P192 Field Trial of a Fibre Optic Ocean Bottom Seismic System

Transcription

1 P192 Field Trial of a Fibre Optic Ocean Bottom Seismic System M. Thompson* (Statoil ASA), L. Amundsen (Statoil ASA), H. Nakstad (Optoplan AS), J. Langhammer (Optoplan AS) & M. Eriksrud (Optoplan AS) SUMMARY Through a joint collaboration between Statoil ASA and Optoplan/Weatherford a fibre-optic ocean bottom seismic acquisition system has been developed and qualified. The system has been qualified in different settings ranging from 40m to 300m water depth. An array of two fibre-optic OBS cables was initially deployed in the harbour in Trondheim, Norway, and later a single fibre-optic OBS cable was deployed adjacent to the Statoil methanol refinery at Tjeldbergodden, Norway. Seismic data were acquired at both locations. The seismic data generated in Trondheim has been analysed and compared to data from a test cable based on MEMS technology, which was deployed at the same location. Analysis of the data from both Trondheim and Tjeldbergodden confirms the system s high degree of vector fidelity, high signal-tonoise ratio, very good ground-station coupling, reliability and excellent response in general to wave modes in connection with ocean-bottom seismic. These tests confirm that fibre-optic sensor technology, for applications within permanently buried seismic acquisition systems, is viable and can be introduced to the industry in connection with life of field seismic projects.

2 Introduction Multi-component fibre-optic sensors have previously been demonstrated in connection with permanent in-well applications (Bostick et al., 2003; Keul et al., 2005). One major advantage of fibre-optic sensor technology over electrical sensor technology is that the fibre-optic sensors are passive and do not contain electrical, or moving components which over time may be prone to degradation and failure. This result in a significant reliability improvement compared to electronic systems. Instead, all of the sophisticated instrumentation is located at the surface, which makes it easy to maintain, and upgrades to the system will not require expensive and risky field operations. A permanent, buried ocean-bottom system, using electrical sensors, has previously been tested out at the Valhall field in the North Sea (Barkved, 2004). The introduction of fibreoptic sensors to this acquisition concept may contribute to significantly lower cost in-sea equipment as well as more reliable sensors and better data quality over time. Fibre-optic ocean bottom acquisition system A multi-component fibre-optic seismic acquisition system has been developed and qualified as a joint venture project between Optoplan AS and Statoil ASA. The seismic sensors are fully based on fibre-optic sensor technology, i.e. each 4-C ocean bottom seismic station consists of a 3-C fibre-optic accelerometer unit and a fibre-optic hydrophone. Data acquisition Two qualification experiments were carried out. The first experiment (Thompson et al, 2006) was carried out in 40m water depth in harbour area of Trondheim, Norway (Figure 1). During this first experiment two fibre-optic cable units were deployed and for comparison a system based on MEMS technology was deployed in parallel. All three cables were buried at a depth of about 1m into the seafloor. Seismic data were generated using a selection of single airguns towed from the Norwegian Geological Survey (NGU) vessel M/V Seisma. The second experiment was carried out in approximately 300m water depth adjacent to the Statoil methanol refinery in mid Norway. This fibre optic OBS cable was buried to a depth of 1m into the seafloor and the fibre-optic lead-in cable was brought ashore into the methanol plant where the active top-side instrumentation was placed. Seismic data was later generated using a full size seismic source array commonly used in commercial seismic OBC acquisition settings. Data Examples and Analysis The data from the fibre-optic OBS system was compared in quality with data from an electrical system, which was deployed in conjunction with the first experiment. Key issues addressed in the data analysis were vector fidelity, system frequency response, intrinsic system generated noise, signal-to-noise ratio, and ground-station coupling. Analysis showed clean and high quality raw data (Figure 2), where both the frequency content and the quality of the data from all stations appear to be uniform. This indicates very good coupling of the buried stations to the seafloor and high signal-to-noise ratio. Hodogram analysis was carried out on the data with excellent results (Figure 3), and the data were successfully rotated using vector rotation based principles. Both the hodogram analysis and vector rotation indicate that the fibre-optic OBS system possesses good coupling to the sea floor and excellent vector fidelity.

3 Conclusions We have demonstrated that fibre-optic sensor technology can be utilised in ocean bottom seismic applications. The technology has been tested in two different settings and exhibits excellent vector fidelity, very good ground-station coupling and flat broad-banded frequency response. The data from this system show low noise and exhibit similar degree of vector fidelity to the reference system deployed in the first experiment. The results are promising and represent a great step towards utilizing fibre-optic sensor technology in ocean bottom seismic for reservoir time lapse studies. Acknowledgments The authors would like to thank Statoil ASA and Optoplan/Weatherford for permission to publish this paper. We also wish to thank our colleagues at Statoil ASA and Optoplan AS for their hard work throughout the project, as well as NGI, NGU and BOLT Technologies for support throughout the project. The Trondheim Harbour Authorities are highly acknowledged for their permission to conduct the test in the harbour area. References Barkved, O., 2004, Continuous seismic monitoring, 74 th Annual International Meeting, Society of Exploration Geophysicists, Expanded Abstracts, p Bostick F. X, Knudsen S, Nakstad H, Blanco J, and Mastin E., 2003, Permanently installed fiber-optic multi-station 3-C in-well seismic trial at Izaute field, 65 th EAGE Conference & Exhibition Keul, P. R., Mastin, E., Blanco, J., Maguérez, M., Bostick, T., and Knudsen, S., 2005, Using a fiber-optic seismic array for well monitoring, The Leading Edge, 24, No.1, p Thompson M., Amundsen L., Karstad P. I., Langhammer J., Nakstad H., and Eriksrud M., 2006 Field trial of fibre-optic multi-component sensor system for application in ocean bottom seismic. 76 th Annual International Meeting, Society of Exploration Geophysicists, Expanded Abstracts, p Blue Red Green cable : FO cable : FO cable : MEMS Figure 1: The survey area in Trondheim harbour for the first experiment.

4 Figure 2: Common receiver gathers from a representative fibre-optic station. Components are: Inline (top left), Crossline (top right), Vertical (bottom left) and Hydrophone (bottom right). Source used was the BOLT 40 cu.in. air gun and the shot line is parallel and straight over the receiver cable. Filter: 3 Hz (12dB/oct) 160 Hz (72dB/oct). Figure 3: Hodogram analysis of data from a fibre-optic station which is tilted about 45 degrees in the vertical-crossline plane.

5 Figure 4: The common receiver gathers for a fibre-optic station at the top are inline crossline and vertical accelerometer data and bottom are rotated inline, crossline and vertical accelerometers. Source used was the BOLT 40 cu.in. air gun and the shot line is parallel and straight over the receiver cable. Filter: 3 Hz (12dB/oct) 160 Hz (72dB/oct).