The Microlaterolog

Abstract

Abstract A new electrical logging method, called MicroLaterolog, is described, whereby the resistivity Rxo of the invaded zone close to the wall of the bore hole is measured. This method essentially utilizes a system of concentric circular electrodes imbedded in an insulating support which is applied to the wall of the hole. A beam of current of very small diameter is focused horizontally into the formations by means of an automatic control device, and then opens widely at short distance from the wall. With this method, Rxo most often can be recorded directly, except when the mud cake is very thick, in which case a correction is easily provided. The basic role of factor Rxo in the quantitative analysis of electrical logs in terms of fluid saturation and of porosity is explained. The paper is illustrated with field examples. Introduction In electrical logging, the resistivity of that part of the permeable and porous formations which is invaded by mud filtrate is an important factor in the interpretation. Measurements made with the conventional devices - normal, lateral- and also with improved systems as the Laterolog and induction logging - are very often more or less affected by the presence of the invaded zone, and the knowledge of the resistivity of this zone is useful in the evaluation of the true resistivity of the beds, which itself is a basic element for the determination of fluid saturation. Moreover, the comparison of the resistivity of the invaded zone with the resistivity of the mud filtrate gives valuable indications on the magnitude of the formation resistivity factor - which in turn is necessary for the quantitative interpretation of the logs, both in terms of fluid saturation and of porosity. On the other hand, it is generally admitted that the invaded zone is not a homogeneous medium separated from the uncontaminated part of the bed by a well defined cylindrical boundary, but that the fluid distribution - filtrate, connate water, hydrocarbon - and hence, the resistivity in the invaded zone varies progressively with the distance from the wall of the hole. The term "resistivity of the invaded zone" therefore corresponds to an average value which is a function of the distribution of the fluids. Inasmuch as the law of this distribution is not exactly known, the resistivity of the invaded zone is not a well defined factor.