The energy sector needs better, faster data. From shale oilfields in North America, to desert sites in the Middle East, fully understanding down well events is key to quickly addressing well integrity issues and boosting production.
Offshore energy, however, has long presented a unique set of challenges for operators. This includes data acquisition. The assumptions underpinning infrastructure installation and data processing onshore do not always hold for subsea assets, requiring deep geophysical expertise to unravel the complex problems involved.
The increased costs and safety risks associated with offshore energy assets, securing accurate, detailed data such as Distributed Acoustic Sensing (DAS) to inform operational decisions is particularly crucial.
LYTT recently supported a project in the Gulf of Mexico to successfully implement a world-first subsea fiber optic installation for seismic monitoring– read more about the project here.
This blog will explore my key takeaways from the project for the wider O&G industry, outlining the challenges in installing fiber optic cables at subsea assets, and showing how LYTT was able to support an innovative solution by working collaboratively with a diverse group of stakeholders.
Challenges of accessing DAS insights from subsea assets
The technological complexity of a subsea DAS installation stems from two key factors – the limitations of the fiber optic cables due to optical losses and the long interrogation distances.
Challenge 1: Technical limitations of fiber as a signal carrier
Fiber optic cables suffer optical losses over long distances. Subsea installations require a greater length of cable to transfer information from the subsea well to the surface-based production quarters therefore requiring compensations for this additional risk to be considered at the design level.
Additionally, all the technical output needs to be aligned. A single mismatch or improperly spliced cable in the optical chain can lead to less accurate insights, compromising DAS measurements for seismic. Subsea installations also use umbilicals and termination assemblies to support the fiber optic cables from the sea floor to the production center, adding a further layer of complexity.
With so many variables to consider across the project as a whole, creating a holistic solution required expertise and collaboration across many different disciplines relating to the subsurface, wells, umbilical, topside, seismic, technology and leadership.
At the Gulf of Mexico project, we drew on our extensive field trial work to advise on a system design to compensate for optical losses. This was achieved by using engineered fiber with a gauge optimized to improve signal quality in subsea environments and also increase the detection threshold for future 4D seismic monitoring.
This ensured that more accurate data reached the onshore processing center in near real-time, while the connection quality and noise threshold were optimized for accurate high volume data transfer.
Challenge 2: Ensuring a clean signal with limited background noise
Geophysical expertise is critical to understanding and characterizing what types of flow noise are acceptable to take high quality measurements to be used in subsurface updates. Operators also need to distinguish clearly between production noise and well seismic events. Doing this enables useful reflectivity data to be extracted in order to gain an accurate dynamic understanding of down well seismic events around the wellbores.
We therefore managed the real-time validation of the system with seismic data, providing technical assurance for the methodology employed during the project. The evaluation showed that the data quality was excellent across the length of the well, with high fidelity measurements and signal-to-noise-ratio in the expected downhole array aperture for borehole seismic imaging in actively producing wellbores. The impact of production noise on the end output was minimal.
Operators sometimes consider implementing fiber to pose additional technical risks. Our experience enabled us to manage and mitigate risk, ensuring a successful installation of fiber optic cables for use with DAS. For this new technology, a common language had to be developed across multi-disciplinary teams.
The benefits to the industry are significant. With a permanent installation of a subsea DAS system, the operator can have ‘seismic on demand’. Carrying out pioneering projects like this ensures that those that follow can use hard won insights to deliver efficiently, and successfully.