Prediction of Seismic Time-Lapse (4-D) Effects of the Reservoir Based on Petrophysical and 3D Seismic Data in the Turaco Prospect, Semliki Basin, Western Uganda

Abstract

Time-lapse or 4D-seismic refers to the repetition of a seismic survey after a period of time, time being the ‘fourth dimension’. Integration of seismic and well log data enabled the prediction of time-lapse effects by creating and comparing monitoring surveys to represent reservoir impacts due to production at different production scenarios within Kakara and Kisegi Formations. This enabled the understanding of flow behavior of the reservoir during production. Horizon interpretation and well log data analysis enabled the identification of zones of interest. Missing logs were synthesized including Vs from Vp, Vshale from gamma log and porosity from the density log. Fluid substitution was performed on Vp, Vs and Density at different reservoir scenarios involving 0, 40, 80 and 100% gas saturations. These were used to generate Amplitude Versus Angle (AVA) synthetics (monitor surveys). Monitor surveys were subtracted from the base survey to obtain AVA difference hence predicting the effects of saturation changes on seismic. There is a slight decrease in positive seismic amplitude to almost zero at 40% gas saturation in Kakara and Kisegi formations, phase reversal, strong decrease in seismic amplitudes at 80 and 100% gas saturation was noted. Increasing gas saturation resulted into decreasing Vp from 3,500m/s to 3,000m/s at 100% gas saturation. The Poisson ratio (δ) decreased from about 0.35 at 100% brine saturation to range of 0.2-0.3 at 100% gas saturation. Formation density decreased from 2.40g/cc to 2.17g/cc with increasing gas saturation and slight increase in Vs was noted. Fluid substituted well-log data from this formation was used to demonstrate the power of a Rock Physics Template (RPT) in reservoir monitoring and characterization. Two rock physics models were tested that is the cemented and consolidated sand models. The most suitable model for Turaco was the unconsolidated sand model. This model was fairly in agreement with the saturation, lithology and porosity well log data as compared to cemented sandstone model which over and under predicted all well log data.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018