A Tale of Two Fields - Impact of Acquisition and Geology on 4D Seimic Data Quality

A 4D project was completed over Erha Main and Erha North Fields in OML133, offshore Nigeria. A 2009 monitor survey was compared to a 2005 baseline to determine gas and water movement in the reservoirs and to improve geological and reservoir simulation models. The Erha 4D dataset is an example of two extremes in terms of detectability and seismic repeatability. A strong 4D response was expected at Erha North and a marginal 4D response was expected at Erha Main with additional complications due to poor repeatability around the surface facilities and the effects of a large shallow channel. Mitigation of repeatability issues was achieved via extensive QC of the 4D binning approach, and application of 3D Surface Related Multiple Elimination during processing. Additionally, amplitude preserving controlled beam depth migration resulted in further improvement in 4D data quality with removal of non-repeatable higher-frequency noise and migration smiles that are visible on the Kirchhoff migration differences. The resulting 4D data have revealed high degrees of reservoir heterogeneity along channel axes and connectivity between channels and channel complexes. The 4D results have been used to evaluate the current inventory of Erha infill wells, to work over existing wells, and to identify new opportunities.