Andrea Comerlati

Can CO2 help save Venice from the Sea

On 14 May this year, Italian Prime Minister Silvio Berlusconi cut the ribbon on a multi-billion-dollar project named MOSE that is aimed at solving the problem of “acqua alta,” the increasingly frequent floods that jeopardize the survival of Venice. Cost is estimated (a few say conservatively) at 3 billion euros and construction time (a few say optimistically) at 8 years. MOSE involves building mobile barriers at the Venice Lagoon inlets to prevent severe Adriatic Sea storms from flooding the city. Although the Italian government and the local administrations have given their final approval, MOSE still has several opponents who believe it will cause severe threats to the lagoon ecosystem, and will soon become obsolete because of the expected sea level rise due to global warming.

A comparison of projective and direct solvers for finite elements in elastostatics

The Finite Element Method (FEM) is widely used in civil and mechanical engineering to simulate the behavior of complex structures and, more specifically, to predict stress and deformation fields of structural parts or mechanical bodies. In the former case, the coupling between different types of elements, such as beams, trusses, and shells, is often required, while in the latter fully 3D discretizations are typically used. For both, FEM leads to symmetric positive definite (SPD) matrices that, depending on the type of discretization and especially on the topology of the nodal connections, may be efficiently solved by either the Preconditioned Conjugate Gradient (PCG) or a direct solver such as the routine MA57 of the Harwell Software Library. Numerical experiments are shown and discussed where the effect of spatial discretization, different solution techniques, and a possible nodal reordering, is explored. The PCG preconditioner used is a variant of the incomplete Cholesky factorization with variable fill-in. It is shown that for structures with 1D or 2D connections, such as for example a bridge, MA57 performs usually better than PCG. In this case it is noted that some reorderings specifically designed and implemented for direct elimination methods can be very helpful for PCG as well as they yield a cheaper preconditioner and lead to a much faster PCG convergence. The main disadvantage is the need for an appropriate degree of fill-in for the preconditioner which turns out to be problem dependent and must be found empirically. However, in fully 3D problems, arising for example from the FE discretization of structural components or geomechanical structures, PCG outperforms MA57 while also requiring much less memory, and thus allowing for the use of much refined grids, if needed. With the aid of a large geomechanical problem it is shown that direct solvers may not be (even) used on serial computers due to their prohibitive computational cost with PCG the only viable alternative solver.

A preliminary numerical model of CO2 sequestration in a normally consolidated sedimentary basin

A widespread concern among the scientific community is the increase of the greenhouse gases, especially CO 2 , which may yield an increase of earths temperature. To reduce the CO 2 emission into the atmosphere an option which is attracting a growing attention is the anthropogenic CO 2 sequestration in deep geologic formations. Numerical models help much in the design of the injection system, management and control of the operations, and efficiency of the confinement. One major process to be addressed is the dynamical simulation of the two phase flow CO 2 -groundwater if CO 2 is sequestered in deep saline aquifers. Codes based on finite elements are developed to predict a fundamental process underlying the CO 2 subsurface confinement, i.e. two-phase flow with dissolution of CO 2 in the liquid phase. Preliminary simulations of the injection from a point sink into a deep aquifer located at about 1000 m depth in the normally consolidated sedimentary Northern Adriatic basin shows the relative importance on the amount of the sequestered CO 2 of such factors as the actual nature of the gas (accounted for by the CO 2 supercompressibility), the formation anisotropy, the injection pressure, the aquifer elastic storage and the CO 2 solubility in the groundwater. It turns also out the need for the accurate representation of the dependence of the degree of saturation and the hydraulic conductivity on the capillary pressure.

Stochastic layering effect on the upward migration of anthropogenic CO2 sequestered in saline aquifers.

A widespread concern among the scientific community is the increase of the greenhouse gas emissions, especially CO2, which may yield an increase of earths temperature. To reduce the CO2 released in the atmosphere an option which is attracting a growing attention is its sequestration in deep geologic formations. Deep saline aquifers may offer a great potential for the storage of large volumes of carbon dioxide. However, because of the relatively small practical experience with CO2 disposal in brine formations, more research is needed to assess the feasibility of the operation. In particular more studies are needed to identify aquifers sealed by continuous clay caprock over a regional scale. For this reason it is important to study the heterogeneous character of the sealing layers, i.e., the probability of having small fractures or zones of high permeability in the aquitards. In this paper we try to evaluate the effects of these potential heterogeneities on a CO2 sequestration problem. Aquitard properties are generated using a stochastic approach based on the binary field generation so that higher permeability inclusions are produced in the caprock with a given distribution. Montecarlo simulations are then needed to calculate the probability distribution of the escape of CO2 through a sequence of aquifer/aquitards that are representative of the Upper Adriatic system. Stochastic modeling appears to be a very powerful tool to take into account in a CO2 sequestration project the degree of continuity of the sealing aquitards. This issue is of importance to understand how potential discontinuities in the caprock can affect the upward migration of the injected carbon dioxide and to quantitatively assess the risk related to possible CO2 escape to the ground surface.

CO2 injection below the Venice Lagoon: a numerical study*

The existence of the historical setting of the city of Venice and the environmental ecosystem of her lagoon is jeopardized by the increasing frequency of floods occurred over the last decades. The endless debate on the most appropriate solution for the Venice safety seems to have recently come to a final conclusion with the official approval of MOSE, an impressive engineering project consisting of 79 mobile barriers planned to close the lagoon inlets during the most severe storms, thus preventing the sea from flooding the city. However, MOSE has still many opponents who do not believe it can work effectively to preserve both the city and the lagoon ecosystem. We present here a new solution which has none of the environmental consequences charged to MOSE and can complement it in duration and safety. This is concerned with the injection of anthropogenic CO2 in a brackish sandy aquifer lying 600–800 m below the lagoon. Based on new recent hydrogeological and geomechanical information of the Northern Adriatic basin, a numerical study is performed with the aid of multiphase flow and geomechanical models showing that a set of vertical injection wells properly located in the lagoon area can uniformly raise Venice up to 12 cm over a 10 year time, with a partial mitigation of most of the high tides that threat the existence of the city. Moreover, CO2 sequestration in a safe geological formation can also contribute to meet the requirements of the 1997 Kyoto protocol on the reduction of greenhouse gas emissions. The proposed solution appears to be a promising strategy to be investigated with further analyses.