Time-lapse seismic (4D seismic) is an integrated part of the well process on Troll, where its uses range from planning of infill-wells between already existing producers, geo-steering and verification of the Troll geological model. Through 4D modeling, a workflow aiming at mapping the gas-oil-contact (GOC) after production is developed. Inputs for forward modeling are saturations of gas, oil and water, and their changes in the individual zones as well as pressure changes. Convergence is met when attributes derived from synthetic 4D signal matches those derived from measured 4D signal. The workflow includes building of petro-elastic model, estimation of background pressure effect, extraction of GOC from 4D signal, fluid substitution, time-depth conversion, attribute generation and convergence/updating GOC.
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