This paper describes 4D interpretation results in a very challenging Middle East carbonate context.
It consists of a 4D pilot (OBC/OBC) over a giant field divided into two phases. For Phase1 the monitor seismic survey has repeated the geometry of the base survey (parallel shooting) to get started in the best possible 4D conditions. For Phase2 the monitor seismic survey design is a modern source spread acquisition for wide azimuth objective. It is clearly different from the base survey design, and so starts with a worse 4D repeatibility.
In this paper, we describe the challenges attached to both phases of this pilot and explain how in the end it has been successfully interpreted.
A focused study is made on a particular 4D anomaly corresponding to an increase of impedance in the reservoir. This is interpreted as the water front movement (due to water injection) during the interval between base and monitor surveys. Petrophysically this is consistent, if water replaces oil in the reservoir then impedance increases. However, 4D interpretation can be ambiguous and other production phenomena can explain impedance increase, so the interpretation must be assessed carefully. This is done through calibration of the interpretation to well data (time evolution of the water cut).
The final interpretation is robust. Other elements not described in the paper could have been added to consolidate the interpretation like the fact that interpreted 4D anomalies (and so water rise-up) stop vertically on a well known impermeable barrier (anhydrite level).
Once the 4D signal is calibrated, unexpected phenomena (i.e. heterogeneities not predicted by the reservoir model) are highlighted as potentially providing added value to reservoir understanding. As a deliverable, 4D anomalies are interpreted as 3D geobodies and a water rise-up top surface is picked. This information will be key to position new wells and to update the reservoir model.
Though 4D seismic techniques are very mature and widely illustrated in clastic reservoir environments, it is still rarely used operationally to monitor carbonate fields. This paper proves the concept that a reliable 4D signal can be extracted over such Middle-East carbonate reservoir.