Formation Electrical Anisotropy Derived from Induction Log Measurements in a Horizontal Well

Abstract

An existing theory describes how electrical anisotropy in the formation affects the response of resistivity logging tools. We have related this theory to the processing of LWD induction logs, and are thus able to calculate the anisotropic resisticities directly from the logs. The method has been demonstrated by application to logs from a horizontal well section. Anisotropy ratios of 2–5, and occasionally higher values, were obtained for this formation. We also addressed the accuracy of these numbers by using independent sets of input logs. The results indicate that the logs are influenced by factors like invasion in addition to the anisotropy. Our approach provides a fast and efficient computer algorithm. The output is calculated at the depths of the input logs; hence, the resulting anisotropy becomes a depth-dependent formation property. Introduction Electrical anisotropy has gained considerable attention over the last years. If present in the formation, neglection of this property when interpreting resistivity logs may lead to erroneous saturation estimates, and may thus have great consequences upon development and production strategies and the overall economics. Electrical anisotropy denotes that the resistivity shows directional dependence. In sedimentary formations, it is commonly assumed that the anisotropy is caused by the deposition process, which yields different small-scale (grain and pore size scale) structural properties in the vertical and the horizontal direction. Anisotropy may also occur on "lithology scale", i.e., as a result of thin layers (compared to the extension of the electric field) having individual isotropic properties. Since the effect is determined by the sedimentary structure, a formation can be expected to show anisotropy in several properties simultaneously, like electric, acoustic, and fluid flow resistance (permeability) properties. A common way of describing anisotropy is to distinguish between the vertical direction and directions in the horizontal plane. In this paper, we shall denote the resistivities in these directions by RV and RH, respectively. However, the terms "vertical" and "horizontal" refer to the original deposition process, and may no longer correspond to the actual orientation of the formation, due to small- or large-scale geological activity. For dipping beds, it is common practice to assume one resistivity (RH) in the bedding plane, and one (RV) in the direction normal to the bed, unless evidence of intra-bed disturbances suggests other orientations of the anisotropy. Numerous publications have addressed the influence of electrical anisotropy upon resistivity logs. Among the effects that have been studied are anisotropy in dipping and thinly laminated formations1–3, and in crossbedded formations4. Effort has been put on theoretical tool response modelling and simulation5–7, and on anisotropy corrections to logs8,9. From field cases, anisotropy ratios (RV/RH) up to the order of 5–10 have been reported7,8,10. In this paper, we demonstrate a method for calculating the electrical anisotropy directly from well logs, based upon the theory developed by Hagiwara6. The method has been implemented and applied to log data from a horizontal North Sea well. Theory Hagiwara6has analysed the resistivity log's response in anisotropic formations. According to this reference two different measurements are sufficient to determine the anisotropy unambiguously, as long as the anisotropy orientation is known. The measurements may differ with respect to one or more of the following:electrode spacing (which is a prerequisite for phase- and attenuation- derived resistivity);frequency;deviation angle between tool axis and anisotropy orientation.

Imparting Directional Dependence on Log-Derived Permeability

“…2 are the k V :k H data from a recently published North Sea study. 9 Similar to the data published by Keelan 7 , the local k V :k H ratio based on paired plugs is 0.5, although the ratio of 0.1 is not unusual, and some plugs exhibit a much smaller permeability ratio of 0.01.…”