Fiber-optic sensors show promise in offering cost-effective multicomponent seismic data in deep water.

The goal of optimum reservoir recovery has moved a step closer following the formation of a transatlantic partnership among four major oil and gas companies and two world-leading technology providers. This Joint Industry Program (JIP) aims to develop reliable, cost-effective seabed seismic acquisition systems based on advanced fiber-optic technology. Comprising up to 40,000 sensors at depths of up to 7,000 ft (2,134 m), this new generation of fully optical systems could make permanent seabed seismic arrays an everyday reality in hydrocarbon exploration and production.

Technology originally conceived to defend NATO ports against submarine attack could soon be helping to improve recoveries from oil and gas fields. The JIP, backed by US $600,000 from BP, ChevronTexaco, ConocoPhillips and Shell, aims to deliver commercial seabed fiber-optic systems within 3 years. According to Tim Jackson, program manager of multicomponent seismic technology for BP, "Full-wave fiber-optic systems could provide the cost breakthrough required to accelerate the adoption of permanent reservoir monitoring systems in the marine environment."

The world's first fully optical seabed systems are being designed to operate with no electronic components or moving parts under water. This innovation promises to increase reliability and reduce life-cycle costs compared to conventional, cable-based reservoir monitoring systems deployed on the sea floor. The JIP is led by QinetiQ and Input/Output (I/O), which formed a strategic alliance more than a year ago to advance fiber optic-based seabed seismic technology.

The first phase of the JIP involves the development of a prototype fiber-optic sensor package and the benchmarking of its performance against the best available seismic sensors, including both conventional geophones and I/O's digital full-wave VectorSeis sensor.
The JIP's second phase, planned for 2005, will see construction and offshore testing of a larger four-component (4-C) array, with more than 100 sensor packages.

Optimal reservoir drainage

Permanent seabed seismic arrays are a key feature of so-called "intelligent" fields, which promise to increase hydrocarbon recoveries by enabling real-time field measurement and improving reservoir management. "Such 'life of field seismic' (LoFS) systems do exist already, but until now the potential of permanent arrays has been limited by high installation costs and concerns about the long-term viability of conventional electronic seismic systems on the ocean bottom," said Roger Hunt, QinetiQ's Fiber Optics business development manager.

The seabed is one of the most hostile environments on Earth for the installation of sensitive electronics. The extreme pressures and corrosive composition of seawater can cause high failure rates in conventional systems should water intrusion occur. But, according to Hunt, "The application of fiber optics has the potential to produce a step-change in seismic acquisition technology. It is capable of delivering a new generation of affordable systems with fewer electronics components and reduced deployment costs."
I/O officials agree that fiber optic seismic holds tremendous promise for future oilfield development and permanent reservoir monitoring. "Fiber optics offer the potential to significantly improve reliability and drive down the life-cycle costs of permanent seabed seismic installations," Bjarte Fageraas, I/O's vice president of Marine Imaging Systems, said. "If we can demonstrate that a step-change in costs can occur, we believe permanent reservoir monitoring will then become viable in a wider range of reservoirs worldwide. This is a joint win for all parties as the number of field deployments will increase, costs will come down, recovery rates will improve and life cycle economics will be significantly enhanced for the asset owners."

New generation of optical sensors

The system under development will feature a new generation of optical sensors that will allow JIP participants to acquire full-wave (multicomponent) seismic data from the seabed. According to Fageraas, "By acquiring and processing full-wave data, oil and gas companies are able to develop better subsurface images that are generally higher resolution, more repeatable, and more suited to delineating subtle structural and stratigraphic features along with fluid type and movement."

A further major advantage of fiber-optic systems is a theoretical capability to operate in far greater water depths than conventional seismic systems. In underwater tests lasting more than a decade, QinetiQ has already demonstrated that optical sensors can continue to perform satisfactorily over long periods. Situated off the coast of southern England, the company's permanent test array was finally withdrawn only after the shore cable was accidentally dragged up by a fishing boat. "These trials have shown that fiber-optic acoustic sensors continue to function even after sea water ingress," Hunt said.

Fiber optic systems are now being designed for operation in water depths over 7,000 ft (2,134 m), with a working life-span of at least 15 years. Since lightweight cables containing the sensors are less than half an inch in diameter; deployment is achievable from reels onboard standard service vessels or from an underwater vehicle. These systems also lend themselves to large multiplexed architectures, which means many thousands of sensors can be built into an array. This keeps cable size and weight down for easier deployment, especially in deeper water.

Seismic sensor breakthrough

The key to the new generation of seismic technology has been the fiber-optic seismic sensors, originally conceived for submarine defense. "These devices work by producing tiny alterations in a light signal as it passes through the sensor in response to infinitesimal vibrations and pressure changes," explained Phil Nash, QinetiQ's Fiber Optic technical leader. "The sensors are essentially formed from tightly coiled optical fibers, which are fractionally distorted by seismic pressure waves."

These seismic waves are generated conventionally using ship-mounted air guns to produce a pulse which is reflected by the geological substructure. The light signal is produced by a laser onboard the host platform and then compared to the returning signal. The laser light is communicated to and from the sensor array by fiber-optic cables. An "interrogator" infers the amplitude of the seismic pulse by measuring alterations in the path lengths or phase of the laser light traveling to and from the coil, hence the name "interferometry."

Working with I/O, QinetiQ has developed a 4-C sensor package which fits into a proprietary pressure housing. The lightweight packages are about one quarter of the size of most existing packages (Figure 1).

Data processing

With a single optical fiber capable of supporting up to 500 sensors, a cable holding several fibers can clearly incorporate many thousands of sensor packages spread over the seabed in a wide array. But, "Handling the mass of data that emanates from these sensors and properly integrating the data streams is another part of the fiber optic seismic challenge," cautioned Nash.

For this, QinetiQ has developed a system multiplexing architecture in two levels.

To increase the number of sensors, the second-level architecture uses wavelength division multiplexing, which splits the laser light into up to eight different wavelengths, or colors. Each color is sent along the single fiber from the interrogator unit, but the sensors are arranged on the fiber as parallel subarrays, each of these being interrogated by only one of the colors.

Once the colors have passed through the subarrays they return to the optical receiver and phase demodulator on the surface, which can distinguish between each array and each sensor. In this way, a single fiber can support 128 4-C packages. Bringing several fibers into a cable means that the overall number of sensors can be increased significantly to match the size of array required in the offshore field (Figure 2).

If the JIP succeeds in delivering commercially viable equipment, fiber optics will make "intelligent fields" an everyday reality. This new generation of seismic technology will then have a positive net impact on the world's recoverable hydrocarbon reserves.

For more information about QinetiQ, visit www.QinetiQ.com. For more information about I/O, visit www.i-o.com.

Comments