1,721,037 research outputs found
Land Subsidence due to Gas/Oil Production in Inhomogeneous Transversally Anisotropic Half Space By a Boundary Element Method
No full textIn a previous paper1 the authors have developed and implemented a new boundary element (BE) model to simulate and predict land subsidence occurring over three‐dimensional gas/oil fields in a homogeneous and isotropic half‐space. The approach relies on Betti's reciprocal theorem and makes use of the classical fundamental solution of Boussinesq in the framework of the theory of linear poroelasticity. The BE method is here extended to inhomogeneous, transversally anisotropic soils by the aid of a two‐dimensional finite element (FE) model which provides a fundamental numerical solution for the actual multi‐layer setting of the subsurface system. The new FE–BE approach is then used to simulate the subsidence caused by gas production over the deep reservoir of Campo Ravenna Terra, Ravenna (Italy) from 1950 to 1980. The results compare very favourably with the outcome from a full more expensive three‐dimensional FE model of the same occurrence. Copyright © 1990 John Wiley & Sons, Lt
Una procedura iterativa di ottimizzazione per la determinazione degli autovalori di matrici simmetriche sparse
No full textAn Accelerated Conjugate Gradient Method for the Solution of Sparse Symmetric Eigenproblems
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Domain Decomposition for Randomly Heterogeneous Porous Media
No full textA domain decomposition algorithm using triangular subdomains has been developed to solve steady flow problems in randomly heterogeneous porous media. In each triangle three adjoint solutions are sought by traditional finite elements. These solutions, which incorporate the actual non-homogeneities of the permeability field and may be evaluated simultaneously on a parallel machine, provide adjoint fluxes to be subsequently integrated over the subdomain boundaries and properly assembled over the nodes of the macromesh. A final algebraic unsymmetric system is thus arrived at, whose solution gives the head on the nodes of the subregions or macromesh. The formulation fully includes the effects of the heterogeneity distribution at the microscale. The approach appears to be particularly suited to randomly heterogeneous media
An Orthogonal Accelerated Deflation Technique for Large Symmetric Eigenproblems
No full textAn improvement in accelerated conjugate gradient iterations is presented for the evaluation of several of the leftmost eigenpairs of large sparse symmetric positive definite matrices. The approach relies on an orthogonal deflation procedure and is based on the subsequent preconditioned conjugate gradient optimization of Rayleigh quotients over the restricted space orthogonal to the set of eigenvectors previously computed. Comparison with the accelerated simultaneous iterations performed over large finite element problems (with size up to 4500) shows that storage requirement is significantly less and CPU times may be reduced by a factor of two or more. © 1992
3D geomechanical modelling for CO2 geological storage: A case study in an offshore northern Adriatic reservoir
No full textOne of the six CO2 carbon capture and storage (CCS) demonstration projects recently selected within the European Energy Programme for Recovery (EEPR) is located in Italy. In the framework of the feasibility study, the selection of a geological formation suitable to store the required 1 Mt/yr of CO2 over 10 years and the safety of the CO2 disposal are two major issues. In the present modeling study, we investigate the role played by geomechanics in assessing the maximum CO2 amount that can be sequestered into a 2000 m deep multi-compartment reservoir seated in the off-shore northern Adriatic sedimentary basin. We use a three-dimensional finite element–interface element geomechanical model to simulate the possible mechanical failure in both the injected formation and caprock, the fault reactivation, and the ground surface displacement. The faulted geological structure is reproduced based on detailed seismic surveys, with petrophysical/geomechanical properties based on the several well-logs available from several oil/gas explorations in the area. The pore pressure distribution due to two injection wells is provided by a fluid-dynamic simulator and a sensitivity analysis is carried out to investigate the role of the major uncertainties in the geomechanical setting. The modeling results suggest that a safe and permanent containment may be secured over a few years only. Afterwards, mechanical failure by shear stress is likely to be experienced by a significant portion of reservoir's injected compartments. Shear failure and fault reactivation can occur much before attaining the hydraulic fracturing pressure, hence represent two major issues in assessing the maximum allowable CO2 injection overpressure
Effective Preconditioning for the Simultaneous Evaluation of the Smallest Eigenpairs of Sparse Matrices
No full textA preconditioned simultaneous iteration method is described for the solution of the generalized eigenproblem Ax=lBx, where A an B are are real symmetric positive definite matrices. The procedure relies on the optimization of the Rayleigh quotient over a subspace of orthogonal vectors by a conjugate gradient technique effectively preconditioned with the pointwise incomplete Cholesky factorization. The numerical experiments show that, while the simultaneous conjugate gradient scheme fails to converge, the accelerated iterations yield accurate results in a number of steps which is much smaller than N
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