CCUS (Carbon Capture, Utilisation & Storage) is a collective term used to describe various methods of preventing CO2 from entering the atmosphere or scrubbing already emitted CO2 from the atmosphere.
When we look at the ‘storage’ part of the equation, in 2021 there were only 27 CCS projects in operation in the world, while hundreds would be needed to live up to its promise as required by different IPCC (Intergovernmental Panel on Climate Change) scenarios. There are several reasons for the limited take-up of CCS technology so far, but some reasons relevant to this discussion are:
The cost associated with the long-term monitoring (30+ years) of CCS sites to meet regulatory requirements
In the EU, CO2 storage will be admitted to the EU ETS (Emission Trading Scheme) only if monitoring and verification can be carried out ‘satisfactorily’
The potential risks involved, such as CO2 leakage and induced seismicity
The limited public acceptance for these projects
What is DFOS?
In the 1980s, it was suggested that sending laser light through fibre optic cables, originally developed for telecommunications purposes in the 1970s, could also be used as a sensor to measure physical, chemical, and biological parameters because they would modify a property (e.g., intensity, polarisation, or phase) of the laser light travelling through the cable. Several techniques were tested, and this led to what is commonly known as DFOS (Distributed Fibre Optic Sensing) technology, where the entire length of the glass fibre(s) inside the fibre optic cable is used as a continuous sensing element.
Depending on what is measured, DFOS can be divided into DAS (Distributed Acoustic Sensing), DTS (Distributed Temperature Sensing), DSS (Distributed Strain Sensing) and so on.
DAS measures acoustic (sound) waves traveling through the fibre optic cable, DTS measures temperature over the full length of the fibre optic cable, whilst DSS measures strain in the cable. The fibre optic cable can be several kms long, 50 km and more are not unheard of depending on the application.
The system components are a box, i.e., the interrogator, consisting of all the optical and electronic components to make the system work (laser, optical detector), and one or more fibre optic cables, data storage, and measurement, visualisation, and data analysis software.
Figure 1: DFOS system components
Why and where is DFOS a useful monitoring technology?
DFOS has several interesting properties for industrial applications, mostly because of the unique properties of fibre optical cables. Optical fibres:
Are resistant to electromagnetic interference (EMI), unlike copper cable or other sensors, making it an ideal solution for environments that contain variable frequency drives
Do not conduct electricity and do not ignite in the presence of open flames
Can withstand extremely high and exceptionally low temperatures
Are highly resistant to chemicals and radiation
Are lightweight, small, and low in cost
Have a low attenuation compared to electrical cables, supporting long measurement ranges
These properties make DFOS useful for continuous, non-destructive monitoring in several industrial applications where the intrinsic toughness of fibre optic cabling in harsh environmental conditions is an asset.
Selected applications for DFOS measurement technology are:
Table 1: Key application of DxS technologies
DFOS can be a game changer for CCS site monitoring
To meet regulatory requirements and subsequently cultivate acceptance from the public for CCS, it is vital to ascertain that injected CO2 stays in place in the subsurface as modelled. This means that the CO2 plume needs to be monitored in near real-time, allowing for any potential leakage to be promptly detected allowing corrective actions to be initiated. Traditional (4D) seismic studies, as employed in the oil & gas industry, are not sufficient for CCS monitoring. They are typically only done every two to three years because of their prohibitive cost so other monitoring technologies that can be deployed on a continuous basis are needed.
Figure 2: Example of an offshore CCS site in Japan monitored with a multitue of small seismic sources in challenging conditions [i]
DFOS sensing technologies, and more especially DAS and DTS, deployed on the surface and/ or downhole in a monitoring well, will be an important, even indispensable component of any CCS monitoring plan. It is a technology that fits nicely within the requirements of a CCS project, because:
Traditional environment monitoring (e.g., 4D surveys) are prohibitively expensive if they must be repeated for 30+ years for permanent installations such as CCS sites; DFOS monitoring solutions can be an order of magnitude less expensive compared to traditional monitoring solutions and monitoring will be a significant part of the total cost of any CCS project.
DFOS monitoring is very robust. Fibre optic cables can survive for an exceedingly long time (50+ years), interrogators and other ‘active’ components have lifespans of tens of years and can be easily replaced or upgraded at a predictable cost.
DFOS can address much of the regulatory requirements with a permanent installation and only requires light technical supervision that can be done remotely (in the cloud).