Real-time Estimation of Resistivity Anisotropy Using Array Lateral and Induction Logs
Authors: Frenkel Michael A. , Geldmacher Ingo M. 
Published: October 2008

Conventional resistivity tools often miss hydrocarbon pay zones in thinly laminated sand-shale sequences. Incorporation of formation resistivity anisotropy into a petrophysical model allows for a significantly more accurate means to estimate hydrocarbon reserves in low-resistivity reservoirs. To correctly determine the anisotropy distribution around the borehole, an interpreter must use the multi-component induction logs acquired with the application of the new 3DEX tool.

If the multi-component induction measurements are not available, an interpreter should consider some alternative ways to determine anisotropic properties of the formation. In this paper, we propose two new methods to estimate resistivity anisotropy using vast logging data from the Array Lateral Log (High-Definition Lateral Log - HDLL) and Array Induction Log (High-Definition Induction Log - HDIL) tools:The first method is based on a sequential interpretation of the HDIL and HDLL data. It does not require layer selection for formation model definition. It is very fast because it uses sequential induction and galvanic data processing as well as short inversion windows.The second method utilizes borehole-corrected lateral resistivity (LR) logs and vertical resolution matched (VRM) focused logs. The method does not require application of inversion-based processing and provides anisotropy values at every logging point. This approach is extremely fast. We evaluate and compare the anisotropy interpretation results derived from the application of these methods. We use a 150-foot portion of a single data set acquired in a vertical offshore well in the Mediterranean region. This unique logging data set contains various different array resistivity logs. We show that both methods supply an interpreter with anisotropy estimates within time limits imposed by real-time well-site processing.

Background

A significant part of the world's estimated hydrocarbon reserves is contained in thinly-bedded, low-resistivity formations containing hydrocarbon producing laminar sands. In these formations, the transverse (vertical) resistivity, Rv, perpendicular to the bedding plane is greater than the longitudinal (horizontal) resistivity, Rh, parallel to the bedding plane. This dependence of resistivity on the direction of current flow is called anisotropy.

The anisotropy coefficient is defined as a ratio of Rv over Rh: Ratio Rv over Rh (Available in full paper)

The traditional laminar shaly sand saturation equations are based on a simple parallel conductivity model that typically results in significant underestimation of hydrocarbonsin- place. The resistivity anisotropy based interpretation methods, when combined with petrophysical analysis, lead an interpreter to a better identification and a more accurate quantification of hydrocarbon reserves.1

An accurate determination of formation anisotropy could be achieved via the application of unique multi-component induction (3DEX) technology. This approach was developed to provide both improved identification and quantification of hydrocarbons. Our 3DEX tool provides all necessary data to accurately compute both horizontal (Rh) and vertical (Rv) resistivities.1-4 Enhanced shaly sand reservoir characterization models utilizing multi-component induction data lead interpreters to a significant increase in the estimates of hydrocarbons-in-place.1

If the multi-component induction measurements are not available, or if use of this instrument under given environmental conditions is not recommendable, an interpreter should consider alternative ways to det