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Fibre-optic sensors for sea-bed testing

07 Mar 2005

Project Stingray, which aims to commercialize fibre-optic seismic sensors for mapping oil and gas reservoirs under the sea-bed, begins testing this month. James Tyrrell reports.

Extracting hydrocarbons from deep below the ocean floor is an expensive operation for which small improvements in reservoir management can mean large cost-savings. UK firm QinetiQ thinks its fibre-optic technology could bring big benefits to the industry and, working together with US seismic equipment specialist Input/Output (I/O), the company is hoping to bring a seismic sensor to market.

QinetiQ's concept has captured the interest of oil and gas majors BP, ChevronTexaco, ConocoPhillips and Shell, who have come onboard to fund the prototype assembly and testing. With phase-one testing beginning this month, the joint-industry programme known as "Project Stingray" could lead to the deployment of the world's first fibre-optic sea-bed monitoring system.

Conventionally, electrically powered sensors are used to scan the sea-bed, but as QinetiQ's fibre-optic business group-manager Michael Gill explained, these have a major disadvantage. "Electronic components and seawater don't mix very well," he told OLE. "Stingray uses fibre optics for both the sensors and the means of capturing the data, with an interrogator system on the dry end."

Over the past decade, QinetiQ's seaborne fibre-optic technology has been refined for defence applications such as underwater and surface target-detection. As a result, the firm has developed a good understanding of how the sensors and architecture behave. Gill feels that it is the culmination of this knowledge, in combination with the falling cost of telecoms-type optoelectronic components, that makes now the right time to commercialize the system.

QinetiQ's seismic sensors consist of optical hydrophones and accelerometers that map the ocean floor by measuring the pressure and shear components of underwater shockwaves. An acoustic source is fired in a pattern around the sensor array to produce shocks that are reflected by the strata. It is the timing and the strength of these reflections that provides the clue as to what lies beneath the seabed.

"Basically, what is happening [in our sensors] is that a coil of fibre is distorted by the seismic signal," Gill explained. "We are capturing the phase change that this distortion creates in a pulse of laser light." The light pulse, provided by a fibre laser on the surface, reflects back from each detector within the sensor array in turn, through a modulation and demodulation scheme. An interrogator unit then examines the phase change to create a seismic map of the ocean floor.

The sensors themselves are based on a strand of high-numerical-aperture silica fibre with a diameter of 80 μm, while standard 125 μm singlemode fibre transmits data to the surface. In theory, a complete system can be tens or even hundreds of kilometres long, with an array containing thousands of sensors. Ultimately, cables could be laid into the seabed by a remotely operated vehicle of the type commonly used for deploying transatlantic telephone cables.

Phase-one testing is planned for this month and the Stingray team is busy building its array at QinetiQ's Windrith site in the UK. QinetiQ is working closely with I/O - its alliance partner on the project - which is supplying some of the hardware that will be used for testing, such as the recording unit.

"We will deploy a prototype hybrid array on the sea-bed just off the south coast of England," revealed Gill. "What we are doing is benchmarking our system and our sensors with the I/O sensors - a MEMS-based system which is pretty much acknowledged as state-of-the-art technology." For the phase-one testing, the array will be positioned at depths of up to 18 m. It measures 2 km in length with 20 detector stations spaced at 50 m intervals. In total, around 8.5 km of fibre is used for the data transmission and sensing.

Featuring I/O's proven design of cable and pressure housing, the phase-one sensing stations are actually much larger than the envisaged final product. However, if the core sensor design works well, the team will shrink everything down to a much smaller size before beginning phase two of the development programme. Currently, the accelerometers are 40 mm in diameter and 40 mm high, while the hydrophones are housed in 30 mm-diameter cylinders of about 50 mm in length.

With phase one of the project focusing on benchmarking, phase two is about scaling-up and on-site testing and is scheduled to start in the summer, running for 14-15 months. "The idea is to demonstrate that a near-engineering-product level of design deployed on a real reservoir can provide the right quality and detail of data," said Gill. "[When] we prove that the system architecture works, and the interrogator system works and can be linked in to provide the right kind of data, we hope to move forward with the four sponsors to a second phase in which we will build a much larger array."

When in place, such an array could bring immediate benefits. "The high-quality data can be interpreted to show where the hydrocarbons are in the reservoir at any given time," Gill said. "If you repeat the survey fairly frequently, you can track movement and plan the reservoir extraction programme to maximize the rate of extraction from the reservoir."

Typically, the extraction from an oil or gas field is only around 30-35% of the actual hydrocarbon in the reservoir. If it were possible to improve that figure by just a few percent, the returns would be significant. "You can do that in a number of ways, but by getting a better image throughout the exploitation of the reservoir, you can cut out some of the uncertainties in the reservoir model," explained Gill. "You can put injection wells in the right place and you can drill less dry wells."

Having recognized the potential of its fibre-optic technology to transform seismic sensing, QinetiQ has already captured the basic intellectual property behind Project Stingray. "The advantages of fibre are that you can increase the reliability, longevity and simplicity of the system and reduce the cost, while still deriving the right kind of quality data," said Gill. "With fibre-optics, you can reduce the size, weight and cost of the sea-bed component by a significant amount." These are comments that the oil industry has been heard to echo.

"Most [seismic monitoring] systems share a similar design based on ocean-bottom cables and geophone sensors, which can be costly both to deploy and to maintain," commented Tim Jackson from BP. "We think fibre-optic systems could provide the cost breakthrough required to accelerate the adoption of permanent reservoir-monitoring systems in the marine environment."

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