Determination of Fine Structure of Shale from Logging Data
Irina Bayuk and Evgeni Chesnokov
Institute For Theoretical Geophysics, College of Earth and Energy, University of Oklahoma
A theoretical method has been elaborated allowing the determination of pore/crack geometry of illite-rich shale from logging data. Two pore/crack systems are assumed to be present in shale: (1) clay-related voids and (2) vertical cracks. The method is based on the inverse problem solution incorporating a minimization of misfit between experimental values of elastic wave velocities provided by sonic (including dipole cross sonic) measurements and the theoretical values. For the minimization, methods of nonlinear optimization with limitations imposed on the sought-for parameters are applied.
When calculating the theoretical values of elastic wave velocities, the general singular approximation (GSA) method of effective medium theory (EMT) is used. In such a modeling, pores and cracks are assumed to be ellipsoids of revolution whose shape is characterized by aspect ratio. The connection between the pores and cracks is taken into account by a parameter called friability that is also found from the inverse problem solution. The theoretical values of elastic wave velocities result from the five-step modeling incorporating the following steps:
(1) placing the illite single crystals and thin cracks filled with bound water in kerogen matrix,
(2) imbedding the clay-related pores and cracks,
(3) rotation of the “illite-kerogen” cracked particles in the polar plane,
(4) imbedding the grains of minerals (quartz, calcite, dolomite, and so on),
(5) adding the vertical cracks to the medium.
The logging data necessary for solving such a problem are: compressional and two shear elastic wave velocities, density, porosity, and mineral composition of shale (at least, average, for the depth interval under study).
As a result of the inverse problem solution the following parameters are found: two aspect ratios characterizing the shape of clay-related and vertical cracks, amount of vertical cracks, friability, and angle of deviation of “illite-kerogen” cracked particles from the vertical direction. Besides, all components of elasticity tensor of shale are determined, which makes it possible to theoretically predict the elastic wave velocity propagating in any direction.
This method has been applied to real logging data for shale. It was found that the clay-related voids exhibit variability of aspect ratio values varying from 1e-5 to 0.1. The friability parameter resulting from the inverse problem solution indicates high pore/crack connection. At some depths, the shale under study contains vertical cracks (no more than 1%) having aspect ratio up to 0.01. The anisotropy of elastic wave velocities theoretically calculated form the elasticity tensor components is around 8 – 10% both for compressional and shear wave velocities.
AAPG Search and Discover Article #90065©2007 AAPG Southwest Section Meeting, Wichita Falls, Texas