Determining Anisotropic Formation Resistivity at Any Relative Dip Using a Multiarray Triaxial Induction Tool

Abstract

Abstract Thin-bedded sand-shale sequences frequently exhibit resistivity anisotropy. That is, resistivity measured perpendicular to the bedding (Rv) is significantly higher than the resistivity measured parallel to the bedding (Rh). It is difficult to determine water saturation in these sequences using standard induction logs that respond primarily to Rh, regardless of relative dip angle. If conductive shale laminations are present, the water saturation is over estimated; equivalently, hydrocarbon in place is underestimated. A recently introduced triaxial induction tool provides several 3x3 tensor measurements that are sensitive to Rh, Rv and formation dip. A fast and rigorous inversion of triaxial induction data provides logs of Rh and Rv without shoulder bed effect, and also provides logs of formation dip. The robustness of the inversion algorithm has been validated with synthetic log data. A petrophysical model has been developed that computes both sand resistivity (Rsand) and shale resistivity (Rshale) from Rh and Rv logs. Fraction of shale, determined using established log interpretation techniques, is an input to the model. Since the induction measurement is relatively deep, the uncertainty in water saturation using the model is greatly reduced compared to interpretation models that use microresistivity logs. Case studies from oilfields around the world demonstrate the utility of triaxial induction measurements for formation evaluation. Data were acquired in a wide variety of borehole and formation environments. These include oil-based mud, water-base mud, dipping formations, vertical wells, deviated wells, and a wide-range of borehole sizes. Low resistivity pay zones were identified that might otherwise be missed with standard induction logs. Microresistivity images confirmed the presence of thin laminations and confirmed the dip measurements from the triaxial induction. Introduction H.G. Doll introduced the standard induction tool in 1949.1 An alternating current of constant intensity is sent through a transmitter coil with its magnetic moment along the axis of the tool (axial coil). The alternating magnetic field induces currents in the formation surrounding the borehole. These currents flow in a formation ring that behaves like a transmitter coil and generates its own secondary magnetic field, which induces an electric signal measured as a voltage in the axial receiver coil. This voltage is proportional to the rock conductivity and is normalized in terms of apparent conductivity. With the axial coil arrangement in standard induction tools (dual induction tools and array induction tools), the induced formation currents are orthogonal to the tool (and borehole) axis. The limitations of standard induction measurements can be expressed in several ways:When the thicknesses of individual beds are less than the vertical resolution of the induction (e.g., several feet), the formation can be considered as an equivalent anisotropic formation.Standard induction logs are dominated by conductive layers and are relatively insensitive to resistive layers that may be associated with hydrocarbon bearing sands. This occurs at all relative dip angles but is especially pronounced at zero relative dip. For the case of zero relative dip, induced currents do not cross bed boundaries and the measured conductivity is essentially a measurement of conductivity parallel to formation bedding. The measurement is insensitive to conductivity perpendicular to formation bedding.When the thicknesses of individual beds are comparable to the vertical resolution of the induction, the tool response is affected by conductive shoulder beds. The shoulder bed effect is especially pronounced at high relative dip angle. Dip and shoulder bed effect can be corrected using advanced post-processing algorithms (e.g., maximum-entropy processing2,3). This type of processing requires an external measurement of dip from, for example, a borehole imaging tool.