ABSTRACT

Directional wells, especially horizontal wells, are commonly drilled today to enhance reservoir productivity and minimize unwanted production of water or gas. At the steep apparent dip angles encountered, logging tool response characteristics change. Induction tools become more sensitive to bed boundary location. They also detect resistivity anisotropy, which remained largely invisible in vertical wells. The interpretation of induction logs in directional wells poses several challenges. Like all logging tools, induction tools were developed for wellbores perpendicular to the bedding planes. The measurements provide several radial depths of investigation. Separation of the logs is generally caused by invasion, and this separation provides a radial resistivity profile. However, in directional wells, a cap shale or an aquifer can cause induction curves to separate because the multiple depths of investigation have different sensitivities to beds adjacent to the zone of interest. Thus curve separation no longer indicates invasion exclusively. In anisotropic formations, induction tools sense a weighted average of the horizontal and vertical resistivities. This observed resistivity may differ considerably from the resistivity of a nearby vertical reference well where induction tools sense only the horizontal resistivity. In these complex formation geometries, forward modeling can provide a reliable resistivity interpretation of tool response in layered, anisotropic media. The computer modeling program generates logs for either dual induction or array induction tools. The tool can be tilted at any user-provided angle against a layer-cake formation. Each layer may have an intrinsic resistivity anisotropy. A major advantage of the program is that computer run-time is independent of the number of beds modeled. The modeling program is used to study the sensitivity of both array induction and dual induction tools to anisotropy. In thick beds, anisotropy alone can cause the induction curves to separate because the mixing of the horizontal and vertical resistivities distorts the skin effect correction. Curve separation is most noticeable in conductive zones and at high dip angles. At steeply dipping bed boundaries, polarization horns appear, similar to those occurring on 2-MHz resistivity logs. These horns are most prominent on the long arrays. A field log example is used to illustrate the use of tool response modeling in an iterative inversion for Rt. The induction interpretation is corroborated by 2-MHz resistivity measurements obtained while drilling. Logs are modeled in both vertical and near-horizontal wells in the same layered formation.

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