A proven method for acquiring highly repeatable towed streamer seismic data

Abstract

Seismic reservoir monitoring has become an important tool in the management of many fields. Monitoring subtle changes in the seismic properties of a reservoir caused by production places strong demands on seismic repeatability. A lack of repeatability limits how frequently reservoir changes can be monitored or the applicability of seismic monitoring at all. In this paper we show that towing many streamers with narrow separation, combined with cross‐line interpolation of data onto predefined sail lines, can give highly repeatable marine seismic data. Results from two controlled zero time lag monitoring experiments in the North Sea demonstrate high sensitivity to changing water level and variations in lateral positions. After corrections by deterministic tidal time shifts and spatial interpolation of the irregularly sampled streamer data, relative rms difference amplitude levels are as low as 12% for a deep, structurally complex field and as low as 6% for a shallow, structurally simple field. Reducing the degree of nonrepeatability to as low as 6% to 12% allows monitoring of smaller reflectivity changes. In terms of reservoir management this has three important benefits: (1) reservoirs with small seismic changes resulting from production can be monitored, (2) reservoirs with large seismic changes can be monitored more frequently, and (3) monitoring data can be used more quantitatively.

Tools and Techniques: Marine Seismic Methods

“…Kragh and Christie (2002) reviewed nonrepeatability and looked in detail at the two metrics, the normalized root mean square (NRMS) difference and the predictability, which are now routinely used to measure the time-lapse repeatability. However, it is now well established that well-repeated surveys with careful processing can show residuals well below 20% NRMS, for example, Day et al (2010), Li et al (2004), and Eiken et al (2003). Repeatability metrics such as NRMS are highly sensitive to the smallest differences in the seismic measurements.…”

Assessing 4D, New Wonder Tool, Or More Work Needed?

“…[Eiken 2003] If feathering, which is largely controlled by nature does not repeat then only near offsets will repeatable. Good practice is to repeat the shot locations and tow closely spaced multiple streamers.…”

Field test of a permanent in-well fiber-optic seismic system

“…Risks associated with 4D seismic analysis include a difficulty in separating pressure and fluid saturation effects and misinterpretations between the desired reservoir properties and other effects (such as formation compaction and changes in surface conditions such as tidal and temperature effects). Since repeatability is an issue between different vintage seismic surveys (Ronen et al, 1999;Landrø et al, 2001;Eiken et al, 2003), Landrø (1999) profile (VSP) surveys by acquiring a 3D VSP survey over a period of two days. Shot locations were reoccupied at different times during the survey, and the recorded traces from those shots were analyzed.…”