Strange Induction Logs- A Catalog Of Environmental Effects

The field testing of the Phasor induction tool revealed a number of situations where Phasor logs differed from conventional induction logs. These differences have been satisfactorily explained by using computer modeling in conjunction with log analysis. However, during the process of resolving the differences the authors became aware that the extent of environmental effects on conventional induction tools was not widely known. In order to alert log analysts to these problem areas of induction interpretation, several examples of unexpected, or "strange" environmental effects are examined in this paper. In high resistivity formations, a "thin bed" for induction can be 50 feet or more. Errors in interpretation caused by failing to recognize extreme shoulder effect in resistive beds can lead to underestimating the amount of hydrocarbons present in a reservoir. A more precise understanding of the severity of shoulder effect based on a systematic examination of computed logs for various bed thicknesses helps avoid these errors. At the low end of the resistivity range, horns and overshoots at bed boundaries are the normal response of the conventional deep induction tool, while these artifacts are absent from Phasor logs. Computed logs generated from known formation parameters remove ambiguities in interpreting low resistivity formations. All sorts of strange responses that have no other apparent cause have been attributed to dipping beds. Using a computer code to model tool response in multi-layer formations with dip, assumptions concerning dip can be tested, and the correct interpretation rules for induction response in dipping formations can be formulated. The original purpose of the dual induction tool was to give a better estimate of Rt in the presence of resistive invasion. However, the concepts of the shallow, medium and deep measurements can be upset if induction tools are run out of necessity in formations where the invaded zone resistivity is considerably less than that of the uninvaded zone. Modeling tool response in different invaded formations again allows for formulation of the proper interpretation rules.