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Approche des mécanismes de l'injection sableuse per descensum

Abstract : The outcrops of Bevons, Nyons and Rosans in south-eastern France, like the Numidian ones (Sicily, Tunisia,Morocco...) or Tourelle ones (Canada) are excellent spots for the observation of many sandy injectites supplied by turbiditic channels. Two types of injectites are present : sills (horizontal) and dykes (vertical), dykes resulting from sills. Depending on the authors, their creation processes can either be upward (per ascensum), postdepositonal, or, probably more often, downward (per descensum), contemporaneous of the sand feeder set up and object of this study. The study and the modeling the mechanisms at work in this fractured network has two major benefits. First, the ability to predict the hermeticity of the argillaceous materials, with the prospect of using them as waste storage sites. Second, since sandy injectites can be found between hydrocarbon-richreservoirs, they could help predict fluid circulations and cross-flows during the exploitation of those reservoirs. A geometrical model has been established from the observation of the outcrops and has led to certain assumptions on the mechanisms being taken into account in the modeling of either the fracturings or the injections. Several approaches have been carried out in parallel : - the study of the compaction of argillaceous sediments prior to injection : for rather high sand concentrations (35 %), the weight of the injection mixture column is higher than the vertical stress in the superficial part of the sedimentary succession. This reversed density can be responsible for the fact that the injection is more likely to propagate downwards in the case of dykes or along stratigraphic discontinuities (ash layers, for example). Deeper in the sediments, the pressure in the sandy mixture column varies linearly with depth whereas the stress in the sediments does not (because of an exponential porosity curve). From a certain depth on, the fluid pressure becomes lower than the stress in the sedimentary succession : the injection cannot exceed this depth. Thus, the propagation of the dykes is limited by the weight of the host material. The use of the hydraulic fracturing as injection mechanism : rock mechanics and more precisely the hydrofracturation mechanisms have been tested to try to model these large scale observations. The force responsible for the initiation and the fractures' propagation is the pressure due to the turbiditic filling of the channel. The model takes into account the fact that the injection must raise the overlying sediments. The first results have shown that as soon as a horizontal fracture is initiated, the propagation is fast. The opening that is obtained (sill thickness) is decametric in size : it is the same order of magnitude as the outcrops' observations. - the use of a yield stress fluid in a stochastic network (with probabilities adjusted so that the realizations are consistent with those actually observed) has allowed the determination of the maximum extensions of injection. The results of the model are in agreement with the observations : - a single sill can propagate on 2400 m in a few tens of hours, - a single dyke can go down to 400 m under the paleo-sea-floor in a few seconds, - a network of connected sills and dykes (connected by a sill to the main channel) can have an extension of 1200 m horizontally and 400 m vertically, and will form over a few hours. The formation of the sills is longer than that of the dykes, as the formation of a single sill takes several hours whereas that of a single dyke only takes a few seconds. When sills and dykes are connected, the network propagation time is approximately 2 hours. Dykes are always formed in a few seconds and have an effect on the propagation of the sills, which are propagated half as far as if they were alone.
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Submitted on : Tuesday, October 2, 2007 - 8:00:00 AM
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  • HAL Id : pastel-00002979, version 1


Rosalie Vandromme. Approche des mécanismes de l'injection sableuse per descensum. Planète et Univers [physics]. École Nationale Supérieure des Mines de Paris, 2007. Français. ⟨pastel-00002979⟩



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