54 PRODUCTION/PROCESSING/HANDLING
Monitor surface deformation with Earth-orbiting satellites
Alessandro Ferretti and Brian Young look at how the permanent scatterer technique is being used to measure surface deformation.
W hether from primary or enhanced oil
recovery techniques, or from CO2 sequestration, surface deformation arising from injection or production
practices can become an issue for reservoir engineers. Apart from the environmental impact of subsidence and uplift phenomena, recent optimisation techniques ask for timely information about many geophysical
parameters, both downhole and on the surface. Surface deformation measurements are lately gaining increasing attention within the reservoir engineer community, which is searching for new monitoring tools to complement seismic surveys. These monitoring technologies are relatively low in cost and their information content adds significant value, if properly interpreted and integrated with more conventional data. One such technology – InSAR – can provide high-quality, remotely sensed data about surface deformation.
InSAR uses radar signals to generate images of
the earth’s surface. Sensors mounted on satellites circumnavigate the globe on a N-S orbit. These sensors broadcast signals toward the earth, some of which are scattered back to the satellite. The returned signals are integrated to form radar scenes reflecting the surface profile of the earth. Specialist software analyses these images to detect and measure change in the surface profile, such changes representing deformation in some form. Satellites have been accumulating data since the early 1990s and, as a result, this technology is the only technology that is able to provide an history of movement, retroactively. Of course, it is possible to look forward to monitor reservoir behaviour into the future. The use of radar sensors mounted on
board Earth-orbiting satellites started about two decades ago. Over time, these early algorithms have been significantly upgraded and are much more powerful, today.
One of these algorithms – the Fig. 1. The basics of the permanent scatterer technique.
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Permanent Scatterer (PS) technique (PSInSAR), developed by the Politecnico di Milano, in 1999, is able to determine mm-scale displacements of features on the ground surface. Using the PS technique, it is possible to resolve surface motion of ~0.5 mm/yr on small-scale objects on the landscape, including individual targets/ metallic structures, outcrops, or features on buildings, not previously recognised in traditional SAR interferometry. The motion behaviour of these objects, when situated in an oil field, reflects the surface deformation impacts of reservoir operations. For instance, deformation of the material overlying a CO2 injection
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