4D seismic for oil‐rim monitoring

Abstract

In the central North Sea ‘Gannet‐A’ field, a 50 ft oil rim is overlain by a gas cap of variable thickness. Oil is produced from horizontal wells which initially produced dry oil, but as the field became more mature, a significant water cut was seen in several wells. A dedicated 4D seismic monitor survey was acquired in order to assess the remaining distribution of oil reserves. By forward modelling the synthetic seismic response to parameters such as contact movement and residual saturations (using 2D and 3D wedge models), and comparing the results with real seismic data, we are able to decipher the contact movements across the field. It is shown that, in one part of the field, the increased water cut is caused primarily by the vertical displacement of the entire oil rim into the initial gas cap. This oil‐rim displacement produces a very different 4D seismic response from the case of a static gas–oil contact and rising oil–water contact (normal production). As a result of these observations, we are able to optimize field production by both re‐perforation of existing wells and by drilling sidetracks into the displaced rim: a brown‐field development opportunity that might otherwise be missed.

Integration of 4D Seismic Data and the Dynamic Reservoir Model - Revealing New Targets in Gannet C

“…Where the flood zone is so thick that no interference occurs between the original and the producing contact, some interpretation was also performed on reflectivity difference data, with the producing oil-water contact shown as a hardening reflector. At Gannet C, this method was considered both fit-forpurpose and appropriate (given the poor up-dip imaging on this survey), although elsewhere, in Gannet A with better imaging, a stronger DHI, and interference between a moving OWC and GOC, detailed quantitative studies of tuning 4D signals have been performed (Staples et al, 2005). Fault communication A pre-production oil-water contact was visible in many areas on 3D seismic, although due to its interference with lithology and variations in time-depth conversion, there was some uncertainty over the premise that it was completely flat.…”

Time lapse (4D) and AVO analysis: A case study of Gullfaks field, Northern North Sea

“…In most cases, time lapse can distinctively and accurately separate the effect of pressure and saturation changes. This has been made possible by the world-leading marine 4D repeatability (Normalised Root-mean square (NRMS) noise, which is kept at as low as 7%), sparse acquisition systems (enabling high quality 4D from 10-fold data), cheap acquisition and intense effort in simulator synthetic and geomechanical modelling processes (Staples et al, 2006). To extract fluid types or saturations from seismic, crosswell, or borehole sonic data, we need a procedure to model fluid effects on rock velocity and density.…”