CGG Delivers Advanced 4D Images for BP Angola Survey in 8 Weeks

Added: 15th October 2019 by CGG

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CGG Subsurface Imaging, part of CGG’s Geoscience division, has delivered state-of-the-art broadband 4D seismic results ahead of schedule from BP Angola’s latest monitor survey offshore Angola. This achievement builds on previous 4D seismic processing projects undertaken for BP Angola.

The enhanced imaging volumes from the Greater Plutonio development in the Lower Congo Basin were delivered four weeks early for this time-critical BP project. Faster than expected completion of this workflow, which included the latest advanced proprietary deghosting and demultiple technology, was made possible by continued investment in CGG’s high-performance computing capacity and the advance of its technology, along with close collaboration with the BP team.

What is 4D seismic?

Once wells have been drilled and oil is being produced, the challenge for engineers is to understand the behaviour of the reservoir, identify locations for infill drilling and design enhanced oil recovery programs. 4D seismic obtained by repeating 3D seismic surveys at regular time intervals and looking for differences in the reservoir response, allows any change to be monitored at high resolution across the whole field.

The act of taking oil and gas out of the reservoir causes changes in fluid flow, saturation and pressure and these alter the elastic properties of the affected rocks, which in turn causes measurable changes to the travel-time and amplitude of seismic waves. In order to visualise and quantify these changes a repeat seismic survey using the exact same acquisition parameters (even down to shot location) is acquired. This new survey is known as the monitor and the original is labelled the baseline. This gives us our fourth dimension which is time.

Sometimes the monitor is processed in exactly the same way as the baseline even down to resurrecting old versions of programs used in the original processing; often both surveys are processed with the latest available technologies. The aim of 4D processing is to attenuate the 4D noise caused by changes in acquisition parameters such as more modern recording equipment or from different environmental conditions such as water currents, and to emphasise the actual differences, the 4D signature, caused by the changes in fluid, pressure and stress.

A key philosophy in 4D processing is to optimise for the 4D difference. Often if you optimise the processing of each individual dataset you end up with a worse 4D result. 4D cost functions are often used to allow a trade-off between what is best for processing and what gives the best 4D difference. The image shows the 4D difference at the top of a reservoir into which fluid has been injected in order to extract more oil with the entry point indicated by the strong red area.

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