Feasibility Study for Geophysical Monitoring Renewable Gas Energy Compressed in Pore Storages

Most renewable energy sources are intermittent and need buffer storage (e.g., compressed air energy storage, CAES) to bridge the time-gap between power supply and demand peaks. Replacing pore brine with CAES causes changes in electro-elastic properties and density, and justifies applications of multi-geophysical approach. In this numerical study we apply techniques of the elastic full waveform inversion (FWI), electric resistivity tomography (ERT), transient electromagnetic induction (TEM) and gravity to detect and monitor CAES in deep reservoirs and possible leakages in shallow groundwater aquifers of North Germany. For different subsurface model scenarios of CAES reservoirs and leakages, synthetic data sets are generated and inverted using constraints on the initial model. Results reveal principally the capability of our applied approach to resolve the CAES plume in deep saline reservoirs and shallow groundwater aquifers. The ERT resolution for leakages is highly enhanced for the combined surface-borehole survey compared to the individual surface and borehole surveys. The applied gravity technique is highly sensitive to the mass deficit caused by CAES plume. The detect ability limit of the technique is determined by the least CAES volume causing an anomaly with amplitude just above the accuracy range of modern micro-gravimeters. The FWI technique can map the shallow CAES leakage by anomalies in the reconstructed ΔVp, ΔVs and Δdb tomograms within the background aquifer. However, these tomograms contain inversion artifacts and smearing effects related mainly to the dominance of the Rayleigh wave in the data. Obviously, applied multi-techniques complement and confirm each other. CAES plumes cause strong mass deficits and moderate resistivity highs and thus are more sensitive for gravity and FWI methods. Applying constrained inversion minimizes interpretation ambiguities and helps recovering almost realistic electro-elastic parameters that can be applied in adequate petrophysical equations to quantify CAES saturations.