Reconciliation of Resistivity Log Readings in Vertical and Horizontal Boreholes

Abstract

Two examples are discussed of a vertical pilot hole and a horizontal side-track, in which the resistivities recorded in the vertical hole are significantly different from those recorded in the horizontal hole. It is concluded that the differences may be caused by (1) variations in reservoir quality, (2) nearby layers, (3) resistivity anisotropy or (4) borehole and invasion effects. Resistivity tool modelling is found to be an essential tool in the analysis. However, the presently available computer codes are capable only of handling the effect of nearby, parallel layers. Full 3D modelling will be required to account for borehole and invasion effects and more complex layering. In addition, core data or data from "directional-resistivity" logging tools will be required to quantify the effect of resistivity anisotropy. As long as it is impossible to fully quantify the above-mentioned effects, it is recommended to treat hydrocarbon saturations from horizontal well logging data with great care, especially when they are to be used in oil-in-place and/or reserves calculations. This applies in particular when no nearby vertical calibration well is available. Introduction With the maturing of horizontal well drilling technology, future oil-in-place and reserves estimates are likely to become more and more dependent on logging data from (near) horizontal wells. The prime means to determine hydrocarbon saturations is via resistivity logging, and one might expect that resistivity log interpretation in a horizontal well is similar to the interpretation in a vertical well. However, a comparison of resistivity log readings obtained in several horizontal wells with those from nearby vertical wells shows that the recorded resistivities can be substantially different. Two examples will be discussed in detail, involving both a vertical pilot hole and a horizontal side-track. In the analysis of this phenomenon, the resistivity tool responses in both the horizontal and vertical wells were simulated using forward modelling computer codes. The codes calculate tool response by solving Maxwell's equations for a simplified logging environment: borehole effects and invasion are neglected and the formation is described by a layered-earth model. The codes can, however, handle any dip between the borehole and the layers, which allows analysis of the above-described phenomenon insofar as dip is an issue. In order to take borehole effects into account, an approximate procedure was applied in which the modelled tool response was compared with the actually measured response, corrected for borehole effects according to the charts provided by the logging contractor. The error that is introduced by this procedure is probably quite small as the borehole corrections were found to be negligible for the two examples discussed in this paper. Unfortunately, it is not possible to apply a similar procedure to account for invasion. In a horizontal borehole, the invasion profile is expected to be asymmetric. Since the invasion correction charts provided by the logging contractor are based on symmetric (circular) invasion profiles, they are not applicable here. P. 151^