MA-1-5

What can 3D Seismic Data Contribute to a Mature Oil Field with 1000 Wells?

Olabode Olatoke, Matthew Pointing, Katherine Totton, Kenneth Tough, Christoph Lehmann, James Gardner

BP, Sunbury-on-Thames, UK

Introduction – The Rumaila Field

The ‘super-giant’ Rumaila oil field in South East Iraq contains multi-billion barrel oil accumulations within multiple clastic and carbonate reservoirs. The field has been on production since 1954, with over 1000 wells drilled to date.

The first full-field 3D seismic dataset was acquired in 2012 to support field re-development. Over 1800 km

2 of high-fold, wide-azimuth seismic data using the Independent Simultaneous Source

(ISS®) technique with cableless Nodes (ISSN™) was safely acquired in less than eight months (Harrison et. al., 2013). The dataset was subsequently taken through a bespoke processing sequence that included Kirchhoff Pre-Stack Time Migration (PreSTM) imaging in 2013 (Dvorak et. al., 2014).

In mature fields like Rumaila, with dense well population, an abundance of historical data, and where subsurface studies have been routinely performed in the past without 3D seismic, the value of 3D seismic is often challenged. This paper demonstrates that despite a wealth of subsurface data, 3D seismic can play a significant role in developing subsurface understanding, and can directly influence both well planning and reservoir management activities.

ISSNTM

: Safe, efficient acquisition in a challenging environment

Harrison et al, (2013) describe the acquisition of the ISSNTM

dataset over the Rumaila field and the significant operational and access challenges that were overcome. The choice of ISSN

TM as the

acquisition technique for the Rumaila seismic survey was driven by access restrictions due to almost 60 years of oil field infrastructure, villages, farms and marshlands (Figure 1), and safety concerns caused by the presence of significant explosive remnants of war (ERW). This technique reduces the volume of equipment required, hence reducing crew size and exposure to ERW. The survey geometry consisted of a 200m x 200m nodal (receiver) grid and a shot grid of 50m x 50m, using up to 15 vibrator trucks shooting simultaneously. This design facilitated the acquisition of a high quality wide azimuth dataset with a maximum fold of over 460 at 5000m offset (Figure 1, right image). High productivity rates (over 5000 vibrator points per day) were achieved, with acquisition (including infill) completed safely in less than eight months.

A seismic processing sequence was adapted from a previous ISS™ project, to address the specific challenges of the Rumaila dataset, and was completed by a leading seismic processing contractor. The sequence included ISS interference removal, surface-wave noise attenuation in the receiver domain, surface consistent processing in the CMP domain, 5D regularisation, offset-vector-tile domain noise attenuation and Kirchhoff Pre-Stack Time Migration. A significant effort was directed at noise and multiple attenuation, with several processes applied at various stages of the sequence to address these issues. Full-stack and Amplitude-Variation-with-Offset (AVO) volumes were among the key deliverables, and are deemed to be of variable but generally good quality at most levels of interest.

Figure 1. Left image shows map of Rumaila oil field showing seismic area (purple polygon) and concession (blue polygon) as well as general infrastructure. Right image is the same as the left image with a fold map at 5000 m offset overlain.

So, what has 3D seismic contributed to the Rumaila field?

The contribution of 3D seismic in Rumaila has evolved significantly over the last eighteen months following the completion of detailed seismic interpretation and analysis work, and the integration of this with log, core and production data. Currently, seismic is supporting the following activities:

1. Well Planning a. Interpreted and depth calibrated seismic has provided the framework for a new full

field structural model, which is used routinely for depth prognosis, and high angle well (HAW) planning.

b. Prediction of geological related Non-Productive drilling Time (NPT) in the overburden.

c. Successful prediction of overburden NPT has led to seismic use in the identification of water disposal targets.

2. Reservoir Characterisation and Target Optimisation a. Seismic has facilitated updates to the Mishrif Reservoir Depositional Elements

(RDE) Maps and facies interpretation. b. Seismic attributes have led to an improved understanding of the Mishrif reservoir,

thus presenting target optimisation and prioritisation opportunities.

1a. Structural Model

Interpretation and depth calibration of the first 3D seismic on the Rumaila field enabled the building of a new full field structural model with significantly improved structural control. Although one might expect depth and structural control to be very good on a field with over 1000 wells, the majority of these wells are located on the crest, with limited penetrations on the flanks or in the saddle between the two domes at North and South Rumaila.

The previous structural model was predominantly well derived, with the seismic contribution limited to only 4 interpreted horizons from a relatively sparse network of 2D lines from various vintages. The 3D seismic provided a much more robust framework for the new (92 layer) structural model through; 9 calibrated depth horizons from the surface to the Top Gotnia (in the Upper Jurassic); the incorporation of key reservoir and drilling horizons; and additional constraining horizons of good seismic event continuity and associated high confidence well picks. This model is now routinely used for depth prognosis in all new wells, and is of particular importance to the planning of an increasing number of high angle and horizontal wells to both the Mishrif and Main Pay reservoirs targeting:

- Identified intervals of good reservoir quality within the Mishrif - (Often thin) intervals of bypassed oil in the Main Pay

Indeed, previously planned high angle well trajectories have been updated based on the new structural model to reduce the risk of limited completion length in the target interval, or well TD in the wrong interval. The new structural model also provides the basis for structural contribution to the reservoir static and dynamic models, and has been used in Gross Rock Volume calculations and uncertainty studies.

1b. Dammam Non-Productive Time (NPT)

All wells drilled on the Rumaila Field pass through the Dammam formation; a shallow carbonate interval containing sections of karst. As new wells are drilled into zones of extensive karst, drilling fluids are partially or completely lost into the formation. This disrupts and delays operations and in some cases can lead to well control events related to loss of hydrostatic head. Losses are cured with Loss Circulation Material (LCM) or cement plugs, depending on the severity of the losses. Total and partial Dammam losses account for 25% of all NPT experienced whilst drilling wells on Rumaila Field, highlighting the scale of the problem.

The 3D seismic data has been integrated with logs and a losses database. This has resulted in a new geological model describing the origin, distribution and nature of the Dammam karst. The karst comprises interconnected networks of vugs, constrained within two key intra-Dammam zones. These zones relate to periods of time when the Dammam formation was exposed due to locally fluctuating sea levels and slow growth of the Rumaila Field structure. Loss onset data, superimposed on Dammam seismic data, shows the location of the two primary loss zones to be in the middle of the Dammam formation (Figure 2). Gamma ray and density logs show erratic character through the same zones, indicating variable density and lithology profiles as would be expected in a karstified interval.

Specific seismic reflection events have been tied to these karst indicators and careful seismic character interpretation has been undertaken (Figure 2). Absent or chaotic seismic reflection character has been linked to increased karstification whereas continuous seismic reflection character is linked to less karstified areas. Results from recently drilled water injection wells show that wells drilled into intensely karstified areas are far more likely to suffer complete losses than wells drilled into other areas. Risk maps have been created based on this reflection character interpretation. Composite Common Risk Segment (CCRS) maps have been created based on the seismic character interpretation, which are routinely used in conjunction with local NPT data to make predictions on whether losses are likely to occur in new wells.

Comparison of the CCRS map with historic loss data indicates a match between occurrence of losses and high risk areas 70% of the time (n=169), indicating that the method captures extent and

distribution of Dammam karstification successfully. This new seismic-based product allows subsurface and drilling teams to plan and successfully mitigate for Dammam drilling losses.

Figure 2. Top image shows seismic data, Dammam depth surfaces and points from the Dammam losses database – with total losses in red and partial losses in yellow. Bottom image shows interpretation of different seismic reflection characters, with overlay traces of drilling fluid loss rate for three example wells

2a. Mishrif Reservoir Characterisation

The Upper Cretaceous Mishrif reservoir is one of the key producing reservoirs in the Rumaila oil field. It is comprised of five third-order progradational and aggradational sequences that can be subdivided into two main reservoir zones. Both reservoir zones include heterogeneous shallow marine carbonate reservoirs adjacent to distal deep marine carbonate non-reservoirs.

Two key aspects where seismic attributes contribute to the Mishrif reservoir development are:

- Improving the geological understanding – captured in Reservoir Depositional Elements (RDE) maps. This is based on the identification/ refinement of features such as facies boundaries, tidal channels, palaeokarst features and faults. These RDE maps are not only a distillation of our subsurface understanding but will provide guidance in building subsequent geomodels.

- Acting as a tool in the well planning process – mainly for local well target optimisation and prioritisation, as well as in choosing local well analogues for reservoir quality and performance prediction (2b).

Prior to 3D seismic attributes, the characterisation of the Mishrif reservoir was conducted using mainly the extensive well and core data – and most recently captured in the current geomodel and RDE maps. The current geomodel was built using petrophysical logs from over 700 wells, mainly drilled along the crest of the structure – as well as horizons from the interpreted fast-track 3D seismic data. Away from well control, the geomodel was driven by conceptual depositional trends and modelling parameters. The availability of seismic attributes and recently drilled wells not used in the geomodel presents a unique opportunity for a blind test to compare the predictive abilities of the geomodel and seismic attributes. It highlights the need to refer to both the seismic attributes and geomodel in predicting reservoir quality, especially away from well control – and the need to update the RDE maps.

Multiple seismic attributes, underpinned by comprehensive seismic-to-well ties and drawing on the current geomodel, are being used together to highlight depositional trends and reservoir quality changes across this super-giant field. These seismic attributes are generated using medium-to-high confidence field-wide interpretation of the top and base of the two main reservoir zones within the Mishrif. Using a single field-wide seismic attribute for reservoir quality prediction in both zones is challenging due to heterogeneity in the overlying stratigraphy and within the reservoir zones. A clearer indication of reservoir quality variation can be resolved by using a variety of seismic attributes (e.g. reflectivity, coloured inversion, spectral decomposition, coherency, etc.) extracted over different windows. This has led to the identification of important geological features (Figure 3) such as facies boundaries, tidal channels, palaeokarst features, interpreted as sinkholes and faults.

These observations have led to an improved geological understanding and updated RDE maps.

Figure 3. Example of different seismic attributes highlighting different geological features - faults (left – on windowed coherency), and channels (right – windowed RMS amplitude) that have been integrated in the RDE maps.

2b. Mishrif Reservoir Well Target Optimisation

The Mishrif reservoir is being depleted by pattern drilling, and seismic attributes are now routinely used as one of the available tools for geological description in the target selection process. These tools (including maps of current geomodel porosity, RDEs, pressure and seismic attributes) are used for well target prioritisation and in choosing local well analogues for reservoir quality and performance prediction. Seismic attributes are also being used in well target optimisation, where the opportunity arises. Figure 4 shows a recent example where seismic was used for well target optimisation. In this example, the reservoir quality and performance within the reservoir sequence of interest at wells either side of a target location were significantly different. Well 1, to the North of the target, was characterised by a good reservoir quality grainstone carbonate, while Well 2 to the South had a poor reservoir quality mud-rich lagoonal carbonate. Without seismic attributes, the initial planned well location (PW) is likely to have encountered poor reservoir quality like Well 2 to

the South – probably resulting in poorer reservoir performance. Seismic attributes and sections show that the grainstone channel encountered in Well 1 can be mapped to the revised location (PW - Rev) a few hundred metres north of the initial location. This led to a seismic attribute driven target optimisation that is expected to improve reservoir quality.

Figure 4. Bottom left: Spectral decomposition 25Hz iso-frequency map capturing the sequence of interest (Blue rectangle on seismic sections) showing sinuous channel features encountered by wells showing good reservoir quality. Green arrow highlights the initial (orange) and optimised (light green) well locations on the map and section A-A’ (Top left). The seismic sections (A-A’, B-B’ and D-D’) show the channel feature, with the edges highlighted with the orange arrows (B-B’ – also marked on the map) as well as the white arrows. The yellow-filled curve is porosity (reversed – so that high porosity is to the left).

Conclusion

The first full-field 3D seismic data, acquired using the innovative ISSNTM

technique over the densely penetrated super-giant Rumaila oil field, is playing an increasing role in well planning and reservoir management activities:

- Improving depth prognosis and well planning for high angle wells - Reducing NPT while drilling through a karstified carbonate interval - Improving the geological understanding of the Mishrif reservoir - Assisting in the Mishrif well planning – as an optimisation and prioritisation tool, as well as

being a tool in choosing local offset analogue wells.

These activities demonstrate the contribution of 3D seismic on a mature field with a 1000 wells.

References

Dvorak, M., Tough, K., Eller, M. and Smith, M., 2014, Imaging a Mature Oil Field Using Simultaneous Source Technology – the Processing of the Iraq Rumaila ISSN Survey. 76th EAGE Conference & Exhibition, Expanded Abstracts Th EL12 06.

Harrison, D., Eller, M. and Cave, N., 2013, 3D Seismic Acquisition in the Modern Battlefield – Innovative Use of Technology to Reduce Safety Risks. 75th EAGE Conference & Exhibition, Expanded Abstracts TH1616.

Acknowledgements

We would like to thank BP and the Rumaila Operating Organisation (ROO) Joint Venture partners PetroChina and South Oil Company (SOC) for permission to publish this abstract.

ISS® is a registered trade mark of BP p.l.c. ISSN™ is a trade mark of BP p.l.c.