RELATIVE PERFORMANCE OF FOCUSED AND UNFOCUSED DEVICES IN A 2-DIMENSIONAL BOREHOLE ENVIRONMENT

Borehole direct current (dc) resistivity problems are solved using fin ite difference method (FDM) primarily to extend the log interpretation in the two-dimensional (2D) domain. Transitional invaded zone, flushe d zone, uncontaminated zone, shoulder beds and borehole filled with mu d are simulated simultaneously in the model. Linear variation of resis tivity is assumed for transitional invaded zone. Normal, lateral and m odified laterolog7 (LL7(4)) electrode configuration (discussed in the text) responses were computed using expanding rectangular grid system. LL7(4) responses with variable geometric factor did show some superio rity over the conventional normal and lateral for conductive target be ds sandwiched between the resistive beds. For relatively small borehol e diameter, all three probes can detect resistive target bed satisfact orily. However, for highly conductive mud both LL7(4) and normal elect rode configuration fail to detect any signal from the target bed. Bypa ssing effect of the current for LL7(4) is more than that in normal log . Lateral log shows a signal of conductive invaded zone generated by t he conductive mud. Bucking current ratio peaks and the geometric facto r peaks in LL7(4) can detect the bed boundaries.