Repeatability issues of 3-D VSP data

Increased repeatability is recognized as one major issue for improving the time-lapse seismic technology as a reservoir management tool. A 3-D vertica l seismic profiling (VSP) data set, acquired over a period of two days, is used to analyze how repeatable a permanent installed geophone array can be and how repeatability changes with inaccuracies in source positioning. It i s found that for a frequency range between 3.5 and 50 Hz, the difference ro ot-mean-square (rms) level between two recorded traces belonging to two dif ferent shots is about 8%. This fact shows that there is a potential for acq uiring very accurate time-lapse seismic data by using a permanently install ed downhole geophone array. Repeatability variation with increasing shot se paration distances is analyzed, showing a rapid decrease in repeatability a s the accuracy of the positioning of the repeat survey decreases. Horizonta l geophone components show approximately the same degree of repeatability c ompared to the vertical component, but horizontal geophone data is slightly more sensitive to positioning errors. The results show that repeated 3-D V SP surveys (preferably using permanently installed geophone arrays) might b e an efficient tool for detailed and precise monitoring of fluid and pressu re changes within a hydrocarbon reservoir.