November 2015

What's new in production

Now hear this
Don Francis / Contributing Editor

It’s the sound of data and of what could be the Next Big Thing in completion and production operations. Distributed Acoustic Sensing (DAS), has useful upstream applications, and numerous companies are developing this technology. A thumbnail description by one of them, Silixa, of its DAS flavor may be helpful.

In essence, it’s an “optoelectronic” system that employs a fiber-optic cable. When a pulse of light travels down this cable, a small amount of light is naturally scattered (through Rayleigh, Brillouin and Raman scattering) and returns to the sensor unit. The nature of this scattered light is affected by tiny strain events within the optical fiber structure, which themselves are determined by the localized acoustic or seismic environment.

Silixa’s DAS system collects the true acoustic signal at all points along an optical fiber. This allows the system to determine the speed of sound in the material surrounding the sensing cable, which enables applications such as flow quantification.

Time-lapse seismic. The company offers a lengthy list of applications and benefits. They claim that using a permanently installed, fiber-optic sensing system to acquire time-lapse seismic in a producing well enables the operator to describe changes occurring in the reservoir over time, as a result of production. This could help to incrementally improve recovery.

In general, DAS systems do not employ downhole electronics or gadgets with moving parts. They should be more reliable than conventional geophone-based methods, where DAS advocates say electrical noise, component failure, and mechanical fatigue can reduce measurement quality and system life.

Permanent production logging. British company Optasense describes its DAS technology, as treating the entire length of existing or new fiber-optic cable as an array of thousands of high-resolution sensors.

According to the company, its production flow monitoring service helps operators to understand inconsistent production, well-to-well or stage-to-stage. Using a fracture profiling technique, its DAS system identifies perforations that contribute to production, in real time. Flow through each perforation or port in the well can be quantified.

Permanently installed fiber and topside equipment creates a flow profile of the well, on demand. Flow data are generated at each perforation cluster, and historical waterfall data are recorded and processed. When repeated, these measurements can be used to monitor the effectiveness of each perforation entry point, for the life of the well. Production flow across control valves can be determined, to monitor the condition of ESPs, and to confirm opening and closing of gas lift valves in smart-well completions.

The company sees DAS permanent production logging as a superior alternative to conventional production logging tools. The attraction here, is the elimination of well intervention, at least for production logging purposes, along with minimal interference with flow.

Thousands of single-point sensors. Ireland-based Omnisens’ fiber optic technique provides distributed pressure, temperature and strain monitoring. The company says a single optical fiber [can be the equivalent of] thousands of single-point sensors, providing a significant reduction in installation, calibration and maintenance costs. In addition, assets can be monitored in real time, where previously this was impractical, due to distance, location or environment. The company describes optical fiber as cheap, lightweight, pliable, immune to electromagnetic interference with a life expectancy of more than 40 years. These advantages make it a cost-effective, flexible, durable and inert sensor medium.

For fiber-optic enthusiasts, here’s a brief excursion into some technical arcana: the company prefers Brillouin Scattering over other backscattering phenomena. To begin with, it can be stimulated by special optical processing, dramatically increasing the magnitude of the Brillouin interaction and making it significantly more efficient for sensing purposes.

Its technique exploits the sensitivity of the Brillouin frequency shift for temperature and strain-sensing applications. Standard low-loss single-mode optical fiber offers several tens of kilometers of distance range from a single interrogator, and compatibility with standard telecommunication components.

Since there is no dispersion effect in single-mode optical fibers, the optical signal characteristics are not affected, with the result that spatial resolution is maintained all along the sensing length. In contrast, the dispersion effects of multi-mode, fiber-based Raman sensing systems cumulate along the sensing length, degrading the spatial resolution.

Brillouin scattering can be stimulated optically, resulting in the greatest intensity of the scattering mechanism, and thus an improved signal-to-noise ratio, providing large optical budget and long-distance capabilities.

Being a frequency-based technique, Brillouin is inherently more accurate and more stable in the long term than intensity-based techniques, which suffer from a tendency to drift.

Brillouin scattering is an intrinsic phenomenon in optical fibers, and temperature and strain fluctuations impact the silica density. In this aspect, Brillouin-based measurements are independent of the measuring device, as it is an intrinsic property of the silica fiber that is analyzed. This absence of calibration drift guarantees accurate and reliable measurement over the long term.

There’s a lot more to say about DAS, but this glance suggests that if the industry is to be driven by Big Data, it will first need Big Measurement. This is certainly something that DAS is well-suited to provide. wo-box_blue.gif

About the Authors
Don Francis
Contributing Editor
Don Francis DON@TECHNICOMM.COM / For more than 30 years, Don Francis has observed the global oil and gas industry as a writer, editor and consultant to companies marketing upstream technologies.
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