Distributed temperature sensing heats up
  from: Offshore Engineer
  by: Jennifer Pallanich
  Thursday, October 02, 2008

Distributed temperature sensing (DTS) is crossing the threshold between operator interest and operator acceptance for use in monitoring and troubleshooting well operations. The technology, pioneered in the 1980s, has been in use in the oil & gas sector for about 10 years. One company is working to improve the technology and the way operators deal with its accompanying abundance of data as a means of preventing interventions, as Jennifer Pallanich reports.

The appeal of distributed temperature sensing is large: lots of data available in real-time to allow better management of operations.

In addition to providing conventional production monitoring, the technology allows an operator to identify problems before they become serious and to troubleshoot wellbore conditions without intervention, such as locating tubing and equipment leaks, detecting flow behind casing and identifying cross flow. It also enables an operator to optimize multi-zone production over life of a well by identifying water or gas break-though and monitoring changes in zone contributions as well as steam flood and SGD efficiency. Finally, it allows operators to verify effectiveness of downhole operations in real-time by confirming injection profiles, monitoring cementing jobs and verifying gas-lift valve operations.

SensorTran’s Jack Angel, vice president of oil and gas business development, says it comes down to: ‘Having information versus not having information. What’s it worth? If you defer an intervention, what’s that worth?’

SensorTran, a NASA technology spin-off based in Austin, Texas, has been designing and supplying fiber optic DTS systems since 1998.

DTS works via a pulsed laser paired with the optical fiber through a directional coupler. Light backscatters as the pulse propagates through the fiber, and because the velocity of light propagation is known, distance can be determined from the timing of the returned backscattered light. Certain components in the backscatter – called Raman components – are affected by temperature, so it becomes possible to calculate temperature by figuring the ratio of the signals that are affected by the backscatter. By combining this temperature measurement technique with distance measurement using the speed of light, the DTS provides temperature measurements incrementally along the length of the fiber.

The fiber optic DTS method using the Raman-effect was developed in the early 1980s, and it is based on optical timedomain reflectometer technology, using a technique derived from telecommunication cable testing.

Traditional temperature measurement techniques tend to provide non-continuous information or data about single points down the well. Because DTS provides data over thousands of points in real time, says Mikko Jaaskelainen, SensorTran’s chief technical officer, it’s important to have access to a system that can handle the deluge of information in real time.

‘How do you handle this data overflow?’ he asks.

Using a systems approach, he adds, SensorTran provides a solution that can handle the large amounts of data, visualize that data, and provide automatic alarms as defined by the user.

At its most basic, changes in temperature measured through DTS can alert one to the location of a problem, but combining it with PerfectVision gives insight into what the problem is, Jaaskelainen says. SensorTran’s PerfectVision system combines data from a number of locations to pinpoint where problems exist, he says. He adds that it enables correction of certain effects, such as fiber aging, changes in connector losses over temperature, or non-uniform cable manufacturing. ‘It removes most of the uncertainty and unreliability problems previously associated with DTS,’ he says.

SensorTran’s PerfectVision uses dual laser technology, dynamically and automatically calibrating each of the measurement points. The technology eases deployment and improves the life of the fiber. For now, SensorTran serves as an original equipment manufacturer and sells its sensors to service companies.

According to the company, using DTS makes sense when there is a need for a more complete picture of a well’s performance.

A typical DTS well monitoring solution consists of a DTS unit connected to an optical fiber installed in a control-line within the well. They provide real-time dynamic temperature data with high accuracy, very fine resolution, and fast measurement speeds for up to 40,000 discrete points along a single optical fiber. One surface unit can control up to 50 wells.

‘There’s a lot of opportunity unrealized yet in thermal profiling and distributed sensing,’ Angel says.

Applications include leak detection, stimulation, and observation wells, and it can be used in various offshore locations.

‘You can plan your interventions, whether they’re light or heavy, you’re not surprised,’ Angel says.

SensorTran says its DTS can be used not just in single wellbores but also across fields to provide insight about the condition of the reservoir. ‘If you’re going to do reservoir monitoring, you have to see into the reservoir,’ Jaaskelainen says.

Using DTS provides a chance to obtain a more complete picture of the reservoir, Jaaskelainen says. In the past, operators have only used instruments in a few high end wells in each field because of cost. Cost-effective sensing solutions allow operators to place instruments in a larger percentage of wells in a field so they can draw conclusions about the reservoir. Such an understanding helps operators avoid leaving behind pockets of hydrocarbons, he adds, and allows optimization of the fields.

Given that bill of goods, why hasn’t use of DTS become more widespread?

‘Sensing technology is proven, yet there’s a reluctance to embrace it,’ Angel says.

Jaaskelainen adds: ‘It’s related to the packaging and deployment, as it’s always been in the oil industry.’ Many of those issues have been addressed, he says. Industry associations such as the Subsea Fiber Optic Monitoring (SEAFOM) group, in which SensorTran participates, are addressing any remaining deepwater challenges.

‘There is a cycle of adoption and performance that we are moving through,’ Angel says.

While companies are starting to understand the technology and value, adoption has been slow. People ask how fiber optics can measure the temperatures, Angel says.

‘Our challenge is to take the magic out of this perception. The reality is it delivers on the promise.’ OE

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