|2D/3D DUAL-SENSOR STREAMER CASE STUDIES (NORTH SEA, UK AND NORWAY ATLANTIC MARGIN): THE NEW STANDARD FOR EXPLORATION/PRODUCTION AND RESERVOIR CHARACTERISATION|
The “simple” traps have already been discovered and the oil & gas industry is moving towards more challenging areas where we need to detect and properly image very complex reservoirs as well as resolve very thin remaining hydrocarbons columns. For these reasons, geoscientists are always looking for a seismic bandwidth as wide as possible, extending both at the low and high end of the spectrum, to achieve a detailed reservoir characterisation analysis.
Achieving a broader seismic bandwidth from seismic acquisition, has been the subject of numerous discussions and trials. In 2007, PGS introduced a dual-sensor towed marine streamer (Tenghamn et al. 2007 and Carlson et al. 2007). This dual-sensor streamer enables the recording of the scalar pressure field and vertical particle velocity component. By recording those two components, the immediate benefit is the removal of the ghost reflections, which expands the seismic resolution by extending the seismic frequency bandwidth.
The broader seismic bandwidth from the dual-sensor streamer will have the following benefits:
· At the low end of the amplitude spectrum:
o Better depth penetration of the seismic signal
o Improved seismic inversion results (Whitcombe and Hodgson, 2007) as this improved low frequency content allows the inversion to be mainly based on seismic data and less dependent on a low frequency model “inserted” by the geoscientists into the inversion scheme. The direct consequence of this would be that the seismic inversion results would be more reliable away from the well control and therefore will provide a better predictor tool than a conventional streamer acquisition system.
· At the high end of the spectrum:
o Optimal vertical seismic resolution throughout the available depth range and more importantly at the reservoir level. Every reservoir geoscientist would like to see as much detail of the reservoir as possible.
· Better estimate of the earth attenuation can be obtained (Cambois et al., 2009)
· Increased signal to noise ratio across the full seismic bandwidth, since the streamer is towed deep, in a quiet marine environment.
Figure 1: Theoreticalamplitude spectra for conventional and dual-sensor streamer data. The extended bandwidth of the dual-sensor streamer data has a significant effect on the low end of the amplitude spectrum without suffering any losses at the high end of the spectrum.
The analysis of the dual-sensor streamer acquisition impact presented in this paper is based on some 2D and 3D datasets that PGS has acquired recently: North Sea, UK and Norway Atlantic margin and the Carnarvon basin in North-West Australian shelf.
All case studies illustrate the added value of this new acquisition system at the reservoir level.
|2D UK AND NORWAY ATLANTIC MARGIN|
The industry has tried many different seismic acquisition techniques to overcome the challenge of imaging below basalts. Several different source configurations have been tried; some very large volumes, deep tow and bubble-tuning, to mention a few. OBC and towed streamer seismic have been compared and seismic offsets have been extended even beyond 30km in search for high-amplitude reflection energy from large incidence angles, as well as converted waves. These seismic acquisition projects have all been highly specialized and have taken a large effort in planning and execution. Whereas some of them have given new insight, the specialized nature clearly implies there are trade-offs in terms of how suitable the techniques are for multi-target imaging and operational efficiency. The environmental impact of the largest sources may also be an issue.
Discoveries in the Faroe-Shetland and Møre Basins have so far been made in areas not covered by extensive basalt flows. A significant untapped potential may be present in equivalent leads, and good quality seismic data is required to image and analyze these areas. Bounding the proven discoveries are vast areas where the basalt flows seismically obscure the geology and structure of the underlying section. Very few wells have been drilled, and the sub-basalt plays are largely untested and represent an intriguing exploration upside.
With the above in mind, PGS decided to acquire a set of 2D lines to explore what data quality uplift could be gained from dual-sensor towed streamer technology. A dual-sensor streamer combines data from collocated pressure and particle velocity sensors to separate the wavefields into up- and down-going parts, thus eliminating the effect of the receiver ghost. Removal of the receiver-side ghost allows the streamers to be towed deep without loss of resolution, thereby enhancing the low frequency signal-to-noise. The intention with the Corona Ridge 2D survey was to use operationally efficient source and streamer parameters which should enable both high resolution imaging of post- or no-basalt targets as well as deep penetration and improved imaging below basalt. A total of 20 lines were acquired in the geographic area from Corona Ridge to Møre Basin, with a source of 4130cuin towed at 9m and a dual-sensor streamer of 8100m length, towed at 25m depth. The data was processed with special attention paid to multiple attenuation and velocity picking with geological model input, especially sub-basalt. The results (Figure 2) show very high resolution in the sequence above/outside basalt flows, and clearly improved imaging below basalt due to the increased content of low frequency signal. The analysis showed that the acquisition setup was fit for the purpose of multi-level exploration in this seismically challenging area, and the survey was extended in 2010.
Figure 2: Line through the provisional drilling location of Lagavullen ( 6201/10), note the well imaged low frequency events below the basalt and improved bandwidth in lower basalt section, where potential intercalations with sediments may occur (analogous with Rosebank)..
|3D DUAL-SENSOR STREAMER IN THE NORTH SEA|
In 2009, the first ever North Sea 3D dual-sensor towed streamer surveys were acquired in three different areas of the Norwegian sector of the Central and Northern North Sea. Since the dual-sensor towed streamer technique was utilized in close proximity to conventional seismic surveys, direct operational comparisons were possible and will be discussed in this paper.
Prospects and fields of greatly varying age and depth of burial were imaged with a superior data quality at all levels. Targets described in this paper vary from very shallow Neogene channel systems to sub-salt Permian structures within one dataset, through producing Paleocene sands to a Jurassic discovery at more than 4500m depth in another dataset, to significant discoveries in Jurassic sands and fractured basement in the third area. Comparisons with vintage data will be shown and analyzed in this paper.
Figure 3: The line shows a comparison between a conventional and a dual-sensor streamer acquisition over a North Sea dataset. A seismic imaging improvement with the dual-sensor streamer is clearly visible throughout the section: from the Paleocene done to the Jurassic level. At the Jurassic level the tilted fault blocks seismic response are significantly improvde leading to new potential exploration areas.
One of the three datasets has been used to carry out an extensive reservoir characterization study. The broader bandwidth and especially at the low frequency side of the seismic amplitude spectrum represents a key step forward into the seismic reservoir property estimation as the need of a-priori information is considerably reduced compared with a conventional towed marine streamer acquisition. Figure 4, illustrates the benefit of having a significant amount of low frequency from the seismic to estimate elastic properties. The example demonstrates that the acoustic impedance estimated at the Well B is nearly identical whether the well is included as a priori information in the model or not. This observation shows that our ability to predict the reservoir properties in well B is much improved due to the availability of a broader seismic bandwidth offered by the dual-sensor streamer.
Figure 4: Comparison of two acoustic impedance results using the dual-sensor technology and with two different initial models. The results of the acoustic impedance (represented by the colored image) at the well locations are extremely similar using either three wells (top figure) or instead two wells (bottom figure) in the initial model. The results at Well B between the top and bottom figure are identical demonstrating that, the reliability of the estimated reservoir properties are dramatically enhanced with the dual-sensor streamer technology.
|CARNARVON BASIN IN NORTH-WEST AUSTRALIAN SHELF|
The next case study is from the North West Australian Shelf (Carnarvon Basin). The seismic line shown was acquired with conventional and dual-sensor streamer technology (Figure 5). This area is a world class gas province with minor oily sweet spots in the Permo-Triassic sediments overlain by Jurassic to Cenozoic syn and post rift successions (Longley et al., 2003).
Figure 5 presents the result of a relative acoustic inversion of the conventional and the dual-sensor streamer data. A relative inversion (inversion without any low frequency or a-priori model) was chosen to avoid any bias from any well information. Therefore, the inversion results represent only the seismic contribution. The dual-sensor streamer provides a clearer image observed in the following aspects: definition of the flat-spot is greatly improved, the top and base of the reservoir is more clearly defined and improved delineation of the geo-bodies. From a seismic inversion point of view, we should expect to retrieve the layering of the earth (removal of the wavelet effect) and this is better achieved with the dual-sensor streamer technology as opposed to the conventional streamer which looks like the seismic.
Figure 5: illustrates two versions of the same reservoir image. A large gas discovery sits in the middle of each image. The top seismic images correspond to conventional streamer and dual-sensor seismic response. The bottom images correspond to the acoustic impedance results from the conventional and the dual-sensor streamer. The difference between the conventional and dual-sensor technology data can easily be evaluated with this side by side comparison
Since the launch of the dual-sensor streamer technology in 2007, it has been used worldwide to acquire more than 100,000 line km of 2D seismic and more than 10,000 square km of 3D in a variety of geological settings. In all cases, the operational and seismic data benefits have been clear and substantial. It has now been demonstrated that the dual-sensor streamer technology offers added value not only in terms of seismic acquisition operational efficiency, or improved seismic imaging, but also improved seismic reservoir characterisation. Reservoir delineation and geobodies detection is improved thanks to an increased signal to noise ratio and broader bandwidth. This broader bandwidth, especially at the low frequency side of the seismic amplitude spectrum represents a key improvement in the seismic reservoir property estimation. The need for a-priori information is considerably reduced compared with a conventional seismic streamer.
Cambois, G., B. Osnes, A. Day and A. Long, 2009, Dual Sensor Streamer Increases Data Bandwidth Leading to Improved Penetration and Higher Resolution. EAGE Marine Seismic Workshop - Focus on Middle East & North Africa
Carlson, D., W. Söllner, H. Tabti, E. Brox, and M. Widmaier, 2007, Increased resolution of seismic data from a dual-sensor streamer cable: 77th Annual International Meeting, SEG, Expanded Abstracts, 994–998.
Longley, I. M., C. Buessenschuett, L. Clydsdale, C. J. Cubitt, C. J. Davis, R. C. Johnson, M. K. Marshall, N. M. Murray, A. P. Somerville, R. Spry, and N. B. Thompson, 2003, The Northwest shelf of Australia—A woodside perspective, in M. Keep and S. J. Moss, eds., The sedimentary basins of Western Australia 3: Proceedings of the Petroleum Exploration Society of Australia Symposium.
Söllner et al, 2007, Surface-related multiple suppression in dual-sensor towed-streamer data, SEG Expanded Abstracts.
Tenghamn, R., S. Vaage, and C. Borresen, 2007, A dual-sensor, towed marine streamer; its viable implementation and initial results: 77th Annual International Meeting, SEG, Expanded Abstracts, 989–993.
Whitcombe, D. and Hodgson, L. 2007, Stabilizing the low frequencies, The Leading Edge, 26, 66-72