Global Histories of Work

Global Histories of Work is the first title in the new series Work in Global and Historical Perspective. This collection of selected articles written by leading scholars in different disciplines provides both an introduction and numerous insights into themes, debates and methods of Global Labour History as they have been developed over the last years. The contributions to the volume discuss crucial historiographical developments present different professions that have gained new attention in the context of an emerging Global Labour History critically engage the boundaries of free labour and the ambiguities contained in this concept and take up and historicize current debates about informal labour. Global Histories of Work will familiarize readers with a burgeoning fi eld of high academic, social, and political relevance

Micro-Annulus Generation under Downhole Conditions: Insights from Three-Dimensional Staged Finite Element Analysis of Cement Hardening and Wellbore Operations

A micro-annulus (MA) is defined as a high permeability zone or gap initiating/occurring at the casing-cement and cement-formation interfaces during the wellbore life span. An MA can significantly compromise wellbore integrity by establishing enhanced fluid flow pathways. This study uses a staged finite element approach to simulate wellbore integrity during various loading steps of wellbore operations under downhole conditions. Particular emphasis is placed on the processes of cement poro-elastic property evolution, volume variation, and pore pressure variation as part of the cement hardening step. The resulting state of stress during the life cycle of a typical injection well (i.e. hardening, completion, and injection) is analyzed to assess the onset and evolution of micro-annuli at various interfaces of the composite wellbore system under downhole conditions. The results show that cement shear failure is observed at the casing-cement interface during pressure testing (excessive wellbore pressure); and tensile debonding failure initiates at the cement-formation interface due to cement shrinkage during hardening and injection-related cooling (thermal cycling). Sensitivity analyses considering several parameters show that: (1) the degree of poro-elastic bulk shrinkage has significant implications for both shear and tensile failure initiation - the less the cement shrinks, the less likely the failure initiation is; (2) cement integrity increases with increasing depth; (3) cement pore pressure evolution has significant implications for tensile failure - if cement pore pressure decreases more, higher temperature differences can be sustained before an MA occurs; and (4) cement temperature fluctuations during hardening promote initiation of debonding failure. In summary, the results presented indicate that establishing downhole conditions to quantitatively analyze MA generation is necessary. The results are different compared to laboratory studies without considering/simulating downhole conditions. The knowledge from this study can raise the awareness of predicting and evaluating MA under downhole conditions and can be used to supplement and improve future laboratory experiments

Large-Scale Mechanical Buckle Fold Development and the Initiation of Tensile Fractures

Tensile failure associated with buckle folding is commonly associated to the distribution of outer arc extension but has also been observed on fold limbs. This study investigates whether tensile stresses and associated failure can be explained by the process of buckling under realistic in situ stress conditions. A 2-D plane strain finite element modeling approach is used to study single-layer buckle folds with a Maxwell viscoelastic rheology. A variety of material parameters are considered and their influence on the initiation of tensile stresses during the various stages of deformation is analyzed. It is concluded that the buckling process determines the strain distribution within the fold layer but is not solely responsible for the initiation of tensile stresses. The modeling results show that tensile stresses are most dependent on the permeability, viscosity, and overburden thickness. Low permeability (\u3c10-19 m2), high viscosity (≥1021 Pa s), and low overburden pressure can explain tensile failure at the fold hinge. Tensile stresses in the limb of the fold cannot (in general) be explained by buckling. Rather, it develops due to a combination of compression and erosional unloading. The modeling results show that erosion of high permeability rocks can explain the generation of tensile stresses at significant depths (∼2 km) both at the hinge of the fold and throughout the limb of the fold. This study shows that tensile stresses and associated failure within buckle folds is directly dependent on the distribution of material parameters but moreover to the strain history of the geologic system

125 Jahre Berliner Afrika-Konferenz: Bedeutung für Geschichte und Gegenwart

"Die Berliner Afrika-Konferenz vom 15. November 1884 bis zum 26. Februar 1885 im Reichstagspalais in der Wilhelmstraße jährt sich zum 125. Mal. Das öffentliche Echo ist gering; das offizielle Gedenken ist weitgehend ausgefallen. Das Auswärtige Amt sieht von Gedenkveranstaltungen ab. Kritiker wiederholen, dass eine kritische Aufarbeitung des Kolonialismus immer noch ausbleibe. In Afrika ist bis heute die Ansicht verbreitet, dass die Konferenz das 'freie Afrika' zerstört habe, künstliche Grenzen eingeführt und die Menschen brutal unterdrückt habe - wovon sich der Kontinent bis heute nicht erholt habe. Das suggestive Bild, Afrika sei auf der Berliner Konferenzwie ein Kuchen unter den imperialistischen Mächten aufgeteilt worden, ist irreführend. Zum Zeitpunkt der Konferenz hatte die Aufteilung des Kontinents längst begonnen. Erst in den drei folgenden Dekaden legten die Europäer im gegenseitigen Wettstreit in Afrika jene internationalen Grenzen fest, die noch heute weitgehend Geltung haben. Die langfristige Folge der Konferenz war die 'Grenzziehung nach innen': der territoriale Verwaltungsstaat wurde zum allein gültigen staatlichen Organisationsmodell. Die Bedeutung der kolonialen Grenzen sollte - zumindest für die Zeit bis zum Zweiten Weltkrieg - nicht überbewertet werden. Im Kontext der Dekolonisation änderte sich dies. Der Unabhängigkeitskampf, die Auseinandersetzungen um Ressourcen sowie die Etablierung politischer Klientelsysteme fanden innerhalb der kolonialen Grenzen statt. Die Führer der jungen afrikanischen Staaten blieben beim Status quo, da sie fürchteten, eine Revision der kolonialen Grenzen würde zu Kriegen führen. Die Mobilisierung politischer Ethnizität und die Zunahme gewaltsamer Grenzkonflikte nach dem Ende des Kalten Krieges stellen die territorialen Grenzen afrikanischer Staaten nur ausnahmsweise in Frage. Gegenwärtige Konflikte in Afrika sind nicht Folge der Grenzziehungen durch europäischen Kolonialmächte, sondern Resultat der Schwäche postkolonialer Staaten. In diesem Sinne steht die Berliner Afrika-Konferenz weniger für eine andauernde Grenzproblematik, sondern symbolisiert das ambivalente Erbe des Kolonialismus in Afrika." (Autorenreferat

Lars Kreye, „Deutscher Wald“ in Afrika. Koloniale Konflikte um regenerative Ressourcen, Tansania 1892–1916. (Umwelt und Gesellschaft, Bd. 23.) Göttingen, Vandenhoeck & Ruprecht 2021

Dieser Beitrag ist mit Zustimmung des Rechteinhabers (De Gruyter) frei zugänglich.Peer Reviewe

Sebastian Bischoff / Barbara Frey / Andreas Neuwöhner (Hrsg.), Koloniale Welten in Westfalen. (Studien und Quellen zur Westfälischen Geschichte, Bd. 89.) Leiden, Brill 2021

Dieser Beitrag ist mit Zustimmung des Rechteinhabers (De Gruyter) frei zugänglich.Peer Reviewe

Numerical Modeling of Geomechanical Processes Related to CO{sub 2} Injection within Generic Reservoirs

In this project generic anticline structures have been used for numerical modeling analyses to study the influence of geometrical parameters, fluid flow boundary conditions, in situ stress regime and inter-bedding friction coefficient on geomechanical risks such as fracture reactivation and fracture generation. The resulting stress states for these structures are also used to determine safe drilling directions and a methodology for wellbore trajection optimization is developed that is applicable for non-Andersonian stress states. The results of the fluid flow simulation show that the type of fluid flow boundary condition is of utmost importance and has significant impact on all injection related parameters. It is recommended that further research is conducted to establish a method to quantify the fluid flow boundary conditions for injection applications. The results of the geomechanical simulation show that in situ stress regime is a crucial, if not the most important, factor determining geomechanical risks. For extension and strike slip stress regimes anticline structures should be favored over horizontally layered basin as they feature higher ΔP{sub c} magnitudes. If sedimentary basins are tectonically relaxed and their state of stress is characterized by the uni-axial strain model the basin is in exact frictional equilibrium and fluids should not be injected. The results also show that low inter bedding friction coefficients effectively decouple layers resulting in lower ΔP{sub c} magnitudes, especially for the compressional stress regime

RDF validation requirements - evaluation and logical underpinning

There are many case studies for which the formulation of RDF constraints and the validation of RDF data conforming to these constraint is very important. As a part of the collaboration with the W3C and the DCMI working groups on RDF validation, we identified major RDF validation requirements and initiated an RDF validation requirements database which is available to contribute at http://purl.org/net/rdf-validation. The purpose of this database is to collaboratively collect case studies, use cases, requirements, and solutions regarding RDF validation. Although, there are multiple constraint languages which can be used to formulate RDF constraints (associated with these requirements), there is no standard way to formulate them. This paper serves to evaluate to which extend each requirement is satisfied by each of these constraint languages. We take reasoning into account as an important pre-validation step and therefore map constraints to DL in order to show that each constraint can be mapped to an ontology describing RDF constraints generically