3-D Seismic Interpretation: Time-lapse seismic
We saw in chapter 5 how seismic data can be used in favourable cases to infer the nature of fluid fill (gas, oil or brine) in a reservoir. An application of this is to follow the way that fluids move through a reservoir during production, by carrying out a baseline seismic survey before production begins and then repeat surveys over the production lifetime. Where 3-D surveys are repeated in this way, they are often referred to as 4-D seismic , with the idea that time is the extra dimension over standard 3-D. Differences in seismic amplitudes or travel-times between the surveys can reveal the movement of fluid contacts (e.g. where produced oil has been replaced by brine) or the extent of pressure changes that affect reservoir properties. As sketched in figs. 8.1 and 8.2, it is not always straightforward to separate out the causes of the changes. At a producing well, the pressure drops, and if it drops far enough then gas will come out of solution. This will result in a decrease in both P velocity and in density, resulting in a drop in acoustic impedance. On the other hand, the drop in pore pressure causes an increase in P velocity and density. At a water injector (fig. 8.2), we replace oil by water, which in itself would increase P velocity and density; however, the injection also increases the pore pressure, which causes the reverse effect on P velocity and density.
Feasibility of time-lapse seismic reservoir monitoring; the physical basis
As reservoir fluids are produced or fluids are injected during enhanced oil recovery, the reservoir’s seismic properties may change. If these changes are large enough, repetitive surveys can map fluid distributions, pressure/fluid fronts, and temperature changes which means, at least in theory, that we can perform time‐lapse seismic reservoir monitoring (herein called TLSRM instead of the popular, but inaccurate, term 4-D seismology).
Time‐lapse seismic monitoring: Some shortcomings in nonuniform processing
Conceptually, repeat or time‐lapse seismic monitoring is straightforward; seismic data sets acquired at different times are calibrated, compared, and the intersurvey differences analyzed and interpreted in terms of fluid changes or movement.