Ekofisk Life of Field Seismic: 4D Processing

Transcription

1 Ekofisk Life of Field Seismic: 4D Processing Sebastien Buizard 1, Alexandre Bertrand 2, Karl Magnus Nielsen 1, Sylvain de Pierrepont 1, Andrea Grandi 3, Henning Hoeber 1, Geir Oexnevad 1, Alain Gresillaud 1 1 CGGVeritas, 2 ConocoPhillips, 3 Total E&P Norge Introduction We present the 4D processing sequence for the Ekofisk Permanent Reservoir Monitoring project. Four vintages of data were acquired between the end of 2010 and summer 2012 using 200km of trenched fibre optic seismic cables covering about 60 km² and with a total of C sensor stations. We discuss key aspects of the processing sequence, turnaround, and the integration of expertise between client and contractor. This delivers outstanding seismic repeatability with clean, well resolvable 4D signals and low residual 4D noise (NRMS of 3-5%).

2 Introduction 4D seismic is an important tool for reservoir management at the Ekofisk field in the southern part of the Norwegian North Sea. As subtle 4D seismic changes related to production and injection develop rapidly, frequent and highly repeatable 4D seismic monitoring is required to increase understanding of reservoir depletion zones and intra-reservoir injected water fronts (Hermansen, 2008). In 2008 ConocoPhillips decided to install a permanent reservoir monitoring (PRM) system at Ekofisk. An Optowave fibre optic system was chosen as the best long term solution to support the intensive drilling program planned for the next 15 years and for fast delivery of high fidelity 4D seismic products (Folstad et al., 2011). The system was installed according to plan and was fully functional in October Four Life of Field Seismic (LoFS) surveys have been acquired so far and two further surveys are planned in CGGVeritas is currently responsible for all the main elements of the seismic delivery chain for the Ekofisk LoFS project: equipment supply (Nakstad, 2011), acquisition and processing. The involvement of a single contractor group enables good coordination between the different phases of the project and has facilitated its realisation, reduced processing turnaround and enabled timely mitigation of any arising issues. Integration is further enhanced by co-location of the acquisition QC and processing teams in the operator s offices in Stavanger (Hoeber et al., 2011). This paper describes the P-wave data processing and shows examples of the resulting high-quality 4D seismic products. LoFS acquisition The permanent seismic recording system at Ekofisk consists of 200km of trenched fibre optic seismic cables covering about 60km², using a total of C sensor stations (15864 channels). The receiver array and the burial depth of the sensors are shown on Figure 1, overlaid on the map of infrastructure at the sea bottom (70-80m). The cables are spaced 300m apart and the receiver station interval is 50m. The seismic acquisition is performed with a containerized source system operated from one of the Ekofisk supply vessels. The shotlines are acquired parallel to the receiver lines, with 25m between shotpoints and 50m between shotlines. The shot effort extends over 2km beyond the limits of the receiver layout, giving approximately 143km² shot coverage. Table 1 shows the dates and durations of the four LoFS surveys acquired so far. Survey Acquisition period Duration (days) LoFS1 Nov 2010-Jan LoFS2 May-June LoFS3 Sept-Nov LoFS4 June-July Table 1 Acquisition periods and durations of LoFS1-4 surveys. Figure 1 (left) The Ekofisk PRM receiver array of fibre optic cables with overlay of the receiver station burial depth. During acquisition, raw data are transmitted near real-time via a dedicated fibre optic link to the ConocoPhillips offices in Stavanger where the contractor s acquisition QC and dedicated processing teams are located. The major benefit of this arrangement within the client s offices is the short communication path, a key factor in achieving the rapid turnaround required for the 4D processing. LoFS on LoFS 4D processing With fixed sensors, the high degree of acquisition repeatability on Ekofisk is ideal for repeat processing: the processing flow optimized from the base and first monitor surveys is applied to each

3 LoFS monitor vintage with a minimum of change between surveys. Our design of the LoFS 4D processing sequence was guided by the need for robustness and efficiency. By the second monitor acquisition (LoFS3) turnaround needed to be reduced to 1 month after the last shot, without loss of 4D resolution. Dedicated modules and solutions were developed for the processing of the LoFS data using expertise across divisions and via joint workshops at key stages. In order to achieve the required turnaround, optimizing the processing flow with regards to minimizing data sorting is essential. Shot domain processing on the Ekofisk project starts immediately after each line is acquired. Upon completion of the acquisition, full 3D receiver gathers are produced. True 3D processing, which optimally addresses the 3D nature of the LoFS acquisition, is then applied. Following this, the data is sorted to the common offset domain for Kirchhoff migration. Some of the key elements of the 4D processing sequence are described below in more detail. Shot domain processing As the acquisition proceeds, the data is quality checked and reformatted onshore in real time, and by a team co-located with the processing in the client s offices. Shot domain processing is initiated as soon as nav-seis merged data is available to the processing team, typically half a day after a sail line was acquired. Key steps of the shot domain processing include: - Pressure recording harmonization: analysis of several repeated lines of the first LoFS survey highlighted that a certain number of pressure recordings had degraded over time. Early detection and a close interaction with CGGVeritas/Optoplan R&D staff proved essential in designing a processing solution to this issue. - 3C Rotation and PZ summation: parameters and operators were established during the processing of the first LoFS survey and kept unchanged for subsequent monitors. The PZ summation consists of the following three-step procedure (Soubaras, 1996): calibration of the geophones (crossghosting technique); separation of the up- and down-going wavefields by summing the hydrophones and the calibrated geophones (ghost elimination); and lastly application of a source-side de-pegleg which amounts to a surface consistent gapped deconvolution in the receiver domain. - Denoise 1: The large concrete tank (100m diameter) which stands in the centre of the Ekofisk platform complex is a source of strong back-scattering noise affecting all sail lines (Figure 2a). The tank noise is removed in the 2D receiver domain by applying a high resolution linear Radon after tank noise flattening with the water velocity (Figure 2b). Another source of noise is seismic interference (SI) from other crews shooting in the vicinity of Ekofisk and recorded at several occasions during the LoFS surveys. This is removed by f-x prediction filtering. At the end of the shot processing the now significantly reduced data volume is transferred to CGGVeritas computer hub in the UK for the more computer intensive part of the processing sequence. 3D receiver domain processing After sorting the data into 3D receiver gathers, the following key processes are applied: - Denoise 2: within a 3D receiver gather sorted in the crossline direction the non-repeatable noise generated by vessels (and platforms) operating constantly in the Ekofisk area is randomized and can be removed by f-x projection filtering, as shown in Figures 2b-g. - 4D environmental corrections: tidal statics and water velocity corrections are derived from data measured during the acquisition. - 3D tau-px-py deconvolution is then applied, after interpolating shots down to 12.5m to reduce spatial aliasing. A mute is also applied in the tau-px-py domain (Figure 2h). - 4D trace editing is performed, instead of the more usual 4D binning, which is only suitable for 4D parallel processing (not repeat processing). A small number of traces are rejected based on a geometrical criterion fixed for all surveys and based on a threshold for source, the distance between actual and pre-plot shot coordinates. - Data regularization with missing trace restoration (midpoint regularization in the 3D receiver domain) ensures identical number and position of traces on every vintage.

4 Figure 2 3D receiver gather at different denoise stages, sorted in inline/crossline (red surround) or crossline/inline (green surround). Before (a) and after (b) tank noise removal; before (c) and after (d, g) denoise; denoise difference (e, f); after 3D tau-p deconvolution and mute (h).green (resp. red) dotted lines indicate the locations of the displayed shotpoints (resp. shotlines). Offset domain and post-migration processing - The data is subsequently sorted to 50m offset classes prior to static binning (with fold weighting) and Kirchhoff pre-stack time migration. Pre-stack post-migration processing consists of RMO and Radon demultiple. The raw full offset stack was delivered one month after the last shot for LoFS3 and 3.5 weeks for LoFS4. The final full offset stack with Q compensation and dip consistent filtering is available shortly afterwards. Other than a small bulk time shift no further 4D spectral matching is applied. 4D results The LoFS 4D seismic data has remarkable repeatability, with extremely low NRMS values, on the order of 3 to 5%. In the NRMS displays (Figure 3) we have blanked an area of higher 4D noise at the centre of the field, the seismic obscured area (SOA), which is due to an overburden gas cloud. Figure 3 LoFS1/LoFS2 (left), LoFS2/LoFS3 (middle) and LoFS3/LoFS4 (right) NRMS maps (computed on final stacks in a ms window) Outside of the SOA clear 4D signals can be observed at injector and producer wells operating between LoFS surveys. The detection level for the 4D time shifts is below 0.2ms, a precision which would have been unachievable using streamer data. Figure 4 shows an example of the high quality 4D signal highlighting amplitude changes of the order of 5% caused by gas coming out of solution around a new producer (slightly more than a month of production at the time of LoFS3).

5 Figure 4 LoFS2 (left), LoFS3 (middle), 4D difference (right) scaled by a factor of 10. The two horizons are top and base reservoir. The SOA is visible on the left hand side of each panel. Potential further improvements in seismic processing include an additional denoise step to better address VZ noise prior to PZ summation, and the use of pre-stack depth instead of pre-stack time migration. We intend to use the improved P-velocity model updated in the overburden by Full Waveform Inversion, which significantly improves the imaging in the SOA (Bertrand et al., 2013). Reprocessing of the P-wave LoFS data with these improvements has started and is hoped to further enhance our confidence in the extremely subtle 4D effects detected so far. Conclusions We presented a 4D friendly processing sequence for the Ekofisk PRM project that delivers highquality 4D seismic products with very rapid turnaround. Key to this is the integration of expertise between client and contractor and transversely across all aspects, such as acquisition, acquisition QC, processing and imaging, and including R&D. The Ekofisk LoFS project is delivering outstanding seismic repeatability with clean 4D signals and low residual 4D noise (NRMS of 3-5%). This powerfully demonstrates the potential of Permanent Reservoir Monitoring. Acknowledgements We thank ConocoPhillips Norge AS and the PL018 Partnership (Total E&P Norge, ENI Norge, Statoil, and Petoro), as well as CGGVeritas management for their permission to publish this work. We acknowledge the work of all our colleagues involved with the Ekofisk LoFS project, and especially Habib Alkhatib, Julian Holden, Erlend Rønnekleiv and Bjarne Lyngnes. We dedicate this paper to the memory of our colleague and friend Haakon Haugvaldstad, ConocoPhillips Norge Operations Geophysicist. References Bertrand, A. et al., [2013], Wide-azimuth PP/PS depth imaging at Ekofisk using Full Waveform Inversion, submitted to the 75 th EAGE Conference, London. Folstad, P.G. et al., [2011] Ekofisk PRM The technical case for this brand new installation. EAGE Workshop on Permanent Reservoir Monitoring (PRM) Using Seismic Data, Extended Abstracts. Hermansen, H., [2008]. The Ekofisk Field: Achieving Three Times the Original Value, 19th World Petroleum Congress, June 29 - July 3, Madrid, Spain Hoeber, H. et al., [2011] The Ekofisk Life of Field Seismic An Integrated Operation, EAGE Workshop on Permanent Reservoir Monitoring (PRM) Using Seismic Data, Extended Abstracts. Nakstad, H., Langhammer, J. and Eriksrud, M., [2011], Permanent Reservoir Monitoring Technology Breakthrough in the North Sea, 73 rd EAGE Conference, Extended Abstract. Soubaras, R., [1996] Ocean-bottom hydrophone and geophone processing. 66 th annual SEG meeting, Extended Abstracts.