Downhole fiber optic measurement allows longer term evaluation

Open or closed loop systems can be used, with closed loop systems yielding better quality information [4,833 bytes]. The separation of the Anti Stokes Raman Band from the rest of the frequency spectrum make it ideal for detailed frequency analysis [15,924 bytes].

From single point to many points

Kevin McMillin
Engineering Editor
Fiber optics have been used in the oil and gas industry for over a decade as a component in measurement and monitoring systems. Wireline service companies incorporated the technology into their measurement operations, and hold numerous patents related to the use of fiber optics in oilfield measurement tools. However, until recently, application of this technology was limited to invasive, sequential single-point measurements for which the measuring tool must be physically manipulated through the wellbore environment.

Pruett Industries International of Bakersfield, California has now developed a passive fiber optic temperature profiling system for petroleum industry applications, using fiber optic distributed temperature sensing (DTS). The significance of this development is that the system provides a continuous, real-time temperature versus depth profile along the entire length of an optical fiber. Laser technology eliminates the need for any physical manipulation of tools through the wellbore environment. The system is capable of bi-directional temperature measurement in permanent or temporary service equipment configurations.

Well temperature surveys to date have been primarily taken with sequential single point measurements, sampled using highly accurate quartz-gauge technology to give a temperature profile versus depth. Logging units and personnel arrive on location and stay there while performing the survey.

In the past this was considered acceptable and a cost of doing business. But electronics and fiber optic technology have improved, allowing for more automated systems to be designed. The added costs of personnel and temporary measurement equipment can be eliminated. Fiber optic distributed temperature sensing brings automation to well temperature surveying operations, allowing for optic fiber to be run into a well, linked to a remote acquisition system, and left alone - ideal for offshore applications.

This remote controlled system is a viable alternative for multi-well monitoring. A virtually unlimited number of wells may be monitored with a single remote surface unit, and radio or cellular links to a client's office. The increased presence of minimal-use offshore platforms in marginal, high well density fields is an ideal environment for application of these remote monitoring systems.

Measurement fundamentals

The DTS technique uses optical domain reflectometry (OTDR) to measure temperature effects along a fiber optic cable. Reflections from a laser beam, fired from a surface control unit, are generated by temperature "excited" variations in conducting medium composition and density, molecular vibrations, and bulk variations. These reflections are detected and analyzed for frequency phase shifts to yield a temperature measurement.

The three most prominent reflection methods to be considered in the measurement are Rayleigh, Brillouin, and Raman Scattering. Rayleigh scattering is characterized by strong signal strength but exhibits limited temperature sensitivity. Brillouin scattering is characterized by a strong signal strength and high temperature sensitivity, but is too close to the Rayleigh wavelength to separate without expensive spectrum analyzing electronics. Raman scattering, the weakest signal strength of the three bands, is temperature sensitive and resides outside the Rayleigh and Brillouin wavelengths. The lower frequency peak, known as the "Anti-Stokes" Raman Band, is the most temperature sensitive and is the band utilized for the actual temperature measurement.

Strong reflective signals in relation to cable "noise" increase the accuracy of the measurement. Signal strength losses can be generated by bad connector splices, tight bends, and of course, mechanical damage in the cable. A broken fiber will terminate the signal completely. Acceptable losses within any given length of cable are approximately less than 20dB. Any configuration of connectors and splices can be used as long as the cumulative loss is not greater than 20dB.

To obtain a temperature gradient a measurement of depth is required. The depth of any particular sample is computed by measuring the time of the return signal and using the speed of light as the velocity of the signal.

Conveyance methods

Several different fiber deployment methods are available. The traditional conveyance method via wireline is widely used because of a thriving market for one-time surveys. The initial survey after a well completion, and sequential, periodical production evaluation surveys over the life of a well, still exists. Coiled tubing has been used to deploy the optic cable in existing production tubing for a one-time temporary survey.

An optical Fibertube™ is installed within conventional coiled tubing, and the coiled tubing is deployed to the maximum desired survey depth and the temperature profile obtained. There is no limitation on tubing sizes in which the optic cable can be deployed. The most unique, and the most permanent, method is deployment on new production tubing as it is run into well. An empty 1/4-in. stainless steel tube is directly attached to the tubing.

This looped system allows for permanent, protected installation of the fiber optic cable, or temporary housing for less frequent one-time surveys. Centralizers are installed on the string to protect the empty cable tube from extensive, damaging contact with the wellbore. Optical fiber is then installed in the 1/4 in. tube. Looped or single-end system configurations offer long-term monitoring or periodic survey options. Permanent monitoring installations are achieved by cementing the 1/4 in. tube behind the casing string.

Future applications

Applications such as perforation response and electrical submersible pump monitoring have already been achieved with this technology. Future applications include leak detection and valve integrity in offshore gas lift operations. The capability of permanent installation in high deviation wells attached to production casing and easy deployment of temporary services makes this technology very flexible for future development in a fast, ever-changing area of the oil and gas industry.

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