Wavelength
Anti-Stokes process Stokes process
Brillouin
(distributed temperature
and strain sensing)
Pulsed laser signal
Rayleigh
(distributed
acoustic sensing)
Raman
(distributed
temperature sensing)
Downhole sensing applications
enhanced by specialty optical fibers
GEORGE OULUNDSEN, DANIEL HENNESSEY, and MIKE CONROY
As the worldwide demand for ener-
gy increases, it becomes more impor-
tant to efficiently extract and recover oil. In response, the oil industry
continues to find new oil reservoirs
and has developed enhanced extraction methods to recover oil from nontraditional wells such as shale oil (e.g.
Eagle Ford and Bakken formations)
and heavy oil sands (McMurray formation and the Orinoco Belt). In addition, only about a third of the po-
tential oil is recovered from existing
reservoirs, and enhanced oil recovery
methods could increase the production of existing wells.
1 Enhanced oil
recovery methods have required new
technologies for the continuous monitoring and optimization of the recovery process.
These enhanced oil recovery meth-
ods typically include cyclic steam stim-
ulation (CSS) and steam assisted gravi-
ty drainage (SAGD).
2, 3 In both SAGD
and CSS, the use of high-tem-
perature steam (often along
with other fluids) is used to
heat and reduce the viscosity
of the oil, improving flow and
facilitating extraction to the
surface. Efficient operation
and optimization of
the recovery process
requires continuous
process feedback.
Continuous monitoring of the well-temperature profile
is important for optimal operation of the well. However,
extremely harsh conditions found in
oil wells complicate the measurement
and monitoring of the downhole environment. As an example, in SAGD
and CSS wells, the temperatures typ-
ically reach 200–350°C; pressures
can reach 5000 psi; and the wells can
contain high levels of hydrogen with
partial pressures reaching 1–2 atmo-
spheres (atm).
The high temperatures and harsh
environmental conditions in oil wells
challenge traditional electrical-based
temperature sensors. In addition, tra-
ditional temperature sensors only give
discrete local temperature values and
usually cannot be left in the well for
continuous process feedback. Starting
in the 1990s, as enhanced oil recovery
methods became
more prevalent, the
oil industry start-
ed experimenting
with fiber optic sensors to overcome
the challenges of traditional sensors.
One of the first types of fiber optic
sensors developed for oil-well monitoring was a temperature sensor based on
distributed temperature sensing (DTS),
where the optical fiber is the sensor.
Today, DTS is gaining significant popu-
larity for continuous temperature mon-
itoring in oil wells and it has become
a valuable tool for optimizing and im-
proving the operation and efficiency of
oil recovery. Additional sensing technol-
ogies using optical fibers include dis-
tributed strain sensing (DSS) and dis-
tributed acoustic sensing (DAS), and
the combination of temperature and
strain called distributed temperature
and strain sensing (DTSS). In all cases,
the optical fiber is the sensor element.
The fiber is the sensor
Optical fiber-based distributed sensing
is based on monitoring changes to the
intrinsic properties of the light with-
in the fiber when it is exposed to environmental changes, such as tempera-
ture or pressure. All of the distributed
Distributed temperature sensing (DTS)
is the most common use of optical fiber-
based sensors in oil recovery; the fibers
are highly specialized and must withstand
the high-temperature, hydrogen-rich
environments found in oil wells.
