oa Value-add and safe drilling from 4D AVO evaluation in the Enfield oil field, North-West Shelf, Australia

The Enfield oil field, on the Australia’s Greater North-West Shelf, started production in July 2006, and has already seen significant benefit from 4D seismic monitoring. 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. Full-field monitor surveys in February 2007 and December 2008 have made major contributions to reservoir management and identification of infill drilling opportunities.

This paper highlights the 2009 development drilling in the field’s Sliver Block, where 4D not only supported the business case for a new production and water injection well, but also provided important information to enhance safe and efficient drilling. 4D AVO analysis involved extraction of changes over various source-receiver offset ranges, driven by a rock property model in which nears and mids tend to show pressure changes, while fars and ultra-fars show saturation changes.

A clear 4D AVO signal on the early monitor showed the drilling target area to be pressured up by a distant injector, contrary to pre-4D expectations. The later monitor confirmed this, and also showed the oil target intact – with no significant water sweep. Using 4D, significant overpressure in the target was predicted. Accordingly, casing schemes for the wells were redesigned to enable safe reservoir entry; the distant water injector was shut in prior to drilling; and appropriate heavy brine was ordered for completion operations. Actual pressures fell within the predicted range, and a safe and successful drilling campaign was achieved.