Monitoring with permanent systems
The choice of which monitoring technique to use depends on a number of factors, including the expected life of the field and the required intervals between repeat surveys. In fields with more than 10 years of production left, or those where difficult reservoir conditions imply that it would be advantageous to carry out frequent surveys, typically at intervals of about one year or less, permanently installed ocean bottom seismic (OBS) cable systems is the cost-effective alternative to conventional streamer surveys.
Achievements with such Life of Field Seismic (LoFS) systems have been demonstrated by BP (GEO ExPro No. 2, 2004). However, despite its successes with the projects at the Valhall field offshore Norway (2003) and the Clair field pilot offshore UK (2006), permanent seismic installations have not been widely adopted on a larger scale. Can new developments based on fibre-optics make the future more attractive?
Life of Field Seismic (LoFS)
Monitoring with non-optical permanent sensor technology
BP's Valhall field, in production since 1982, is one of the North Sea giants. The first 4D seismic data was acquired in 2002. The 4D data revealed significant production-induced changes, and resulted in the Valhall partnership's decision in 2003 to purchase and permanently trench an OBS system into the seabed. This allowed for ‘seismic-on-demand', and an unprecedented potential to acquire high quality seismic data for reservoir monitoring. The project was named Life of Field Seismic (LoFS) Valhall project (see GEO ExPro September 2004).
The cost (excluding installation) and the huge amount of data are illustrated by the key statistics of the LoFS Valhall project:
- Investment $US 40 - 45.
- Receiver coverage 45 km2, 120 km of seismic cables, with more than 10,000 sensors.
- Shooting area 125 km2, with 50,000 shots per survey.
- 7 Terabytes of data per survey, transfer of data onshore via optical link.
- Survey duration 15 days.
- Total weight of in-sea equipment 355 tons.
In 2006, BP trenched 40 km of cables over a 13 km2 pilot area of the Clair field west of the Shetland Islands. BP commissioned the cable systems for their two LoFS applications from Geospace Engineering Resources International (GERI), an OYO Geospace company.
The seabed part of LoFS systems is entirely buried into the seabed. A riser cable is installed from the topside down to the seabed where it connects to the seabed system by means of subsea connectors. The subsea plant consists entirely of cables. Typically, there are two specific types of cables:
- Umbilical cables, which contain electric and fibre optical internal cables.
- Seismic array cables, which contain electrical cables and seismic sensor pods usually every 50 metres.
Other players in this segment of the conventional seismic cable market are the French company Sercel (the CGGVeritas seismic acquisition manufacturing arm), today owner of Optoplan, and the Norwegian company OCTIO Geophysical.
OCTIO Geophysical co-operates with ION who has pioneered the development of a MEMS (Micro-Electro-Mechanical System) based 3 axial accelerometer called VectorSeis. OCTIO has acquired an exclusive license to use VectorSeis for permanent installations offshore in their ReM (Reservoir Monitoring) system. The company has StatoilHydro Venture as investor and strategic partner.
A new generation of technology
In 2004, Optoplan and Statoil started a research collaboration to develop a fibre optic seismic array sensor system, today called Optowave. This co-operation combined the knowledge and experience of Statoil, a leader in time-lapse (4D) seismic and multi-component ocean bottom seismic (OBS), with Optoplan's knowledge and experience in the field of optical sensing in the oil and gas industry, in order to develop a reliable, low-risk, cost-effective monitoring technology.
A key advantage of optical sensor technology is that the subsea components are completely passive, providing greater durability and reliability when compared with systems that use electronic or moving-coil sensors. Fibre optic sensors aim to provide data over the life of the field with lower maintenance or replacement costs than other technologies. The fact that only a small number of optical fibres are used to collect data from many thousands of channels distributed over the reservoir means that the system is more compact, lighter and thus should be easier to install.
The Optowave system is made up of fibre optic lead-in cables and a laser interrogation?instrumentation system that is placed on a platform or some other surface facility. In techno-language, a highly advanced multiplexing technique allows thousands of sensors to be interrogated by the laser instrumentation through the subsea optical fibre lead-in cable.
A long way from lab to seabed
Two significant field tests of fibre optic seismic sensor technology were conducted during 2006. The first test was carried out in Trondheim harbour in Norway, in which fibre optic cables were deployed at 40 m water depth and buried to a depth of one metre in the seafloor sediments. An electrical MEMS (Micro-Electro-Mechanical Systems) cable was installed for comparison. The test verified excellent performance of the optical system.
The second field test was carried out at Tjeldbergodden (also Norway) where StatoilHydro operates Europe's largest methanol plant. Here a fibre optic cable was trenched into the seafloor at a water depth of approximately 300 m. The main motivation of this test was to confirm the procedures required for upcoming offshore installations. The trials went smoothly and seismic surveys verified the excellent performance of the fibre optic sensor system.
First in the world
Early in 2007, Optoplan signed a contract with Statoil for a permanent reservoir monitoring pilot installation at their Snorre field in the North Sea. Previously, Snorre had created a focused seismic monitoring (FSM) project, which identified some of the actions required in order to achieve Snorre's ambitious recovery factor of 55%. One action identified as having significant potential was the use of frequent time-lapse seismic over restricted areas. This could be realised through the implementation of a permanent seismic solution, in this case utilizing fibre optic sensor technology. As a result, StatoilHydro installed 10 km of seismic array cable with 200 stations in the summer of 2008.
The breakthrough contract for fibre optic technology, won against tough competition from rival systems, was the award to supply the Optowave system to monitor the ConocoPhillips' operated Ekofisk field in the North Sea.
The Ekofisk project will involve the supply of 240 km of seismic cables, covering an area of 65 km2, the largest area yet for a permanent installation. The cost of the seabed monitoring system, including the laser interrogation and data recording equipment, will be upwards of $US 40 million. Marine installation is planned for 2010 but the contract has yet to be awarded and the project is understood to require costly specialist marine handling equipment. Installation cost, which in this case may exceed $US 40 million, is always a significant part of permanent monitoring costs.
In the next issue of GEO ExPro, we will report results from the Snorre pilot.
Thanks to Hilde Nakstad (Optoplan) and Mark Thompson (StatoilHydro) for sharing their insight into the validation of fibre optic technology.
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