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Soutenance de thèse: « Effect of mechanical compaction and biphasic saturation on the frequency dependence of elastic wave velocities in porous rock »


École Normale Supérieure


Séminaires ENS


Sun Chao

ENS-PSL / Université du Pétrole de Pékin Abstract The aim of this thesis is to investigate the dispersion of elastic wave velocities in porous and saturated rocks. An introduction is given in Chapter 1. Chapter 2 presents two stress-strain devices, which can both capture the evolution of the Young’s modulus and Poisson’s ratio in the seismic frequency band. In addition, we analyzed the resonance effect of the apparatus using a numerical modeling approach. In particular, the numerical model indicated that a modification of the shaker boundary can move the first resonance frequency to higher frequency. Using this method, the frequency-band of the apparatus installed in Beijing was improved to 1-2000 Hz compared to the original ones of 1-100 Hz. On the other hand, we were able to show that the apparatus installed in Paris have similar resonance contaminations but at frequency higher than 1000 Hz. In addition, this second apparatus was modified to cut-off the pore fluid dead volumes using new ‘micro-valves’. Chapter 3 presents a generalized global-drainage-flow model based on the three-dimensional diffusion equation. In addition, the three-dimensional model can also be used to predict the influence of the « dead volume » on the dispersion. In the chapter 4, we investigated the influence of microscopic heterogeneity on the global flow and local flow at the scale of strain gauges. The observation of the local flow is influenced by the position of the strain gauges. It is due to that although the sample is homogeneous in terms of porosity and crack density, it is not the case in terms of crack aspect ratio, which may slightly vary along the sample. A 3D diffusion model coupled with a simple squirt model was built to further interpret the data. In the chapter 5, we investigate the effect of mechanical compaction on squirt-flow. The Bleurswiller with a porosity of 25% was mechanically compacted through a triaxial cell, and a stress-strain device was used to measure Young modulus dispersion and attenuation. Measurements show that pore collapse and grain crushing increase crack-fraction and the mean crack aspect ratio. The critical frequency of the squirt-flow therefore moves towards higher frequencies. As a consequence, for the compacted sandstone, Gassmann equation can be applied to wider frequencies, even to the logging band. These results have potential applications in reservoir monitoring, well-log interpretation, and time-lapse seismic analysis. Finally, in the chapter 6, we conducted an experiment to investigate the influence of saturation on the elastic wave velocities. An Indiana limestone was chosen due to that no dispersion related to squirt flow has been observed in the sample. Two saturation method-imbibition and drainage-were used to saturate the sample. Only for the drainage case, a significant attenuation and dispersion are observed. This is the consequence of the fluid distribution. In the drainage case, as the saturation increase, the critical frequency moves to lower frequencies. Finally, a model was built in the frame of Biot’s theory to predict these last measurements. Jury Professeur Ba (Hohai University) : Rapporteur / Dr Sarout (CSIRO) : Rapporteur / Pr. Leroy (Imperial College) : Examinateur / Dr. Adelinet (IFPEN) : Examinateur / Pr. Gueguen (ENS) : Examinateur / Pr. Tang (China University of Petroleum) : Examinateur / Pr. Wang (China Unversity of Petroleum) : Co-directeur de thèse / Dr Fortin (ENS/CNRS) : Directeur de thèse