Time-Lapse Seismic Interpretation Method of Thin Reservoir and Its Application in Monitoring Channel Reservoir of Deep Water in West Africa

Abstract

Few methods of time-lapse seismic interpretation are suitable for thin reservoir monitoring. Based on forward modelling, a new method is proposed to analyze changes of thin reservoir using differential waveform of time-lapse seismic data. The first phase of this study is modelling acoustic response of thin reservoir to simulate acoustic impedance changes. Base seismic data minus monitoring seismic data equals differential data. Simulating results indicate that differential waveform is independent of the initial impedance of reservoir and surrounding rock and controlled by impedance changes of reservoir. Increase of acoustic impedance of thin reservoir leads to positive waveform (wave trough on top of wave crest) and decrease of acoustic impedance of thin reservoir leads to negative waveform (wave crest on top of wave trough). Amplitude of differential waveform becomes stronger with the increasing of absolute value of impedance change of the thin reservoir. According to the conclusion above, interpreter can quickly determine whether the impedance of thin reservoir increase or decrease. Target reservoir selected for this study is the shallowest of a series of stacked oil pays and any possible effects of production in overlying reservoirs are eliminated. Two 3-D seismic surveys have been acquired before and after initial production – one in 1998 and the other 2011. Differential Waveform is interpreted as single positive waveform or superposition of positive waveforms within the scope of reservoir, which indicates the impedance of the thin oil layer increased during water-flooding development. Analysis of production history and logging data interpretation proves the conclusion above. In this study, there is no evidence of degassing. Small changes of pressure and temperature in this reservoir led to little effect on the velocity and density change of reservoir. Increase of water saturation caused by water injection led to the positive waveform. Water saturation, porosity, shale content, velocity and density of reservoir in wells were determined by interpretation of well logs. Velocity and density increase with water saturation when porosity and shale content are fixed. It is estimated that there will be an average of 14 percent of impedance increase of the reservoir when water saturation varies from 0 to 100 percent. Current oil saturation of reservoir can be qualitatively estimated according to the amplitude of differential waveform.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015