September 1999

What's happening in production

September 1999 Vol. 220 No. 9 

Perry A. Fischer, 
Engineering Editor  

Smartwell technology gets an IQ boost

Somehow, this editor missed an exciting development that occurred during the last 20 years. Although well known in the optical-engineering community, a cursory survey of colleagues and peers revealed that many others are likely to be unaware of this invention, probably because they’ve had their hands full keeping up with changes in the oilfield industry. The invention is an optical device with ramifications that may prove to be as far-reaching as the transistor: the fiber-optic Bragg diffraction grating.

The cursory survey also showed that a brief introduction to diffraction gratings is called for.

Everyone is familiar with a prism — that triangular piece of glass that bends light into a rainbow of colors. A diffraction grating does the same thing, except that the result is very narrow, discontinuous, wavelength(s) of light, unlike the prism’s smeared-rainbow effect. They can be designed to this by transmitting or reflecting light through / from the grating, as well as breaking light into many, a few, or even just one wavelength(s), depending on grating design and wavelength of the incoming light.

Fig. 1

Although input is broadband, output from each Bragg grating sensor comprises a narrow bandwidth, shown multiplexed. Small variations in each pulse allow measurement of temperature (DT), pressure (DP), flow (DF) and perhaps sound (DS).

A conventional grating comprises many extremely small slits or etchings on glass or metal. It can now be "written" into the core of an optical fiber, through variations in the fiber’s matrix. To be sure, this introductory definition is an oversimplification, but it comprises the gist of what a fiber-optic Bragg diffraction grating is.

They have been used in aircraft, telecommunications and the Space Shuttle. Materials can be coupled to Bragg gratings that, via adsorption, will induce stress, and thus measurement of gases. Their application is branching into permanent measurement of stress in buildings, bridges and roadways, and application patents will soon number in the hundreds. Potential uses are unlimited, and they will soon be coming to an oil well near you.

In their paper, Rugged telemetry system for coiled-tubing earth drilling, authors R. S. Weis and B. M. Beadle of Texas Christian University describe their idea for using Bragg gratings, coupled to piezoelectric transducers, to permit telemetry during coiled-tubing drilling operations.

United Technologies Corp. (UTC) holds fundamental patents in the area of multiplexed distributed sensing. They recently licensed these patents, on an exclusive basis, to CiDRA, a new venture in the smartwell technology arena. UTC also took a stake in the new company. With the help of experienced oilfield service company employees, CiDRA has married fiber optic, intra-core, Bragg grating sensors with well-completion and monitoring techniques. The aim is permanent installation of downhole sensors to measure temperature, pressure, flow and acoustics.

These gratings have some very desirable properties. First, the optical fiber sensors can withstand a high-pressure / high-temperature (302–392°F) environment. Second, the fiber Bragg grating can be considered a universal transducer via the mechanism of strain. The slightest amount of strain changes the grating length, which in turn changes the wavelength of the diffracted light. Third, they can transmit multiplexed data, i.e., many data types can be simultaneously transmitted on the same fiber. Finally, they require no electricity.

Next month, World Oil will feature a technical article detailing use of this new technology.

Preventing produced water. When the new smartwell sensors detect water breakthrough, what’s the next step? Ordinarily, water production occurs beneath the oil zone, so simply setting a plug at the oil / water contact usually solves the problem. However, when production comes from multiple zones, and water breaks through between these zones, another solution is required.

Just such a situation occurred on Shell UK’s North Sea Brent Alpha platform this July. They called Schlumberger. After running Reservoir Saturation and Production Services Platform tools, fluid type and contribution from each zone were identified, as well as two additional oil zones for perforation. Analysis further revealed that most of the water came from two, 4-ft intervals, situated above deeper producing zones.

The solution was to use the company’s new PatchFlex technology. A flexible, composite sleeve, comprising carbon fiber, thermosetting resins and a rubber skin, is set via an inflatable setting tool that is run in on electric wireline. After positioning, a pump within the setting tool begins inflating the cylinder with well fluid. Pressure is maintained as heating hardens the resins. After hardening, the setting tool is deflated and withdrawn to surface.

After applying the tool as described, the new zones beneath the patches were brought into production. Before patching, the well was producing 85% watercut; this was reduced to 10%. At present, the thermosetting compound has a short shelf life, so each sleeve must be custom-built for the job. However, the company is working on compounds with longer shelf life, as this would allow standardization and help reduce lead-times and costs for clients. WO

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