THEORETICAL MODELLING OF RESISTIVITY TOOL RESPONSE USING FINITE DIFFERENCE METHOD

Abstract

Finite difference method (FDM) can be used for solving direct current resistivity problem in borehole environment. Log interpretation can be extended in 2D environment after taking into account transitional invaded zone, flushed zone, uncontaminated zone, shoulder beds and borehole filled with mud simultaneously. Theoretical responses of normal and laterolog7 (LLT4) have been calculated using the expanding rectangular grid system. LL7 responses did show some superiority over the conventional normal and lateral for conductive target beds sandwiched between the resistive beds. For relatively small borehole diameter, all three probes can detect resistive target bed satisfactorily. However, for highly conductive mud both L17 and normal, electrode configuration fail to detect any signal from the target bed. Bypassing effect of the current for LL7 is more than that in normal log. Lateral log shows a signal of conductive invaded zone generated by the conductive mud. Bucking current ratio peaks and the geometric factor peaks in LL7 can. detect the bed boundaries.