4D seismic for oil-rim monitoring

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.