Exposing the 4D Seismic Time-Lapse Signal Imbedded in the Foinaven Active Reservoir Management Project
Authors: Bouska Jack G. , O&apos;Donovan Andrew R. 
Published: October 2008

Abstract
An illustration of high resolution, time lapse (4-D) seismic information which provides valuable insight into connectivity and pressure communication within the turbiditic sand reservoir. Analysis of the Foinaven Active Reservoir Management (FARM) 4-D test pilot study, and subsequent implementation across the entire field, demonstrate the applicability of 4-D data as an additional tool for non-invasive reservoir surveillance.
The Foinaven field is located 190 km west the Shetland Islands, in a water depth of 500m. The reservoir consists of turbidite sands, in complicated stacked channel systems, intersected by faults and up-dip sand pinchouts. The (light, sweet) oil is very near the bubble point, at in-situ reservoir pressures, and consequently exhibits good AVO-DHI. This feature of direct hydrocarbon detection makes the reservoir an ideal candidate for the use of time lapse (4-D) seismic as a relatively inexpensive surveillance method.
When the oil bearing sands are placed on production, and the reservoir pressure drops below bubble point, a dramatic time lapse signal can be detected using repeat seismic. Analysis of the recorded time lapse anomalies assists with detection of pressure barriers, localization of sealing faults, and inference of compartmentalization within the reservoir.

Method
Optimizing reservoir understanding requires bringing a number of technologies to bear on the problem, including time lapse (4-D) seismic, depth imaging, and reservoir simulation. Integration within a consistent geologic framework, and codification into a competent reservoir model complete the unifying steps.

Examples
The study area, shown in the bottom right hand corner of figure 1, is a self contained portion of the reservoir, bounded by sealing faults to the north and south, and stratigraphic pinch out in the up-dip direction. Highlights from the reservoir model, and history match are also illustrated in figure 1. The current reservoir model provides a tight match to the well production history, and is a valuable tool in the reservoir management arsenal for this compartment of the field. The current quality of the history matched reservoir modeling work places extra demand on any additional Geophysical data to provide new information which is significant enough to influence the reservoir management decisions. For instance, a 4-D seismic survey would need to show dramatic new insights about the subsurface to justify the extra expense.
The resulting value of 4-D seismic can be illustrated very effectively with figure 2. This image shows amplitude maps, extracted along the T32 horizon for both the 1995 and 1998 Foinaven 4D surface towed streamer volumes. These horizons show significant amplitude change following ten months of oil production. The Oil passed through bubble point, as the initial production dropped the reservoir pressure by 500-700psi, releasing gas out of solution.
The far end of the producing (horizontal) well bores are indicated with the letter "P" on the image. Note a strong increase in negative (trough) amplitude between the 1995 and 1998 surveys surrounding these well bores. This effect is attributed to gas evolving out of solution from the oil, and lowering both velocity and density of the fluid.