4D Seismic Survey

PGS has acquired more than 20 000 sq. km of 4D data using GeoStreamer technology. The results have demonstrated superior repeatability with streamer and source steering, accurate elimination of rough sea noise with multisensor data, and compatibility with all other seismic data acquisition techniques.

Periodic, high-density 3D monitor surveys are used to detect production-related changes in a reservoir. The process starts with a baseline dataset, then each 3D dataset is compared with the last. Time-lapse reservoir monitoring surveys can be useful to identify bypassed reserves, place injector wells, locate unidentified permeability conduits, and monitor depletion or manage CO2 sequestration.

See also: Processing 4D seismic data

4D monitoring can be carried out in virtually all geological and environmental settings, and our project experts can select from a range of acquisition solutions to address environmental challenges such as marine fauna, or variable currents and weather, or geographical variables like busy oilfield traffic, very deep water, and remote locations.

4D Basics

  • Small production-related changes in the reservoir are revealed by comparing time-lapse seismic surveys
  • 4D seismic can inform production intervention decisions
  • To spot the difference caused by production, variations in acquisition or imaging must be minimized
  • External noise and sea-state must be removed

 

Acquisition Repeatability

One of the basic requirements for 4D marine seismic surveys is to repeat the acquisition configuration as closely as possible. To achieve this we use advanced acquisition controls that optimize vessel, streamer and source positions, minimizing the effects of currents on the acquisition equipment. This also improves survey efficiency. Most PGS 4D projects deliver all of this as part of a standard 4D solution. We also actively control and monitor the wavefield emitted from the source.

For the best time-lapse seismic results, optimal reservoir characterization and high-resolution imaging, we recommend deep-tow, high-density spreads on acquisition projects.  Towing deeper takes advantage of a quieter recording environment and GeoStreamer increases the bandwidth of the data. High-density streamer spreads ensure high 4D repeatability and resolution.

PGS has processed numerous 4D projects, successfully matching high-quality GeoStreamer data with a wide range of data acquired with other technologies, including conventionally-acquired monitor or baseline data. Usually, this means downgrading the GeoStreamer data, but these processes can also be used to upgrade the legacy data in a 4D-friendly manner, to achieve a better image of the 4D difference. GeoStreamer data acquired on top of OBN or slanted streamer solutions can also provide reliable 4D results.

Multisensor monitor on multisensor baseline 4D surveys have been assessed. They deliver full bandwidth P-UP 4D data that is free from rough sea surface effects and provides an ideal starting point for high-resolution 4D and QI analysis.

Imaging the 4D Difference

Safeguarding repeatability is equally important during seismic processing, although 4D seismic processing workflows can mitigate some causes of a non-repeatable signal. This is not the only concern, turnaround time is often a crucial factor, as answers must be delivered on time for maximum value.

Experience has shown that GeoStreamer 4D provides the best comparative data for production decisions. However, the realities of field production are such that seismic processing must be able to deal with different types of data. That requires a robust 4D processing toolkit. 

  • 4D processing can match GeoStreamer with lower quality data or upgrade legacy data in a 4D-friendly manner
  • Turnaround time is important in 4D processing
  • QC is essential to highlight and mitigate the causes of non-repeatability