Reducing structural uncertainty on the Azeri Field using Ocean Bottom Seismic: Offshore Azerbaijan Thomas Lyon*, Jack Bouska, Rodney Johnston, Michael Mueller, Leon Thomsen : BP Exploration Summary The interpretation of 3D Ocean Bottom Seismic survey over part of the Azeri field, offshore Azerbaijan has delivered significant reduction in structural uncertainty. The crest and South/Central flank of the field was poorly imaged on towed streamer data; this led to “no data areas” with large depth uncertainty and a poorly resolved structural model. The depth migrated PZ products from the OBS survey have facilitated a choice between alternative structural interpretations and provided improvements in depth prediction for development drilling. Introduction The Azeri field is the largest oil accumulation in the Azerbaijan sector of the Caspian Sea, containing over 8 billion barrels (1.3 billion m3) of oil in place. It is part of the ACG (Azeri, Chirag, Gunashli) development located approximately 100km east of Baku. The field will produce from 4 fixed platforms at a maximum rate of 800,000 stb/d (127,000 m3/d), using up to 300 reservoir penetrations to do so and a combination of gas and water injection. The Azeri field is under development by AIOC and operated by BP with first oil expected at the start of 2005. The Azeri field is the most easterly culmination of the elongate ACG anticline extending in a northwest to southeast direction Water depth over the accumulation ranges from 120 to 350 metres with platforms located in the range of 120 to 175 metres. The structure is asymmetric with steep dips on the north flank ranging from 30 to 40 degrees and a less steep south flank dipping mostly in the range 15 to 25 degrees. Hydrocarbons are found within a series of 9 laterally extensive stacked sandstone intervals; the reservoirs are Pliocene in age, occurring in the Pereriv and overlying Balakhany Formations. The high structural relief combined with excellent top seal gives an oil column of 1000m on the north flank and circa 600m on the south flank. Cross-sections through the structure reveal an anticline with compressional features at depth transitioning to extensional faulting in the shallow overburden. Mud volcanoes of varying size penetrate the structure at discrete points. The mud volcanoes are characterized by debris cones on the seabed fed by over-pressured shale from strata below the target reservoirs. Surface tow 3D seismic data recorded over the structure is generally of high quality, often revealing direct evidence of hydrocarbon/water contacts in the form of structurally

conformable amplitude anomalies (figure 1). Nevertheless significant areas of poor imaging occur towards the crest around and between the mud volcanoes (figure 1).

The cause of poor imaging has been ascribed to a combination of the following-: • Absorption of the P-wave signal in gas charged

sediments of the overburden. (i.e. “gas chimney” effect) • Poorly understood lateral velocity changes associated

with gas charged sediments. • Structural and stratigraphic complexity in the near

surface associated with the formation of mud volcanoes and the ACG structure.

• Seismic noise backscattered from near surface heterogeneities

Construction of a structural image in the area of no coherent reflectors was the primary objective of the Azeri 4COBS survey. Pre-survey feasibility studies suggested that converted (P-S) waves would deliver the best chance of shrinking the no-data area. P wave results were expected to make only a marginal impact on the extent of good data. In practice the results from PZ imaging (hydrophone and vertical geophone summed) have been very significant while the C-wave has proved more challenging. Economic justification for the survey

This attribute extraction shows the area of incoherent imaging on surface tow data in orange as well as the presence of a direct hydrocarbon indicator at the oil-water contact. Figure 1: Amplitude extraction from streamer tow data

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Reducing structural uncertainty: Offshore Azerbaijan

The economic justification for the Azeri OBS survey (ref: Mueller & Lyon 2002) was based on quantifying the expected impact of structural uncertainty on drilling costs and project value. Drilling times and cost for the Azeri development were based on analogue data from offset drilling activity. The drilling time and costs from the analogue data were divided into various categories reflecting whether they are productive or non-productive activity. Key outputs for our purpose are the -: • number of geological sidetracks/pilot holes needed to

mitigate depth uncertainty. • number of wells that are re-drilled due to structural mis-

positioning (e.g fault presence/absence or compartmentation).

• Non-productive time in the overburden due to unexpected geo-hazards.

• An assessment was made of the relative level of structural uncertainty in the analogue data compared to the target project.

The full field reservoir simulator was initialized using drilling times that reflected drilling with or without mitigation of structural uncertainty. Full cycle economic analysis of the resulting production and cost profiles provided a means of communicating the value of mitigating structural uncertainty over the life of the Azeri development. Only after undertaking this economic analysis could the benefits be clearly articulated. • The value of mitigating structural uncertainty is

significant. The development calls for accurate placement in terms of stand-off from contacts, most of the producers are high angle wells with long sand faces so tolerance to depth uncertainty is relatively low.

• The highest value lies in the acceleration of production rather than in the capital drilling cost savings. This is largely because the drilling program spans in excess of 15 years. (Drilling capex savings would assume more significance for a shorter drilling campaign)

• Structural uncertainty is greatest early in the development and could directly affect production ramp-up.

In assessing the value of OBS seismic it was recognized that while some 2D testing of 4C OBS could be done (Probert et al 2002), the technique was unproven in the South Caspian. It was also recognized that OBS was unlikely to provide an image that was entirely free of structural uncertainty. Despite discounting for these factors there was significant net benefit to warrant proceeding with a 4C OBS survey on Azeri. Method The acquisition and processing were planned from a holistic viewpoint so that the data recorded was optimized

for both existing, and untested, processing options. The Azeri OBS survey incorporated a number of specific design elements, driven by BP’s experience in multi-component seismic. The design and processing is discussed more fully in two companion papers (Bouska et al and Johnston et al), the most important features are -: • Interlaced receiver deployment to maximize coverage on

the difficult crest of the structure and to deliver cost savings on the more easily imaged flanks

• Uniformly sampled shot wave field in a wide patch over each deployment.

• Multi-pass pre-stack noise attenuation, one in the common receiver domain, the other in the single fold common offset domain.

• First break refraction tomography used to estimate near a surface velocity model as well as P- wave and S-wave receiver statics.

• Iterative, adaptable approach to processing that allowed learning from the previous iteration. Important when delivering the first large scale deployment of 3D OBS in a new area.

• The parameterization of anisotropy in the P-wave depth migration.

The processing of the seismic data was phased in the following way. a) Priority area time domain processing b) Fastrack depth migration of the full survey area c) Pre-stack time migration of the full survey area d) Final depth migration of the full survey area. Results Interpretation of the 4COBS PZ priority area time migration revealed that a withdrawal feature exists at all levels from the deepest reservoir targets to within 200 metres of the seabed. This feature is illustrated clearly in figure 2. Although some previous towed streamer based

seismic interpretations show indications of a depression

Figure 2: 3D view of depression around central Azeri mud volcano

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Reducing structural uncertainty: Offshore Azerbaijan

east of the central Azeri mud volcano only one quadrant (quarter) of the feature could be tracked and its structural context was unclear; the rest of the area around the mud volcano was mapped by ghosting an interpretation across what was effectively a no data area. The PZ data clearly reveals a near circular withdrawal feature with the Central Azeri mud volcano eccentrically located within the circle. Prior to the OBS data interpretation two alternative models were postulated, 1) the area had some form of ill-defined depression near the mud volcano or 2) the apparent structure was a velocity pushdown effect from the overburden.

(figure 3 compares the surface tow data to the OBS PZ) In the interpretation of the towed streamer seismic, the velocity push down model was adopted with the alternate model being acknowledged in the depth uncertainty estimate. Figure 4 shows the Top Pereriv B horizon mapped from interpretation of the PZ data compared to the pre-existing towed streamer. There are still areas of poor image within and around central Azeri but the overall structural shape is now unambiguous.

Even depth migrated seismic data commonly needs calibration using well data to generate depth horizons that tie the well markers. By comparing depth residuals at the wells we can assess the extent to which a new velocity field or migration approach improves our ability to predict depth away from the well control. The Azeri OBS depth migration was parameterized for anisotropy using initial values estimated from walk-away VSPs, it was also based on an iterative layer-by-layer velocity modeling approach. Figure 5 compares the Towed streamer depth misties with the OBS depth misties. The scatter in depth errors is less for the OBS over most of the survey area. Instead of a + - 150 metre range the depth errors away from the crest are in a + -75 metre window after removing an average shift for a given horizon. Fluid contacts are evident on attribute extractions from the surface tow data but are significantly enhanced in some areas on the OBS data. Figure 6 is an example comparing extractions on the same event from OBS and towed streamer; the OBS fluid anomaly is much less ambiguous.

Figure 3: Comparison of OBS data to existing streamer data

Figure 4: Top Pereriv B reservoir maps: OBS v Towed Streamer

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Reducing structural uncertainty: Offshore Azerbaijan

The improved image and signal/noise of the OBS data is considered to derive from a combination of the following-: • PZ summation – the summation of Hydrophone and

vertical geophone provides improvements to the frequency spectrum especially at the low end.

• Coupling and quiet environment – The placing of receivers on the seabed allows the recording of lower frequencies than is possible with the towed streamer.

• Wide Azimuth – The combination of different azimuths provides the possibility of undershooting overburden features that disrupt or attenuate the signal.

• Fold – Signal to noise enhancement derive from the relatively high fold especially over the crest where the fold exceeds 300.

Conclusions Interpretation of the PZ components of the Azeri OBS data has significantly reduced structural uncertainty in South Central Azeri. Key results to date are -: • The area of incoherent imaging has been significantly

reduced giving the opportunity to calibrate the structural interpretation with one or two wells rather than slowly evolving a model in the poor data area at high cost through pilot holes and side-tracks of a large number of wells.

• A peripheral sink to the central Azeri mud volcano is clearly mapped. Integration of data from the first south flank development well has proved consistent with this mapping.

• Three wells retargeted and successfully drilled on the south flank following interpretation of OBS data. Circa 12 to 15 other wells will directly benefit from structural mapping using the new data.

• More consistent residual depth errors when calibrating the raw depth migration to the wells. This holds the promise of less depth uncertainty

• Better S/N gives cleaner and more extensive attribute response (DHIs).

References Michael Mueller & Thomas Lyon, ,AZERI FIELD 2D 4C OBS TEST RESULTS AND 3D 4C OBS BUSINESS CASE EVALUATION, Baku Geophysical Conference 2002 Probert, T., Bryan, R., Underwood, D., Mueller, M., Lyon, T. and Rowson, C., 2002, Multicomponent Seismic Challenges on a Mud Volcano - Imaging the Azeri Field, 64th Mtg.: Eur. Assn. Geosci. Eng., F017. Acknowledgments BP operates the ACG field on behalf of the shareholders of the Azerbaijan International Oil Company (AIOC) which include the following companies: BP 34.14%, UNOCAL 10.28%, SOCAR 10%, INPEX 10%, Statoil 8.56%, ExxonMobil 8%, TPAO 6.75%, Devon 5.63%, Itochu 3.92% and Amerada Hess 2.72%. The authors would like to thank the AIOC shareholders for permission to publish this case study and their input to the planning and execution of the project. We also acknowledge the dedication and skill of those individuals in Caspian Geophysical and Western Geco who acquired and processed the OBS survey. Many colleagues in BP helped to make the Azeri OBS a successful project; in particular we thank Dan Ebrom, Jan Kommendal, Richard Seaborne, Dave Buddery and Dave Howe.

Figure 5: Surface Tow v OBS : horizon depth errors at wells

Figure 6: Comparison of attribute map : OBS to towed streamer

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