oa Quantitative 4D interpretation – Relating the seismic to production changes at Enfield, North West Shelf, Australia

4D seismic monitoring at the Enfield oil field has proved to be a key tool for integrated reservoir management since early in the field’s history. Two monitor surveys have been acquired over the field at 7 and 30 months after starting production in July 2006. These datasets enabled a better understanding of injector pathways, pressure and water front movement, depletion gas and baffles. The 4D has also been used to optimise well positioning and well design.

Production data together with 4D seismic attributes has been used to calibrate the initial rock physics model that relates rock properties to production effects. It was found that the initial model under-predicted the seismic response to reservoir pressure changes in the field. The updated rock physics model has been used in an extensive modelling exercise to determine the best seismic attribute to quantify changes in reservoir pressure and saturation. Results show that 4D AvO attributes are crucial in discriminating areas of increased reservoir pressure from free and depletion gas effects. The most accurate method is utilising seismic inversion attributes, followed by AvO intercept and gradient, with the least quantitative being seismic amplitude from partial stacks.