Time-Lapse Seismic Analysis of the North Sea Fulmar Field
Authors: Johnston D.H. , McKenny R.S. , Burkhart T.D. 
Published: October 2008

Abstract
Time-lapse seismic analysis has been applied to two 3-D seismic surveys acquired over the Central North Sea Fulmar Field -- a pre-production survey shot in 1977, reprocessed in 1987, and a 1992 survey. The Upper Jurassic reservoirs in the field have been under production since 1982. Water is the main drive mechanism, supported by flank injection. Although the field is currently at over 80% water cut, there are infill opportunities. Petrophysical analyses for Fulmar indicate that water replacing oil will result in an increase in seismic impedance. In addition, a pressure decline of about 1000 psi during the time between the two seismic surveys will result in a further impedance increase. These impedance changes are observed between the two seismic surveys. In order to overcome inherent differences in the seismic data due to acquisition and processing differences, the data are equalized and then inverted to obtain impedance which is then averaged between the top of the reservoir and the position of the original oil-water contact. Differences in averaged impedance between the 1977 and 1992 surveys clearly show the effects of water influx and pressure decline. The changes observed in the seismic data are overall consistent with predictions obtained from a full-field, history-matched flow simulation. Differences in details may suggest areas of bypassed oil. However, data quality is not sufficient to serve as the sole basis for drilling decisions.

Introduction
In the later phases of a field's life, reservoir surveillance is a key to meeting goals of reduced operating costs and maximized recovery. Differences between actual and predicted performance are typically used to update the geological model of the reservoir and to revise the depletion strategy. The changes in reservoir fluid saturation, pressure, and temperature that occur during production also induce changes in the reservoir acoustic properties of rocks that may be detected by seismic methods under favorable conditions. The key to seismic surveillance is the concept of differential imaging using time-lapse measurements. While one seismic image of a reservoir may not show any obvious production- related effects, differences in repeated surveys may be able to detect even subtle changes in reservoir properties. Acquisition of a seismic survey before production or intervention establishes the baseline conditions of the reservoir. Subsequent monitor surveys are differenced from the base survey. The result is a seismic difference volume which, when integrated with reservoir characterization and flow simulation, may be used to track the movement of fluid in a reservoir between well control. However, the difference between two seismic surveys is not only sensitive to changes in reservoir rock properties but is also sensitive to differences in acquisition and processing, and errors in navigation. As a result, the repeatability of seismic data is a key issue. For legacy seismic data, differencing the horizon-keyed average of attributes such as impedance is more robust in the presence of noise and data artifacts.

The Fulmar Field
The Fulmar Field lies in the Central North Sea approximately 270 km southeast of Aberdeen. The field was discovered in 1975 and is between 9900 and 11000 feet TVSS. It consists of an eroded triangular anticline (Figure 1) with a relatively small areal extent.