Abstract
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.
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