Geomechanics of subsurface water withdrawal and injection

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 \u2018\u2018the cause of subsidence is to be found in the extensive extraction of fluid from beneath the affected area.\u2019\u2019 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 \u2018\u2018effective intergranular stress.\u2019\u2019 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

Simulazione numerica del flusso e trasporto di contaminanti in mezzi porosi a saturazione e densità variabile

La legislazione riguardante la salvaguardia e la tutela delle risorse idriche, e tra queste le acque sotterranee, è in continua crescita in tutti i paesi industrializzati. La protezione delle acque di falda dal sovrasfruttamento e dalla contaminazione di origine diversa (rifiuti urbani e industriali, pesticidi e fertilizzanti, scorie nucleari, ecc...) richiede la previsione degli effetti indotti dalle attività umane sulla quantità e qualità delle risorse sotterranee, previsione che si può conseguire solo attraverso l'impiego di idonei modelli matematico-numerici. Un problema di stringente attualità in tutti i paesi che si affacciano sul Mediterraneo è l'inquinamento degli acquiferi costieri per intrusione di acqua di mare. La simulazione della penetrazione del cuneo salino comporta lo sviluppo di modelli accoppiati di flusso e trasporto che possono essere accuratamente ed efficientemente risolti col metodo degli elementi finiti (FEM) che viene qui implementato in un mezzo poroso tridimensionale a saturazione variabile, e che è quindi in grado di simulare sia la zona insatura (suoli superficiali) che quella satura (falde in pressione). Le non linearità che scaturiscono dall'accoppiamento e dalle leggi costitutive della permeabilità e del coefficiente di immagazzinamento nella zona insatura sono risolte con le tecniche di Picard e di Newton parziale. I modelli discreti finali linearizzati sono trattati col metodo dei gradienti coniugati opportunamente precondizionati per le matrici simmetriche di flusso (PGC) e quelle non simmetriche di trasporto (GMRES, Bi-CGSTAB, TFQMR). Le procedure descritte sono implementate nel codice FEM CODESA-3D (COupled variable DEnsity and SAturation) di cui è offerto un esempio applicativo.In the industrialized countries subsurface water resources are increasingly subject to regulations for protection from over-exploitation and from contamination arising from urban, industrial, nuclear, military, and agricultural activities. Prediction of the effects of anthropogenic impacts on water quantity and quality is an important part of proper aquifer management, and can be achieved through the use of mathematical models. As an example, seawater intrusion in coastal aquifers represents a serious environmental problem, especially in the countries of the Mediterranean basin, and can be simulated using coupled models of water flow and solute transport. Sophisticated groundwater models such as these can be accurately and effciently solved numerically via finite element discretizations of the three-dimensional porous medium. Both saturated (groundwater) and unsaturated (soil water) zones can be represented, and nonlinearities arising from storage-pressure head and conductivity-pressure head dependencies in the unsaturated zone and from coupling of the two equations can be resolved using Picard, Newton, or partial Newton methods. The resulting linearized systems of equations can be solved using a variety of preconditioned conjugate gradient-like methods applicable to symmetric and non-symmetric systems. The mathematical formulation and numerical procedures to be described form the basis of the CODESA-3D (COupled variable DEnsity and SAturation) model

A Novel Factorized Sparse Approximate Inverse Preconditioner with Supernodes

AbstractKrylov methods preconditioned by Factorized Sparse Approximate Inverses (FSAI) are an efficient approach for the solution of symmetric positive definite linear systems on massively parallel computers. However, FSAI often suffers from a high set-up cost, especially in ill-conditioned problems. In this communication we propose a novel algorithm for the FSAI computation that makes use of the concept of supernode borrowed from sparse LU factorizations and direct methods

On the equivalence of total stress and pressure gradient formulations for predicting land subsidence above compacting gas/oil fields

The solution of the poroelastic equations for predicting land subsidence above productive gas/oil fields may be addressed by the principle of virtual works using either the effective intergranular stress, with the pore pressure gradient regarded as a distributed body force, or the total stress incorporating the pore pressure. In the finite element (FE) method both approaches prove equivalent at the global assembled level. However, at the element level apparently the equivalence does not hold, and the strength source related to the pore pressure seems to generate different local forces on the element nodes. The two formulations are briey reviewed and discussed for triangular and tetrahedral finite elements. They are shown to yield different results at the global level as well in a three-dimensional axisymmetric porous medium if the FE integration is performed using the average element-wise radius. A modification to both formulations is suggested which allows to correctly solve the problem of a finite reservoir with an infinite pressure gradient, i.e. with a pore pressure discontinuity on its boundary

Finite element analysis of land subsidence above depleted reservoirs with pore pressure gradient and total stress formulations

SUMMARY The solution of the poroelastic equations for predicting land subsidence above productive gas/oil "elds may be addressed by the principle of virtual works using either the e!ective intergranular stress, with the pore pressure gradient regarded as a distributed body force, or the total stress incorporating the pore pressure. In the "nite element (FE) method both approaches prove equivalent at the global assembled level. However, at the element level apparently the equivalence does not hold, and the strength source related to the pore pressure seems to generate di!erent local forces on the element nodes. The two formulations are brie#y reviewed and discussed for triangular and tetrahedral "nite elements. They are shown to yield di!erent results at the global level as well in a three-dimensional axisymmetric porous medium if the FE integration is performed using the average element-wise radius. A modi"cation to both formulations is suggested which allows to correctly solve the problem of a "nite reservoir with an in"nite pressure gradient, i.e. with a pore pressure discontinuity on its boundary

Subsidence due to peatland oxidation in the Venice Lagoon catchment

Abstract. The Venice Lagoon is characterized by a fast morphodynamics appreciable not only over the geological scale but also in historical and modern times. The lagoon environment proves very sensitive to even minor modifications of the natural and anthropogenic controlling factors. An important human endeavor accomplished in the past century is the reclamation of the southernmost lagoon area that has been turned into a fertile farmland. The reclaimed soil is reach in organic matter (peat) that may oxidize with release of carbon dioxide to the atmosphere. The continuous loss of carbon is causing a pronounced settlement of the farmland that lies below the present sea/lagoon level. This enhances the flood hazard and impacts noticeably on the maintenance and operational costs of the drainage system. Total peatland subsidence is estimated at 1.5 m over the last 70 years with a current rate of 1.5-2 cm/year. The geochemical reaction is primarily controlled by soil water content and temperature, and is much influenced by agricultural practices, crop rotation, and depth to the water table. A small (24 km2) controlled catchment located in the area has been instrumented for accurately monitoring the basic parameters and recording the ground motion. The in situ measurements have been integrated with the combined use of remote sensing data to help cast light on the process and identify the mitigation strategies.Published81-906A. Monitoraggio ambientale, sicurezza e territorioope