Unique data is the key to oil market

29 Jan 2003

For makers of fibre-optic sensor systems, the oil and gas industry has proved tougher to crack than expected. But optics holds the key to so-called "intelligent" oil wells and could become a critical technology in future production. Michael Hatcher looks at the latest developments.

From Opto & Laser Europe February 2003

Photonics has had less of an impact on the oil and gas industry than many people predicted 10-15 years ago. However, the recent installation of the world's first permanent fibre-optic seismic array by Norway-based Optoplan (see Opto & Laser Europe January p12) could signal a change.

One of the many challenges facing the modern oil industry is the ageing of its reservoirs. The problem is this: an oil reservoir is typically more productive early on in its life, and its output inevitably declines with age. This is where the big opportunity lies for optical sensing - increasing the productivity of mature reservoirs.

Ultimately, the oil industry wants to build "intelligent" wells in which a variety of technologies are employed to continuously acquire real-time key data from every part of the reservoir to maximize its output.

Optics should be central to this development, but companies in the sensing business have had to fight a long-standing prejudice against deploying in-well fibre-optic systems. This has arisen from fears about both the fragility of the fibre and the complexity of repairing the systems.

Making an impact This has changed over the past few years, claims Glynn Williams, the joint founder of UK industrial sensing specialist Sensa. He says that fibre-optic sensing is now an accepted technology in the energy industry. He adds that while temperature and pressure sensors have already made a significant impact, the key to really opening up this market is to offer oil companies systems that generate data that cannot be acquired using conventional sensors. "Creating new measurements is of greater value to the oil and gas industry [than offering a replacement for electrical sensors]," Williams told Opto & Laser Europe.

The profile of optical technology within the sector has increased following recent acquisitions by two major players in the oil and gas industry. In the past 18 months the global industrial technology firm Schlumberger has bought Sensa, while its US-based rival Weatherford has snapped up the optical-sensing business of photonics specialist CiDRA, US. Weatherford also owns the Trondheim-based Norwegian University of Science and Technology spin-out Optoplan.

Prior to this, Sensa had kicked off the consolidation with buy-outs of fellow UK firms York Sensors and Sensor Highway.

Whether these acquisitions reflected an increased acceptance of optical technology in the industry or simply an opportunity for large companies to acquire high-quality R&D at a knock-down price in the depressed photonics sector is up for debate.

Aggressive stance According to industry magazine Offshore Engineer, the big firms are taking optical technology seriously: "After looking at more traditional electronic equipment, Weatherford has taken an aggressive stance to promote the use of fibre-optic pressure and temperature sensors, and flow meters," it reported in September 2002.

Intelligent oil wells may still be a long way off, but what is possible with today's optical-sensing technology? Sensa has certainly enjoyed great success with its distributed temperature-sensing systems - now installed in some 250 oil wells across the globe. A key application for these systems is in "heavy oil" extraction. This oil is typically difficult to raise from the reservoir and demands a technique known as "steamflood" extraction. The advantage of the optical-fibre sensor here is that it can monitor the temperature at a number of points along the fibre, which cannot be achieved using more conventional sensing techniques without becoming extremely costly.

This is possible because the temperature measurements are based on Raman scattering in the transmission fibre itself. Local temperature changes induce a subtle but measurable change in the wavelength of Raman-scattered light. Because the fibre effectively acts as the sensor, it is possible to monitor the temperature along the fibre at 1 m intervals to an accuracy of 0.1 ºC.

In a recent application in a Californian oilfield, temperature sensors have been installed at a depth of more than 600 m in 21 steamflood wells to monitor reservoir temperatures that exceed 230 ºC. In one well, the fibre-optic system has already detected early fluid migration through a poorly constructed cement casing.

Unfortunately for Sensa, heavy oil only accounts for about 6% of the world's total production. For the remaining 94% of reserves, fibre-optic sensors must prove themselves similarly advantageous.

Apart from the distributed temperature-sensing system, other fibre-optic solutions include: distributed strain measurements to monitor the flexibility of risers (the steel threads used to raise oil from a reservoir); pressure-monitoring systems that are useful for down-hole applications; systems that can monitor the phase fraction in a reservoir (the relative proportions of oil, gas and water); and sand-detection systems. The latest innovation is seismic data collection, which is used to image an oil reservoir.

Weatherford and its subsidiary Optoplan certainly appear to be making some headway in their development of in-well seismic array sensors. In December 2002 Optoplan engineer Sverre Knudsen and colleagues installed the world's first permanent such multistation array in a well at Izaute Gas Storage in southwestern France for oil firm TotalFinaElf. The system will give engineers an improved knowledge of the reservoir's oil and water content and rock porosity.

Detailed imaging Six sensing stations are used in the system. It is based on fibre-optic accelerometers that measure seismic waves that are induced on the well externally. Using a wavelength-multiplexed system just like that used in optical telecoms, the speed of particles displaced by these seismic vibrations are measured using interferometry.

Five of the stations perform vertical seismic profiling to characterize the reservoir, with the final station recording microseismic activity. Interferometry gives engineers a more detailed profile of the reservoir than the current "noise logging" technique that uses geophones to monitor seismic motion.

Optoplan has also developed an in-well sensor to determine the individual flow rates of oil, water and gas in a reservoir and a prototype sand-monitoring system.

It is applications like this that Williams believes will attract the attention of oil firms, but he warns that certain technological developments must precede widespread deployment. The industry must develop optoelectronic detectors that are less sensitive to ambient temperature changes. A key development, says Williams, will be fibre-optic connectors for underwater use. These will need to withstand high temperatures and pressures and deal with "difficult" fluids, such as oil containing a lot of sand.

Optical pressure sensors must also improve to displace their quartz-based electronic rivals, which are currently more sensitive and stable, says Williams. Advances in fibre technology are also needed to increase back-scatter for improved Brillouin strain measurements and to ensure that fibres used for chemical sensing are better able to withstand the effects of exposure to hydrogen and hydroxide, which degrade the fibre and reduce transmission.

The integration of different sensors is also key. This is necessary because while currently installed systems like distributed temperature sensors provide useful data, the interpretation of those data is somewhat intuitive. For example, a sudden rise or drop in temperature is not always attributable to the same cause. Because of the highly complex environments of oil and gas reservoirs, the true value of distributed fibre-optic sensing systems will probably only become apparent when a single network can reliably measure a range of physical variables throughout a reservoir.

With all of this demanded, Williams says that it will be at least five years before the industry sees a fully distributed sensing system deployed. "The onus is on the fibre-optic sensor developers," he concluded.

Writing in the oil industry journal Hart's E&P, Weatherford's chief executive Bernard Duroc-Danner says that the oil industry can no longer rely on consolidation for wealth generation. He contends that improved technology will be the key to its future prosperity. Fibre-optic sensors should be at the heart of this technology, says Williams: "It will become a ubiquitous part of the oil and gas industry."