PDM has warned data managers about the onslaught of data which 4D "time lapse" seismics is likely to produce over the coming years. The problem is not only one of data volumes, but field seismics now has a temporal component that data managers and modelers will need to handle. Western have now added yet another dimension to the equation with the advent of four component (4C) seismic monitoring of the reservoir. We dealt with the 4D time lapse part in PDM Vol. 1 No. 6. The essentials are that a repeat 3D survey on an oilfield that has been in production for a number of years is acquired and then carefully compared with the data acquired before production started. Differences between the two surveys may show the extent to which fluids have moved through the reservoir in the intervening period. Of course you should not try this on an oil reservoir at great depth, but for oil and gas phases in shallower fields the technique can produce spectacular results.
As in direct prospecting for hydrocarbons, some seismic indicators can be misleading. Many a brightspot has turned out to be hard rock rather than gas and to reduce the ambiguity in seismically derived rock properties, shear wave prospection has been devised. To the geophysically untutored, normal seismics involves compressional waves, where the particle motion is in the direction of wave travel, where shear waves have particles moving perpendicularly to this direction. A land seismic vibrator wobbles a baseplate up and down, whereas one shear wave source actually consists of a large articulated hammer, which wallops a big chunk of iron sideways.
While this works on land, shear waves unfortunately do not travel through water and hence, in Western's innovative Oseberg survey for a group headed by Norsk Hydro, the use of a ocean bottom cable which is moved around the survey area. The full seismic waveform is measured by a hydrophone plus three seismometers mounted at right angles - hence the 4 Component tag. Claiming a world first for this study, Western will be applying techniques developed in previous tests in the North Sea and West Africa during late 1996 and early 1997. Western Geophysical will be deploying one of its recently announced fleet of purpose-built ocean bottom cable vessels the C-Centurion which has just been mobilized to the North Sea. Hydro's objective is to increase the life of the Oseberg Field and allow more accurate wells to be drilled into the reservoir channel sands, which can not be fully imaged using existing methods.
The Statfjord study for Statoil involves reprocessing 3D surveys from 1980, 1991 and 1997 to determine fluid movements in the reservoir and generate hydrocarbon saturation maps at three different stages of production. This will be a multidisciplinary project, involving both Western and Statoil and the approach is expected to significantly improve oil recovery. Processing will be performed at Western Geophysical's Stavanger computer centre using proprietary time-lapse software modules with project management provided by Western's London 4-D Reservoir Monitoring Services group. The resulting 4-D data will be analysed using a number software tools including Lamont 4-DTM software under license from Columbia University.
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