Using time-lapse analysis to predict reservoir changes in a carbonate pool: a case study of the Rainbow B pool
Hannah T. Tran, Laurence R. Bentley, Edward S. Krebes
The Rainbow B pool is a carbonate pool in the process of tertiary production. Miscible gas and solvent were injected into the top of the reservoir in an attempt to extract the remaining oil. The oil in the pore space is being replaced with the gas and the solvent. Two 3D seismic surveys were recorded for the Rainbow B pool. One was acquired in 1987, shortly after the start of tertiary production and the other survey was acquired in 2002. A comparison of the two surveys reveals seismic differences that can be attributed to production-related changes within the reservoir. This will help determine the areas of bypassed oil. Time sags in the seismic data, as well as amplitude changes at the top of the reservoir, reveal areas where gas and solvent were injected.
Before the two surveys can be compared to one another, they must be made to match each other in terms of geometry, time, phase, and frequency. Then subtraction between the two 3D surveys should only reveal production changes. This process is referred to as time-lapse analysis. Preliminary time-lapse analysis in the Rainbow B pool shows the time sags underneath the reservoir, which correspond to the injection zones. However, the amplitude changes at the top of the reservoir are small and poorly correlated to the time sags. The time-lapse analysis results for the Rainbow B pool were also compared to the results the reservoir engineers obtained from simulations with the CMG application. Comparing the geophysical and the engineering results highlight interesting similarities and differences. It was also noted that Gassmann analysis appears to underpredict changes due to solvent and gas substitution.