ESTIMATION OF THE INTERNAL STRUCTURE AND ANISOTROPY OF SHALES FROM BOREHOLE DATA

摘要

Johansen, T.A., Jakobsen, M. and Ruud, B.O., 2002. Estimation of the internal structure and anisotropy of shales from borehole data. Journal of Seismic Exploration, 11: 363-381. A strategy for estimating the grain scale alignment properties and .anisotropy of shales on the decimeter scale from borehole data is presented. The method is based on a rock physics model which takes into account the effects of rock composition and texture to the acoustic properties. The shale is considered as an aggregate of so-called building blocks (regions of fully aligned clay platelets and pores), having an axial symmetric orientation distribution, in which inclusions of isolated (spherical) silt particles are embedded. The internal orientation properties are estimated from the vertical stiffnesses of the shale aggregate. The vertical stiffnesses are obtained from the density and P- and S-wave velocity data adjusted for the effects due to presence of silt. To do the inversion the properties of the building blocks have to be known. These are predicted from the composition data assuming a fixed shape of the clay platelets. Tests have been performed on both synthetic data and on borehole data. The choice of stiffness parameters for the clay-minerals is seen to influence on the inversion results; the stiffer the clay-minerals are, the stronger is the predicted alignment and anisotropy. The examples emphasize that the physical properties of the clay-minerals seem to play an important role for the estimation of the large scale anisotropic properties.