Downhole data acquisition has evolved with many well completions now integrating fiber optic technology so that engineers and operators may gain a better understanding of well and reservoir behavior.

While there are increasing the number of wells equipped with permanent fibers, the distributed fiber-optic (DFO) sensing is being deployed in wells that lack permanent fibers. These sensors are installed below packing units where permanent fiber may not reach, and achieve results where permanent fiber optic systems have failed.

In 2014, IHS CERAWeek featured Silixa Ltd., a provider of distributed fiber optic sensing solutions. The company's intelligent distributed acoustic sensor (iDAS) was found to enable high-quality digital recording of acoustic waves along kilometers of fiber optic cable. Silixa’s distributed sensor offers the finest temperature of 0.01oC and spatial resolutions of 25cm over a wide operating temperature range. With this the technology proved to open opportunities in downhole sensing.

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By integrating DFO sensing, operators like Lime Rock Partners, Chevron Technology Ventures and Statoil Technology Invest found solutions for flow profiling, seismic imaging, hydraulic fracture monitoring and monitoring of well integrity.

The Society of Petroleum Engineers explains how recent advances in technology take DFO sensing to the next level. Carbon rod intervention systems developed by Ziebel seek to identify sand ingress zones as well as provide real-time feedback during stimulation treatments that is not only accurate but economical.

(Click to enlarge.) Ziebel sensing rod sensing rod was parked in the wellbore for acquisition. Source: Ziebel(Click to enlarge.) Ziebel sensing rod sensing rod was parked in the wellbore for acquisition. Source: ZiebelThe two methods developed by Ziebel include a composite carbon rod, known as Z-System, and a gravity deployed composite carbon line, called Z-Line. While both systems have been used to optimize production, monitor well integrity and to allocate water and polymer injection, the idea is to use the technology to monitor sand production and downhole injection processes in real-time.

Alternative methods used to detect sand ingress zones include surface acoustic sensing and running acoustic tools on a wireline, well tractor or coiled-tubing unit. The pitfalls of surface sensing stem from the tool's general inability to accurately detect the depth at which sand ingression takes place. Acoustic tool surveys can provide accurate point measurement, but are costly and time-consuming as areas of interest must be investigated, often with different production rates and choke settings.

The concept Ziebel is building has been proven using permanent fiber installations. In a permanent installation, fiber-optic cable remains partially sheltered from the direct flow. The acoustic energy signal at sand entry intervals was shown to be evident.

In practice, pressure fluctuations are also monitored during production. This provides verification as to when problematic sand production begins, and identifies the sand ingression zone. Acoustic visualization coupled with pressure fluctuations allow operators to control sand production by adjusting the surface choke and remediate the failure area of any sand screen completion.

Another intended application of Ziebel’s carbon rod intervention system is to provide real-time monitoring of stimulation treatments. Monitoring delivery of stimulation treatments to target zones will help producers track fluid placement and identify if a repeat treatment is necessary.

To do so, the deployable system is designed to withstand pressures and flow rates of bull-headed stimulation treatments. Incorporating saver sleeves at the fracturing head decreases the potential for carbon rod erosion. The carbon composite rod or line is placed in the central producing bore and remains stationary.

Outside of the turbulence experienced at the fracturing head, erosion tests reportedly confirm minimal loss of material from the outside diameter of the carbon rod. The integrity and sealing ability of the intervention system remains uncompromised and able to protect the DFO during retrieval.

The problem experienced with permanent installations is that devising a method that avoids damaging the fiber can be both complex and expensive. Intervention with the carbon rod could eliminate these complications and reduce expenses.

Applications especially well-suited for the carbon rod intervention system include chemical fluid diversion in refracturing treatments. Real-time identification of ineffective diversions may well enable responsive adjustments to downhole events.

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