Modelling of coupled fluid‐mechanical problems in fractured geological media using enriched finite elements

Geological environments, such as petroleum reservoirs, normally exhibit physical discontinuities, for example, fractures and faults. Because of the reduced thickness of these discontinuities, finite element formulations with strong discontinuity have been applied to the numerical modelling of geological environments. Until now, two relevant characteristics of petroleum reservoirs have not been addressed by these formulations. The first is the pore pressure jump in the direction normal to a discontinuity in a fluid‐mechanical coupling condition, which is present primarily in sealing faults owing to the contrast of permeability with the porous medium. The absence of this jump can affect the prediction of the deformability of a physical discontinuity. Furthermore, reservoir models frequently use coarse meshes. Thus, the method used to evaluate the pore pressure in the discontinuity may exhibit a strong dependence relative to the mesh refinement. Based on these characteristics, in this study, a formulation of an enriched finite element for application to coupled fluid‐mechanical problems with pre‐existing physical discontinuities saturated by a single fluid is presented. The formulation employs discontinuous interpolation functions and enables the reproduction of jumps of displacement and pore pressure associated with a discontinuity inside the element without the need to discretise it. An approximation to estimate the pore pressure in the discontinuity was developed, one which seeks to minimise the influence of refinement. The element's response is verified by comparison with a one‐dimensional analytical solution and simple examples that are simulated using commercial software. Copyright © 2015 John Wiley & Sons, Ltd.     

Factors controlling the vegetation distribution and coal‐forming environments in a strike‐slip basin. The Pennsylvanian Peñarroya‐Belmez‐Espiel Basin,southern Spain

Coal‐forming environments require humid to perhumid conditions. Tectonics governs the size, location and availability of coal seams developed in such environments. While large Pennsylvanian paralic basins generated thick and continuous coal seams, many other small coeval basins, which were tectonically active, developed a puzzling succession, with carbonaceous deposits that varied in size, thickness and the nature of the coal‐forming flora. This study, conducted in the Peñarroya‐Belmez‐Espiel coalfield, a Variscan strike‐slip basin in the south of Spain, provides insights into this subject. The coal seams analysed, generated in different depositional environments, have quantitatively different palynological assemblages. Lacustrine coals are dominated by lycopsids; distal alluvial plain/marginal lacustrine coals are dominated by sphenophytes and tree ferns, and middle alluvial fan coals are dominated by sphenophytes, tree ferns and lycopsids. This means that when conditions were favourable for peat accumulation, peat accumulated regardless of the nature of the available flora.     

Core Jamming in Unconsolidated Formations

Geotechnical and Geological Engineering - In coring operations, core jamming is a permanent concern, taking place when the rock core sample becomes stuck within the inner core barrel, thus...     

Ray-traced slant factors for mitigating the tropospheric delay at the observation level

Three-dimensional ray tracing through a numerical weather model has been applied to a global precise point positioning (PPP) campaign for modeling both the elevation angle- and azimuth-dependence of the tropospheric delay. Rather than applying the ray-traced slant delays directly, the delay has been parameterized in terms of slant factors, which are applied in a similar manner to traditional mapping functions, but which can account for the azimuthal asymmetry of the delay. Five strategies are considered: (1) Vienna Mapping Functions 1 (VMF1) and estimation of a residual zenith delay parameter; (2) VMF1, estimation of a residual zenith delay and estimation of two tropospheric gradient parameters; (3) three-dimensional ray-traced slant factors and estimation of a residual zenith delay; (4) using only ray-traced slant factors and no estimation of any tropospheric parameters and; (5) using both ray-traced slant factors and estimating a residual zenith delay and two tropospheric gradient parameters. The use of the ray-traced slant factors (solution 3) showed a 3.8% improvement in the repeatability of the up component when compared to the assumption of a symmetric atmosphere (solution 1), while the estimation of two tropospheric gradient parameters gave the best results showing an 7.6% improvement over solution 1 in the up component. Solution 4 performed well in the horizontal domain, allowing for sub-centimeter repeatability but the up component was degraded due to deficiencies in the modeling of the zenith delay, particularly for stations located at equatorial latitudes. The magnitude of the differences in the mean coordinates between solution 2 and solution 3, and the strong correlation with the differences between the north component and the ray-traced gradients (coefficient of correlation of 0.83), as well as the impact of observation geometry on the gradient solution indicate that the use of the ray-traced slant factors could have an implication on the realization of reference frames. The estimated tropospheric products from the PPP solutions were compared to those derived from ray tracing. For the zenith delay, a root mean square (RMS) of 5.4 mm was found, while for the gradient terms, a correlation coefficient of 0.46 for the N–S and 0.42 for the E–W was found for the north–south and east–west components, suggesting that there are still important differences in the gradient parameters which could be due to either errors in the NWM or to non-tropospheric error sources leaking into the PPP-estimated gradients.     

Identifying Controls on Water Chemistry of Tropical Cloud Forest Catchments: Combining Descriptive Approaches and Multivariate Analysis

We investigated controls on the water chemistry of a South Ecuadorian cloud forest catchment which is partly pristine, and partly converted to extensive pasture. From April 2007 to May 2008 water samples were taken weekly to biweekly at nine different subcatchments, and were screened for differences in electric conductivity, pH, anion, as well as element composition. A principal component analysis was conducted to reduce dimensionality of the data set and define major factors explaining variation in the data. Three main factors were isolated by a subset of 10 elements (Ca²⁺, Ce, Gd, K⁺, Mg²⁺, Na⁺, Nd, Rb, Sr, Y), explaining around 90% of the data variation. Land-use was the major factor controlling and changing water chemistry of the subcatchments. A second factor was associated with the concentration of rare earth elements in water, presumably highlighting other anthropogenic influences such as gravel excavation or road construction. Around 12% of the variation was explained by the third component, which was defined by the occurrence of Rb and K and represents the influence of vegetation dynamics on element accumulation and wash-out. Comparison of base- and fast flow concentrations led to the assumption that a significant portion of soil water from around 30 cm depth contributes to storm flow, as revealed by increased rare earth element concentrations in fast flow samples. Our findings demonstrate the utility of multi-tracer principal component analysis to study tropical headwater streams, and emphasize the need for effective land management in cloud forest catchments.     

A new computational strategy for solving two‐phase flow in strongly heterogeneous poroelastic media of evolving scales

We develop a new computational methodology for solving two‐phase flow in highly heterogeneous porous media incorporating geomechanical coupling subject to uncertainty in the poromechanical parameters. Within the framework of a staggered‐in‐time coupling algorithm, the numerical method proposed herein relies on a Petrov–Galerkin postprocessing approach projected on the Raviart–Thomas space to compute the Darcy velocity of the mixture in conjunction with a locally conservative higher order finite volume discretization of the nonlinear transport equation for the saturation and an operator splitting procedure based on the difference in the time‐scales of transport and geomechanics to compute the effects of transient porosity upon saturation. Notable features of the numerical modeling proposed herein are the local conservation properties inherited by the discrete fluxes that are crucial to correctly capture the fingering patterns arising from the interaction between heterogeneity and nonlinear viscous coupling. Water flooding in a poroelastic formation subject to an overburden is simulated with the geology characterized by multiscale self‐similar permeability and Young modulus random fields with power‐law covariance structure. Statistical moments of the poromechanical unknowns are computed within the framework of a high‐resolution Monte Carlo method. Numerical results illustrate the necessity of adopting locally conservative schemes to obtain reliable predictions of secondary recovery and finger growth in strongly heterogeneous deformable reservoirs. Copyright © 2011 John Wiley & Sons, Ltd.     

Preliminary paleolimnological study of rocha lagoon, SE Uruguay

Physical, chemical and biological data from a sedimentary profile of Rocha lagoon (Uruguay) are presented. Analyses of silica remains showed that opal phytoliths, diatoms, sponge spicules and chrysophyte cysts were the most important contributors to biogenic silica. Organic matter, total nitrogen and total phosphorus were used to infer past trophic development. Physical analysis of sediments allowed us to identify five lithological units related to sea level variation and climatic events.     

Using historical source data to understand urban flood risk: a socio-hydrological modelling application at Gregório Creek,Brazil

ABSTRACT

The city of São Carlos, state of São Paulo, Brazil, has a historical coexistence between society and floods. Unplanned urbanization in this area is a representative feature of how Brazilian cities have developed, undermining the impact of natural hazards. The Gregório Creek catchment is an enigma of complex dynamics concerning the relationship between humans and water in Brazilian cities. Our hypothesis is that social memory of floods can improve future resilience. In this paper we analyse flood risk dynamics in a small urban catchment, identify the impacts of social memory on building resilience and propose measures to reduce the risk of floods. We applied a socio-hydrological model using data collected from newspapers from 1940 to 2018. The model was able to elucidate human–water processes in the catchment and the historical source data proved to be a useful tool to fill gaps in the data in small urban basins.     

The precambrian evolution of South America

Zusammenfassung Mit Hilfe von ca. 1500 radiometrischen Zeitmessungen wird eine Synthese der prÄkambrischen Entwicklung Südamerikas aufgestellt. In Abb. 1 wird versucht, die Lage der spÄtprÄkambrischen orogenen Gürtel sowie ihre Plattformen darzustellen.Der grö\te alte Kern des Kontinents umfa\t den Guyana-Schild, das Basement des Amazonas-Sedimentbeckens und den Guaporé-Kraton im Süden, einen Raum von ungefÄhr 4,5 Mio. qkm. Die meisten Gesteine wurden von dem transamazonischen orogenen Zyklus erfa\t, dessen radiometrisches Alter etwa 2000 M. J. betrÄgt. Der etwa gleichaltrige SÃo-Francisco-Kraton in Ostbrasilien umfa\t ungefÄhr 1 Mio. qkm. Kleinere Kerne, die ebenfalls die Ereignisse des transamazonischen Zyklus' widerspiegeln, wurden nahe der Atlantikküste, östlich der Mündung des Amazonasflusses (SÃo-Luis-Kraton-Gebiet) und in der Umgebung des La-Plata-Flusses (Rio de la Plata-Kraton-Gebiet) gefunden.Die Kratone sind getrennt durch metamorphe Gürtel, die zum brasilianischen orogenen Zyklus spÄtprÄkambrischen Alters gehören. Der Caririan-Gürtel und die Sergipe-Geosynklinale liegen in der Nordostecke von Brasilien, und der Ribeira-Gürtel erstreckt sich entlang der Atlantikküste im Süden. Zwei symmetrische geosynklinale Einheiten wurden im zentralen Teil des Kontinents erkannt: der Brasilia- und der Paraguay-Araguaia-Gürtel.In den brasilianischen orogenen Gürteln treten an vielen Stellen transamazonische oder sogar Ältere Serien auf, Anzeichen für aufgearbeitetes altes Basement. Dies scheint zu zeigen, da\ die Sialkruste des südamerikanischen Kontinents vor 2000 M. J. schon eine rÄumliche Ausdehnung von mehr als 10 Mio. qkm hatte.

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A general synthesis of the precambrian evolution of South America has been made with the aid of about 1500 radiometric age determinations. In Fig. 1, the position of the late precambrian orogenic belts, as well as their platforms, is tentatively outlined.The largest ancient core of the continent includes the Guyana Shield, the basement of the Amazon sedimentary basin, and the Guaporé craton, to the south, covering an area of about 4.5 million square kilometers. Most of the rocks were affected by the Trans-Amazonian orogenic cycle, whose radiometric ages are close to 2000 m. y. The SÃo Francisco craton of similar age outcrops over an area of about one million square kilometers, in eastern Brazil. Smaller ancient nucleii, also reflecting the events of the Tranz-Amazonian cycle, were found near the Atlantic coast, east of the mouth of the Amazon river (SÃo Luis cratonic area), and surrounding the La Plata river (Rio de la Plata cratonic area).The old cratonic areas are separated from each other by metamorphic belts which belong to the Brazilian orogenic cycle of late precambrian age. The Caririan belt, and the Sergipe geosyncline, occur at the northeastern corner of Brazil, and the Ribeira belt along the Atlantic coast, to the south. Two symmetrical geosynclinal units were recognized in the central part of the continent: the Brasilia and the Paraguay-Araguaia belts.Within the areas of the Brazilian orogenic belts, in many places Trans-Amazonian or even older ages occur, indicating remobilized ancient basement. This seems to demonstrate that the sialic crust of the South American continent, 2000 m. y. ago, already exhibited an areal extent of more than 10 million square kilometers.

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Résumé Une synthèse générale de l'évolution précambrienne de l'Amérique du Sud a été faite à l'aide de 1500 déterminations d'âge radiométrique. La fig. 1 présente un essai sur la répartition des zones orogéniques du Précambrien supérieur et de leurs platesformes.Le noyau ancien du continent, qui est le plus vaste, comprend le bouclier de la Guyane, le socle du bassin sédimentaire de l'Amazone et le Craton de Guaporé, au Sud, couvrant une aire d'environ 4,5 millions de Km². La plupart des roches ont été affectées par le cycle orogénique Trans-Amazonien dont l'âge radiométrique est d'environ 2000 millions. Le craton de SÃo Francisco d'âge semblable, affleure sur une étendue d'environ 1 million de Km², dans l'Est du Brésil. Des noyaux anciens plus petits, affectés également par le cycle Trans-Amazonien, ont été trouvés près de la cÔte Atlantique, à l'Est de l'embouchure de l'Amazone (région cratonique de SÃo Luis), et aux environs du fleuve la Plata (région cratonique du Rio de la Plata).Les régions cratoniques anciennes sont séparées les unes des autres par des zones métamorphiques appartenant au cycle orogénique brésilien, d'âge Précambrien supérieur. La zone caririenne et le géosynclinal de Sergipe affleurent dans l'extrémité NE du Brésil, et la zone de Ribeira, le long de la cÔte Atlantique au S. Deux unités géosynclinales symétriques ont été reconnues dans la partie centrale du continent: les zones de Brasilia et de Paraguay-Araguaia.Dans les régions occupées par les ceintures orogéniques Brésiliennes, il existe en beaucoup d'endroits des roches d'âge Trans-Amazonien et mÊme plus ancien, indices d'un socle ancien remobilisé. Ceci semble démontrer que la croûte sialique du continent Sud Américain montrait déjà, il y a environ 2000 millions d'années, une étendue de plus de 10 millions de Km².

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1500 . . 1 , , . , Guaporé , . . 4,5 . , 2000 . Sao Francisco 1 . . , , , ( Sao Luis) la Plata ( (Rio de la Plata). , . Caririan Sergipe , Paraiba . : Brasilia Paraguay-Araguaia. , . , , , - , 10 . 2000 .

     

Uncertainty Estimate in Resources Assessment: A Geostatistical Contribution

For many decades the mining industry regarded resources/reserves estimation and classification as a mere calculation requiring basic mathematical and geological knowledge. Most methods were based on geometrical procedures and spatial data distribution. Therefore, uncertainty associated with tonnages and grades either were ignored or mishandled, although various mining codes require a measure of confidence in the values reported. Traditional methods fail in reporting the level of confidence in the quantities and grades. Conversely, kriging is known to provide the best estimate and its associated variance. Among kriging methods, Ordinary Kriging (OK) probably is the most widely used one for mineral resource/reserve estimation, mainly because of its robustness and its facility in uncertainty assessment by using the kriging variance. It also is known that OK variance is unable to recognize local data variability, an important issue when heterogeneous mineral deposits with higher and poorer grade zones are being evaluated. Altenatively, stochastic simulation are used to build local or global uncertainty about a geological attribute respecting its statistical moments. This study investigates methods capable of incorporating uncertainty to the estimates of resources and reserves via OK and sequential gaussian and sequential indicator simulation The results showed that for the type of mineralization studied all methods classified the tonnages similarly. The methods are illustrated using an exploration drill hole data sets from a large Brazilian coal deposit.