There is approximately 6,000 km of CO2 pipeline globally, the majority of which is in North America and is used for enhanced oil recovery (EOR) purposes. Compared to other pipeline distances for natural or hydrocarbon pipelines this is a relatively small experience base.
Since 1972 in North America, CO2 has been used for EOR from some man-made sources, but the majority of transported CO2 is from naturally-occurring gas fields along the mid-continental mountain ranges and Mississippi Basin. The maturity of the systems is still limited, but experiences with these pipelines has formed a nucleus upon which to build CCS.
Current European experience rests with Statoil at Norway’s offshore Sleipner and Snovhit fields, although there is some CO2 experience on the continent. The expectation is that European CCS will evolve to target offshore storage sites, and due to the potential for CCS clusters, develop in a network format.
Requirements in CCS systems
The infrastructure needs for CCS are different from those of simpler EOR schemes. In the existing schemes the CO2 is a by-product or natural emission that is used to drive crude oil out of the ground. The economic driver is therefore the demand for oil and when the demand is not there the extraction of natural CO2 does not need to occur.
Future CCS schemes will not have the same relationship. While the sensitivity of the store is an issue, the requirements here such as flow, pressure and temperature set the downstream conditions. The emitter in CCS provides another set of upstream conditions in terms of flow rates, ramp rates, composition, temperature and pressure. Each of these systems impose conditions on the transport infrastructure. This is multiplied when considering networks.
Transportation systems have little ability to respond to variances – essentially even the flow rate is dictated by the power station or store. The dynamics of the whole chain system, with drivers at both ends, is therefore more complex.
What have the US learned?
The experience in the US has highlighted the critical issues that must be considered for CO2 service. Particular learning can be drawn from experience in the areas of inspection, corrosion, material specification, operational safety and thermodynamics.
For inspection there have been problems with dense phase or supercritical pipeline inspection and cleaning using pigs. Simple rubber scraper pigs used in the cleaning of pipelines need a lubricating fluid such as diesel, and they are adversely affected by the CO2, which damages some non-metallic materials. The use of intelligent pigs is routine for pipeline inspection, however US experience has shown that the CO2 penetrates non-metallic components. As the pig is depressurised in the receiver the systems are often subject to rapid gas decompression, destroying the pig.
Corrosion of CO2/water systems has been studied extensively, not just for the CO2 EOR industry but for processes involving the production of ammonia, urea, steam reforming and the sweetening of acid gas.
Care always has to be taken with regard to corrosion however it is clear from continued operation that carbon steel is acceptable as a material for CO2 pipelines. Although, care must be taken not only to ensure a water content limit in the entry specification, but also in the water introduced during hydrotesting, commissioning and maintenance events.
In specifying water content, the industry accepted level is conservatively specified as between 288-480 milligrams per cubic metre. In addition, the presence of other additional “˜acid gases’ such as hydrogen sulphide, and nitrogen and sulphur oxide (NOx and SOx) compounds needs to be considered.
Non-metallic components such as seals, valve seats, O-rings and even greases have also shown to be affected by CO2. Petroleum-based seals can become saturated with the high-pressure fluid and rapidly decompress when the pressure is reduced or structurally weakened. Some greases are also known to become hard and no longer effective. Inorganic materials and greases are therefore more often recommended.
Operational safety
In the US, the operational problems most reported are safety incidents. In the period from 1986 to March 2008 there were 42 reported incidents to the US Department of Transportation Pipeline and Hazardous Materials Safety Administration (PHMSA). This relates to approximately 0.36 incidents per 1,000 km per year (assuming a US pipeline distance average of 5,000 km over the period), compared to 2,447 incidents on the US gas transmission network of 488,000 km, which relates to 0.22 incidents per 1,000 km per year.
There are two other operational issues to be considered. The first is valve operation. It is recommended that all valves are slow-opening. This avoids damage to the valve and surging in the pipeline, which is particularly important for blow down valves. Where segments are above ground, thermal relief should be provided and the valve must be capable of seating under high CO2 pressures. The current practice is not to work on pressurised pipelines at all, and where necessary, sections and valves bodies are blow down before removal of a valve or other equipment item.
The thermodynamics of the fluid are generally unknown and based on the determination of properties by using equations of state. The correct selection of the equations is therefore critical. Experience has also shown that, for key operations such as start-up and blow down, the thermodynamic characteristics require much longer periods to avoid the very low temperatures that are possible with CO2.
There are other key lessons to consider: transient fluid effects, leak proving with air or nitrogen is not sufficient, spacing of block valves, the high level of sensitivity to temperature and pressure, and the attendant effect on pipeline operations.
Major differences
There are major differences, technically and economically, to be considered in CCS schemes. The EOR schemes are demand-led – the need for oil production places a requirement on the provider and the provision of CO2. The compressor operating regime is dictated by the production rate required and the geological configuration of the reservoir. In CCS schemes the same geological factors dictate the operating conditions and flow rates of the transportation system, but there is also the influence of the upstream technology. The power plant or industrial process does not fit the same operating profile as a geologic store. CO2 from a CCS-enabled plant must be accommodated or the emitter will have to free vent, incurring penalties and making the idea of a CCS scheme redundant.
This has an impact on the design of the infrastructure, particularly the downstream configuration at the storage site. While the influences from the storage sites are common with EOR, the addition of deep saline formations to the mix adds another level of complexity and another series of unknowns.
The upstream processes also vary, and while not relevant for a bespoke source to sink design, the processes are relevant for network considerations. There are a number of issues that need to be addressed in the system design. Key amongst these has to be the composition of the CO2 stream entering any transport system. Two considerations have to be made here; the first being safety, and the second being technical. In terms of safety, the impact of contaminants needs to be considered alongside CO2. It is not enough to model the dispersion of a CO2 stream; the constituent parts also need to be modelled.
In terms of design safety the methods are almost global. However this is one of the major issues in the industry today. While we can draw methods and experience from US and Canadian pipelines, there is an underlying issue. It can be argued that the EOR-based systems are more tolerant, more conservative in their approach, and with lower population densities and income-generating oil revenue, EOR can afford to be. A more defined understanding of key risks such as fracture propagation and dispersion modelling is needed for CCS pipeline networks.
In terms of system dynamics and the interactions of all the processing elements, CCS has a series of different operating modes to consider at both ends. The preference for stores is to generally be a constant flow, whereas power station emitters are cyclic and diurnal. There is little scope for change between the two. The driver here is economics and the problem affects the capture plant as well. The emitter can vent if a store closes or requires a reduce inlet rate or pressure, but there will be an associated cost. It is likely that the store will have to be flexible enough in terms of dispersed entry points or multiple store options.
For investment to move forward in CCS the issue needs to be clearly understood. The need for networks is fundamental. CCS may not succeed if the reliance is on multiple source to sink solutions. Cluster networks enable significant savings over the collected costs of A to B solutions.
Issues and approaches
There are a number of issues that need to be considered in transportation and pipelines. These issues do not prevent development but their resolution would aid CCS projects technically and economically.
Flow assurance guidelines for CO2 and a clearer understanding of the fluid behaviour is needed. Experience in the US indicates that there is an issue with surge and transient pressure. The US experience is in onshore pipelines only. Offshore pipelines present a concern are few mitigation measures can be added to offshore pipelines, particularly those that terminate in subsea completions. In this discipline there are also concerns that even at low water content in the fluid CO2 clathrates (hydrates) may form.
Importantly, the resolution of physical properties of possible fluid streams remains an issue. Empirical data to support the assumptions and outputs of the predictive methods is needed. The data must cover the range of possible contaminants and process conditions for a pipeline system.
Dispersion modelling is already acknowledged as an issue and some experimental work has been completed by BP and Scottish and Southern Electricity. Clear dispersion modelling and the behaviour of a depressuring pipeline are critical elements in determining the major accident response, but also important is the definition of safe distances. The behaviour of CO2 at the release point needs to be defined and the computational models validated against it. Efficient and accurate modelling at an early stage for the purposes of safety cases and route definition is key to efficient, practical and safe design.
Human factors
The critical technological issues discussed aid design, safety and integrity of the pipeline, however the human factors should not be ignored. Important in the learning from the US is that the schemes there are operated in an oil and gas production environment. The changes that the emitters, particularly power generators, face in the addition of a capture plant, pipeline and a store are considerable. The current staff will have new processes to control and monitor, chemical stocks to maintain and dispose. While these processes are common to the energy industry, they are not common to the power generation companies.
At the other end of the scale, the storage companies need to understand that the power utilities and emitters may dictate the system, not the store. The dynamics of these systems are different and the future operators and owners need to understand this.
When considering resources there needs to be an understanding of the market size and rate of deployment. Any evaluation of the CCS market is made with caution due to the number of dependencies involved. Projects rely on three things; the supply chain, engineers to design, and skilled technicians to build.
Perhaps the biggest potential issue for the industry to face is the public. There is a growing need to address public education and perception. Failure to engage with the public will have a devastating effect on project development. There needs to be project-specific information, but also a wider education program.
Conclusion
It is apparent that the knowledge gained from current CO2 pipelines will prove vital, not only with regard to experience and solutions, but also for those issues that have not been fully addressed, such as flow assurance, dispersion, properties and engagement. These need to be resolved to enable deployment.
Despite these critical areas that need to be addressed, the body of evidence, experience and proven design methodology, codes and regulation enables the design of CCS pipelines and infrastructure. This experience may prove to be conservative and more costly, but it does not prevent pipelines from moving forward. The industry must maximise the transfer of knowledge from CO2 and hazardous liquid pipeline design. In parallel, research must address the issues discussed and generate resources and tools that can meet the challenge to bring to wide scale deployment.