Whether 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 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 field is related to pressure change at depth, a key parameter in reservoir management. The ability to map the subsidence or uplift patterns, using satellite data, powerfully complements ground-based techniques and enables measurement in areas where access is difficult or expensive.
Observation of the spatial patterns of vertical displacement can provide insight into the structural geology of a reservoir by highlighting the location of active faults or fractures. This information can be integrated with other types of data as input to reservoir models and can assist in designing recovery strategies.
This capability is illustrated by examining the behavior of ground movement at the Carbon Capture and Storage project at In Salah, Algeria.
PSInSAR has recently proven itself as a powerful tool in tracking the subsurface migration of CO2. Measurements of well-head samples and 3D seismic techniques have correlated well with PS data in tracking the CO2 plume.
The Krechba field was developed in July 2004 as a part of a joint venture with BP, Sonatrach and Statoil. Here CO2 is re-injected into the water leg of the Krechba Carboniferous Sandstone gas producing reservoir (about 20m thick) via three horizontal wells at a depth of 1900 metres.
Satellite image data acquired since the injection started in August 2004 and processed with the PSInSAR technique has identified uplift around the injectors, while the production area shows an interesting subsidence pattern.
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