MONITORING OF CHANNEL RESERVOIRS THROUGH INSAR MEASUREMENTS

Channel sandstones have a particularly important place among sandstone reservoirs. Their exploitation is especially challenging because of the impact of reservoir heterogeneities, channel connectivity and well position. Understanding which channels are drained or swept during water flooding would greatly improve the possibility to increase oil recovery. Although ground movements induced by pressure sinks generated in the reservoir by production are generally very limited, surface deformation is known to reflect the pressure distribution in the reservoir. Thus, deformation monitoring of onshore reservoirs can provide useful information about which portions of the reservoir are actually being produced or where pressure is supported by water encroachment, or by water injection in the case of oil reservoirs. Standard ground surface monitoring techniques (such as: levelling campaigns, tiltmeters or GPS) are usually time-consuming in providing accurate measures for a limited set of benchmarks. However, the Interferometric Synthetic Aperture Radar (InSAR) has proven to be a cost-effective tool to measure surface displacement with millimetric precision over wide areas. The possibility to measure surface effects due to reservoir exploitation depends chiefly on the depth and extension of the reservoir, magnitude of pressure depletion within the reservoir, formation heterogeneity and geomechanical properties. This paper presents the results of a study aimed at assessing the effects of these parameters on the surface deformation pattern and magnitude for fluvial reservoirs. Since meandering and braided rivers often generate poorly connected reservoir sand bodies of different size and sinuosity, the approach taken was to build a number of geological models representing fluvial depositional environments at different depths. Then production and water flooding in alternating channels was simulated. Eventually, three-dimensional geomechanical models were developed for each reservoir so as to calculate the vertical and horizontal (longitudinal and transversal) components of the displacements. In order to assess whether these displacements could be detected via InSAR, the three different components of displacement were combined to calculate the Line Of Sight (LOS), to simulate the corresponding satellite displacement measurement; the latter was then compared with the sensitivity of the InSAR technique. Results show that, depending on reservoir depth, extension and pressure decline, a correspondence between the distribution of sand bodies within the reservoir and the surface displacement pattern can be established. Conversely, for the typology of reservoir under analysis, geomechanical properties turn out to play a minor role in the clear detection of surface deformations.