Oilfield Technology - May/June 2020

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Oilfield Technology

May/June 2020

QI studies for reservoir development and near-field exploration aswell as

for 4Dproductionmonitoring.

Since the receivers are coupledwith the seabed, OBN systems record

not only the pressure but also the particlemotion, where the horizontal

componentmeasures the shearmodes. This provides a second S-wave

image of the subsurface, which complements the P-wave image and helps

to constrain rock and fluid properties.

Theright time for theCNS

OBNacquisition has historically only been considered for a small

proportion of the total number of seismic surveys conducted by the

industry despite highly attractive aspects of the solution. Node surveys

have generally beenmore expensive than streamer surveys because of the

large amount of time spent deploying and retrieving nodes on the seafloor.

For this reason, surveys have been limited to focused reservoir imaging

andmonitoring applications.

OBN contractors have respondedwith a new focus on resolving the

factors limiting the efficiency of the seabed seismic solution, both on

the source and receiver side. In the last fewdecades, the efficiency of

OBN technology has improvedwith longer node battery life and faster

deployment/retrieval methods.

Ongoing engineering efforts, combinedwith

operational innovation, have increased the potential

to achieve a step-change in seabed seismic efficiency

and cost effectiveness, e.g. the number of seismic

traces acquired per day by OBN surveys in the

Gulf of Mexico has tripled over the last 20 years.

OBN

is entering a newphase of maturity and has nowbeen

adopted globally as a leading imaging technology in a

variety of settings.

Large-scaleOBN–theright strategy for future

CNSdevelopment

When the objective for the OBN data is to achieve

high-resolution images of complex structures, the

size of the survey is no longer comparable to the

standard node patches deployed for 4Dmonitoring.

The imaging of complex salt diapirs requires an

extended shooting area around the deployed nodes. However, with

a reasonably well sampled, large-area node survey it is possible to

efficiently acquire larger source-receiver offsets (and therefore imaging

apertures) and achievemore efficient acquisition per square kilometre

through an economy of scale. By including adjacent geological targets

and increasing the survey size, OBN surveys becomemore competitive

than the typical small patch surveys generally acquired on a proprietary

basis. In addition, amore comprehensive analysis of the lateral extension

of complex structures joining up specific targets provides a wider

geological context.

This iswhy CGGMulti-Client, in conjunctionwithMagseis-Fairfield,

have designed the CornerstoneOBNprogramme to deliver subsurface

images of unprecedented quality in themost challenging areas of

theUKCNS. The first phase, which started inMarch 2020, will provide

approximately 2500 km

of full-azimuthdata suitable for fielddevelopment

and near-field exploration (Figure 6).

Thematurity of commercial OBN technology for large-scale, dense

exploration surveys is a key enabler behind this strategic shift. Specifically

for the shallowwaters of theNorth Sea, the development andmaturity

of efficient node-on-a-rope (NOAR)/node-on-a-wire (NOAW) acquisition

systems using reliable autonomous long-duration nodes ensures the

productivity required tomake such a survey economically viable. In this

specific case, the use of ZNodal OBN technologywill make it possible

to acquire high-quality seismic datawithminimal HSE risk. The system

is lightweight, making deployment faster and economical, with no

troubleshooting requiredwhile recording.

Thismulti-phase, multi-client programme covers reservoir targets in

HPHT areaswheremultiple complexities in the sedimentary overburden

pose challenges for seismic imaging using existing streamer data. OBN

datawill complement the existing high-quality Cornerstone streamer data.

Mastering the cutting-edge data-driven processing of OBNdatawill raise

the bar for the coming years in theNorth Sea.

Note

The author wishes to thank CGGMulti-Client, in collaboration with Magseis-Fairfield, for

giving their permission to publish this article. All images are courtesy of CGGMulti-Client.

References

WILSON, A. J. S., and DUTTON, D. ‘How tomake a Step Change in Seismic Image

Quality – Experience from the Golden Eagle Field’, 81

EAGE Conference & Exhibition,

(2019).

ZHANG, Z., MEI, J., LIN, F. , HUANG, R., andWANG, P., ‘Correcting for salt

misinterpretation with full-waveform inversion,’ 88

SEG Annual International

Meeting, (2018).

WANG, P., GOMES, A., ZHANG, Z. andWANG, M., ‘Least-squares RTM: Reality and

possibilities for subsalt imaging,’ 86th SEG Annual International Meeting, (2016).

LI, Q., SLOPEY, W., ROLLINS, F., BILLETTE , F., UDENGAARD, C., and THOMPSON, B.,

‘Leading a new deep water OBN acquisition era: two 2017-2018 GoMOBN surveys,’

SEG Annual International Meeting, (2018).

Figure 5.

Seismic line along the CNS superimposedwith the inverted Vp/Vs attribute. The arrows

indicate themost difficult challenges driving the need for OBNdata.

Figure 6.

LargeOBNmulti-client survey in the CNS designed to provide

exceptional imaging of deepHPHT reservoir targets obscuredby salt

diapirs.