A zero offset VSP survey was acquired over a thick carbonate interval. The objectives of the repeat survey were to assess repeatability of the measurement, robustness of the time lapse VSP processing mainly to account for time lapse noise and quantification of the time lapse signal in the reservoir due replacement of water by gas caused by gas injection that occurred between the baseline and repeat surveys.
A long side of the two VSP's, an array sonic (SDT) and the sonic scanner tools were run. The SDT was run to recover the monopole compresssional and shear waves, while sonic scanner to acquire both the dipole and monopole shear and compressional.
Data processing was focused on isolating the time lapse signal in the reservoir section by comparing the baseline and repeat VSP surveys and sonic logs. Both the baseline and repeat surveys were processed with identical parameters to ensure that the datasets are comparable at every processing step. The processed data shows a good degree of repeatability, and deterministic deconvolution as expected was enough to take care of source wavelet variations between the two surveys.
The time lapse VSP and sonic logs were run in carbonate reservoirs where gas is being injected to assess the effectiveness of recovering more oil as part of Enhanced Oil Recovery (EOR). The current recovery mechanism in the reservoirs is to inject water to enhance pressure support. The gas pilot objective is to increase the recovery factor to recover more oil and enhance productivity.