that improved accuracy can be achieved by using three coupled gravimeters placed at the sea floor. Tis technical success led to a full field programme at the Troll field, in the North Sea. Another successful field example is a time- lapse gravity monitoring of an aquifer storage recovery project in Leyden, Colorado (Davis et al., 2005), essentially mapping water influx. Obviously, this technique is best suited for reservoirs where significant mass changes are likely to occur, such as water replacing gas. Shallow reservoirs are better suited than deep ones. Te size of the reservoir is a crucial parameter, and a given minimum size is required in order to obtain observable effects. For monitoring volcanic activity, gravimetric measurements might help to distinguish between fluid movements and tectonic activity within an active volcano. Since the breakthrough of active EM (Electromagnetic)
Figure 4.15: Schematic plot showing how 4D refraction analysis can be used to detect shallow gas leakage.
surveys (see Chapter 6) at the beginning of this millennium (Ellingsrud et al., 2002) this technique has mainly been used as an exploration tool, in order to discriminate between hydrocarbon-filled and water-filled rocks. Field tests have shown that such data are indeed repeatable, so there should definitely be a potential for using repeated EM surveys to monitor a producing reservoir. So far, frequencies as low as 0.25 Hz are being used (and even lower), which means that the spatial resolution will be limited. However, as a complementary tool to conventional 4D seismic, 4D EM might be very useful. In many 4D projects it is hard to quantify the amount of saturation changes taking place within the reservoir, and time-lapse EM studies might be used to constrain such quantitative estimates of the saturation changes. Another feature of the EM technique is that it is not very sensitive to pressure changes, so it may be a nice tool for separating between saturation and pressure changes. Tis is in contrast to conventional 4D seismic, since seismic data is sensitive to both pressure and saturation changes. By using the so-called interferometric synthetic aperture
radar principle obtained from orbiting satellites, an impressive accuracy of the distance to a specific location on the earth’s surface can be measured (see Chapter 1.6). By measuring the phase differences for a signal received from the same location for different calendar times, it is possible to measure the relative changes with even higher accuracy. By exploiting a sequence of satellite images, it is possible to monitor height changes versus time. Te satellites used for this purpose are orbiting the earth at a height of approximately 800 km. Several examples of monitoring movements of the surface
above a producing hydrocarbon reservoir have been reported. For reservoir monitoring purposes there are, of course, limitations on how much detailed information such images can provide for reservoir management. Te obvious link to the reservoir is to use geomechanical modelling to tie the
movements at the surface to subsurface movements. For seismic purposes, such a geomechanical approach can be used to obtain improved velocity models in a more sophisticated way; if you need to adjust your geomechanical model to obtain correspondence between observed surface subsidence and reservoir compaction, then this can be used to distinguish between overburden rocks with high and low stiffness for instance, which again can be translated into macro-variations of the overburden velocities. Te deeper the reservoir is, the less frequent will be the surface imprint of the reservoir changes, and hence this method cannot be used directly to identify small pockets of undrained hydrocarbons. However, it can be used as a complementary tool for time-lapse seismic since it can provide valuable information on the low frequency spatial signal of reservoir compaction. Seismic refraction methods are among the oldest methods
in seismic exploration. Te term refraction is not unique or precise. According to Sheriff (2002) it has two meanings, first to change direction (following Snell’s law) and second to involve head waves. Here we will use a relaxed interpretation, which means that the term includes head waves, diving waves and reflections close to critical angle. An attractive application of time-lapse refraction is to detect changes in shallow sediments. One such example is sketched in Figure 4.15, where an underground blowout (the same as shown in figures 4.2 and 4.3) causes gas to be trapped in a thin sand layer. In this case, the most robust 4D refraction attribute is the time shift. Tis will be further discussed in Section 4.3.
4.1.8 CO2 Monitoring Interest in CO2 injection, both for storage and as a tertiary
recovery method for increased hydrocarbon production, has grown significantly over the last decade. Statoil has stored approximately 0.9 million tons of CO2
- per year since 1997 in
the Utsira Formation at the Sleipner field, and several similar projects are now being launched worldwide. A key focus has been to develop geophysical methods to monitor the CO2
injection process, and particularly to try to quantify directly from geophysical data the volume injected. One way to improve our understanding of how the CO2
flows in a porous rock is to perform small-scale flooding experiments on long core samples. An example of such a flooding experiment and corresponding X-ray images for various flooding patterns is shown in Figure 4.16. By
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