4D GNS™ acquisition in deep and shallow waters

 

High costs of new hydrocarbon discoveries and declining production in the mature fields requires continued efforts to ensure the longest possible production life of oil wells.

4D seismic is a well-established technique that makes a valuable contribution to enhance recovery from mature fields.

The basic idea behind 4D seismic is quite simple. The multiple surveys acquired at different points in time over a production field allow to monitor production-induced fluid movements within the production layer, optimizing the recovery strategy, increasing oil yields and production efficiency.

 

4D GNS™ acquisition advantages

 

Providing good repeatability of offshore 4D seismic surveys is the major challenge in ever-changing marine environment. It is very difficult to re-position in the same locations the sources and receivers and to re-produce the original (base-line) survey parameters.

Shifting the measurements to the seabed would stabilize the receiver positions. The repeatability of a source location is a more simple problem, since the air gun towing distance is from 30 to 50 meters, which is incommensurably with the towed receivers array up to 6 km long. In fact, even detailed knowledge of currents, tides, weather and equipment changes not always provide reliable repeatability of hydrophone cables positioning. The GNS™ Autonomous Self Pop-up Nodes (SPNs) are located on the sea bottom at stable points reproducing the original survey positions. The problem becomes simpler given that the vertical position of the receivers is pre-determined by the water depth and is not affected by sea currents and tides, so, we have to maintain only fixed coordinates of the receiver.

 

The SPN sea-bottom position can be determined in three ways:

 

Firstly, for water depth up to 500m the SPN free-sinking time is rather small (8 minutes with 1 m/sec sinking speed), so the SPN's position does not deviate considerably from the surface deployment point.

Secondly, denser spread geometry for the repeated surveys gives us a sufficient number of common offset traces for CMP processing to obtain CMP gathers with the same offset azimuth and range as for the base-line survey, even with positioning deviation.

And thirdly, in deep water the SPN can be planted onto the defined position on the seafloor with the ROV. A specific tailor-made solution can be chosen depending on a particular survey conditions.