Reservoir Monitoring Technology

Author Lasse Amundsen og Martin Landrø

Permanent seismic monitoring is an emerging market with significant potential. Who are the major players and what has been done to validate the Optowave technology? This article has been designed to get you up to speed with this important development.

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 fibre optic OBS system. Thousands of sensors are trenched in the seafloor and connected back to topside facilities for reservoir monitoring.

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.

Qualification of fibre-optic OBS sensors (left) in Trondheim harbour. The sensors were trenched to 1 m (top right). The Norwegian Geological survey provided the source vessel (bottom right).

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.

Monitoring with re-deployable OBS systems OBS systems need not necessarily be permanently trenched into the seabed. The advantage of a re-deployable system is that it can be moved around the field as required. One option is simply to order the OBS monitoring survey from a contractor. Another is to purchase dedicated equipment that can be deployed to chosen survey areas. Statoil was the first company to apply OBS monitoring with re-deployable cables, using it in 2001 at its Gullfaks field. At Gullfaks, about 25 per cent of the field is devoid of seismic data since production started in 1986, because survey vessels towing lengthy hydrophone streamers have to steer well clear of permanent field installations (particularly platforms) due to safety requirements. Statoil has successfully applied this technique together with streamer seismic in a number of monitoring surveys acquired by various service companies. The Gullfaks 4D seismic has contributed directly to more than 15 infill wells and generated a net present value of around US$ 1 billion, while the costing about US$ 60 million. To monitor the Azeri-Chirag field offshore Azerbaijan, BP purchased a 120 km re-deployable OBS system from Geospace Engineering Resources International (GERI) in 2007, at a cost of $16.2 million. Baseline surveys were acquired in 2007, and repeat surveys and a further baseline have been acquired in 2008.

Fibre optic seismic array for the Snorre FSM project prior to loading on the installation vessel. Photo: Tom Reidar Guttormsen

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.

Common receiver gathers for a fibre-optic station. Top = inline, crossline, and vertical accelerometer data. Bottom = rotated inline, crossline and vertical accelerometer data.

Fibre optic players There are three players offering fibre optic seismic systems: Optoplan, Stingray Geophysical and Petroleum Geo-Services (PGS).   The Norwegian company Optoplan, a pioneer over the past 25 years in optical fibre sensing within oil and gas applications, was spun out in 1985 from the optic technology group at the Norwegian University of Science and Technology (NTNU) in Trondheim. From 2002-2007, Optoplan became a subsidiary of Weatherford International Ltd. In 2007, Wavefield Inseis entered into an agreement with Weatherford to commercialise the Optowave system. Upon completion of the commercialisation period, Optoplan became a subsidiary of Wavefield Inseis. Late 2008, Wavefield Inseis was acquired by CGGVeritas. Optoplan today is part of Sercel.   The UK company Stingray was established in 2006 to commercialise fibre-sensing technology licensed from QinetiQ, a former UK Ministry of Defence's research arm (see GEO ExPro October 2007). Its Fosar system was trenched and tested in late 2008 at Tjeldbergodden offshore Norway, where Optoplan's seismic equipment was also trialed. No results from this experiment have been reported in the literature. Stingray is in partnership with Bergen Oilfield Services for equipment deployment. Investors in the company are Energy Ventures, Chevron Technology Ventures and StatoilHydro Ventures. Stingray runs a joint industry project sponsored by BP, Chevron, ConocoPhillips and StatoilHydro.   PGS' permanent monitoring system is called Opto-Seis. In 2006, PGS and Chevron ran a pilot in the Gulf of Mexico offshore Galveston but the cable was not trenched. PGS has set up a dedicated facility for the manufacture of fibre optic cable, as well as R&D, in Austin, Texas.

Acknowledgments

Thanks to Hilde Nakstad (Optoplan) and Mark Thompson (StatoilHydro) for sharing their insight into the validation of fibre optic technology.  

References

Foster D, S Fowler, J McGarrity, M Riviere, N Robinson, R Seaborne and P Watson 2008 Building on BP's large-scale OBC monitoring experience-The Clair and Chirag-Azeri projects: The Leading Edge 27 1632-1637  

Nakstad H and J T Kringlebotn 2008 Probing oil fields: Nature photonics 2 147-149  

Thompson M, L Amundsen, P I Karstad, J Langhammer, H Nakstad and M Eriksrud 2006 Field trial of fibre-optic multi-component sensor system for application in ocean bottom seismic: SEG Expanded Abstracts 25 1148-1151  

Thompson M and M Andersen 2008 Focused seismic monitoring: The Leading Edge 27 1626-1631  

Van Gestel J P, J H Kommedal, O I Barkved, I Mundal, R Bakke and K D Best 2008 Continuous seismic surveillance of Valhall Field: The Leading Edge 27 1616-1621

Martin Landrø is a professor in Applied Geophysics at the Norwegian University of Science and Technology (NTNU), Department of Petroleum Engineering and Applied Geophysics, Trondheim, Norway.   Lasse Amundsen is Chief Scientist Exploration Technology at Statoil. He is adjunct professor at the Norwegian University of Science and Technology (NTNU) and at the University of Houston, Texas.