Reconciling Time-lapse Seismic Data and Production Data: Application to a Gas Field

Abstract

Abstract In general, integration of time-lapse seismic and dynamic data requires extensive work to build a "common" earth model1,2 honoring both types of data. This typically leads to longer turnaround times for time-lapse seismic projects. This paper demonstrates a method to shorten this time by reconciling production data with time-lapse seismic data in the data domain rather than in the earth model domain. The results from this method demonstrate that this quick evaluation can reveal whether or not the observed seismic differences represent reservoir changes associated with production, and can provide a basis for a more extensive model based evaluation. This method was applied to a gas reservoir in the Gulf of Mexico where preliminary rock physics modeling showed that water replacement would lead to more than 10% impedance change for approximately 30% of gas saturation change. Model fluid substitution studies using an approximation of the local geologic setting yielded seismic impedance changes significantly lower than those observed in field measurements. This discrepancy is demonstrated through further model studies to be the result of thin bed tuning. Rebinning and cross-equalization with time-varying crosscorrelation were applied to normalize the 1987 and the 1995 3D seismic volumes. The seismic difference map was then transformed into a calibrated gas volume through reconciliation of fluid production volumes with seismic difference volumes. The saturation-amplitude relation used to threshold the seismic difference volume was calibrated at well locations. These results were validated using material balance and production data analysis to reconcile engineering data with base and monitor seismic survey volumes. Hence, the final difference map maximizes the probability that the seismic differences are representative of saturation changes in the reservoir. This work: (1) provides a new procedure for gas reservoir time-lapse analysis under certain geological conditions and production mechanism; and (2) demonstrates a new first order method to reconcile time-lapse seismic data and production data without employing time consuming reservoir characterization and flow simulations. This approach greatly reduces the turnaround time involved in use of time-lapse seismic information for reservoir management. Further, the result could lead to better subsequent model based analyses1,2. This work also shows that seismic tuning effects need to be taken into account when relating petrophysical changes to seismic amplitude differences in time-lapse seismic studies.