[1] Underground gas storage (UGS) in depleted hydrocarbon reservoirs is a strategic practice to cope with the growing energy demand and occurs in many places in Europe and North America. In response to summer gas injection and winter gas withdrawal the reservoir expands and contracts essentially elastically as a major consequence of the fluid (gas and water) pore pressure fluctuations. Depending on a number of factors, including the reservoir burial depth, the difference between the largest and the smallest gas pore pressure, and the geomechanical properties of the injected formation and the overburden, the porous medium overlying the reservoir is subject to three-dimensional deformation with the related cyclic motion of the land surface being both vertical and horizontal. We present a methodology to evaluate the environmental impact of underground gas storage and sequestration from the geomechanical perspective, particularly in relation to the ground surface displacements. Long-term records of injected and removed gas volume and fluid pore pressure in the "Lombardia" gas field, northern Italy, are available together with multiyear detection of vertical and horizontal west-east displacement of the land surface above the reservoir by an advanced permanent scatterer interferometric synthetic aperture radar (PSInSAR) analysis. These data have been used to calibrate a 3-D fluid-dynamic model and develop a 3-D transversally isotropic geomechanical model. The latter has been successfully implemented and used to reproduce the vertical and horizontal cyclic displacements, on the range of 8-10 mm and 6-8 mm, respectively, measured between 2003 and 2007 above the reservoir where a UGS program has been underway by Stogit-Eni S.p.A. since 1986 following a 5 year field production life. Because of the great economical interest to increase the working gas volume as much as possible, the model addresses two UGS scenarios where the gas pore overpressure is pushed from the current 103%p i , where p i is the gas pore pressure prior to the field development, to 107%p i and 120%p i . Results of both scenarios show that there is a negligible impact on the ground surface, with deformation gradients that remain well below the most restrictive admissible limits for the civil structures and infrastructures. Citation: Teatini, P., et al. (2011), Geomechanical response to seasonal gas storage in depleted reservoirs: A case study in the Po River basin, Italy,

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[1] The Emilia-Romagna coastland south of the Po River delta, Italy, has experienced a dramatic land settlement mainly due to the large groundwater withdrawal related to the local economic and tourist development started in the early 1950s. Although the use of surface water has reduced the settlement rate over the last three decades, anthropogenic land subsidence still continues in a few kilometer wide coastal strip at a rate larger than the natural one. The occurrence is reconstructed since 1946 with the aid of advanced finite element flow and poromechanical models implemented with a realistically detailed geology of the regional shallow multiaquifer system. The models have been calibrated against the piezometric, leveling, and extensometer records observed over the last 50 years, and a land subsidence prediction in 2016 is performed. The results show that the extensive groundwater pumping that occurred in the past is most likely the main cause of the recent land settlement as well because of the delayed compaction of the clay aquitards comprised between the depleted aquifers. However, the available pumping data do not allow for a thorough understanding of the current local settlement process along the coastline, which is the most vulnerable area of the Emilia-Romagna region from an environmental viewpoint. If the planned scenario of groundwater resource management will be implemented, anthropogenic land subsidence is bound to become a marginal problem for the central and northern portion of the Emilia-Romagna coastland.Citation: Teatini, P., M. Ferronato, G. Gambolati, and M. Gonella (2006), Groundwater pumping and land subsidence in the EmiliaRomagna coastland, Italy: Modeling the past occurrence and the future trend, Water Resour.

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[1] Deltas are highly dynamic coastal systems that over the last few decades have generally experienced a substantial area loss caused by trapping of river sediments in upland drainage basins as well as land subsidence due to natural and anthropogenic causes. A major example is the Po Delta in the Mediterranean in northeastern Italy. This area has experienced as much as 3 m of land subsidence from the 1930s to the 1970s primarily because of the extraction of gas-bearing waters. However, present subsidence rates are largely unknown and the ground settlement is supposedly controlled by natural long-term deep processes. We have combined radar Interferometric Point Target Analysis (IPTA) with previous geomorphological investigations on aerial/satellite images and seismic surveys, and geochronological data from core samples and geomechanical in situ tests, to assess the current sinking of the delta and to understand the processes controlling the vertical movement. The high density of the measurable point targets (more than 15,000) allows characterization of the spatial variation in the vertical land motions (VLM), ranging from −1 to −15 mm/yr. We find that subsidence rates are significantly correlated with the age of highly compressible Holocene deposits that compose the shallowest 30-40 m of the sedimentary sequence. A typical log-type consolidation equation applicable at the scale of the entire delta has been obtained. We conclude that the consolidation of late Holocene sediments is the major cause of the present land subsidence in the Po River delta. This finding has significant impact on the understanding of many other modern deltas that were formed in the lower Holocene epoch.Citation: Teatini, P., L. Tosi, and T. Strozzi (2011), Quantitative evidence that compaction of Holocene sediments drives the present land subsidence of the Po Delta, Italy,

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