Estimating 4D Velocity Changes and Contact Movement on the Norne Field
Authors: Aarre Victor 
Published: October 2008

Abstract
Hydrocarbon production and injection programs induce changes in elastic properties in a reservoir. These elastic changes can be monitored over time using 4D seismic. Monitoring of changes in saturation and pore pressure within a reservoir is of particular interest, because these properties have a strong impact on recovery rates. How the reservoir responds to these changes over time can also be used to derive a better understanding of the permeability within the reservoir. This case study will attempt to quantify 4D velocity changes and contact movements on the Norne field, based on four vintages of seismic surveys. These properties are often related to pressure and saturation changes in a reservoir, and it can be argued that for the Norne reservoir there is a clear relation between changes in pore pressure and changes in velocity (Osdal., 2006).

Introduction
The Norne Field is situated in the southern part of the Nordland II area in the Norwegian Sea. The size of the field is approximately 9 km × 3 km. The reservoir rocks are sandstone of Lower and Middle Jurassic age. The hydrocarbon resources consist of a gas cap (25 m.), mainly situated in the Garn formation, and an oil leg (110 m.), mainly situated in the Ile and Tofte formations. Production started in 1997 (Osdal and Alsos, 2002). The reservoir was initially at near hydrostatic pressure, and production has been driven by water and gas injection (mainly into the water column below the reservoir). Porosity and permeability are 24-28% and 0.1-2.0 Darcy, respectively, but shale and calcite permeability barriers and faulting have a major impact on reservoir production.

The role of 4D seismic data
In order to achieve oil recovery above 50%, it is necessary to monitor the drainage efficiency. 4D seismic monitoring can be very important in order to detect oil/water contact movements in the different faulted segments.
Rock physics modeling also indicates that there is a clear relationship between pore pressure and compressional velocity in this reservoir setting. As the pore pressure increase the effective pressure will decrease, reducing the acoustic coupling between sand grains in the rock matrix, inducing a 4D velocity decrease. 4D velocity changes can thus be an effective indicator of changes in reservoir pore pressure (but, we know that other mechanisms also have an effect on acoustic velocity, most notably fluid saturation changes and the presence of gas).
Very good repeatability and high signal-to-noise ratios are very important in order to monitor these effects. Streamer steering is also important on this field, as this enables seismic acquisition closer to the Norne production vessel, thus reducing the area of limited seismic coverage.
Initial production was based on a seismic survey acquired in 1992, before production assets were positioned in the field. Seismic monitor surveys have then been acquired at regular intervals after production started. The first monitor survey was acquired in 2001, with repeated surveys in 2003, 2004 and 2006. Workflow results from the 2001, 2003 and 2004 surveys are also discussed by Aarre, 2006.