Water contact mapping and saturation estimation using simultaneous inversion of time-lapse seismic data constrained by fluid substitution and Leverett J-function analysis

Abstract

The integration of a Leverett J-function analysis with the Gassmann and mass-balance equations allows the time-lapse change of seismic reflectivity to be expressed explicitly in terms of the change of the free water-contact (WC) level, reservoir porosity, and initial water saturation. Given the initial 3D distribution of water saturation and porosity and a well-log derived height-saturation-porosity function as a hard inversion constraint, synthetic modeling suggests that time-lapse seismic data can potentially be directly inverted for the change in free-water level, and time-lapse changes in the spatial variation of water saturation can thus be predicted. This integrated inversion approach is applied to a real time-lapse seismic difference volume from the Maui-B field in the Taranaki Basin, New Zealand, where the change of WC level from a baseline model is the only free parameter to be inverted and is obtained by searching for the global error minimum between tmodeled and real time-lapse seismic differences as the modeled WC level is varied. The method correctly predicts the gas depletion and a measurable rise of the water contact on the southern flank of the field.