Example 4D difference section, generated by subtraction of monitor data from baseline data. Changes in the reservoir stand out clearly.

The basic premise of 4D is a simple one. The method involves the acquisition, processing and interpretation of repeated seismic surveys over a producing field with the aim of understanding the changes in the reservoir over time, particularly its behaviour during production.

This understanding has very real budgetary consequences as increasing the recovery factor of a reservoir, even by a few percent, has significant revenue implications.

The ability to monitor the behaviour of a reservoir during its production lifetime now allows engineers to observe changes in the subsurface beyond the limited windows provided by wells. The field-wide information provided by seismic is now considered key in optimizing the recovery of remaining reserves.

4D seismic has evolved from a qualitative tool to identify producing zones and bypassed oil, to become an integral part of quantitative reservoir management.

The commercial success of a field can be determined by the smallest of margins; 4D makes a significant contribution towards understanding the dynamic behavior of the reservoir and thus plays a part in maximizing the return on investment. Improving recovery rates by a few percent, or extending production for a few years can have a crucial impact on that margin.