[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,

[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.

No abstract

No abstract

Abstract. Uncoupling between the flow field and the stress field in pumped aquifers is the basis of the classical groundwater hydrology. Recently, some authors have disputed the assumption of uncoupling with regard to both fluid dynamics and porous medium deformation. The issue is very important as it could undermine the traditional approach to simulate subsurface flow, analyze pumping tests, and predict land subsidence caused by fluid withdrawal. The present paper addresses the problem of coupling versus uncoupling in the Po river plain, a normally consolidated and normally pressurized basin which has experienced in the last 50 years a pronounced pore pressure drawdown because of water and gas removal and where a large hydromechanical database is available from the ground surface down to 4000 m depth. A numerical study is performed which shows that the matrix which relates flow to stress is very similar to the capacity matrix of the uncoupled flow equation. A comparison of results obtained with the finite element integration of the coupled and uncoupled models indicates that pore pressure is rather insensitive to coupling anywhere within the pumped formation while in the adjacent aquitard-aquifer units, coupling induces a slight overpressure which quickly dissipates in time with a small initial influence on medium deformation, and specifically on land subsidence. As a major consequence the uncoupled solutions to the fluid dynamic and the structural problems appear to be fully warranted on any timescale of practical interest in a typical normally consolidated and pressurized basin. IntroductionWhen an aquifer, an oil/gas reservoir, or a confining bed experiences a change of the internal flow and stress fields (typically due to fluid withdrawal), the incremental effective stresses and the fluid dynamic gradients that develop within the porous medium are intimately connected. This complex interrelation was first mathematically described by Biot [1941]. A model of flow and stress based on the Biot equations is said to be a coupled model.Groundwater hydrologists and petroleum engineers who are mostly concerned with the fluid dynamic aspects of the coupled process have developed the uncoupled flow theory, whose most widespread and used equation, the so-called diffusion equation, was originally derived by Theis [1935] more than 60 years ago. This equation incorporates the rock structural behavior into a lumped mechanical parameter (the elastic storage coefficient) and is solved separately and independently for the pore pressure p. Once p is obtained, it may be used as an external source of strength in a poroelastic model of the porous system to provide the medium deformation, typically, land subsidence, i.e., the vertical displacement at the surface boundary. This is the uncoupled, or two-step, approach followed by many authors to simulate and predict land settlement due to

We introduce a class of block preconditioners for accelerating the iterative solution of coupled poromechanics equations based on a three-field formulation. The use of a displacement/velocity/pressure mixed finite-element method combined with a first order backward difference formula for the approximation of time derivatives produces a sequence of linear systems with a 3 ×3unsymmetric and indefinite block matrix. The preconditioners are obtained by approximating the two-level Schur complement with the aid of physically-based arguments that can be also generalized in a purely algebraic approach. A theoretical and experimental analysis is presented that provides evidence of the robustness, efficiency and scalability of the proposed algorithm. The performance is also assessed for a real-world challenging consolidation experiment of a shallow formation

[1] The frequency of flooding in Venice has drastically increased over the last 50 years as a major consequence of natural and anthropogenic land subsidence, mean sea level rise, and a more active lagoon hydrodynamics induced partly by deepening of the largest navigation channels. Subsurface fluid injection is a well-established technology that is currently used either to enhance oil recovery from oil fields or to reduce land settlement due to hydrocarbon production. To help mitigate the inundation events in Venice, a numerical study of seawater injection into a 600-800 m deep geologic formation is performed with the aid of advanced numerical fluid dynamic and geomechanical models. A number of parametric scenarios are addressed, consistent with the basic geological configuration derived from the lithostratigraphy of nearby areas in the northern Adriatic basin. Preliminary quite encouraging results show that a set of 12 vertical injection wells, strategically located within the lagoon, may raise Venice from 11 to up to 40 cm over a 10 year period, thus offsetting or mitigating the vast majority of the high tides that occasionally plague the city. Further ad hoc geological and geophysical investigations of the lagoon subsurface are required before the present prefeasibility study can be turned into a design project of anthropogenic Venice uplift.

No abstract