Detecting remaining gas reserves based on coupled modelling of subsidence and reservoir depletion processes

Depletion of large volumes of natural gas, oil and water from hydrocarbon reservoirs may lead to movements of the Earths surface (tilt or subsidence). Observations of these movements (Interferometric Synthetic Aperture Radar (InSAR), leveling measurements) can provide a better insight on subsurface processes like reservoir compaction or the aquifer strength. A simple inversion approach does not usually provide a sensible solution due to the non-uniqueness of the inverse problem and the sensitivity of the inverse problem to small fluctuations in the data. This necessitates the use of all available prior information (geologic model, reservoir model) in the inversion procedure to better constrain the subsurface parameters. A time-dependent inversion scheme is adopted for resolving the spatial and temporal reservoir pressure drop from the surface subsidence observations. The innovative inversion method is effectively applied to one synthetic and two field cases (one from The Netherlands, one from North Africa). The results suggest that incremental gas reserves due to reservoir compartmentalization can be detected successfully.