Time-lapse seismic surveys: Rock physics basis

John Jianlin Zhang

ABSTRACT

The major factor that determines the elastic moduli of a rock is the pore geometry. Based on the inverse Kuster-Toksoz modelling of sandstone samples, it is found that the concentration of round pores is in direct proportion to porosity while that of cracks is small and has no correlation with porosity or clay content. The variability in the elastic moduli at a given porosity can be attributed chiefly to the variability in the crack concentration. Moreover, the effect of cracks on the elastic moduli decreases with increasing porosity. Consequently variation of the elastic moduli with changing effective pressure will generally become smaller at high porosity.

The effect of fluids on the elastic moduli is most important in the rock of low dry elastic moduli. Effective pressure has a greater impact on the dry elastic moduli at low effective pressure and the rate of change of the dry elastic moduli with respect to effective pressure can be modelled as the quasi-exponential function. Temperature has a small effect on the dry elastic moduli unless it changes drastically.

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