Correcting the Induction Log for Dip Effect

Abstract

Abstract Determining the true formation resistivity Rt from induction logs in the presence of either dip or well deviation is complicated by the distortions in the log introduced as the apparent dip angle increases. This effect is well documented. The effect on traditional ID logs is to make shoulder effect somewhat worse, causing resistive beds to read even lower. The effect of dip on Phasor* induction logs is more readily apparent, since the shoulder effect has been removed. As the dip angle increases, the Phasor ID logs read more nearly like the traditional ID log. A correction algorithm has been developed which produces the correct curve shape and resistivity reading in resistive beds, but which does not attempt to reduce the apparent thickening of resistive beds. The charge-buildup effects which lead to this apparent thickening are quite nonlinear, depending strongly on the conductivity contrast across the boundary. The increase in shoulder effect with dip angle, however, is approximately linear in both contrast and conductivity magnitude. This near-linearity allows a correction algorithm to be developed using the inverse-filter method. The algorithm as currently developed gives greatly improved resistivity accuracy in resistive beds at angles up to 70°. Tests on a large number of computed logs confirm the apparent linearity of the main part of the dip effect.