HP/HT wells have always been and remain at the frontier of the E&P business, requiring the application of new technology to provide the economic justification for field development. Through this sector, advanced drilling and completion technologies have been conceived and then proven, demonstrating the true spirit of the industry.

In 2015, the need has never been more pressing to reduce the cost of HP/HT field development. Distributed fiber-optic (DFO) technology for use in the E&P business has been gaining acceptance over the past three years, with applications proven for seismic, production profiling and unconventional well monitoring. This technology is now in a position to allow a step-change in data acquisition and reservoir monitoring in high-value HP/HT wells, allowing cost-effective reservoir management.

Optical fibers are the established medium for the transmission of high volumes of digital data, especially in the telecommunications business. However, there is a small but growing optical fiber sector where the fiber itself becomes the sensor. When a laser is used to pulse light into the fiber, a small amount of the light is naturally backscattered (through Rayleigh, Brillouin and Raman scattering) and returns to the sensor unit. This returning signal is processed to give a measurement of physical properties such as temperature, acoustics and chemical composition. Therefore, applications are being introduced into sectors as diverse as medicine, turbomachinery and the E&P sector.

Silixa has developed two fiber-optic interrogators for the E&P sector: the intelligent distributed acoustic sensor (iDAS) and the XT distributed temperature sensor (XT-DTS).

The iDAS utilizes a novel optoelectronics architecture that measures the modulation of the backscattered light. The acoustic field around the fiber exerts tiny pressure and strain changes onto the fiber. These changes are measured optically at a rate of up to several kilohertz. This allows observation of the acoustic field at discrete positions along any standard optical fiber (up to many kilometers) with a resolution of about 1 m (3.2 ft)—in other words, a microphone every meter. This has allowed seismic, production profile and integrity services to be developed.

The DTS measures the Raman scattering along the optical cable. Specific spectra created by the laser pulses within the fiber are proportional to the localized temperature. This allows continuous temperature measurements along any standard optical fiber (up to 30 km [18.6 miles]), achieving temperature resolution of less than 0.01 C (0.02 F). Monitoring absolute temperature as well as change in temperature across a producing interval has led to the development of sophisticated production profiling.

Optical fiber cables
Many lessons were learned from the fiber installations for the DTS in the late 1990s and in the early 2000s, specifically that with high temperatures, the fibers (which are fundamentally “flexible glass” with a very specific refractive index) exhibited darkening in the presence of hydrogen, which attenuates the light passing through the fiber and hence affects the quality of the measurements.

The industry responded with the development of new hydrogen-resistant fibers and improved coatings (applied to the outside of the fiber) for high-temperature applications. Consequently, the fiber life is in excess of 10 years. For some specific applications such as steam-assisted gravity drainage (SAGD) oil wells, where temperatures can exceed 400 C (752 F), pump-down systems have been developed to allow easy replacement of fibers with more chemically resistant coatings.

Benefits of optical fiber
Once the optical fiber is packaged in a metal tube, known as fiber in metal tube (FIMT), the fiber is suitable to be installed in oil and gas wells, is mechanically protected and is pressure-isolated. The FIMT can be encapsulated inside ¼-in. cable that can be strapped to tubing or cemented behind casing. Fiber also can be run on coil or stiff rod for interventions.

There are immediate advantages to the subsurface engineers once the optical fiber is installed:
• The fiber is there for the life of the field;
• As the laser interrogator is at surface, DFO services are operationally nonintrusive;
• After installation, the fiber is “on depth” and therefore useful for repeat measurements;
• Data acquisition is often performed remotely, thereby reducing offshore personnel;
• Data can be recorded “on demand,” thereby allowing true monitoring; and
• Monitoring can be done under flowing conditions.

Fiber for HP/HT
Data acquisition in HP/HT wells is notoriously difficult. Due to the high temperature, many acquisition systems do not function, whether permanent or interventionbased. Additionally, completions are complex and subject to high pressure, making reentry hazardous and leading to major HSE concerns.

Intervention is required to obtain data, but this is very costly and presents a serious risk to the well with loss of production.

Installed optical cables are therefore perfect for HP/HT wells since once they are installed, no further intervention is required. The coatings on the fiber are now developed to allow long fiber life. The active element (the interrogator) is on the surface. Finally, the permanently installed fiber allows “data on demand,” either by a specific quantitative survey or by qualitative monitoring of the well.

HP/HT examples
A North Sea multiple-well oil and gas production platform was producing from formation depths of 4,500 m (14,764 ft) and at a temperature of 160 C (320 F). Optical cable was strapped to the production tubing, originally used for pressure and temperature monitoring at the tubing reentry.

Distributed acoustic sensor (DAS) seismic offset vertical seismic profiling (VSP) acquisition was conducted in multiple offshore wells without interrupting production. The system acquired data with full wellbore coverage for every shot, and all acquisition units were time-synchronized using VHF triggering and GPS-linked continuous recording. The survey was repeated two years later in the same three wells for 4-D monitoring evaluation. Again, there was no interruption to platform production or operations.

The surveys enabled seismic data on demand independent of operations/production, expanding the opportunity for future surveys. Future fiber functionality is to shoot seismic on all wells every three months using an airgun from the rig to monitor overburden.

An offshore gas production platform in the Asia-Pacific region was producing from formations at 5,000 m (16,404 ft) and at a temperature of 200 C (392 F). Optical cable was installed in micro-coil and entered into the production liner. Both DAS seismic VSP and production profiling were completed with no degrading of the fiber with temperature.

The customer was able to recover data in a hostile environment to define producing and nonproducing intervals. Future fiber functionality is to permanently install fibers in wells to allow permanent and continual DTS and DAS across the production interval.

In the U.S., a steam injection profile on a horizontal steam injector field featured two wells with true vertical depth of approximately 1,000 m (3,280 ft), both of which had been completed with high-temperature optical sensing cable. Steam quality and injection rates were varied at the surface, and relative steam distribution to each of several flow control devices (FCDs) was estimated using a combination of iDAS and DTS data.

The distributed acoustic data indicated which FCDs were taking significant steam. The distributed temperature quantified the annular steam distribution within slotted liner sections in between the FCDs. The DTS stability of ULTIMA DTS identified temporal anomalies in the steam injection process, while the high sampling frequency used for iDAS made it possible to develop an acoustic fingerprint for the injection activity at each FCD.

Continuous data allowed the customer to manage steam injection to optimize oil recovery. Future fiber functionality is to enhance production profiling and shoot repeat DAS seismic to monitor the progression of the steam cloud.

Continuous production monitoring
Silixa has developed a fiber optic-based acoustic flow metering system. This allows continuous measurement of the flow and composition of both the liquid and gas phase fluids within a flowline.

When wrapped in a continuous length of fiber helically around the exterior of any flowline, the iDAS measures the dynamic radial strain (hoop strain) exerted along a section of a pipe and the convection speeds of eddies. By measuring the speed of sound, the fluid type is identified, and gas volume fraction and water-to-liquid ratio are determined.

By combining the continuous monitoring of the well contribution by DFO together with the continuous flow and fluid composition at the wellhead, complete well monitoring is achieved.

Remotely controlled, with data streamed to the field office and head office and with local alarms to alert any changes, the well can be monitored 24 hours a day. But when there is a change in production, it can be seen immediately through alerts and by review of the production profile. The reservoir engineers will be able to clearly identify the production zone that is changing.