Sensornet installed a digital well integrity solution to monitor and optimize well production during a workover to install a gas lift system in a producing oil well in the Middle East. (Image courtesy of Sensornet)

The pressure grows daily for operators to meet hydrocarbon production targets and maximize recovery within safety and environmental regulations as well as time and budget constraints. These challenges demand monitoring technologies and solutions with the precision to detect the smallest temperature variations, the speed to deliver timely interpretation of critical changes and events, and the reach to operate along the full extent of an asset throughout the life of the field, however remote or hostile the operating environment.

Digital well integrity

First established in the oil and gas industry for downhole well monitoring, distributed temperature sensing (DTS) involves installing fiber-optic sensing cable in the well completion to enable the production engineer to continuously monitor well integrity The sensing cable is permanently installed downhole, and the result is a powerful non-intrusive, real-time monitoring system that allows production and reservoir engineers to analyze underperforming wells and schedule remedial action.

Condition monitoring along the entire completion can identify problems and flow fluctuations that cause fluid slugging. DTS monitoring allows optimization of the entire gas lift process to ensure flow. The distributed flow information provides input for reservoir characterization and the resulting field development plan.

On a sectional basis, the DTS system is able to monitor changes in gas inflow because cooling occurs as injection gas loses pressure entering the production string. This is clearly identified on the DTS profile. By continuously monitoring thermal events in the completion, operators can determine which valves are operating and the sequence in which they are opening. If there is a tubing or packer leak, the operator is able to detect the precise location of the event and determine the most effective course of remediation. Without complete coverage, it is possible to misinterpret well integrity issues and to mistake leaks for improper valve operation. The high resolution of the acquisition unit also allows detection of intermittent valve operation.

FloQuest DTS visualization and interpretation software provides the operator with a tool to analyze the flow profiles and changes to production when combined with other petrophysical data (e.g. porosity, permeability, rock strength, etc.).

An accurate monitoring system can also detect flow assurance issues that not only minimize the amount of production lost but also prevent potentially major environmental or safety incidents from occurring. Early problem identification allows the operator to schedule intervention requirements with planned maintenance downtime. By detecting the temperature change of the surroundings, the DTS can not only detect the presence of a leak, but can pinpoint the location down to 33 ft (1 m).

In the event of a gaseous leak, the temperature drop due to expansion is instantaneous and can be considerable. The DTS can provide measurements every 10 seconds, a frequency that is essential in the case of potentially explosive gases.

The effect of liquid leakage (e.g. oil or water) on temperature is less pronounced and is even more of a challenge to detect along a horizontal well. This system can detect changes of less than 32°F (0°C). By analyzing temperature measurements, the system not only detects leakage but can quantify leaks down to 1-liter (0.26-gal) accuracy.

The monitoring gap

During a workover to install a gas lift system in a producing oil well in the Middle East, Sensornet installed a digital well integrity solution to monitor and optimize well production. DTS data from across the production interval was used to enable better understanding of lift performance over time, resulting in a dramatic reduction in the requirements for traditional pressure gradient surveys. The measured inflow profile and depletion trend have proven invaluable in the design and placement of subsequent wells in the field.

Intervention, production, and remediation planning led to the realization that deferred production each year exceeded 270,000 bbl. In addition, there were gas lift operational issues that reduced daily production and caused associated downstream problems due to slugging.

With an immediate need to provide increased and improved production information to the production and reservoir engineering teams, the operator deployed a Sensornet DTS acquisition system to provide an accurate flow profile along the length of the well to ensure events were detected immediately. Deploying this system allowed the production and reservoir engineers to use limited intervention opportunities to find viable solutions to rectify underperforming wells.

For the sensing cable, used the specially coated optical fiber is pre-inserted into protective metal tube layers with a typical outer diameter of 1?4 in. and encapsulated in an anti-abrasive plastic. This cable was conveyed with the completion on the outside of the tubing and was permanently installed to the toe of the horizontal well. At each tubing joint coupling, a protector clamp secured the optical cable and provided a protective envelope for the cable as it is run into the well.

The deployed SureSight sensing cable ensures years of operation without degradation or loss of signal performance, providing continuous information on flow and temperature events on the targeted zones. This data enables the operator to make informed decisions on stimulation programs. Installing the digital well integrity solution was quick and efficient and resulted in little interruption to the conventional completion deployment.

Once permanently installed, the real-time system measured minute temperature changes and supplied calibrated temperature readings at 3.3-ft (1-m) spacing along the well bore from the surface to the toe, giving a thermal profile of the complete well. This provided flow and diagnostic information from initial well stimulation, startup, and production along the 2.7-mile (4.4-km) well length and gave production and reservoir engineering teams information to better understand lift performance so well production could be optimized and operating costs minimized.

During well startup, the operator estimated six hours of rig time were saved when the DTS temperature profile clearly detected a slugging faulty valve, allowing the component to be identified quickly and immediately replaced. If this system had not been deployed, it would have been difficult to identify which valve was affected.

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