Reducing Intervention in Subsea Wells With Fiber Optic Technology: Deployments and Results
Authors: Lovell John R 
Published: May 2013

Permanently installed fiber optic systems have been commercially available in the oilfield since the early 1990's, with the bulk of the deployments providing fiber-optic distributed temperature. Over 1200 installations have been deployed by one service company alone. Such installations have reduced the need for intervention via logging tools and have given crucial insights into wellbore integrity and reservoir production. Consequently, a prime application of fiber technology should be in deepwater fields, where intervention can be prohibitively expensive, if not impossible - but that has yet to prove true. The reluctance to incorporate downhole fiber appears to be because of limitations of technology relating to cables, deployment, acquisition and interfacing. These barriers are slowly being eliminated and a small number subsea wells have now been implemented with in-well fiber. Specific technology breakthroughs have been pertinent to subsea: cables that withstand hydrogen darkening; fiber connectivity interfaces to subsea trees; and increased deployment options for acquisition hardware. Deployed subsea solutions have included optic monitoring for cementing, integrity and clean-up. A roadmap to eliminate some remaining obstacles includes the needed interfaces for power and telemetry to the SPS and the interpretation of temperature data on complex subsea fields. That roadmap needs to take into account the full range of hardware opportunities available for subsea applications and not exclusively fiber-optic. Indeed, the optimal completion solution need not involve sandface fiber at all.

Introduction

Permanently installed fiber optic systems have been commercially available in the oilfield since the early 1990's, with the bulk of the deployments providing fiber-optic distributed temperature based upon Raman backscattering, (Brown, 2009). This Raman DTS measurement system provides continuous data along the length of the completion without the need for intervention and without introducing restrictions in the ID of the completion tubulars: restrictions that could impede subsequent wellbore access. Typical DTS spatial resolution is better than one data point per meter and with a temperature resolution 0.01oC. Applications of the distributed temperature data have included calculating flow contributions across the sandface, evaluating water-injection profiles, diagnosing the effectiveness of fracture jobs, finding cement tops behind casing, and identifying cross-flow between producing zones.

The growth of fiber installations for dry-tree (land and platform) applications is increasing exponentially. Over 1200 installations have been deployed by one service company alone. The bulk of those installations were single-stage completions, often for thermal or SAGD application, and relied upon the ability to pump fiber along a control line to convey the sensing fiber. About 20 of those wells have fiber pumped along sandface of dual-stage OHGP completions.

A significant advantage of the pumped fiber was that if the fiber should degrade over time, then it could be pumped out and replaced. This was particularly important before the discovery of how to construct fibers with some immunity to darkening from H+ and OH- ions, (Ramos et al., 2008). With hydrogen resistance, came the possibility of longevity of fixed-fiber cables and, since 2006, an alternate to pumped fiber has been available that combines multiple hydrogen-resistant fibers with an electronic line to create a hybrid cable (e.g. Fig. 1). This cable can power high-accuracy quartz gauges and also give the benefits of fully distributed Raman DTS. (Algeroy et al., 2010). More than 60 such systems have been deployed to date. The combination of these pumped and hybrid fiber installations have proven to reduce the need for intervention via logging tools, and provided crucial insights into reservoir production, performance of artificial lift systems, wellbore integrity and production optimization.