“Dual laterolog responses in anisotropic crossing bedding formation,” Petrophysics, Vol. 42, No. 6, pp. 624-632, 2001.
In this paper the response of the dual laterolog instrument to anisotropic beds with cross bedding geometry is studied. In such geometry, the principal axes of the anisotropy of each bed may have different dip and strike angles oriented arbitrarily with respect to the bedding plane. A computer code based on the three-dimensional finite element method (3-D FEM) has been developed to investigate the effects of this kind of formation on the dual laterolog. Dual laterolog responses in cross bedded anisotropic beds may be generally classified in two categories: vertical and deviated wells. Vertical wells refer to the case where the borehole is perpendicular to the formation bed boundaries. In such cases and with negligible invasion, the laterolog response is close to the true formation resistivity in isotropic beds. Its response in anisotropic beds depends largely on the relative deviation angle. The apparent resistivity response in anisotropic beds reaches a constant center-bed response when beds are thicker than five feet. Further, the apparent resistivity at the center of anisotropic beds is slightly higher than the theoretical limit of RI, for low relative deviation angles and lower than the theoretical upper limit of (R,,R/,)"? for high relative deviation angles. The second categoty includes deviated wells where the borehole is deviated with respect to formation bed boundaries. In such cases the apparent resistivity reading in the anisotropic bed is affected not only by the relative deviation but also by the strike angle. Higher relative deviation in the bed increases the resistivity reading because of larger contributions from the R,. value. Despite the complicated conductivity configuration, the laterolog responses are relatively smooth. No prominent horns or spikes are observed in the many examples studied in this work.