Instrumented oil fields have been discussed for many years, and many measurements (e.g. temperatures, pressures, flow rates, etc.) are being made continuously across producing fields. Seismic measurements should be included and the required instrumentation installed early in the life of the field rather than as an afterthought. Although there may be a significant initial cost to permanent seismic installations, this cost is relatively small when compared to the overall cost of the production facilities. The ability to repeat surveys is greatly improved, and the cost of these becomes increasingly lower as the number of subsequent surveys increases because the initial cost would be distributed over the larger number of surveys. Once this step is taken, 4D will truly become an integral part of reservoir management and its benefits maximized.
The first commercial 3D survey acquired in an offshore environment was recorded in 1975 in the Gulf of Mexico. A number of additional 3D surveys were acquired around the world over the next few years with excellent results. Nevertheless, it took many years before 3D surveys became routinely accepted as part of the field development and production cycle.
Similarly, the potential for the acquisition of repeated 3D surveys over the same field (time-lapse seismic, or 4D) to improve the quality and accuracy of the reservoir monitoring process also has been recognized for many years and is becoming an expected step in field development.
Initial 4D field tests were conducted offshore in the late 1980s and early 1990s, and today there are numerous case histories with excellent examples of both the technical and financial benefits that can be attributed to time-lapse seismic. Although the benefits of time-lapse surveys may not be seen until later in the life of the field, the required planning to optimize the surveys should not be omitted in the initial stages of field development.
Three very important issues have been noted by industry from the time-lapse surveys acquired over the last 20 years:
- 1. A feasibility study should be made to evaluate whether the expected reservoir changes will be detectable using the differences between 3D volumes recorded at different times in the life of the field.
- 2. The initial, or baseline, survey should be of very high quality and recorded as early as possible in the life of the field, preferably before any production.
- 3. Significant improvements in the seismic differences are achieved by replicating, as closely as possible, the source-to-receiver offsets and azimuths between the baseline and subsequent monitor surveys.
Much of the 4D recorded to date does not have the benefit of comparison to an early baseline survey since the reservoirs were put into production long before the advent of 3D seismic and are now a considerable way down the decline curve. However, oilfields that are in an early stage of development have a very good opportunity to minimize the difficulties of recording and analyzing the 4D data and therefore to maximize the payback over the life of the field.
Typically, conventional streamer marine 3D surveys are acquired in the exploration phase, before the start of development, and are assumed to be adequate baseline surveys for subsequent 4D studies. However, once the production facilities are in place, it becomes increasingly difficult, if not impossible, to acquire the monitor surveys with a high degree of repeatability. This has led to a considerable effort by the industry in developing ways to use “legacy” streamer surveys for the baseline and to match the data acquisition attributes of subsequent monitor surveys to those data. Some of the repeat surveys already acquired have been very expensive because of this need to match attributes to the original “legacy” data.
When the decision is made to establish full production in a field, the existing 3D survey can be used in the feasibility study. The decision also should be made to emplace a permanent seismic recording system in the ocean floor to record a new baseline 3D survey. Not only will this baseline survey be significantly better, but a much higher degree of repeatability for the monitor surveys will be possible. Even if additional production facilities reduce the access for the source vessel for monitor surveys, the original baseline data set could be reprocessed to include only the same source locations as the monitor and thus enable the seismic data volumes to have the same spatial attributes.
Additional benefits can also be realized:
- The time to process monitor surveys can be dramatically reduced
- The timing of monitor surveys becomes more flexible
- If only a small area within the field is affected by a problem, the recording of partial surveys is possible
- Vessel availability becomes less of an issue if only a source vessel is required
- Weather problems are reduced because the sensors are on the sea floor
- Passive seismic recording, for example of well fractures, is easily performed.
Malcolm Lansley
Sercel
