.
Compared to other substances that are transported by pipeline (oil, natural gas and water), the critical point of CO2 lies close to ambient temperature. This means that even small changes in pressure and temperature may lead to rapid and substantial changes in the physical properties of CO2 (phase, density, compressibility).
In CCUS applications, regulating the temperature and pressure will be a difficult undertaking, particularly over long distances. Pipelines will span hundreds of miles, and be subjected to various climates and conditions, which will naturally affect pressure and temperature. Therefore, not only is there a risk of changing between phases, but also when operating on or close to a phase boundary line, multiphase flow conditions can arise. Phase changes and multiphase flow occurring at measurement points will have a significant detrimental effect on measurement accuracy, where flow meters are designed to operate in one specific phase only.
Another major challenge for measurement will be coping with impurities in the CO2 stream, which will be present and vary depending on the capture process, capture technology and fuel source used. Even trace levels of contaminants will invalidate the CO2 equations of state and phase diagram, which are based on pure CO2. Without knowing the exact phase envelope and physical properties of the CO2 stream, it will be extremely difficult to control the CCUS processes and undertake accurate flow measurement. Specifying the optimum flow metering system is dependent on the operating phase.
There are a number of other factors that may affect the measurement of CO2. The acoustic attenuation properties of CO2 can affect flow measurement using ultrasonic meters. Large pipeline diameters may limit some measurement technologies, and the corrosiveness of CO2 mixtures may, where applicable, have to be considered during the planning of measurement systems and materials.
Measurement needs. Three principal areas are essential to monitor CO2 across the CCUS chain:
- Sampling of the CO2 mixture
- Determining the physical properties
- Flow measurement systems.
Sampling of the CO2 mixture. Sampling of the CO2 stream will be necessary to determine the CO2 concentration and for the regulatory reporting of other non-CO2 components in the CO2 stream. Sampling points will be necessary at the capture plant and at various points throughout the transportation network where the composition can vary. It will be necessary to undertake continuous sampling using continuous emissions measurement systems (CEMS).
Once the composition of the CO2 stream has been measured, the physical properties can then be calculated to provide the necessary data for handling and transporting the CO2 throughout the different parts of the CCUS network and for flow measurement purposes.
Determining the physical properties. There will be a need to establish new equations of state and phase diagrams, due to to the many different impurities in CO2 streams that are likely to arise in CCUS schemes. Physical properties software modelling packages can be used to generate new data for the different CO2 mixtures. However, any such models would have to undergo validation to demonstrate the level of accuracy, as even small errors may result in serious problems during the processing and transport of CO2.
Another issue with relying on physical properties software modelling is that there can be a wide variation in results between different software packages and algorithms when used to model the same CO2 mixture. It may be necessary, therefore, to establish validated industry standards and tools (hardware/software) to minimize inconsistencies and ensure a uniform approach throughout industry. This would be a fundamental requirement in cases where different parties are sharing the same CCUS network.
Flow measurement systems. Flow measurement, in conjunction with the CO2 concentration derived from sampling of the CO2 stream, will be required to calculate the transfer of CO2 on a mass basis across the CCUS chain. For example, the draft CCUS Monitoring and Reporting guidelines under the European Union Emissions Trading Scheme require that the overall measurement uncertainty, i.e., for the combination of flowmeter and composition analyzer, be carried out within measurement uncertainty levels of 2.5%.
To achieve such levels, it will be essential to install the correct type of flowmeter at locations along the network where the flow conditions are stable, and in the single phase under which the flow meter is designed to operate. This may necessitate the use of gas meters at certain locations and liquid meters at other locations along the network. Special consideration should be given to any flowmeter selected to measure in the supercritical phase, to ensure the flowmeter is suitable, of sound design with proven accuracy within this specific phase.
To ensure and maintain a traceable measurement uncertainty for the purpose of regulatory reporting, flow measurement systems should be calibrated, maintained and checked at regular intervals. Flowmeters should be calibrated at traceable laboratories in CO2 under the conditions and ranges for which they will be required to operate. Any secondary instruments used to convert into mass flow, such as pressure, temperature and density instruments, should be calibrated and traceable to national standards and located as close as possible to the flowmeter. However, there is a lack of traceable calibration facilities in the world that can offer CO2 as the test medium. The few facilities that do exist are limited to gas phase CO2.
There are a number of potential flowmeters that may be suitable for use in CCUS schemes, some of these have already been used to measure CO2-rich mixtures in enhanced oil recovery (EOR) schemes and CCUS pilot plants. However, in general, there has not been any real validation of their performance and associated measurement uncertainty, due to the different measurement needs and regulatory requirements.
TYPES OF FLOWMETERS
The following provides a brief overview on some of the types of flow metering technologies and meters that are potentially suitable for CCUS applications.
Differential pressure flowmeters. Orifice plate meters have a long track record of measuring CO2 and are used widely in EOR applications. They can be used over a wide range of pipe diameters. Good knowledge of density and viscosity (when used under stable, single-phase conditions) will provide accurate measurement with uncertainty levels of ±1%. They are of robust design but intrusive in the pipeline, so they incur pressure drops across meter, thus necessitating the need for careful positioning in the CCUS pipeline to avoid phase changes.
Venturi meter and V-cone meters are other types of Differential Pressure flowmeters that could be used in CO2 applications. They are of robust design and can be used over a wide range of pipe diameter sizes. However, there is a lack of experience for these meter types. They induce lower pressure drops than orifice plates but are typically less accurate. However, in optimum conditions, they can achieve ±1% measurement uncertainty under stable, single-phase flow.
Volumetric flowmeters. Turbine meters have a long track record of measuring CO2 and experience in EOR applications, with reported measurement uncertainty within ±1%. They can be manufactured for any given diameter of pipe. They have a large number of moving parts, but are considered robust, reliable and have a good track record in single-phase flow. Turbine meters will work in single-phase gas, liquid or supercritical fluid, if of the correct design, i.e., a liquid turbine meter can only be used for liquid applications, and is not intended to operate in multiphase flow. If a meter encounters a phase for which it is not designed, there is a large risk of mechanical failure. They are sensitive to pulsations and need to be calibrated in the viscosity and conditions of use.
The vast majority of ultrasonic flowmeters use either time-of-flight (ToF) techniques to determine fluid flowrate. Traditionally, ultrasonic ToF meters have not been intended for CO2-rich applications. This is due to the relatively large amplitude loss of the ultrasound waves, referred to as attenuation, that occurs in CO2. The attenuation of an ultrasonic pulse comes from either classic absorption or from relaxation processes. Classical absorption is based on the effects of viscosity and thermal conductivity. The relaxation processes that lead to attenuation are based on the exchange of energy between molecular vibrations and translations.
For CO2, it is the relaxation processes that are the main contributors in terms of causing the meter to lose signal. Over recent years, an ultrasonic meter has been developed to overcome the issues caused by the attenuation. This has included the use of more sophisticated and powerful signal processing features and diagnostics. Subsequently, a number of field trials in CO2-rich applications (60% and upwards) have demonstrated accurate and comparable results with an orifice plate used as a reference. Although further validation will be necessary, and extensive development required on the majority of meters on the market, ultrasonic ToF meters have the potential to provide a high-accuracy, non-invasive CCUS measurement system.
Mass flowmeter. Coriolis meters have a demonstrated track record in EOR applications and are used for the custody transfer measurement of gaseous and supercritical ethylene. Recent developments suggest that selected meters may be able to operate and measure in two-phase conditions, although not to the levels of accuracy required for CCUS regulatory measurement. Their main advantage is the ability to provide a direct mass flow measurement. Coriolis meters are limited to a pipeline diameter of 16 in., which means that for large pipelines, a split manifold to accommodate a number of meters in parallel will be required. In this situation, consideration would have to be given to pressure drop and the impact on pipeline and process conditions.
Integrated measurement system. All of the various measurement parameters in the CCUS scheme will be interdependent of one another; sampling, physical properties, and flow measurement. Sampling the composition of the CO2 stream will provide the necessary data to determine and calculate the physical properties and phase envelope at various points. This will allow the planning of the operational processes to determine the necessary pumping arrangements and conditioning required to transport the CO2 economically and safely in the pipeline. The physical properties data and composition will feed into flow measurement calculations to determine mass flow of the CO2.
To ensure the effective control and management of the overall system from point of capture of the CO2, through its transportation to injection into the storage formation, it will be necessary to have smart measurement systems and interfaces in place to provide online tracking of the key parameters.
This could comprise CEMS systems, which feed into a physical properties calculation tool, based on industry standards that generate the necessary data and phase envelope, which in turn feeds into a flow measurement system. Comprehensive measurement systems could integrate the different parameters to cater for both the process and regulatory reporting needs, including measurement uncertainty throughout the measurement chain. Figure 2 shows an integrated measurement/tracking system in a shared pipeline.
Across the complete CCUS network, accurate measurement of CO2 at temperatures, pressures, flowrates and fluid phases will be required. These measurements require validation through a credible traceability chain. This traceability chain will provide the confidence in meter performance, financial and fiscal transactions and critically, environmental compliance. With the renewed interest in CCUS and its role in enabling large-scale displacement of natural gas by hydrogen in gas grids, it is essential that a robust metrology framework be implemented and deployed as soon as possible. The success of CCUS depends on it.
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