188 Seismic Imaging 7.6 Mechanical properties To prepare for the construction and operation of a deep geological disposal facility, the mechanical behaviour of the rock formations must be well understood. One of the parameters studied for this purpose is the Young’s modulus. A workflow has been developed to estimate static Young’s moduli in claystone and limestone formations on seismic lines. The P-wave and S-wave velocity (Vp, Vs) distributions and the density ρbulk distribution obtained by elastic inversion of the seismic data after calibration on well data (VSP and acoustic logs) enable the computation of dynamic mechanical modules, such as the shear modulus (m), Young’s modulus (E), Poisson’s ratio (ν). The dynamic Young’s modulus is given by the following formula (equation (7.7)): E V v v v bulk = − ( ) + ( ) − ( ) ρ P 2 1 2 1 1 , v V V V V V V V V = ( ) − ( ) − = − − ( ) P S P S P S P S / / 2 2 2 2 2 2 2 2 1 2 2 (7.7) The Vp, Vs and ρbulk distributions for the seismic lines 07EST10, IL405 and XL217 are shown in Figures 7.12 to 7.14 respectively. Mechanical properties of the Callovo-Oxfordian clay formation were characterized in the laboratory (deformation modulus, compressive strength, tensile strength, etc.) using conventional triaxial or uniaxial compression tests. Samples that were significantly desaturated (Sr < 90%) or damaged were eliminated from the analysis. Finally 39 core samples, selected from the Cox in well Est 433, were used to measure the static Young’s moduli ES in the laboratory. At the same depths, logging data (acoustic and density logs) were used to compute the dynamic Young’s moduli ED. The results are shown in Figure 7.19 (top left). A description of methods for determining the relationship between static and dynamic Young’s moduli can be found in a number of standard texts. A synthesis is given by Eissa and Kassi (1988). In the laboratory it has been shown that it is possible to predict the static moduli values ES from the dynamic values ED. The appropriate function relating to the high correlation coefficient between measured and predicted static values is the linear function: ES = a.ED + b (7.8) However, Eissa and Kassi (1988) have shown that the value of the static modulus of elasticity can be best predicted from the relationship: Log10 (ES) = a.Log10 (ρbulk ED) + b (7.9)
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