[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.
Land subsidence and uplift, ground ruptures, and induced seismicity are the principal geomechanic effects of groundwater withdrawal and injection. The major environmental consequence of groundwater pumping is anthropogenic land subsidence. The first observation concerning land settlement linked to subsurface processes was made in 1926 by the American geologists Pratt and Johnson, who wrote that ''the cause of subsidence is to be found in the extensive extraction of fluid from beneath the affected area.'' Since then, impressive progress has been made in terms of: (a) recognizing the basic hydrologic and geomechanic principles underlying the occurrence; (b) measuring aquifer compaction and ground displacements, both vertical and horizontal; (c) modeling and predicting the past and future event; and (d) mitigating environmental impact through aquifer recharge and/or surface water injection. The first milestone in the theory of pumped aquifer consolidation was reached in 1923 by Terzaghi, who introduced the principle of ''effective intergranular stress.'' In the early 1970s, the emerging computer technology facilitated development of the first mathematical model of the subsidence of Venice, made by Gambolati and Freeze. Since then, the comprehension, measuring, and simulation of the occurrence have improved dramatically. More challenging today are the issues of ground ruptures and induced/ triggered seismicity, which call for a shift from the classical continuum approach to discontinuous mechanics. Although well known for decades, anthropogenic land subsidence is still threatening large urban centers and deltaic areas worldwide, such as Bangkok, Jakarta, and Mexico City, at rates in the order of 10 cm/yr.
Land subsidence at Ravenna is the result of aquitard and reservoir compaction caused, respectively, by extensive groundwater withdrawals from the unconsolidated Quaternary basin and gas production from a number of pre‐Quaternary pools scattered over the area. Water pumpage paralleled the postwar industrial development of Ravenna until the middle seventies when consumption was drastically curtailed owing to the economic crisis and the activation of a new aqueduct. Gas production started in 1952. The exploitation of several reservoirs is currently under way and the search for new fields is still in progress. Geodetic records indicate that the maximum cumulative subsidence over the period 1950–1986, including a natural geologic settlement of perhaps 2 mm/yr, has been 1.30 m in the industrial zone of Ravenna. In 1980 the municipality promoted a reconnaissance study with the primary aim of providing the information base needed to reconstruct the actual occurrence, understand correctly the physical behavior and produce the essential input data to a mathematical model which realistically relates the subsidence of the city to groundwater withdrawal and gas removal with an emphasis on their respective influences. The results from the three‐dimensional numerical simulations, performed with the aid of mixed finite element, finite difference and integral models, show that the primary responsibility for the regional land sinking should be placed on the subsurface water overdraft which occurred until the middle 1970s. Gas withdrawal plays a role restricted to the area overlying each reservoir with a magnitude depending on the depth of burial, thickness of mineralized rocks and overall volumetric production. A major environmental impact may be expected where the gas subsidence bowl is intersected by the Adriatic coastline.
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
A review of land subsidence theory and an outline of the justification for our choice of a model to simulate the subsidence at Venice are provided. The review is essentially a search for a mathematical model that can be used to link realistically the occurrence of land subsidence to the groundwater withdrawals that are its cause. The Biot system of equations for three-dimensional consolidation offers the best approach at the theoretical level, but the large number of required parameters precludes application of the approach in practice. Approaches based on the independent solution of the diffusion equation provide a practical alternative, as long as the conditions underlying the development of the equation are recognized, and, if necessary, monitored. At Venice, we have chosen a two-step procedure to analyze the subsidence in the complex aquifer-aquitard system that exists there. First, the regional hydrauhc head drawdowns are calculated •n a two-d•mens•onal vertical cross section in radial coordinates, using an idealized 10-layer representation of the geology. The computations are carried out with a model based on the diffusion equation and solved with a numerical finite element technique. The calculated head values in the aquifers are then used as time dependent boundary conditions in a set of one-dimensional vertical consolidation models solved with a• finite difference technique and applied to a more iefined representation of each aquitard. This approach appears to offer the best trade off between theoretical elegance, data availability, and computer limitation. Its main disadvantage lies in the limitations imposed by the requirements of radial symmetry. In recent years, worldwide attention has been directed toward Venice, where a group of environmental problems threatens the existence of this historic and loved city. The problems are threefold: (1) periodic flooding of the city caused by high tides and the response of the Venetian lagoon to seiches in the Adriatic Sea, (2) subsidence under the influence of heavy groundwater withdrawals in the nearby industrial centers, and (3) air pollution of industrial origin. In this study we present a mathematical analysis of the second of these problems: land subsidence in the viciniW of the Venetian lagoon. The results are presented in two related papers. In this first paper we provide • review of the available theory and an outline of the justification for our choice of a specific model. place for the first time the recent developments in groundwater flow theory and the theory of consolidation as they apply to the problem of land subsidence. Our primary research aim is to use a model that realistically links the groundwater withdrawals (and the resulting piezometric declines) to the vertical consolidation causing subsidence.In 'the second paper we will present some detailed simulations for the Venice case, outlining the somewhat sparse data base that provides the input to our model (geologic configuration, physical properties of the formations, withdrawal rates), and ...
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