Identification of the Electrical Anisotropy on Cretaceous Turbiditics Reservoirs Through Log Simulation in Horizontal Wells
Authors: Soares G.F. , Coutinho M.R. 
Published: December 2007

Begun in the early 80's, the drilling of horizontal wells is a technique whose utilization has undergone a very significant increase this decade. Moreover, as the interpretation of logs was originally developed for the geometry of vertical wells, or those with only moderate deviation, it has become necessary to establish new parameters for interpretation in horizontal or highly deviated ones. In these, the tools are run in a position almost parallel to the bedding, and we actually measure a resistivity that can be decomposed into horizontal (Rh) and vertical (Rv) components.

Using the two curves (phase shift and amplitude attenuation) of a 2 MHz MWD electromagnetic tool in a horizontal well, we calculated the Rh, Rv and the coefficient of anisotropy in Cretaceous turbiditic reservoirs. Toward this end, we used two methods: the Hagiwara (1996), and a log response simulation that uses the Rh from the geologic model of the area, and calculates Rv and the coefficient of anisotropy by matching the computed curves against the acquired ones. Since both methods lead to results with reasonable agreement between the measured and computed logs, they may be applied to obtain Rh and Rv in horizontal, or highly deviated, wells for the purpose of log interpretation.

Introduction

With the increase in horizontal well drilling over the last ten years, a new challenge has appeared, that is, how to understated and interpret the logs from these wells. Previously, the logs were acquired from vertical or slightly deviated wells, and all interpretation was based on a model where the layers were almost perpendicular to the well. Nowadays, in horizontal wells, the tools are positioned parallel, or almost, to the layers, thereby creating a need for new techniques and interpretative parameters.

The need to quantify the horizontal resistivity values (Rh) in horizontal or highly deviated wells so as to enable comparison with the resistivity values derived from vertical wells, is a topic of much ongoing research.

Effects, such as electrical anisotropy important in the interpretation of resistivity curves in highly deviated wells, have been studied since the beginning of the century. Researchers, like Cheamali (1987), Gianzero (1978), Bittar (1994) and Hagiwara (1992/1994), have all developed methods to quantify the anisotropy and to determine the horizontal and vertical resistivities from the logs, combined with knowledge of the relative angle between the well and the layers.

At the same time these methods were being developed, some new log simulation programs were being devized that would allow us to simulate the behavior of logging tools. To simulate resistivity logs it is necessary to load this kind of program with the geologic model of the area, the well trajectory, and the horizontal and vertical resistivities. The proper utilization of these programs not only enables improvements in the geologic model, but also contributes to a better interpretation of the logs. The aim of this work is to determine the horizontal and vertical resistivities, as well as the electrical anisotropy coefficient, in a horizontal well, drilled in cretacic reservoirs of the Caipos Basin, through two different methods: