The sensing cable is attached to the outside of 7-in. casing with a metal cross-coupling protector

Thermal recovery, polymer flooding, and miscible gas injection are some of the methods used to enhance oil recovery. Using cyclic steam stimulation (CSS), a form of thermal recovery, has proven effective in optimizing production without adding excessive costs.

As 70 to 80% of operating costs are attributed to the steam generation and injection process, understanding steam placement and steam chest growth, in addition to understanding the production profile, is crucial to minimizing the steam/oil ratio (SOR). The ultimate aim of Sensornet’s digital well integrity solution is to fully optimize the thermal recovery CSS process in pursuit of a low cumulative SOR.

Digital well integrity

First established in the oil and gas industry for downhole well monitoring, distributed temperature sensing (DTS) involves installing a fiber-optic sensing cable in the well completion to enable the production engineer to maximize output, reduce intervention costs, and increase well integrity. A reliable, life-of-well system is critical, as the injection/production cycle can exceed several years. Permanently installed DTS technology provides continuous information about which zones are contributing to production, where the steam chest growth is occurring, and where the steam front is likely to break through.

The Sentinel DTS system allows production and reservoir engineers to analyze underperforming wells and fully understand which sections of the well are producing to achieve a uniform stable flow, thereby maximizing the SOR.

Any change in flow has an associated temperature change. The DTS system senses changes in temperature as small as 0.007°C from the initial baseline trace along any part of the well bore. More importantly, where large thermal events occur (such as in thermal recovery), this measuring performance translates into an update of the complete well thermal profile every 10 seconds.

Intermittent changes are also detectable. If required, the system can measure to a range of 18.6 miles (30 km). That equates to 15 miles (25 km) along a surface flow line and down into a 3-mile (5-km) well. User-defined alarms can be set to continually police the well bore for changes. A triggered alarm alerts the production engineer by text message or email.

Fiber-optic sensing

The 572ºF (300ºC) rated SureSight sensing cable consists of high-temperature coated fibers contained in multilayer hermetically sealed metal tubes. Great care must be taken to ensure the integrity of the optical path and the fiber’s optimum thermal conductivity. The rugged design protects the sensing fiber during installation and ensures long-term protection against hydrogen ingress. The cable is connected to the surface unit via an optical rotary joint so system integrity can be monitored during deployment.

Intervention in a production well results in deferred production, so minimizing intervention time is desirable. Installing the digital well integrity solution with the completion is quick and efficient, causing little interruption to conventional well deployment.

The DTS system is able to monitor, section by section, any changes in the steam flow as it loses pressure and cools down entering the well bore through the formation. This ensures that the sensing solution is truly distributed along the completion. Without complete coverage, it is possible to misinterpret well integrity issues, mistaking leaks for improper steam operation.

The system at work

Sensornet recently achieved an industry first for a major Middle East operator by installing a DTS system on the outside of casing and cementing it in place. In this
installation, the DTS SureSight cable was installed on the outside of the 7-in. casing string. The system was permanently installed in the well on the outside of the casing and was conveyed to the toe of the well.

Once the casing was at depth, the company used a novel mechanical orientation and alignment system to carry out a tubing-conveyed perforation (TCP) operation.

In these particular wells, the steam injection cycle took approximately six weeks, followed by shut-in for approximately one week further to permit the steam to pervade the formation and allow the resulting hot water to mix with the oil to encourage flow. The well was then put into production, and the oil/water mix flowed to surface. When the oil content reduced significantly, the steam injection process began again ? a process known as “huff and puff.”

As the steam chamber grew during the steam stage, there was great uncertainty regarding when, where, and how much steam was present around the reservoir during injection. During production, there is uncertainty of flow distribution across the producing zones. Steam breakthrough from adjacent injection wells puts the well at risk of catastrophic failure. The DTS provided the continuous temperature profile of the complete well that was required to close the monitoring gap.

DTS and production

The DTS system also can monitor the production cycle, enabling the operator to determine the location and individual flow rates of the producing zones. This information allows steam pressure and steam placement to be adjusted to optimize performance.

The system installed was based on a fiber-optic DTS system and used the company’s proprietary high-temperature sensing cable in conjunction with a TCP gun. This was the world’s first example of a casing-conveyed DTS system run with the company’s oriented perforating system.

The high-temperature SureSight cable range is based on a pre-installed fixed cable construction. The specially coated optical fiber is pre-inserted into protective metal tube layers where the outer tube diameter is typically 0.25 in. At each tubing joint coupling, a stainless steel band holds the optical cable in place to achieve the tight tolerances required. This robust sensing cable was deployed to ensure years of operation without degradation or loss of signal performance. The distributed flow information provides valuable input to the reservoir characterization and ongoing field development plan.

Installing the sensing cable behind the casing protects the DTS fiber from direct exposure to the steam and the mechanical thermal stress effects during the steam injection cycle. During the production cycle, direct inflows are detected rather than a commingled temperature, making flow interpretation less complex.

Once the casing-conveyed DTS sensing cable was installed along the 0.6-mile (1 km) well, the cemented casing had to be perforated using TCP guns. To prevent damage to the sensing cable during perforation, the company used its mechanical oriented perforating system to ensure the lower section of the sensing cable was away from the phased gun orientation so the integrity of the sensing cable was not risked during perforation. Orienting the cable 180° from the direction of the perforations avoided the risk of damage to the cable during perforating. Additional perforating in the future can be performed using the same orientation and alignment.

Measureable performance

This digital well integrity solution provided a continuous thermal profile for the three CSS wells and continues to be used with 100% success to avoid cable damage while perforating. The benefits of this solution are obvious, not only for the identification of issues in the reservoir throughout the lifecycle of a well, but also for future drilling, as the construction of a robust reservoir model can dictate when a well is not needed or has to be relocated.

The Middle East has fully embraced this technology as operators in the key oil and gas producing region look to optimize field development and secure maximum production combining DTS with zonal isolation and remote flow control.

Removing guesswork and uncertainty can potentially save millions of dollars for the operator through reducing or eliminating costly interventions and identifying potentially hazardous leaks to prevent unquantifiable cost of human life or the environment.

Safety, security, environment

Installing this system provided complete security for the well. All thermal events were monitored, which provided an accurate understanding of which zones were taking the most steam, the relative production from each zone, and integrity monitoring of the casing to provide early warning of the onset of steam breakthrough. Continuous monitoring provided crucial information to control where the steam was injected and at what pressure. Not only does the system improve recovery, it reduces water usage as well.

By creating 328 to 394 ft (100 to 120 m) of perforations, it is possible to view exactly where the steam breakthrough points occur and the varying levels of production through the formation. This is traditionally carried out through six or seven observation wells that surround the main injection well. Results from these wells may be unreliable, as they do not monitor the entire length of the installation. Issues around potential points for leakage and dangerous gases traveling to the surface may also be a possibility. Monitoring the complete well provides a simple method of identifying the reservoir layer and understanding the growth of the steam front over time. The trending data gives an indication of the applied steam pressure, and the effectiveness of steam placement, and the ensuing heating of the reservoir zone.