The ability to permanently monitor oil and gas wells is becoming essential given the importance of EOR and increased regulatory control of well integrity. Fiber-optic distributed acoustic sensing (DAS) can provide a solution to this once an optical fiber is installed in a well.
After several years working closely with Shell, Opta-Sense recently signed a further three-year development contract with that company. Now OptaSense is able to offer its DAS technology and services to the rest of the oil and gas industry.
How it works
Using a sensing principle called coherent optical time domain reflectometry, DAS injects a laser pulse down a single-mode optic fiber. As it travels along the fiber, a minute amount of the light is backscattered to a receiver. Any acoustic or vibrational disturbance along the fiber alters the backscattered light from that local region. At the receiver this change in the backscatter is processed, and the acoustic disturbance is recorded. Since the speed of light in a fiber is constant, by accurately timing the arrival of any change to the backscatter at the receiver, users can determine where the disturbance occurred along the fiber.
In practice this means that along a fiber of, for instance, 6 km (3.6 miles), up to 4,000 independent simultaneously sampled acoustic channels at 1.5-m (5-ft) spatial resolution can be resolved. Alternatively, if higher fidelity signals are required, conserving amplitude and phase coherence, 800 channels of 7.5-m (25-ft) spatial resolution can be captured simply by changing the settings in the receiver unit in the control room.
The practical implementation of this technology means that once connected to an optical fiber installed in a well, DAS provides continuous monitoring along that well from top to bottom. For applications requiring permanent monitoring, this provides a unique capability to “listen” to the well during all stages of its life from drilling to abandonment. DAS can be used in a wide variety of applications such as a permanent receiver array for vertical seismic profile (VSP) measurements through completions as a means of listening to the fluid and proppant uptake during hydraulic fracturing or in the production phase where the DAS can be used to measure the flow rates along each producing zone in each well.
DAS for VSP
VSPs are conventionally acquired with geophone arrays to image the seismic setting of a well. DAS provides the ability for the first time to image wells that previously were inaccessible to conventional geophone arrays. Using fiber permanently installed behind casing, semipermanently
installed on the production string, or temporarily installed via wireline or on coiled tubing, it is now possible to make economical VSP measurements directly in wells that are either too hot, too high-pressure, highly deviated, or have restricted access. For instance, high-temperature steam injection wells can be instrumented with DAS using fiber that can withstand temperatures in excess of 300°C (572°F) so that VSP images can be repeatedly made to determine the extent of the steam front. In highly deviated wells where it is impossible to insert a geophone array, VSP images can be made by attaching the sensing fiber-optic cable to the production string. And in producers and injectors where space restrictions prevent conventional seismic sensors from being deployed, low-profile fiber cable can be deployed.
The ability to take repeated time-lapse measurements on a dense array of many hundreds of virtual sensors, with full coverage of the well from top to bottom without well intervention, provides a unique capability for enhanced imaging of the reservoir. This has been used and certified for monitoring of, for instance, CO2 injection.
The DAS-VSP service provided by OptaSense is fully integrated with various seismic source controllers, ensuring accurate timing. If Vibroseis is used as a seismic source, real-time correlation and stacking of the data produces standard seismic data files (in SEG-Y format) with fully populated headers. The data also can be remotely quality-controlled and assessed via a secure live Internet feed in real time.
During the treatment and subsequent production in unconventional oil and gas, DAS can be used to monitor microseismic events, especially those larger magnitude events that originate at existing fault locations. Because of the broad bandwidth and high dynamic range capabilities of the DAS technology, the large low-frequency seismic signals generated at existing fault locations can be accurately measured. Furthermore, the long-array sensing aperture permits geolocation if sensing fibers are deployed in multiple wells.
Hydraulic fracturing is being increasingly used to improve oil and gas productivity in tight reservoirs. Using fiber attached behind the casing and cemented in place or in openhole sections, it is possible with DAS to monitor the entire fracturing process at the treatment zone, including packer deployment, perforation detonation, sleeve opening, fluid and proppant entry, and plug setting. This in situ monitoring provides the ability to visualize and hear exactly what is happening along the well and in particular at the treatment zone.
With a fiber permanently installed behind the casing, it is now possible to monitor the effectiveness of the hydraulic fracture stimulation process at each treatment stage. The DAS hydraulic fracturing profiling service continually monitors and records the acoustic signals measured by DAS, the temperature measured by a fiber-optic distributed temperature sensing system, and the pump rates provided by the pumping contractor on site. This information is combined during each stage treatment to provide an estimate of the fluid and proppant uptake at each of the perforations. This information can be used in real time by the completions engineer in the field or remotely to optimize the treatment and increase well productivity or in post-job analysis.
In addition to the capabilities offered by DAS for reservoir evaluation, permanent production flow logging is now possible with fiber installed on casing or tubing with measurements taken on a continuous basis or at regular intervals using the OptaSense drive-by acquisition service. OptaSense is working to provide the ability to continually measure flow rates along the well without the need for well intervention and with no interference to the flow.
Furthermore, in existing or abandoned wells without fiber permanently installed, DAS can be used on a wireline to detect and locate anomalous behavior such as casing leaks and packer failures.
DAS is a rapidly developing technology that is gaining significant attention in many aspects of the oil and gas industry. OptaSense is helping to pioneer this new technology and has recently been awarded Hart Energy’s Meritorious Engineering Award for its DAS-VSP service.