4-D seismic method
There is a strong resemblence between the techniques used in clinical medicine and geophysical prospecting. Here, I will refer to this resemblence within the context of the 4-D seismic method. A heart patient is monitored from year to year by the patient’s cardiologist to observe and detect changes in the parameters that describe the heart itself and its condition to sustain the patient’s life, and the composition of the patient’s blood. By using an echocardiogram derived from ultrasound waves, the cardiologist measures the size of the heart and observes whether the valves have any leakage. By recording an electrocardiogram, the cardiologist observes the systolic pressure associated with the rythmical contraction of the heart during which the blood is pumped out and the diastolic pressure associated with the rythmical dilatation of the heart during which the blood is pumped in. It is not just the heart’s parameters themselves that are important to the cardiologist, but also the critical changes in those parameters from year to year during the patient’s life span. Eventually, the cardiologist uses the data associated with the patient’s medical history to judge whether or not a surgical intervention is required, at which time an angiogram is taken to make direct observations of the heart to provide the necessary instantaneous information to the surgeon. When evaluating the historical data, the cardiologist is cognizant of the fact that the heart’s parameters are directly influenced by the evolving technologies used in taking the measurements, and the interpretation of the measurements is influenced by the changing medical knowledge and experience of the cardiologist.
Similarly, the reservoir geophysicist uses the 3-D seismic method combined with the direct observations made at well locations to monitor the reservoir conditions that are crucial for optimum development of the field. The objective in optimum field development is to lengthen the life span of the field, prevent water invasion, and recover as much hydrocarbon as possible. By recording 3-D seismic data over the field at various time intervals, which may be from months to years, we introduce the fourth dimension to the the analysis of the data — calendar time, thus the term 4-D seismic method to describe the time-lapse 3-D seismic exploration.
- Monitoring the spatial extent of the steam front following in-situ combustion or steam injection used for thermal recovery,
- Monitoring the spatial extent of the injected water front used for secondary recovery,
- Imaging bypassed oil,
- Determining flow properties of sealing or leaking faults, and
- Detecting changes in oil-water contact.
A demonstrative example of the 4-D seismic method is shown in Figure 11.5-1 . The two seismic sections along the same inline traverse have been extracted from the image volumes derived from 3-D poststack time migrations of two time-lapse 3-D seismic data. One survey was conducted in 1989 before the production commenced in the field and the other survey was conducted in 1998 sometime after the production was started. The oil-water contact (OWC) is distinctly visible in the 1989 section, while it is not apparent in the 1998-section. Additionally, the top-sand reflector is stronger in the 1998 section. This may be attributed to the increased impedance contrast between the overlying shales and the reservoir sands which have higher water saturation as a result of continuing production.
Figure 11.5-1 Time-lapse seismic data: (a) preproduction (1989), and (b) postproduction (1998). (; figure courtesy MacLeod et al., Chevron and Schlumerger Geco-Prakla; data courtesy Chevron.)
Figure 11.5-2 Time lapse-seismic data: six generations of data displayed side by side. (; figure courtesy Lumley, 4th Wave Imaging and Chevron; data courtesy Chevron.)
Figure 11.5-2 shows another application of the 4-D seismic method to steam flooding of a reservoir for thermal recovery. In this field study, six time-lapse 3-D seismic surveys were conducted . The first survey at time T1 was conducted prior to steam injection, while the subsequent surveys were conducted after the steam injection was started. The red, near-circular feature on the time slices correspond to the spatial extent of the injected steam; note how the steam front expands further in the northwesterly direction with increasing calendar time.
- Lumley et al., 1994, Lumley, D. E., Nur, A., Strandenes, S., Dvorkin, J., and Packwood, J., 1994, Seismic monitoring of oil production: A feasibility study: 64th Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, 319–322.
- Lumley, 1995a, Lumley, D. E., 1995a, Seismic time-lapse monitoring of subsurface fluid flow: Ph. D. thesis, Stanford University.
- Lumley, 1995b, Lumley, D. E., 1995b, 4-D seismic monitoring of an active steamflood: 65th Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, 203–206.
- Ecker, 1999, Ecker, C., Lumley, D. E., Tura, A., Kempner, W., Klosnky, L., 1999, Estimating separate steam thickness and temperature maps from 4-D seismic data: An example from San Joaquin Valley, California: 69th Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, 2032–2034.
- MacLeod et al., 1999, MacLeod, M. K., Hanson, R. Bell, R. C., and McHugo, S., 1999, The Alba Field ocean bottom cable seismic survey: Impact on development: The Leading Edge, 1306–1312.