Simulation of multiphase flow in fractured reservoirs using a fracture-only model with transfer functions

We present a fracture-only reservoir simulator for multiphase flow: the fracture geometry is modeled explicitly, while fluid movement between fracture and matrix is accommodated using empirical transfer functions. This is a hybrid between discrete fracture discrete matrix modeling where both the fracture and matrix are gridded and dual-porosity or dual-permeability simulation where both fracture and matrix continua are upscaled. The advantage of this approach is that the complex fracture geometry that controls the main flow paths is retained. The use of transfer functions, however, simplifies meshing and makes the simulation method considerably more efficient than discrete fracture discrete matrix models. The transfer functions accommodate capillary- and gravity-mediated flow between fracture and matrix and have been shown to be accurate for simple fracture geometries, capturing both the early- and late-time average behavior. We verify our simulator by comparing its predictions with simulation results where the fracture and matrix are explicitly modeled. We then show the utility of the approach by simulating multiphase flow in a geologically realistic fracture network. Waterflooding runs reveal the fraction of the fracture–matrix interface area that is infiltrated by water so that matrix imbibition can occur. The evolving fraction of the fracture–matrix interface area turns out to be an important characteristic of any particular fracture system to be used as a scaling parameter for capillary driven fracture–matrix transfer.     

Rock falls in the Mont Blanc Massif in 2007 and 2008

Due to a lack of systematic observations, the intensity and volume of rock falls and rock avalanches in high mountain areas are still poorly known. Nevertheless, these phenomena could have burly consequences. To document present rock falls, a network of observers (guides, mountaineers, and hut wardens) was initiated in the Mont Blanc Massif in 2005 and became fully operational in 2007. This article presents data on the 66 rock falls (100 m³?≤?V?≤?50,000 m³) documented in 2007 (n?=?41) and 2008 (n?=?25). Most of the starting zones are located in warm permafrost areas, which are most sensitive to warming, and only four rock falls are clearly out of permafrost area. Different elements support permafrost degradation as one of the main triggering factors of present rock falls in high mountain areas.     

P- and S-velocity images of the lithosphere–asthenosphere system in the Central Andes from local-source tomographic inversion

About 50 000 P and S arrival times and 25 000 values of t * recorded at seismic arrays operated in the Central Andes between 20°S and 25°S in the time period from 1994 to 1997 have been used for locating more than 1500 deep and crustal earthquakes and creating 3-D P , S velocity and Qp models. The study volume in the reference model is subdivided into three domains: slab, continental crust and mantle wedge. A starting velocity distribution in each domain is set from a priori information: in the crust it is based on the controlled sources seismic studies; in slab and mantle wedge it is defined using relations between P and S velocities, temperature and composition given by mineral physics. Each iteration of tomographic inversion consists of the following steps: (1) absolute location of sources in 3-D velocity model using P and S arrival times; (2) double-difference relocation of the sources and (3) simultaneous determination of P and S velocity anomalies, P and S station corrections and source parameters by inverting one matrix. Velocity parameters are computed in a mesh with the density of nodes proportional to the ray density with double-sided nodes at the domain boundaries. The next iteration is repeated with the updated velocity model and source parameters obtained at the previous step. Different tests aimed at checking the reliability of the obtained velocity models are presented. In addition, we present the results of inversion for Vp and Vp/Vs parameters, which appear to be practically equivalent to Vp and Vs inversion. A separate inversion for Qp has been performed using the ray paths and source locations in the final velocity model. The resulting Vp , Vs and Qp distributions show complicated, essentially 3-D structure in the lithosphere and asthenosphere. P and S velocities appear to be well correlated, suggesting the important role of variations of composition, temperature, water content and degree of partial melting.     

Evidence from the Rio Bayo valley on the extent of the North Patagonian Icefield during the Late Pleistocene-Holocene transition

This paper presents data on the extent of the North Patagonian Icefield during the Late Pleistocene-Holocene transition using cosmogenic nuclide exposure age and optically stimulated luminescence dating. We describe geomorphological and geochronological evidence for glacier extent in one of the major valleys surrounding the North Patagonian Icefield, the Rio Bayo valley. Geomorphological mapping provides evidence for the existence of two types of former ice masses in this area: (i) a large outlet glacier of the North Patagonian Icefield, which occupied the main Rio Bayo valley, and (ii) a number of small glaciers that developed in cirques on the slopes of the mountains surrounding the valley. Cosmogenic nuclide exposure-age dating of two erratic boulders on the floor of the Rio Bayo valley indicate that the outlet glacier of the icefield withdrew from the Rio Bayo valley after 10,900 ± 1000 yr (the mean of two boulders dated to 11,400 ± 900 yr and 10,500 ± 800 yr). Single-grain optically stimulated luminescence (OSL) dating of an ice-contact landform constructed against this glacier indicates that this ice mass remained in the valley until at least 9700 ± 700 yr. The agreement between the two independent dating techniques (OSL and cosmogenic nuclide exposure age dating) increases our confidence in these age estimates. A date obtained from a boulder on a cirque moraine above the main valley indicates that glaciers advanced in cirques surrounding the icefield some time around 12,500 ± 900 yr. This evidence for an expanded North Patagonian Icefield between 10,900 ± 1000 yr and 9700 ± 700 yr implies cold climatic conditions dominated at this time.     

The ice nucleating ability of pollen: Part III: New laboratory studies in immersion and contact freezing modes including more pollen types

Based on earlier experimental studies, the ice nucleating abilities of further pollen types were investigated in the immersion and contact freezing modes. The studies were carried out at the Mainz vertical wind tunnel with freely floating supercooled droplets down to − 28 °C. The pollen had diameters between 26 and 28 μm and correspondingly low sink velocities around 2.5 cm s^{− 1}. The radii of the studied drops were calculated from the recorded wind velocity and for both freezing modes the radii of the observed droplets varied between 315 and 380 μm. Immersion freezing experiments were conducted with pollen particles added to the droplets while in contact freezing experiments supercooled droplets were subjected to a burst of pollen particles. The median freezing temperatures found in the immersion freezing mode were: − 13.5 °C (alder), − 21.5 °C (lombardy poplar), − 21.0 °C (redtop grass) and − 19.8 °C (kentucky blue). The median freezing temperatures in the contact freezing mode were found as: − 12.6 °C (alder), − 17.9 °C (lombardy poplar), − 18.7 °C (redtop grass) and − 16.1 °C (kentucky blue). The results show that the ice nucleating ability of pollen is not restricted to single pollen types but seems to be a general pollen property.     

Intraplate volcanism in New Zealand: the role of fossil plume material and variable lithospheric properties

The geologic evolution of the New Zealand microcontinent was characterised by intermittent Cretaceous to Quaternary episodes of intraplate volcanism. To evaluate the corresponding mantle evolution beneath New Zealand with a specific focus on the tectonic evolution, we performed a combined major and trace element and Hf, Nd, Pb, Sr isotope investigation on a suite of representative intraplate volcanic rocks from both main islands and the Chatham Islands. Isotopically, the data set covers a range between “HIMU-like” end member compositions (²⁰⁶Pb/²⁰⁴Pb: 20.57, ²⁰⁷Pb/²⁰⁴Pb: 15.77, ⁸⁷Sr/⁸⁶Sr: 0.7030, εHf: + 3.8, εNd: + 4.2), compositions tending towards MORB (²⁰⁶Pb/²⁰⁴Pb: 19.01, ²⁰⁷Pb/²⁰⁴Pb: 15.62, ⁸⁷Sr/⁸⁶Sr: 0.7028, εHf: + 9.9, εNd: + 7.0) and compositions reflecting the influence of subducted sediments (²⁰⁶Pb/²⁰⁴Pb: 18.99, ²⁰⁷Pb/²⁰⁴Pb: 15.67, ⁸⁷Sr/⁸⁶Sr: 0.7037, εHf: + 4.4, εNd: + 3.9). Whereas volcanism on the Chatham Islands constitutes the HIMU end member of our data set, intraplate volcanic rocks from the North Island are dominated by MORB-like compositions with relatively radiogenic ²⁰⁶Pb/²⁰⁴Pb signatures. Volcanic rocks from the South Island form a trend between the three end members. Assuming a polybaric melting column model, the primary melt compositions reflect variations in the degree of melting, coupled to variable average melting depths. As the three isotope and trace element end members occur throughout the volcanic episodes, the “HIMU-like” and the sediment influenced signatures most likely originate from a heterogeneous subcontinental lithospheric mantle, whereas an asthenospheric origin is inferred for the MORB-like component. For the South Island, affinities to HIMU wane with decreasing average melting depths whereas MORB and sediment-like signatures become more distinct. We therefore propose a polybaric melting model involving upper asthenospheric mantle and a lithospheric mantle source that has been modified by subduction components and veins of fossil “HIMU-like” asthenospheric melts. The proportion of asthenospheric versus lithospheric source components is controlled by variations in lithospheric thickness and heat flow, reflecting the different tectonic settings and rates of extension. Generally, low degree melts preferentially tap enriched vein material with HIMU signatures. The widespread occurrence of old Gondwana-derived lithospheric mantle beneath intraplate volcanic fields in East Gondwana is suggested by overall similarities between New Zealand intraplate volcanic rocks and volcanic rocks in East Australia and Antarctica. The petrogenetic model proposed here may therefore serve as a general model for the petrogenesis of Cretaceous to Recent intraplate volcanic rocks in former East Gondwana. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Reflectivity measurements in the vacuum ultraviolet wavelength range on technical surfaces for the Wolter I telescope on board the X-ray astronomy satellite ROSAT

The results of reflectivity measurements in the vacuum ultraviolet wavelength region (VUV) on technical surfaces are described which are used in the Wolter I-telescope on board the German X-ray astronomy satellite ROSAT.The materials investigated are the special iron-nickel alloy Invar and a carbon fibre compound (CFK).The centre connecting flange for the parabolic and hyperbolic mirror sections of the telescope is made of Invar. CFK is used for the thermal baffle in front of the telescope. It had to be checked whether the structure of the centre flange and the thermal baffle sufficiently suppress scattered ultraviolet radiation in order to avoid a substantial background level in the detectors which are located in the focal plane of the telescope. The detectors consist of two positional sensitive proportional counters (PSPC) with a spatial resolution of 0.5 arc min and an image converter with a resolution of a few arc sec. The detectors are mounted on a carrousel platform and are intended to be positioned alternatively in the focal plane.