4-D Reservoir Monitoring
Time-lapse seismic in reservoir management is an important technology to help understand petroleum reservoirs and thus to produce more oil and gas from them (Jack, 1997). Four-D reservoir monitoring, an important part of the above, involves the study of two or more 3-D seismic surveys over the same reservoir in the hope of observing changes with time caused by petroleum production. The expected changes in the seismic data are generally small, so good data coupled with careful processing and analysis are required.
Many projects to date have used legacy data, that is 3-D surveys which already exist, acquired at different times over the same area or overlapping areas. Differences in the positioning and acquisition of the seismic surveys are very problematical, so very careful reprocessing is required. Even then legacy data will suffice only where there is a fairly large acoustic change in the reservoir and where the data are very good (Figure 13-1). For lesser acoustic changes caused by production and for less good data areas, deliberately repeated surveys with conformable acquisition parameters will be needed. Some surveys have been and are being recorded by permanently-implanted receivers which greatly improves repeatability and which thus increases the detectability of subtle changes in the reservoir. Clearly this is easier to do on land than at sea. In this chapter you will find six case history examples of 4-D results; they are mentioned by name in Figure 13-1 in their appropriate category. There is a further example reported as Case History 2 in Chapter 14.