Using 4D Seismic to Validate the Geomodel for the South Arne Chalk Field

摘要

The South Arne chalk field is located in the Danish sector of the North Sea. It is a double-dipping anticline extending 12 1/2 km by 3 1/2 km. Production started in the summer of 1999 using horizontal wells and water flooding with a cumulative production to date of 141 MMbbl from the two oil bearing intervals: Maastrichtian (Late Cretaceous) Tor Fm and Danian (Paleocene) Ekofisk Fm. 3D seismic was shot in 1995 (pre-production), in 2005 and in 2011. The matrix permeability of the field is low but greatly enhanced due to natural fractures. Based on the 2005 seismic interpretations a geomodel was built with emphasis on capturing the permeability enhancement due to the occurrence of fractures. The modelling was done using discrete fracture network (DFN) modelling. Fracture density was calculated based on various attributes and the outcome calibrated to well test data. Calibration was performed using full field adjustments on fracture properties enhancing predictability in less data dense areas. The model has been history matched to production data with a good overall result. The seismic shot in 2011 was compared to pre-production seismic data giving valuable information about unswept areas and water cutting as well as differences between the two reservoir intervals. The 4D data also shows that the faults have significant influence on fluid flow as modelled in the geomodel. Using fluid substitution modelling, the strongest signal is expected to come from changes in Sw. Comparing the observed changes from 4D seismic to the history-matched modelled changes in Sw, from pre-production to 2011, a very good match is found, thus validating the geomodel.