November-December
2001
Vol 2 No 6
Xinyou Lu and David L. Alumbaugh
ABSTRACT
An investigation of the sensitivity of two-coil coaxial (2C-40) and focused (6FF40) induction logging sondes shows that for deviated wells, cylindrical symmetry about the borehole vanishes as transverse isotropy is introduced into the formation. For horizontal wells, the sensitivity functions for both tools become increasingly complicated with increasing coefficients of anisotropy. In addition, the sign of the sensitivity changes from predominantly negative to positive with increasing anisotropy. This indicates that for formations with moderate-to-high degrees of anisotropy, the magnetic field amplitude will decrease with a positive perturbation in conductivity, which is the opposite of what occurs in the isotropic case. Increasing anisotropy is also demonstrated to cause greater sensitivity in the axial response function near the source and receivers, thus limiting the depth into the formation that is sensed. The effect of anisotropy on the transverse response function is shown to be direction dependent. In deviated wells, the effect of anisotropy on the sensitivity increases with increasing deviation angle. The sensitivity pattern is skewed and inclined along the deviation direction, but is increasingly out of alignment with the borehole as the relative deviation increases.
Hanming Wang and Liang C. Shen
ABSTRACT
In this paper the response of the dual laterolog instrument to anisotropic beds with crossbedding geometry is studied. In such geometry, the principal axes of the anisotropy of each bed may have different dip and strike angles oriented arbitrarily with respect to the bedding plane. A computer code based on the three-dimensional finite-element method (3-D FEM) has been developed to investigate the effects of this kind of formation on the dual laterolog.
Dual laterolog responses in crossbedded anisotropic beds may be generally classified in two categories: vertical and deviated wells.
Vertical wells refer to the case where the borehole is perpendicular to the formation bed boundaries. In such cases and with negligible invasion, the laterolog response is close to the true formation resistivity in isotropic beds. Its response in anisotropic beds depends largely on the relative deviation angle. The apparent resistivity response in anisotropic beds reaches a constant center-bed response when beds are thicker than five feet. Further, the apparent resistivity at the center of anisotropic beds is slightly higher than the theoretical limit of Rh for low relative deviation angles and lower than the theoretical upper limit of (RvRh)1/2 for high relative deviation angles.
The second category includes deviated wells where the borehole is deviated with respect to formation bed boundaries. In such cases the apparent resistivity reading in the anisotropic bed is affected not only by the relative deviation but also by the strike angle. Higher relative deviation in the bed increases the resistivity reading because of larger contributions from the Rv value. Despite the complicated conductivity configuration, the laterolog responses are relatively smooth. No prominent horns or spikes are observed in the many examples studied in this work.
Chester J. Weiss, Xinyou Lu and David L. Alumbaugh
ABSTRACT
Based on a previously developed analytic solution for induction in anisotropic media, we compute the eddy current paths and their associated magnetic fields for both dipping and horizontal sondes. The complex current paths are displayed using three-dimensional visualization techniques in order to better develop a qualitative understanding of the physics involved. The results indicate that electromagnetic induction in a moderately anisotropic formation is characterized by eddy current paths with significant components not only parallel, but also perpendicular, to bedding planes whenever the source magnetic dipole has a significant horizontal component. The associated magnetic fields have a strong component in the direction perpendicular to the bedding planes—a finding that supports the utility of induction sondes that include transverse-dipole transmitters and receivers to determine formation anisotropy and therefore minimize uncertainty in estimates of hydrocarbon potential.
Enhanced Evaluation of Low-Resistivity Reservoirs Using Multi-Component Induction Log Data
L. Yu, O. N. Fanini, B. F. Kriegshäuser, J. M. V. Koelman, and J. van Popta
ABSTRACT
A new multicomponent induction tool has been developed for detecting resistivity anisotropy in a formation. Existing openhole induction tools comprise co-axial arrays and contain insufficient measurements to resolve and evaluate resistivity anisotropy. In particular, in near-vertical or vertically drilled exploration wells, the measured resistivity will be dominated by low resistivity shale since the current induced will flow parallel to the bedding. Hydrocarbon-bearing sand-shale formation are often misinterpreted.
The multicomponent induction tool comprises three mutually orthogonal transmitter-receiver coil configurations sampling all data necessary to estimate both the horizontal and vertical resistivity components in vertical, deviated, and horizontal wells. Estimating both horizontal and vertical resistivity will greatly improve the accuracy of the water saturation evaluated in hydrocarbon-bearing sands in laminated formations.
We examine various response characteristics of multicomponent instrument responses. We discuss advanced data processing and inversion techniques to estimate the horizontal and vertical resistivities of the formation. We show, using a field test example from Petroleum Development Oman, that hydrocarbon saturation estimates based on data from the new induction tool are significantly higher than those that are based on conventional induction log data.
Michael Zhdanov, David Kennedy, and Ertan Peksen
ABSTRACT
One of the most challenging problems in the field of electromagnetic well logging is the development of interpretation methods for the characterization of conductivity anisotropy in an earth formation. We examine the response of a triaxial electromagnetic induction well-logging instrument in an unbounded, homogeneous, transversely isotropic conductive medium. This instrument detects three components of magnetic field due to each of three transmitters for a total of nine signals. These can be mathematically organized as a tensor array which we call the magnetic induction tensor. The magnetic induction tensor components provide a general description of the electromagnetic field in a transversely isotropic medium. By theoretically analyzing the triaxial induction instrument for its response to the magnetic field components induced in the conductive medium, we derive low frequency approximations for the quadrature components of our induction tensor. Based on this analysis, we find that by measuring the quadrature components of the induction tensor in a deviated borehole, the conductivity anisotropy of the media can be resolved from the instrument response. This information includes not only the vertical and horizontal conductivities, but also the orientation of the logging instrument axis with respect to the tensor principal axes. We introduce the formulas for the apparent horizontal and vertical conductivities sha, sva, the apparent anisotropy coefficient la, and the apparent relative deviation angle aa. These can be used as the basis for a tensor logging instrument response interpretation in unbounded, homogeneous, anisotropic media. The theory is illustrated by numerical examples of induction tensor calculations.