Granulometric and Chemical Composition of the Danube River Sediments, Batina Village, Croatia

The size-fractionated recent sediments of the Danube river, from Batina, were assayed for the total concentrations of 15 elements: Ca, Cr, Cu, Fe, K, Mn, Ni, Pb, Rb, Sr, Ti, V, Y, Zn and Zr. It was found that trace metals increased with decreasing particle size, whereas the highest levels of organic matter from loss on ignition tests were found in the 0.5–1 mm fraction. The exchangeable phase accounted for a very minor proportion of the total heavy metal concentrations. Seasonal patterns of exchangeable Pb, Zn, Cu, Ni, Mn and Fe were more irregular than those of total metal levels, both generally reflecting a decrease in concentrations during spring. Rubidium normalization of the Pb, Cu, Zn, Ni, Cr, Mn and Fe concentrations was applied on the <0.063 mm fraction. A certain amount of the Cu and Ni concentrations may be related to anthropogenic rather than natural processes upstream of the study area and around one of the localities

The Inviscid Limit and Boundary Layers for Navier-Stokes Flows

The validity of the vanishing viscosity limit, that is, whether solutions of the Navier-Stokes equations modeling viscous incompressible flows converge to solutions of the Euler equations modeling inviscid incompressible flows as viscosity approaches zero, is one of the most fundamental issues in mathematical fluid mechanics. The problem is classified into two categories: the case when the physical boundary is absent, and the case when the physical boundary is present and the effect of the boundary layer becomes significant. The aim of this article is to review recent progress on the mathematical analysis of this problem in each category.Comment: To appear in "Handbook of Mathematical Analysis in Mechanics of Viscous Fluids", Y. Giga and A. Novotn\'y Ed., Springer. The final publication is available at http://www.springerlink.co

Variational Phase-Field Modeling of Hydraulic Fracture Interaction With Natural Fractures and Application to Enhanced Geothermal Systems

In every tight formation reservoir, natural fractures play an important role for mass and energy transport and stress distribution. Enhanced Geothermal Systems (EGS) make no exception, and stimulation aims at increasing the reservoir permeability to enhance fluid circulation and heat transport. EGS development relies upon the complex task of predicting accurate hydraulic fracture propagation pathway by taking into account reservoir heterogeneities and natural or preexisting fractures. In this contribution, we employ the variational phase-field method, which handles hydraulic fracture initiation, propagation, and interaction with natural fractures and is tested under varying conditions of rock mechanical properties and natural fractures distributions. We run bidimensional finite element simulations employing the open-source software OpenGeoSys and apply the model to simulate realistic stimulation scenarios, each one built from field data and considering complex natural fracture geometries in the order of a thousand of fractures. Key mechanical properties are derived from laboratory measurements on samples obtained in the field. Simulations results confirm the fundamental role played by natural fractures in stimulation's predictions, which is essential for developing successful EGS projects.Petroleum Engineerin