Impact of 4D Seismic Design on the Quality of 4D Results

Abstract

Abstract After great success of 4D seismic (Time-lapse) in clastic environment lot of 4D projects started 10 years ago in carbonates. Because of a smaller petroelastic response to reservoir modification lot of reservoirs enginners and geophysicists considered that 4D in carbonate has a very little probability of success. To check the capacity of 4D seismic to work in carbonate environment ADMA decided to acquire a 4D pilot on one of their giant field. The design of this pilot has been chosen to mimic as perfectly as possible the base survey. Then another acquisition has been performed with a fully different design that reduces drastically the cost of survey. TOTAL had the great opportunity to process, invert and interpret these datasets. This paper will show some comparison of 4D results obtained from the 2 designs. TOTAL performed a lot of QCs allowing to get a very good appreciation of degradation in repeatability and increase of noise levels linked to acquisition design. The regular QCs as NRMS or predictability are compared but also some new in house TOTAL QCs as SDR (Signal Distortion Ratio), Noise Xplots, etc… Next relevant 4D attributes (like relative changes in P velocities and in P impedances) are computed using in house algorithms that have been developed on numerous Total portfolio cases. They can be compared for both designs using as reference the pilot which is considered closer to reality. A first step was to validate 4D anomalies out of pilot area by comparing them to geological features and production knowledge. So we got the proof of real 4D anomalies related to clear production phenomena, validated by the asset team. Second step was to evaluate the stability of 4D results despite the acquisition design modification and the strong variation of noise level. It is obvious and expected that the "non repeated" design introduces a degradation of the quality of 4D results. Key points are on one hand to check if the 4D processing can handle these differences successfully and another hand to define recommendation to acquire a successful and cost-effective monitoring survey. In conclusion this example shows that 4D seismic in carbonates, oil/water system; with difficult seismic environment is definitely possible under condition that acquisition, processing and inversion of 4D seismic are conducted with care to avoid the weakest link limits value of 4D information