Measuring The Seismic Wave With Photons: Realization of The Optical Oil Field
Authors: Langhammer Jan , Eriksrud Morten , Nakstad Hilde , Berg Carl 
Published: May 2016

Fibre optic sensing technology, which involves no operating electronics at the sensing part, has a potential and significance due to the fact that it will offer higher reliability and longer life time of sensors compared to its electrical counterparts. The passive nature of fibre optic sensors also contributes to that such systems will require a minimum of maintenance during operation over several years. No electronics and fewer components of the in-sea or in-well part of such system may make it easier and cheaper to produce than electrical systems of the same capacity.

All the existing technology which exists in connection with the instrumented oil field can now be changed from electrical to optical sensing technology. Fibre optic receiver systems for 4D purposes, both multi-component ocean-bottom receiver systems and in-well sensors, can now be installed successfully at locations where the oil companies would like exploit the life-of-field seismic concept. We are advocating optical sensing technology to be an important part of the tool box for the oil companies in their work to implement the instrumented oil field in a cost efficient way. The "optical oil field" should represent the next step in technology in connection with reservoir monitoring for increasing the hydrocarbon recovery rate in a cost efficient way.

Introduction

The instrumented oil field has been associated with installation of seismic sensors at the sea floor and in producing/injection wells, in addition to pressure, temperature and saturation gauges to monitor a producing well (Lumley, 2001). On-demand 4D seismic and "intelligent wells" can give information to the reservoir engineers close to real-time implying that decisions connected to optimization of hydrocarbon production can be made a lot faster and with greater confidence than before.

Up to recently, the sensor technology for both seismic and in-well measurements has been based on electrical components. Electrical sensors may experience degradation in the hostile in-well environment.

The telecommunication industry has for several years utilized fibre optic technology in their work to increase band-width and increase life-time of telecom cables. The thin and flexible strands of pure silica glass with thickness of hair has increased the capacity of communication lines within a cable with several factors compared to telecom cables made of copper. Sensors for various measurements, making use of laser light, which is fed through optical fibres, offers a viable and reliable method as an alternative to sensors based on electronics. Fibre optic sensing technology has for the last 5–10 years been available to the oil companies for in-well applications in their work to actively monitor oil/gas flows and injection processes in order to increase hydrocarbon recovery and optimize production. Fibre optic sensors embedded in ocean bottom cables intended for permanent seismic installations at the sea-floor have also emerged as a potential tool for oil companies to utilize information from repeated seismic to better image reservoirs, map reservoir depletion zones and increased understanding of expansion fronts of injection water, drainage areas and connected volumes. This would reduce the risk of encountering water swept areas in a reservoir with future production wells.

Due to their passive nature and no electronics, fibre optic sensor systems can offer a huge reliability potential for several in-well applications including seismic imaging (Bostick et al., 2003, Knudsen et al., 2003 and Keul et al., 2005). The environmental conditions are challenging with temperatures possibly up to 200ºC and pressures exceeding 1000 bars. Conventional electrical sensors will always suffer in reliability at such extreme temperatures and pressures. A successful installation of a fibre optic permanent borehole seismic system in an offshore production well has been executed at Valhall in the North Sea (Hornby et al., 2007).