The Dual Laterolog In Common Complex Situations

Highly dipping beds, washed-out boreholes, and very resistive formations are regularly encountered when running the dual laterolog. Field experience indicates that the tool performs usually well under these circumstances, but no modeling has been given to date to quantify the effect of such anomalies on the tool response. The model for dipping beds is performed with a numerical three-dimensional finite element method. For verification, a degenerate case, ignoring the presence of the borehole, is modeled both analytically and numerically. The agreement between the two approaches confirms the accuracy of the numerical method, The tool response to a dipping interface is then simulated for both deep and shallow devices. An apparent smearing of the boundary, and a small displacement in depth of the log are predicted by the simulation. Computed current patterns help visualize the effect of the dipping interface on the tool. The modeling is extended to dipping beds of varying thicknesses and resistivities, and for different mud resistivity values. Wash-outs in the borehole are modeled using a two-dimensional semi-analytical method. The physical configuration is assumed to exhibit a cylindrical symmetry about the borehole axis. As expected, the deep laterolog shows less sensitivity to hole irregularity than the shallow. Finally, the case of large resistivities is addressed. Standard borehole correction charts are applicable to Rt/Rm = 10000. In a salt saturated borehole fluid, this corresponds to only Rt = 200 ohm-m. By improving the accuracy of a known analytical model the corrections are extended to a resistivity contrast of 10ø. The sensitivity to sonde eccentering in highly resistive formations is also evaluated.