Seismic Monitors North Sea Field
In-Well Optical System at Work
Installation of a permanently placed seismic cable system to acquire time lapse, or 4-D, seismic at the BP-operated Valhall Field in Norway in August 2003 created a goodly bit of buzz -- particularly in the geophysical community. An experimental array had been installed in 1995 by Shell and BP at Foinhaven, but Valhall was the first such system to be financed and purchased by a business unit.
The permanent cable array on the seabed affords a high degree of repeatability -- and repeatability is key to 4-D seismic data acquisition, which is increasingly viewed by operators as a must-have as early as possible in the productive life of a reservoir both for monitoring and management purposes.
Valhall recently was the site of yet another milestone installation of technology geared toward its 4-D program. This time, it’s a downhole application that appears destined to move the 4-D effort forward in giant steps.
A two-year collaboration between BP Norway and Weatherford culminated in the world’s first successful offshore installation of a permanent in-well optical seismic system, according to Weatherford. The system was installed in BP Norway’s G-24 injector well at Valhall.
The installation represents an important extension of the existing permanent seismic monitoring system over the field, according to Olav Barkved, lead geophysicist for BP at Valhall.
The permanent in-well seismic system provides continuous seismic and P/T monitoring data, and it’s interfaced to the permanent OBC system. Both seabed and downhole seismic data are collected simultaneously.
“This is the first time permanent seabed sensor arrays and permanent in-well sensor arrays have been joined and recording both types of permanent sensing at the same time,” said Tad Bostick, vice president of optical sensing systems at Weatherford. “From a permanent reservoir monitoring standpoint, this is a milestone.”
Electronic sensors follow the classic “bath tub” failure characteristic -- early failures, then level reliability, and then a wear-out phase that produces a sudden and sharp increase in failure rates. The higher the operating temperature, the sooner this wear-out phase occurs.
A Better Image
Ocean bottom sensors deliver an image in time only, according to Graham Gaston, business development manager for Weatherford’s optical sensing systems group. It’s necessary to convert time to depth to translate the image such that it represents the real subsurface.
This is routine for individual surveys; but when collecting data over a period of several years, variables exist that make it difficult to compare one survey to the next, Gaston noted.
The in-well seismic sensors provide a constant reference point over the course of the years of 4-D seismic activity; the calibration reference enables the operator to acquire far better comparative images from the seafloor cable system. Another plus afforded by the sensors is production of a detailed image close to the borehole, improving upon the overall subsurface image.
In-well seismic sensors have other applications as well:
- Passive listening for acoustic events to improve understanding of fluid movement, drainage efficiency, active fractures and formation compaction.
- Analyzing reservoir connectivity between wells at a finer scale than possible using surface seismic.
- Optimizing well placement by sensing seismic signals from drill bits.
The Clarion permanent in-well seismic system is made up of highly sensitive, multi-component miniature optical accelerometers. The system features advanced optical multiplexing based on Bragg grating technology.
The optical sensors are made of machined glass that withstands temperatures as high as 345 degrees and pressures as much as 20,000 psi. In fact, the tensile strength of glass is superior to steel. The sensors have no moving parts and no downhole electronic components, and -- unlike traditional electronic sensors, which are susceptible to vibration-induced failure -- they can handle hundreds of Gs of shock stress without degradation or interruption of measurements.
To combat the phenomenon of “hydrogen darkening,” whereby hydrogen gas permeates unprotected fiber at very high temperatures and darkens the glass such that the light-dependent optical sensor is undetectable, uses several layers of protection ensures the hydrogen doesn’t actually get to the glass. Should darkening occur over time, Gaston noted the sensitivity of the system is such they have proved there is no problem over a 20-year life span.
This life span is critical because the reliability of the fiber optics is paramount. After all, permanent monitoring -- by definition -- requires that the sensors never be moved.
Weatherford technicians install the in-well optical seismic system in the North Sea.
Coming to a Field Near You?
It is noteworthy that the brains of the Clarion system are on the surface in readily accessed instrumentation. This means the hardware and/or software can be easily upgraded to improve the performance of sensors, which may have been deployed years earlier.
The use of the sensitive in-well optical sensors, which can actually detect microseismic events, is particularly timely in an area such as the maturing North Sea fields, where operators’ goals today are focused on production optimization through reservoir life extension and increased recoverable reserves.
Expect to see more in-well optical sensor projects fairly soon.
Installations in the Gulf of Mexico and elsewhere are imminent, according to Gaston.
Meanwhile, Bostick gives kudos to BP for being a trail-blazer.
“A key aspect of the project (at Valhall) was the collaboration between BP and Weatherford,” he said. “The field is in a high profile offshore environment where there’s no room for error, and we had strong guidance from BP on delivering technology suitable to them as a lead adopter of this technology.”