Fibre-optic sensors could help oil companies squeeze more from dwindling sub-sea reserves
Optical fibres have become the foundation for our digital world. They are reliable, widely available and can carry huge amounts of data over long distances. They are now a defining technology in modern society and, without them, our daily communications could slow down to what many would feel is an unbearable pace. Below water, however, it has been a different story.
In many applications, fibre optics remain at the periphery of the offshore oil and gas industry; surprising, perhaps, in a sector that claims to be on the leading edge of technology. The problem is that, to prove commercially viable, these systems need to be able to operate at depths of up to 3,000m. Testing is just too expensive and the technical barriers are huge.
According to Martin Bett, chief executive of fibre-optic firm Stingray Geophysical, this is about to change. Currently, only 30 per cent of total hydrocarbons are extracted from the average field. At a time when we could be facing an energy shortage, governments are increasing pressure on oil companies to extract more from existing reservoirs. But, while motivation to increase efficiency is growing, firms are limited by their knowledge of reservoir behaviour.
To prove commercially viable, these fibre-optic systems need to operate at depths of up to 3,000m
Bett believes more information can be gained by developing an ’optical oilfield’, where an array of optical sensors is buried beneath the seabed to provide information on the forces acting on a reservoir. The system would be cheaper, more reliable and easier to install than existing underwater electrical sensors. It’s a vision that Bett has spent more than five years developing into a commercial product and hopes to launch for deep-water trials.
’It’s difficult to know how any reservoir is going to behave,’ explained Bett. ’It’s like taking a sponge full of water and lifting it up to drain the water out. Obviously squeezing is a better option and enhanced oil recovery is exactly the same. While we can’t squeeze the rocks, we can use other methods to get the hydrocarbons out – and knowing exactly where to do that using fibre-optics is key to the whole thing… it could generate billions of pounds for the industry.’
The core technology was originally conceived during the Cold War for anti-submarine warfare applications. It was incorporated into defence group Qinetiq, which later spun out Stingray Geophysical to develop the technology for the oil and gas market. Stingray has since used the basics of fibre-optic communication to develop the Fosar Deep programme. Its central idea is to use a passive array of optical sensor units (OSUs) connected by armoured fibre-optic cables and plowed into the seafloor.
Each OSU contains three fibre-optic accelerometers and a hydrophone. Light is pulsed down the fibres and reflected off mirrors at either end of each sensor, allowing measurement of small changes in the length of the fibre. Changes in length are triggered by seismic waves from an acoustic source, reflecting off subsurface structures and causing the fibre to stretch and relax. These differences are extremely small – around the diameter of an atom, or a tenth of a nanometer.
A recording system on the surface then receives and decodes the information. By acquiring repeatable time-lapse imaging data, the Fosar Deep system can map the fluid movements and pressure changes within the reservoir. The conventional way of doing this would be to tow an array of seismic cables, or streamers, behind a vessel at specified intervals. According to Richard Luff, technical lead on the project, fibre optics provide a better solution.
’If you use electrical systems you obviously need a lot of electrical power and the cables become big and heavy,’ he said. ’This can be a problem during installation and maintenance; but even more critical are the electrical systems, which can be unreliable and prone to failure. Our system uses electrically passive fibre-optic sensors. That means there is no electrical power required in the system that is underwater. The surface electronics do require some power, but that is a very modest amount that you could get from a wall socket.’
“Knowing where to get the hydrocarbons out is key to the whole thing, it could generate billions”
MARTIN BETT, STINGRAY GEOPHYSICAL
By adapting the lasers that are used to pulse light into the array, Luff’s team has improved the signal reliability to better than that of electrical sensors with a much larger dynamic range. As well as this, the high channel count per fibre of the Fosar architecture means that fewer fibres are required in each system, reducing the number of connections that could go wrong. It also means that the cables are smaller and lighter to deploy onto the seabed.
’At 3,000m depths, there are almost two tonnes per square inch acting on an object,’ said Luff. ’Engineers are normally very cautious and conservative people, and their normal approach to that type of design problem is to add more strength. We looked at it in a different way. Rather than getting bigger and heavier as we went deeper, we designed our fibre-optic cables to be very small – down to just a few millimetres. This makes our sensing arrays lightweight and easy to transport to the ocean floor.’
Remotely operated underwater vehicles (ROVs) then distribute the sensors on the seabed. Over the last five years, Stingray has conducted numerous field tests of the system. The next step will be to install a large-scale pilot array in deep water and Bett is in talks with major oil companies to conduct a joint-industry test in 2012.
If successful, Fosar Deep could use predictive data on reservoir performance to delay a peak oil situation. The optical oilfield may be the future of the energy industry and, as it heads towards deeper waters, it could become just as critical in understanding natural processes beneath the seabed. With billions of pounds at stake, fibre optics may be far more important to modern society than we first thought.
Fosar Deep could help prevent deepwater drilling accidents
Stingray Geophysical has wider ambitions for Fosar Deep than enhanced oil recovery. It also plans to use it for safety monitoring of well heads.
According to Richard Luff, much smaller arrays of the system placed around the well head during drilling could have prevented BP’s Deepwater Horizon accident in April last year.
’The arrays could be listening passively to what’s going on,’ said Luff. ’We may, in the future, be able to interpret what happens to the formation fluid as they drill, what happens to the mud and what happens when they cement the job. All of those activities contributed to Deepwater Horizon, which went through multiple failures.’
Luff believes that improvements in technology, processes and safety consciousness will mean we can go into much deeper waters. ’The risks we are taking are not huge, but there is always cost and time pressure. It was probably costing BP in excess of a million dollars a day and people took some short cuts. I don’t think short cuts will be taken in the future and systems such as Fosar could help.’