Application of Optical Sensors in Deepwater Environments
Authors: Hodges Steve , Bilberry Dave , Pleasants Charles , Vozniak John 
Published: October 2008

Fiber optic sensors are currently being deployed in novel completions throughout the world. The first completions being targeted are wells where specific problems associated with temperature need to be addressed. Fiber optics provide unique solutions in challenging environments in both existing fields and new frontiers such as deepwater.

Fiber optics provide a high degree of reliability as no electronics are deployed downhole. Reliability issues are especially important in deepwater applications to reduce risk associated with high failure rates. Having an inherent tolerance for high temperature, fiber optics can provide distributed temperature data at one-meter intervals throughout the wellbore and the flow conduit. Due to their long data transmission length (12km), fiber optic systems have the potential to provide unique solutions to problems encountered in deepwater environments.

This paper discusses the current state of fiber optic distributed temperature monitoring and shows how this technology can be employed in deepwater situations. Temperature measurements at one-meter intervals along the entire flow conduit allow operators to:Define inflow performance without well intervention, thus reducing operating expenseDetect and monitor the progression of water coningDetermine flowing temperature at various positions in the well to help avoid phase change problems such as paraffin or hydrate deposition.

Through early detection of these and other problems, operators can make changes in production profiles or chemical injection programs to minimize the effects of the problem and increase overall wellbore performance. Case histories illustrate the results seen to date and exemplify unique completion techniques that take advantage of fiber optic technology.

Introduction

Fiber optic sensors are widely used throughout many industries and are now becoming established as a way to gather data in the petroleum industry. Considered a superior alternative to conventional electronic sensors in hightemperature pplications (&gt;150°C), fiber optic sensors also have other advantages over electronic sensors, including:Higher sensitivityElectrical passivityHigh temperature tolerance (&gt;350°C)Intrinsic safetyWide band widthImmunity to electromagnetic interferenceSingle point and distributed sensing capabilityMultiplexing capabilitiesExtremely small size.

Among many measurands that currently can be accomplished with fiber optic sensors downhole are temperature, vibration, pressure, acoustics, flow, strain, pH and chemical species.

Principles of Fiber Optic Downhole Temperature Sensing

Fiber optics operate downhole in the following manner. A laser light pulse is sent down a multi-mode fiber optic waveguide. As this pulse travels along the waveguide, specific, temperature-induced molecular vibrations cause a very weak reflected signal to travel back up the fiber to the source. This weak signal is filtered out and measured by the surface opto-electronics system. The surface system compares the launch time of the light pulse to the time taken for the reflected light to get back to the source. The time differential determines the point of the temperature measurement, as the speed of light in the fiber is constant and known.