Advances in 4D Seismic: Application in the North Sea
Authors: Davies D.J. , Wagner H. , Pickering G. 
Published: October 2008

Abstract
This paper describes an integrated approach to 4D seismic, using legacy time-lapse 3D data in the UK North Sea. The baseline 3D surveys are typically 1980s' vintage, with timelapse 3D being acquired after 10 years. The time-lapse surveys have been acquired for structural definition and are therefore only fortuitously 4D. After re-gridding and re-aligning the 3D traces to the same spatial location for each vintage, both poststack and AVO inversion is applied, using the same wells to derive both low frequency models and wavelets in each vintage. This inversion calibrates the seismic in acoustic impedance units relative to the well logs, after removing the (different) wavelet signatures from each vintage. The resulting acoustic impedance volumes are subtracted one from another to provide difference cubes. The seismic analysis is complemented by forward modelling from the reservoir simulator model, i.e. to a range of synthetic acoustic impedance volumes. The synthetic volumes are defined by the spectrum of responses 'allowed' in the current field development scenarios and the comparison with real data tests the reservoir model. In addition, forward modelling into the future predicts the best dates for acquisition of new seismic data. A parallel approach conditions assessments of history matching to the observed seismic response. The method finds application in field development planning, and monitoring of producing reservoirs (oil and gas). History matching and reservoir models are updated with reference to the 4D seismic data. Poorly depleted fault blocks are identified, and flow patterns deduced in water-injection. Pressure variations across fields, including unexpected gas evolution are observed. The paper suggests that 4D seismic can improve development planning and reduce costs over the field lifetime. 4D seismic identifies un-swept oil and improves the recovery factor. We also demonstrate that legacy data-sets are important information for field monitoring.

Introduction
4D seismic has been a 'technology of promise' for several years. The basic idea, i.e. to monitor fluid flow and saturation and pressure changes within producing fields, has been current since the early eighties, when 3D data-sets became widely available. The benefits are clear, to follow the effects of lithological variations and 'baffles' such as sealing faults, within the field, and to alter the field development planning in recognition, producing enhanced recovery as a result. Rock properties modelling and forward modelling from reservoir simulation supports the conclusion that such changes should indeed be seismically visible in many fields during the production lifetime. Integration of the seismic data into the workflow including geological and reservoir dynamic modelling, is a vital element in the 4D study. The 4D data can be used to verify the reservoir model, and limit uncertainties that are carried over into economic modelling. Many problems have stood in the way of an easy realisation of this potential. Foremost has been the difficulty in comparing the often arbitrarily scaled data from different legacy time-lapse 3D volumes, and the difference in acquisition and processing associated with improving seismic technology over the 80s' and 90s' has exacerbated these differences.