James D. Klein
ARCO Oil & Gas Company
ABSTRACT
Electrical anisotropy signifies a dependence of rock resistivity on the direction of current flow. Anisotropy causes induction logs to read too large whenever the well bore is not perpendicular to the plane of anisotropy. The effects depend on the coefficient of anisotropy and the dip angle. The effects are small for angles less than 30°, moderate for dip between 30° and 45°, and large for dip greater than 45°.
The Kalulik and Miluveach shales above and below the Kuparuk River Formation, north slope, Alaska, are highly anisotropic and strongly affect log data from highly deviated wells. The coefficient of anisotropy for these shales is estimated to be equal to nearly 2.7.
A model has been developed for the resistivity of a laminated sand-shale sequence inclined to the well bore. The model assumes isotropic sand layers and anisotropic shale layers, and thus includes two levels of anisotropy. Model parameters are dip angle, sand resistivity, volume of shale, shale longitudinal resistivity, and shale coefficient of anisotropy. This model can be used to correct induction logs for anisotropy.
Application of anisotropy corrections to logs from highly-deviated wells at the Kuparuk Oil Field produces resistivities that are consistent with logs from vertical wells. Formation Microscanner data confirm the electrically anisotropic character of the reservoir.
If corrections are not made, anisotropy may result in erroneous induction log interpretation. These errors are not limited to deviated wells, but may also occur in vertical wells, depending on the geologic structure.