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amplitudes or travel-times between the surveys can reveal the movement of fluid contacts (e.g. where produced oil has been replaced by brine) or the extent of pressure changes that affect reservoir properties.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.
Repeatability issues of 3-D VSP data
Increased repeatability is recognized as one major issue for improving the time-lapse seismic technology as a reservoir management tool. A 3-D vertical seismic profiling (VSP) data set, acquired over a period of two days, is used to analyze how repeatable a permanent installed geophone array can be and how repeatability changes with inaccuracies in source positioning. It is found that for a frequency range between 3.5 and 50 Hz, the difference root-mean-square (rms) level between two recorded traces belonging to two different shots is about 8%. This fact shows that there is a potential for acquiring very accurate time-lapse seismic data by using a permanently installed downhole geophone array. Repeatability variation with increasing shot separation distances is analyzed, showing a rapid decrease in repeatability as the accuracy of the positioning of the repeat survey decreases. Horizontal geophone components show approximately the same degree of repeatability compared to the vertical component, but horizontal geophone data is slightly more sensitive to positioning errors. The results show that repeated 3-D VSP surveys (preferably using permanently installed geophone arrays) might be an efficient tool for detailed and precise monitoring of fluid and pressure changes within a hydrocarbon reservoir.
The Gullfaks 4D seismic study
A time lapse analysis was performed on two 3D seismic datasets (4D seismic) acquired over the northern part of the Gullfaks oilfield in the North Sea. The first survey was acquired in 1985, prior to the production start in 1986, a second survey was acquired in 1995, followed by a third in 1996. The drainage interpretations carried out by means of the repeated seismic datasets are in reasonable agreement with similar interpretations performed the traditional way by reservoir engineers, and have locally resulted in an improved understanding of drainage. The time lapse seismic data are routinely used in well planning projects on Gullfaks. In 1997, two wells were drilled into segments that, based on repeated seismic data, were interpreted to be undrained. Both wells encountered oil-filled reservoirs. Although these wells were not based on time lapse seismic alone, the wells served as a good test of the reservoir management potential associated with the 4D technology. Furthermore, waterfront movements predicted on the basis of time lapse seismic data are confirmed by well observations.