Fiber optic tool continuously logs temperatures in complex, multilateral wells

Fiber optic sensing has gained acceptance in the upstream oil and gas sector for downhole monitoring applications, but this technology has been challenged by E&P activities moving into deeper waters and deeper wells. Recent developments in LIOS Technology’s fiber optic sensors promise to push past the current technical limitations and deliver non-intrusive, continuous temperature logging of wells.

The German company’s evaluation method is an Optical Temperature System (OTS), which continuously monitors the temperature of an optic fiber at all points along its length. The optic fiber functions as a linear sensor, recording temperatures along the sensor cable as a continuous profile.

LIOS Technology employs the Raman Effect for its optic fiber temperature measurement. In the Raman Effect, properties like temperature or pressure affect the glass fibers and locally change the light transmission characteristics in the fiber. In this particular case, downhole temperature changes produce lattice oscillations in the doped quartz glass making up the optic fiber.

The laser light sourced from a semiconductor laser diode passes through the fiber and falls onto these areas of the fiber that are experiencing thermally-induced molecular oscillation. This produces a light scattering effect, and the scattered light undergoes a spectral shift by an amount proportional to the resonance frequency of the lattice oscillation.

The scattered light contains different spectral shares, or bands. Two of these, a temperature-dependent anti-Stokes band and a temperature-independent Stokes band, are compared to arrive at the local temperature of the optical fiber. The temperature resolution is reportedly better than 0.3 K, and the measurement range for a single end fiber setup is 10 km.

The downhole measurement system consists only of the fiber optics package. The more complex processing components that translate the scattered light bands into a temperature profile are located at the surface. LIOS Technology’s commissioning and visualization software, CHARON_02, compresses the large volume of data from downhole and allows for various options in data representation, including:

• Displaying temperature as a function of fiber position,
• Selecting a profile from a user-defined point in time, and fixing this as a reference for comparison with other profile data,
• Comparing temperature against true vertical depth (TVD) and horizontal displacement (HZD) incorporating well trajectory,
• Displaying temperature against an imported geothermal gradient, which can be displayed in the TVD view as well.

Temperature profile data can be transmitted in any desired form or format, using standard interfaces, and can be displayed or further processed by PC, PLC or SCADA systems. LIOS Technology also states that its OTS is the first one with an approved WITSML (Wellsite Information Transfer Standard Markup Language) data interface, which ensures compatible data streams on the TCP/IP level of distributed temperature profiles, backscattering profiles and event data. In addition, the OTS comes with an integrated protocol converter, which safely transfers data and enables integration into customized database structures.

This integrated setup has allowed LIOS Technology to successfully deploy its OTS at various onshore and offshore locations, many performed recently in coordination with Shell International Exploration and Production Company. On some of these deployments, one single evaluation unit with a maximum power consumption of 26 W sensed up to 8 fibers that were each 10 km in length.

LIOS Technology states that continuously measuring downhole temperatures using the OTS fiber optics tool enables full coverage of complex and multilateral wells with a single optical fiber cable. In addition, engineers can use it to monitor the inflow of perforations or injection profiles of injection wells, and to gain insights into production tubing integrity and gas lift performance.

Temperature monitoring using OTS also has benefits at the surface, allowing operators to check for insulation leaks in surface flow lines, monitor the onset of hydrate precipitation or wax deposits, and detect leaks in pipelines.

Overall, OTS helps a producer build a comprehensive understanding of their downhole operations. This in turn will help the producer minimize production disruptions, improve reservoir management strategies, increase production yields and reduce the number of well interventions.

LIOS Technology will be exhibiting its OTS product offerings at Offshore Europe 2007 in Aberdeen from 4-7 September at Booth 1493G in the German Pavilion. To learn more about OTS and to read some case studies on how it has been applied, visit www.lios-tech.com.

Ted Moon is the Technical Editor of SPE.org. He brings information on emerging technologies, R&D successes, new field applications, updates from SPE papers about recent innovations, and more. If you have a question or suggestion for future article topics, email Ted at teched@spe.org.