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4D Time Shift Interpretation and Pressure Calibration for the sub-salt Rotliegend Reservoirs of the Southern North SeaNormal access

Authors: J.P. Brain and T. Lassaigne
Event name: First EAGE Workshop on Practical Reservoir Monitoring
Session: Realising the Value of Data
Publication date: 06 March 2017
DOI: 10.3997/2214-4609.201700008
Organisations: EAGE
Language: English
Info: Extended abstract, PDF ( 1.52Mb )
Price: € 20

Time lapse (4D) seismic in the Rotliegend reservoirs of the Southern North Sea is extremely challenging due to small 4D pressure depletion signals and complex overlying salt geometries. With gas production, pore pressure in the reservoir is reduced, stiffening the rock frame and inducing a velocity increase and reduction in travel time across the reservoir. In order to map depletion away from the producing wells and thereby identify potentially un-depleted areas, we are required to detect time shifts in the order of 1ms or less beneath the reservoir. Here, reflectivity can be variable and signal to noise poor, due to multiple contamination and complex imaging. In addition most baseline surveys in the area date from the late eighties and use two boat towing configurations, requiring additional acquisition effort and processing corrections. This paper describes the processing and interpretation workflows we have developed to measure reliable sub-salt time shifts, calibrate results for pressure prediction and estimate uncertainty. After a multi-field 4D seismic program, Nederlandse Aardolie Maatschappij (NAM) has drilled three wells to date based on 4D results and in all cases pressures were within the pre-drill predicted ranges.

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