Abstract
For compacting reservoirs like the Ekofisk field, travel time shifts up to 18 ms have been observed. In the Sleipner CO2-sequestration project even larger time shifts are observed. For such cases, one finds that pre-stack analysis might be a useful complement for precise time shift estimation. A detailed method for estimation of relative velocity changes within a reservoir layer from pre-stack time-lapse seismic data is presented. By estimating travel time shifts on constant offset gathers between two interpreted horizons, it is possible to estimate the average velocity change within the section between the interpreted horizons. Both from theory and measurements, it is found that the pre-stack method gives smaller velocity changes when estimated between two key horizons. Therefore it is recommended to do pre-stack travel time analysis in addition to conventional post- stack analysis if possible. For a selected seismic inline for the Sleipner CO2 Project, estimate a maximum velocity change of 9% based on pre-stack seismic analysis, while a conventional post-stack analysis gives 14 to 16%.