Reservoir monitoring with PSInSAR™

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. At the In Salah field, in the Algerian central Sahara, a successful project of integration between satellite data and 3D seismic has been recently carried out.

Synthetic Aperture Radar Interferometry (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 analyzes these images to detect and measure change in the surface profile, representing deformation in some form. Satellites have been accumulating data since the early 1990s and, as a result, this 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 such as metallic structures, outcrops, or features on buildings, not previously recognized in traditional SAR interferometry.

Figure 1: basics of the PS technique.

The motion behavior 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. 

The In Salah case study

PSInSAR™ has recently proven itself as a powerful tool in tracking the subsurface migration of CO₂ at the In Salah field.  Measurements of well-head samples and 3-D seismic techniques have correlated well with PS data in tracking the CO₂ plume.

The In Salah field is located in the Algerian central Sahara desert and 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 In Salah Carboniferous Sandstone gas producing reservoir (about 20 m thick) via three horizontal wells at a depth of 1,900 metres.

An exploration 3D seismic survey was designed to image prospects in the Carboniferous and Devonian sequences rather than in the overburden where quality is low. However for sequestration monitoring it is fundamental to understand the structure and characteristics of the overlying section.

Time lapse satellite images were collected since start of injection and processed with the PSInSAR™ technique, which has identified uplift around the injectors, while the production area shows an interesting subsidence pattern. The PS analysis has also confirmed the CO₂ is moving in the direction of preferred fracture orientation at reservoir level.

Figure 2: Ground deformation at Krechba field - 3 years of injection.

Integration of the 3D seismic cubes with the satellite image data has revealed some interesting trends and insight into the structures at In Salah which may be controlling the movement of CO₂ in the subsurface. Initial observations indicate that deep seated (below reservoir) faults may control the field structure at the Carboniferous injection level.

Detailed analyses of the satellite images, and their correlation with the seismic cubes, confirms this interpretation of the probable CO₂ movement controlling mechanism.

These data are now being incorporated into detailed overburden models, incorporating discrete fracture networks, for long term evolution modeling.

Source: A. Mathieson et al. in SEG Las Vegas 2008 Annual Meeting.
See also: D. W. Vasco, A. Ferretti, and F. Novali. "Estimating permeability from quasi-static deformation: Temporal variations and arrival-time inversion". GEOPHYSICS, VOL. 73, NO. 6 November-December 2008.

The company

Tele-Rilevamento Europa - T.R.E. (TRE) was established in 2000 as the first spin-off company of the Politecnico di Milano (POLIMI) technical university and has exclusive worldwide rights to the PSInSAR™ technique.

TRE, together with the research group at POLIMI, is now the largest group specialized in InSAR data processing activities worldwide. The company has more than 20 years of research in the field of radar remote sensing and Earth observation satellites, as well as a team of over 30 staff combining skills in Radar Signal Processing, GIS, Information Technology, Engineering Geology and Civil Engineering, all of them dedicated to InSAR.

In 2008 TRE also opened a subsidiary, TRE Canada Inc., in Vancouver, Canada, to service the North American market.

Authors: Eng. Alessandro Ferretti* and P. Eng. Brian Young**

* Tele-Rilevamento Europa T.R.E. Srl
Via Vittoria Colonna, 7
20149 Milano - Italy
T: +39.02.4343121
www.treuropa.com

** TRE Canada Inc.
#550-409 Granville Street
Vancouver, BC V6C 1T2 - Canada
T: +1.604.331.2513
www.trecanada.com