Pietro Teatini

Importance of poroelastic coupling in dynamically active aquifers of the Po river basin, Italy.

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.

Ill-conditioning of finite element poroelasticity equations

Abstract The solution to Biots coupled consolidation theory is usually addressed by the finite element (FE) method thus obtaining a system of first-order differential equations which is integrated by the use of an appropriate time marching scheme. For small values of the time step the resulting linear system may be severely ill-conditioned and hence the solution can prove quite difficult to achieve. Under such conditions efficient and robust projection solvers based on Krylovs subspaces which are usually recommended for non-symmetric large size problems can exhibit a very slow convergence rate or even fail. The present paper investigates the correlation between the ill-conditioning of FE poroelasticity equations and the time integration step Δt. An empirical relation is provided for a lower bound Δtcrit of Δt below which ill-conditioning may suddenly occur. The critical time step is larger for soft and low permeable porous media discretized on coarser grids. A limiting value for the rock stiffness is found such that for stiffer systems there is no ill-conditioning irrespective of Δt however small, as is also shown by several numerical examples. Finally, the definition of a different Δtcrit as suggested by other authors is reviewed and discussed.

Natural versus anthropogenic subsidence of Venice

We detected land displacements of Venice by Persistent Scatterer Interferometry using ERS and ENVISAT C-band and TerraSAR-X and COSMO-SkyMed X-band acquisitions over the periods 1992–2010 and 2008–2011, respectively. By reason of the larger observation period, the C-band sensors was used to quantify the long-term movements, i.e. the subsidence component primarily ascribed to natural processes. The high resolution X-band satellites reveal a high effectiveness to monitor short-time movements as those induced by human activities. Interpolation of the two datasets and removal of the C-band from the X-band map allows discriminating between the natural and anthropogenic components of the subsidence. A certain variability characterizes the natural subsidence (0.9 ± 0.7 mm/yr), mainly because of the heterogeneous nature and age of the lagoon subsoil. The 2008 displacements show that man interventions are responsible for movements ranging from −10 to 2 mm/yr. These displacements are generally local and distributed along the margins of the city islands.

Coastline regression of the Romagna Region, Italy, due to natural and anthropogenic land subsidence and sea level rise

The Romagna coastal area in the Northern Adriatic Sea has experienced in recent times continuous changes because of its precarious environment and low ground elevation above mean sea level (msl). Major processes that may influence the stability of the coast profile include land subsidence of both natural and anthropogenic origin and the msl rise caused by global climate change. According to the most accredited modeling predictions msl is expected to rise by almost 0.5 m over the next century because of the greenhouse effect. Natural land subsidence is the result of deep downward tectonic movement and consolidation of geologically recent deposits. It may be estimated in the range of 2–2.5 mm/yr in the Ravenna area and twice as much in the Po River delta. Anthropogenic land subsidence is primarily related to groundwater pumping from the upper fresh water aquifer system and gas production from Plio-Pleistocene reservoirs. Geodetic surveys from 1953 to 1990 provide documentary evidence of cumulative land settlement exceeding 0.8 m and 1.2 m at Marina di Ravenna and Cesenatico, respectively. In this study we estimate both natural and anthropogenic land subsidence for the years 2015, 2050, and 2100 with the aid of ad hoc finite element simulation models. The use of these predictions together with the expected msl rise shows that many present lowlands may be permanently submerged at the end of the next century. The extent of the flooded area of the Romagna coastal region can be as much as 690 and 910 km2, using optimistic and pessimistic land subsidence scenarios, respectively. A local detailed analysis indicates that the areas around the cities of Ravenna and Cesenatico may be seriously affected by sea water ingression while the city of Rimini is well protected because of its relatively high elevation above msl.

GIS simulations of the inundation risk in the coastal lowlands of the Northern Adriatic Sea

The Northern Adriatic Coastland, between the cities of Monfalcone and Cattolica, is characterized by locations of great tourist interest, such as the Venice Lagoon and the Romagna Riviera, and areas with a very precarious environmental setting, such as the Valli di Comacchio and the Po River Delta. Therefore, the coastal management and the design of new defence works of the littoral have to be made with the utmost care, possibly with the aid of numerical predictions of the coastal morphodynamics and the flood risk analysis of the lowland involved. In the study area, land may subside due to sediment natural compaction and subsurface fluid (water and gas) withdrawal. At the same time, littoral transport of solid material can contribute appreciably to change the shore morphology. Mean sea level may rise permanently due to global climate change (eustatism) and occasionally due to tides and intensive storm events. The predictions of each individual process is obtained using various ad hoc mathematical models and the outcome of the numerical simulations are managed with a GIS (geographical information system). Coastline evolution until the year 2100 is investigated and risk factor maps of the low-lying coastal areas are generated which account for the hazard of the expected event, and the land economic value and vulnerability.

Peat land oxidation enhances subsidence in the Venice watershed

The southernmost part of the Venice Lagoon catchment was progressively reclaimed from marshland starting from the end of the 19th century and finishing in the late 1930s (Figure 1). As a major result, the area was turned into a fertile farmland. At present, the area is kept dry by a distributed drainage system that collects the water from a capillary network of ditches, and pumps it into the lagoon or the sea. By its very origin, this area lies below sea level and progressively sinks mainly because of bio-oxidation of the histosols (soils with high organic content) that represent a large fraction of the outcropping soil in the area. The bio-oxidation process occurs in close connection with the agricultural practices and is currently responsible for a subsidence rate of between 1.5 and 2 cm/yr.

Simultaneous Monitoring of Soil Water Content and Salinity with a Low-Cost Capacitance-Resistance Probe

Capacitance and resistivity sensors can be used to continuously monitor soil volumetric water content (θ) and pore-water electrical conductivity (ECp) with non-destructive methods. However, dielectric readings of capacitance sensors operating at low frequencies are normally biased by high soil electrical conductivity. A procedure to calibrate capacitance-resistance probes in saline conditions was implemented in contrasting soils. A low-cost capacitance-resistance probe (ECH2O-5TE, 70 MHz, Decagon Devices, Pullman, WA, USA) was used in five soils at four water contents (i.e., from dry conditions to saturation) and four salinity levels of the wetting solution (0, 5, 10, and 15 dS·m−1). θ was accurately predicted as a function of the dielectric constant, apparent electrical conductivity (ECa), texture and organic carbon content, even in high salinity conditions. Four models to estimate pore-water electrical conductivity were tested and a set of empirical predicting functions were identified to estimate the model parameters based on easily available soil properties (e.g., texture, soil organic matter). The four models were reformulated to estimate ECp as a function of ECa, dielectric readings, and soil characteristics, improving their performances with respect to the original model formulation. Low-cost capacitance-resistance probes, if properly calibrated, can be effectively used to monitor water and solute dynamics in saline soils.

Stress–strain analysis in productive gas/oil reservoirs

A numerical study of the stress–strain distribution in a thin disc-shaped reservoir embedded in a poro-elastic half-space and subject to a unit pore pressure decline is presented. The results are then compared with those of a geometrically equivalent porous cylindrical body which is either free to or prevented from expanding laterally (oedometric analogy). The analysis is based on the linear theory of poro-elasticity solved with the aid of the finite element method. The strength source is provided by the pressure gradient generated in a small region surrounding the gas/oil field where pore pressure dissipates. The influence of the burial depth c is also investigated. The results show that the reservoir rock undergoes a vertical compaction δ which is independent of c and very close to the compaction of the equivalent confined cylinder. The confinement factor is also similar. The horizontal displacement is, however, much larger. Its maximum value occurs at the boundary of the field and is of the same order of magnitude as δ. In addition, at the outer reservoir margin shear stresses develop which are totally missing in both the free and the constrained cylinders. It is shown that the vertical displacements of reservoir top and bottom, as well as the radial ones, are sensitive to c, especially in shallow formations. Finally, the largest shear stress is found to be related to the magnitude of the pressure gradient, i.e. to the radial size of the neighbouring volume where pore pressure vanishes. Copyright

Numerical Analysis of Land Subsidence due to Natural Compaction of the Upper Adriatic Sea Basin

We have simulated the evolution of the accreting Quaternary column at three strategic locations (Venice, Po river delta, Ravenna) along the Upper Adriatic coastline. The analysis is performed by a 1-D nonlinear finite element model of soil compaction driven by groundwater flow in an isothermal sedimentary basin subject to a continuous vertical sedimentation process. The constitutive relationship of the most important parameter controlling the event, i.e. the soil compressibility vs the effective intergranular stress, has been derived from a number of oedometer tests carried out on samples taken from shallow and deep exploratory boreholes scattered through the study area. Dating of the sediments has allowed for the estimate of an average depositional rate at selected time intervals spanning the Middle-Upper Pleistocene, the Holocene and the historical times. The model has been run so as to reproduce the present day sediment thickness with a small excess hydrostatic pressure since the Adriatic Sea basin in known to be normally pressured. It is assumed that sedimentation in the last 0.5 My was substantially in equilibrium with subsidence so that the deposition surface remained at a constant elevation. The results of our analysis indicate a natural land settlement in the next century between 0.5 mm/y at Venice and 4÷5 mm/y in the Po delta, with an intermediate value of 2÷2.5 mm/y at Ravenna.

Residual land subsidence near abandoned gas fields raises concern over northern Adriatic coastland

Many gas fields have been detected in the Northern Adriatic basin in the last 50 years. Gas production began in the early 1950s, and today some of the reservoirs are depleted. Other fields are currently under production or awaiting development in the near future (Figure 1a). One major environmental consequence of withdrawing gas from the ground is land subsidence. This can be a matter of great concern if the field is located below or close to low-lying and densely urbanized coastal areas. A ground elevation loss of only a few centimeters in these areas can enhance the ingress of sea water inland and expose the shore to flooding during high tides and severe storm events (Figure 2a).