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See detailExplaining thermal failure in saturated clays
Hueckel, Tomasz; François, Bertrand ULg; Lyesse, Laloui

in Geotechnique (2009), 59(3), 197-212

Failure conditions in soils at elevated temperatures appear to be strongly dependent on the history of the application of stress and temperature. Four cases of such history leading to various modes of ... [more ▼]

Failure conditions in soils at elevated temperatures appear to be strongly dependent on the history of the application of stress and temperature. Four cases of such history leading to various modes of failure are identified and interpreted in terms of thermal Cam-clay models. Particular attention is given to the influence of thermal variability on the coefficient of the critical state, M, or the angle of internal friction. A detailed analysis of the material history offers an explanation of an apparent confusion about whether the soil strength is decreased or increased by temperature. In a companion paper, numerical analysis of the development of axisymmetric thermal and stress fields around a cylindrical heat source suggests that thermal failure may arise in conditions that are far from any mechanically critical situation. [less ▲]

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See detailA deterministic/stochastic model to predict the variation in bulk modulus of chalk
Collin, Frédéric ULg; Schroeder, Christian ULg; De Gennaro, Vincenzo et al

in Geotechnique (2005), 55(2), 135-141

Ekofisk, located 200 km west of the Norwegian coast, is one of the main oilfields in the North Sea. Since the early 1980s severe compaction of chalk reservoir layers has been observed as a consequence of ... [more ▼]

Ekofisk, located 200 km west of the Norwegian coast, is one of the main oilfields in the North Sea. Since the early 1980s severe compaction of chalk reservoir layers has been observed as a consequence of reservoir depletion during oil production. Subsequently, this compaction has been amplified by assisted oil recovery using seawater flooding. The development of our understanding of the inherent mechanisms of this phenomenon has been the objective of extensive experimental investigations in the last two decades. Owing to the very high cost of cored material from the reservoir, experiments are usually performed on chalk samples from an outcrop in Belgium lying at the same stratigraphic level as the Ekofisk reservoir chalks. However, even at the laboratory sample scale,. experimental variability of material response is observed. From a theoretical and numerical point of view, the determination of the mechanical properties of the material is of utmost importance for accurate modelling at both sample scale (laboratory tests) and reservoir scale. The aim of this paper is to introduce a stochastic approach within a deterministic constitutive model of chalk to enable the influence of material heterogeneity to be included in analyses for the range of observed mechanical responses. In modelling the random distribution of material parameters, the time-consuming Monte Carlo simulation method is replaced by a more efficient stochastic modelling technique. The results are given in the form of statistical parameters for the experimental laboratory test responses. The parameters of the distribution law (mean value, range of variation, spatial correlation structure) are fitted to reproduce the range of experimental responses observed at sample scale. Interest is focused on the variability of the chalk bulk modulus, as observed during isotropic compression tests. The proposed methodology provides a satisfactory explanation for the variability of response observed at the sample scale. The potential for extending the proposed approach to reservoir scale is briefly discussed. [less ▲]

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See detailOn the collapse behaviour of oil reservoir chalk
De Gennaro, Vincenzo; Delage, Pierre; Priol, Grégoire et al

in Geotechnique (2004), 54(6), 415-420

Oil exploitation in North Sea Ekofisk oilfield started in 1971, the reservoir is located in a 150 m thick layer of porous chalk (n = 40-50%) at a 3000 m depth. Enhanced oil recovery procedure by sea water ... [more ▼]

Oil exploitation in North Sea Ekofisk oilfield started in 1971, the reservoir is located in a 150 m thick layer of porous chalk (n = 40-50%) at a 3000 m depth. Enhanced oil recovery procedure by sea water injection (waterflooding) was initiated in 1987. Starting from this date, seabed subsidence due to chalk compaction evolves at a fairly constant rate (i.e. 40 cm/year). Nowadays, the decrease of the seafloor level is of about 10 m. Reservoir management and production strategies are at the origin of the growing interest of petroleum industry in disposing of a comprehensive description of the chalk mechanical behaviour. In this note the subsidence due to waterflooding is interpreted within a framework taken from the mechanics of unsaturated soils. By considering oil as the non-wetting fluid and water as the wetting fluid, chalk compaction is depicted as a collapse phenomenon due to oil-water suction decrease. A series of suction controlled tests in the osmotic oedometer cell are presented. Water weakening effects and chalk compaction (collapse) seem likely to occur through the lost of strength of the inter-granular links existing in the oil saturated sample. The nature of these links includes both capillary and physico-chemical fluids-chalk interactions, and is well characterised by the oil-water suction. [less ▲]

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