Evaluating Feasibility of Estimating Attenuation Changes from 4D Seismic

The main problem in measuring of the frequency-dependent attenuation is non-flat and variable spectrum of reflections caused by interference of seismic waves in thin-layered medium. 4D seismic observations provide an opportunity to reduce (almost eliminate) this effect using an assumption that the layered medium does not change (changes slightly) over the time interval between repeated seismic surveys (several months). We analysed 4D seismic data obtained on offshore field with the purpose to study the possibility to use seismic attenuation as a parameter which is sensitive to changes of the amount of the hydrocarbon fluid in pores. The effects of different factors such as changes of seabed depth, repeatability of source position, etc., have been studied. The presented method of 4D attenuation estimation allows us to study block structure of oilfield and separate operating and idle blocks. 4D analysis showed a decrease of frequency-dependent attenuation during oil production. The anomalous zones for all 4D parameters (amplitudes, time, attenuation decrement) are aligned.