Fiber-optic sensors offer a cost-effective solution to borehole and ocean-bottom monitoring.

Oil industry executives faced with a maturing asset base have increasingly relied upon new technologies to close the gap formed by declining reservoir productivity and the resultant increase in recovery cost. To date, this has been a successful strategy as technology, to a great extent, has mitigated production declines. However, there are concerns that this trend may not continue. Witness BP PLC's recent announcement that it is investigating the disposal of several billion dollars of assets, particularly some of its mature North Sea fields, rather than incur the massive investments required to preclude a decline in output at these fields. Clearly new, cost-effective technologies are needed to prevent output declines at the increasing number of fields with depleting reserve and production profiles. 4-D monitoring using fiber-optic sensor systems represent one of those potential emerging technologies.

Improved seismic technology was a key driver in the remarkable improvements in exploration and production success over the past two decades. In the 1990s, the adoption of 3-D seismic led to very significant and measurable increases in drilling success rates. Although 3-D seismic techniques were originally developed in the 1970s, their widespread use was delayed until low-cost digital electronics and improved processors made such systems cost-effective and made interpretation of the data more accurate and efficient. Similarly, the merits of 4-D seismic data acquisition have been heralded for almost a decade, yet its widespread use has yet to occur. Again, despite the growing consensus regarding the advantages of 4-D, the prohibitive costs associated with early systems seem to have stalled its rapid adoption. With recent innovations in low-cost manufacturing, fiber-optics sensor systems may now represent the game-changing technology needed to propel 4-D seismic to widespread use within the industry.

Fiber-optic temperature and pressure sensors being introduced into the oil field are proving their viability in the operating environment. Whereas these static sensor applications have paved the way for the industry acceptance of fiber optics, the real payoff is yet to come. Large-scale fiber-optic acoustic systems that permit continuous and improved imaging resolution (permanent 4-D reservoir monitoring) to facilitate recovering oil from irregular, previously inaccessible and depleted reservoirs will be the real payoff. A new generation of fiber-optic sensor technologies, initially developed for the telecommunications and defense industries, now stands ready to provide the step-change increase in robustness and affordability needed for these oilfield fiber-based sensors systems.

Fiber-optic sensors explained

Optical fibers consist of an extremely thin, ultra-low-loss glass thread that is surrounded with an additional layer of glass called the cladding. The cladding has a different index of refraction than the core, which serves to contain light entirely within the core, even around curves. The cladding, in turn, is typically covered with a comparatively thick polymer- or carbon-based coating for protection. Optical fiber is inherently very strong, capable of withstanding higher stresses than that of steel, and it is inexpensive and reliable.

With fiber-optic sensors, the same fiber acts as the sensor, performs the multiplexing and provides the telemetry path. At the heart of these new fiber-optic sensing systems is the Fiber Bragg Grating (FBG), which turns an ordinary fiber into a sensor. An FBG consists of a piece of virgin fiber where microscopic lines or "gratings" have been inscribed into the core of the fiber with a laser. A special laser permanently inscribes a pattern of periodic refractive index variations in the cylindrical cross-section of the core. A typical grating might consist of a series of these variations over a centimeter of the fiber.

Creating the precise, microscopic FBGs is a technically challenging process. Lasers typically used to write the gratings in the core cannot readily penetrate the polymer coating of the fiber. Consequently, conventional FBGs, including those used in geophysical applications, are made by removing the polymer coating, writing the grating and recoating the exposed fiber, which may then be spliced into longer segments. This process makes the grating itself the weak link in a fiber-optic sensor. In addition, previous generations of fiber-optic acoustic sensors required numerous optical couplers, fiber splices, etc., which also compromised the strength and reliability of the fiber.

A recent breakthrough in fiber technology now enables FBG sensors to be written directly into the core of the fiber without compromising any of these qualities. This process, which received the Photonics Circle of Excellence Award as one of the 25 most technically innovative products of 2002, has now been automated, enabling low-cost manufacturing of long arrays of sensors on a single fiber. Using this new technology, fiber-optic acoustic arrays can be formed onto a single fiber with no splices, stripping, recoating or optical couplers. The resulting arrays are inexpensive and easy to build while retaining the reliability of the virgin fiber.

Fiber-optic acoustic sensor systems consist of two major subsystems, the passive, all-optical sensor arrays and the associated interrogator electronics (Figure 1). The electronics include a laser source with a phase modulator/pulse generator for launching light down the fiber as well as the receiver electronics for demodulating the reflected signals and translating them into a digital electronic signal. When light traveling down the fiber encounters an FBG, a portion of the energy is reflected back toward the source. For one pulse out, there will be N return pulses, where N equals the number of FBGs. The FBGs provide the separation between individual sensor sections.

Permanent monitoring

In comparison with other seismic applications, borehole seismic acquisition systems are relatively immature. This immaturity extends to acquisition, data processing and interpretation. The most popular techniques are 3-D vertical seismic profile surveys using surface sources and downhole receivers, and crosswell imaging using downhole sources and downhole receivers. Today's downhole systems are generally less than 24 levels. Future systems must include three-axis geophone capability, and more than 100 levels will be required. Packaging for the downhole sensors will require ruggedized cabling and operation at elevated temperatures for a time period that supports the 5-plus years between planned interventions.

Permanently installed sensors represent the ideal solution for 4-D monitoring, given that leaving the sensors in a fixed location prevents sensor location inaccuracies, eliminates labor associated with multiple deployments and retrievals and, in the case of borehole sensors, does not require an intervention for the periodic surveys. With the exception of seismic streamers, all other methods of acquiring 3-D data can use permanently installed fiber-optic sensors for 4-D monitoring. Cost will play a pivotal role in the eventual adoption of 4-D and the market share each method acquires.

A look at the numbers is eye opening. As an example, the current price for a proprietary 3-D streamer survey in the Gulf of Mexico is between US $5,000 and $10,000 per square kilometer. A retrievable multicomponent ocean-bottom cable (OBC) survey can run $50,000 to $100,000 per square. Estimates for a permanently installed multicomponent OBC system are projected to be $500,000 to $1 million per square. Thus a price tag for a typical 19-sq-mile (50-sq-km), permanently installed system using today's technology is a staggering $25 million to $50 million, a clear showstopper. Barring a step-change reduction in cost, permanent 4-D appears dead on arrival.

Why fiber optics?

A typical conventional sensor requires electronics to convert its relatively weak output into a usable form that can be transmitted long distances. These electronics must be located in close proximity to the sensor to limit the pickup of noise prior to amplification and digitization. Thus the electronics must be miniaturized and packaged to withstand the environment at the sensor location. The major advantage of fiber-optic sensor systems over existing conventional systems for permanent installations is the complete elimination of electronics at the sensor end. Figure 2 compares the components at the sensor end of a conventional sensing system with a fiber-optic sensing system.

It is readily apparent that removal of electronics at the sensor end can lead to dramatic improvements in reliability. However, less obvious is another tremendous advantage that specifically applies to the remote, time-lapse measurements needed for 4-D monitoring - unlike electronic sensor systems, a fiber-optic sensor system eliminates the requirement for the electronics to be married to the sensor. As a result, the electronics now can be located at a remote location, in a benign environment, many miles from the actual sensor arrays. Consequently, the electronics are always accessible for repair or upgrade. And more importantly, they are only needed when the survey is being performed, allowing them to be shared across numerous permanent fiber-optic sensor installations.

Based upon the new FBG technology, today's fiber-optic sensor arrays can be manufactured for a small fraction of the cost of conventional electronic-based sensor arrays. These low-cost, completely passive optical arrays can be permanently installed without electronics on the ocean bottom, down a borehole or on land (Figure 3). When needed, the electronics can be attached via a single fiber-optic connector or an optical switch.

Future potential

It is often the case that a breakthrough solution exists but the technology to implement the solution is not yet available to make it practical and affordable. 4-D seismic monitoring is a good example. Fortunately, recent advances in fiber-optic sensor technology finally may provide the key to affordable 4-D systems.
What does the future hold? We can expect robust, one-piece, small-diameter solid seismic streamer sections, built using automated equipment to be lightweight, inexpensive and reliable all-optical OBCs, designed for permanent installation and burial. Also becoming available will be permanent borehole seismic arrays that can operate continuously at temperatures to 391?F (200?C) and provide periodic updates without an intervention.

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