4D Seismic Benefits to Development Drilling at Enfield – North–West Shelf – Australia

Enfield oil field is the site of Australia’s first dedicated 4D seismic monitoring program, which has proved to be an outstanding success. The technique is particularly applicable in Enfield due to faulting and baffles within the reservoir, the need for multiple water injection wells, and the presence of a gas cap. Two full-field monitor surveys have together made major contributions to reservoir management and identification of infill drilling opportunities. We highlight the 2009 development drilling in the field’s Sliver Block, where 4D not only supported the business case for new development wells, but also provided important information to enhance safe and efficient drilling. 4D AVO analysis was undertaken, driven by a calibrated rock property model in which near/mid offsets are shown to indicate pressure changes, while far/ultra-far offsets indicate saturation changes. This work showed the target area was strongly overpressured by a distant injector, contrary to expectations, but not swept by water. The two monitors confirmed this was a stable situation, hence the viability of the drilling project. The overpressures required an extra casing in each well for safe drilling. The actual drilling and well completions went smoothly, and confirmed pressures predicted from 4D.