A new era in seismic imaging

Wavefield Inseis ASA

Optowave, a fibre-optic multi-component ocean bottom seismic cable system for permanent installation has been developed, tested in the North Sea and is now a commercial product offered by marine geophysical services provider Wavefield Inseis ASA.

The system is based on a unique fibre-optic technology that has been developed by Optoplan AS out of  Trondheim in Norway. Optoplan  has pioneered optical fibre sensing within the oil & gas industry since 1985 when it was spun out from the optic technology group at the  Norwegian University of Science and Technology (NTNU) in Trondheim.

In 2002 Optoplan became a wholly owned subsidiary of Weatherford and in 2004 the company began a development project, together with Statoil, to develop seismic Ocean Bottom Cables (OBC) based on optical sensing technology and in 2005 started a series of field tests. By 2006 field tests results where such that Wavefield Inseis entered into an agreement with Weatherford to commercialise the system, initially for the permanent seismic reservoir monitoring market. This agreement also involved a purchase option and in December 2007 Wavefield Inseis completed the purchase of 100% of Optoplan from Weatherford.

Permanent seismic installations have emerged as a tool for oil companies to actively monitor oil/gas flows and injection processes in order to increase hydrocarbon recovery and optimize production. A permanently installed system, in comparison with a retrievable ocean bottom cable  system achieves a much higher quality 4D signal since an accurate 4D response relies on, among other factors, repeatability. Permanently installed sensors therefore provide the ultimate in repeatable seismic sensor positioning and earth coupling.

It is also a fact that many reservoirs have characteristics that degrade traditional marine seismic data acquired with arrays of hydrophones towed through the water. For example gas clouds above the reservoir can degrade data from seismic pressure waves  to such as extent that make structural interpretation and hence reservoir management extremely difficult.

The Optowave permanent  system is comprised of  four component (4C)  seismic stations that contain  3 orthogonal accelerometers together with a hydrophone, all of which have been developed using optical sensor technology. This allows shear wave data to also be collected which greatly enhances reservoir imaging leading to improved reservoir monitoring and management. The system is also made up of fibre optic lead-in cables and a laser interrogation  instrumentation system placed at the surface, either on a platform, production vessel or other surface facility.

The subsea components of the system are completely passive in that they have no electronic components whatsoever and so no power is required. The passive nature of fibre-optic sensors, embedded into 4C receiver stations, is an advantage over traditional electrical systems since the in-sea sensor equipment is not prone to electrical noise, leakage or short-circuit. The low power loss and large bandwidth of optical fibres enable extremely high data transmission rates over long lead-in cables and allows the transmission of huge amounts of information over tens of kilometers. This is extremely important when, for example,  the field is operated from sub-sea installations and the platform may be several kilometers away from the producing wells.

Other advantages of fibre-optic sensor systems include the small physical size of the sensors, high sensitivity and chemical/corrosion resistance. In addition, the sophisticated interogation and recording instrumentation is located at the surface, which makes it easy to maintain and upgrade.

An additional and very important factor is that the fibre-optic receiver cables are less expensive to manufacture than the traditional electrical receiver systems. All of these advantages make the fibre-optic sensor technology perfectly suited to be utilized in ocean bottom receiver cables in connection with life-of-field seismic projects.

The system utilizes fibre-optic interferometric sensing technology based on an optical fibre Bragg grating (FBG) interferometric design. Using optical interferometric techniques the fibre length change as a result of any disturbance in the active sensing elements can be measured with extreme accuracy forming seismic wavelets of the particle movement and pressure created by the seismic waves. All sub-sea components are passive and interrogated from the surface using an advanced combined time and wavelength division multiplexing technique.

 In this way the  different wavelengths of reflected light from the FBGs are used to multiplex several stations along a single fibre, which minimizes the required number of total fibre splices. This has the advantage of easing the manufacturing of  large scale systems and allows for pressure balanced receiver stations, which itself enhances the reliability of the system by avoiding the need for pressurized housings. This in turn means that the system is ideal for deep water areas.

The highly advanced multiplexing technique allows many thousands of sensors to be interrogated by the laser instrumentation through the  subsea optical fibre lead-in cable. The  fibre-optic seabed portion of monitoring system is scalable and can accommodate channels counts beyond 10,000. Depending on receiver station interval and receiver line separation, this enables any field operator to cover reservoirs with seabed receivers in order to monitor the production area of interest.

In summary the benefits of  the Optowave seismic array due to the unique use of optical fibres to measure and transmit seismic data from the sea-floor,  include high-precision seismic measurements, much lower power consumption, improved system reliability and lower unit cost compared to other technologies. It is also much lighter in its construction compared to electrical systems which means it is easier and safer to handle and deploy.

Two significant field tests of the technology were conducted during 2006. The first test was carried out in Trondheim harbour in Norway during January 2006. In this test  fibre optic 4C cables were deployed in 40m of water  and buried into the seafloor  sediments at a depth of 1 meter. The test also included the installation of an electrical  cable next to the optical cable so that data from them both could be directly compared. The test verified the excellent performance and vector fidelity of the optical system and repeated surveys since the installation have shown the technology to be very robust and reliable.

Comparisons between traces of the hydrophone, in-line, cross-line and vertical components from both the optical and electrical sensor stations also showed that there is a  high degree of similarity between the two different systems which proved that the fibre optic sensors are able to provide the data quality required of permanent seismic systems for 4D seismic monitoring.

The second field test was carried out in November 2006 at Tjeldbergodden, on  the west coast of Norway, where Statoil operates Europe’s largest methanol plant. Here, a fibre optic cable was trenched into the seafloor in a water depth of approximately 300 m. The main purpose of this test was to qualify the installation procedures required for future planned full system installations in all areas, including deep water. The installation went smoothly and subsequent  seismic surveys again verified the excellent performance and reliability of the fibre optic system.

Repeated tests at two or three month intervals have been at both locations. Key issues addressed in the data analysis were the quality of the raw data plots, vector fidelity, frequency response, intrinsic system generated noise, signal-to-noise ratio and ground-station coupling. The data from both test locations were of very high quality. Both the frequency content and the quality of the data from all stations were uniform. This indicates very good coupling of the trenched receiver stations to the seafloor and high signal-to-noise ratio.

Good vector fidelity of 4C receivers is important in order to reliably record the vector wave field and accurately process the shear wave data to produce reliable images and results that can be interpreted in conjunction with the pressure wave data. A simple approach to evaluate the vector fidelity of a system is to acquire a shot line which is about 45-degrees to the horizontal components (45-degrees in the horizontal inline and crossline plane) of a receiver station. If the receiver gathers from the two different components are approximately equal with respect to amplitude and frequency, we can conclude that vector fidelity is satisfactory. Tests results  from both the fibre-optic systems where the shot line had an approximate angle of 45-degrees azimuth to the cable directions show  the similarity of both the the inline and crossline gathers plus the corresponding frequency spectra is also very good.

The analysis of the data from the pilot tests confirmed the systems high degree of vector fidelity, high signal-to-noise ratio, very good ground-station coupling, reliability and excellent response to wave modes in connection with ocean-bottom seismic. Through the field tests, it was demonstrated that fibre-optic sensor technology are ideal for permanent ocean bottom seismic applications. The Optowave fibre-optic  permanent seismic monitoring system  represents the opportunity for field engineers to optimize production and increase the hydrocarbon recovery rate from existing fields in a cost efficient manner.

In October 2008 Wavefield Inseis were awarded a contract to supply  an Optowave fibre optic  4C seismic reservoir monitoring system for the Ekofisk field in the North Sea. The project will involve the supply of  two hundred kilomteres of Optowave seismic array cables together with the laser interrogation and data recording system. To put that into some type of perspective, if you have been to Bergen in Norway which is home to the operations base of the company, then if you were to drive from the city centre to the airport and back again five times then that is the total length of the cables that will be supplied.

The seismic cables will be permanently installed into the seafloor at the Ekofisk and will represent the first ever full-scale fibre optic permanent seismic reservoir monitoring system implementation. We feel this is fitting since Ekofisk was  not only the first ever discovery offshore Norway back in 1969, but it is also the largest oil and gas discovery in the history of the North Sea.
                                                                                                                              
The Optowave system signals the beginning of a new era in seismic imaging and the next phase in Wavefield Inseis’ evolution as an integrated geophysical  services company. The use of permanent seismic systems is now widely accepted as an alternative to other marine surface seismic or retrievable OBC acquisition techniques for acquiring 4D data. The repeatability from survey to survey achievable with a permanently installed system can aid the operating oil companies in modelling the subsurface and the effect of production, thus maximising the oil recovery rate from the reservoirs. The repeat surveys will also be less costly than with traditional methods as it is only necessary to bring in a single vessel each time to supply the seismic source. It can therefore be the preferred alternative when the economics related to its use over the life of the field are superior.

The introduction of the new Optowave fibre optic permanent seismic system using optical sensing technology is an important breakthrough in many ways, not least that of data quality. The completely passive cable provides, for example, greater durability and reliability compared with systems that use electronic or moving-coil sensors. This means it will provide 4D for the full production life of a field with much lower maintenance or replacement costs. The fact that it uses only a small number of optical fibres to collect data from many thousands of channels means that manufacturing costs and hence the investment required to purchase a system are much lower. It also means that the system is more compact, lighter and hence easier to install. In all it provides the reliable, cost effective, high quality repeatable 4D seismic data that has been long awaited by producers.

For more information about the company, please visit www.wavefield-inseis.com.