Electricity: measuring and testing – Of geophysical surface or subsurface in situ – With radiant energy or nonconductive-type transmitter
Reexamination Certificate
2002-11-22
2004-11-16
Deb, Anjan (Department: 2862)
Electricity: measuring and testing
Of geophysical surface or subsurface in situ
With radiant energy or nonconductive-type transmitter
C702S007000, C175S045000
Reexamination Certificate
active
06819111
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to measuring parameters of interest in a downhole environment. More specifically, the invention is an apparatus and method for determining resistivities values and relative dip in an anisotropic borehole formation.
2. Background of the Art
Electromagnetic induction and wave propagation logging tools are commonly used for determination of electrical properties of formations surrounding a borehole. These logging tools give measurements of apparent resistivity (or conductivity) of the formation that, when properly interpreted, are diagnostic of the petrophysical properties of the formation and the fluids therein.
The physical principles of electromagnetic induction resistivity well logging are described, for example, in H. G. Doll, Introduction to Induction Logging and Application to Logging of Wells Drilled with Oil Based Mud, Journal of Petroleum Technology, vol. 1, p.148, Society of Petroleum Engineers, Richardson Tex. (1949). Many improvements and modifications to electromagnetic induction resistivity instruments have been devised since publication of the Doll reference, supra. Examples of such modifications and improvements can be found, for example, in U.S. Pat. No. 4,837,517 issued to Barber; U.S. Pat. No. 5,157,605 issued to Chandler et al, and U.S. Pat. No. 5,452,761 issued to Beard et al.
U.S. Pat. No. 5,452,761 to Beard et al., the contents of which are fully incorporated herein by reference, discloses an apparatus and method for digitally processing signals received by an induction logging tool having a transmitter and a plurality of receivers. An oscillating signal is provided to the transmitter, which causes eddy currents to flow in a surrounding formation in the frequency domain, transient domain or a combination of both. The magnitudes of the eddy currents are proportional to the conductivity of the formation. The eddy currents in turn induce voltages in the receivers. The received voltages are digitized at a sampling rate well above the maximum frequency of interest. Nyquist sampling criteria specifies the sampling frequency to be at least twice the maximum frequency present in the signal being digitized in order to avoid aliasing distortion of the digitized signal. The digitizing window is synchronized to a cycle of the oscillating current signal. The oscillating current could be a combination of sinusoidal frequencies for a survey in the frequency domain or a repetitive transient current source for a survey in the transient domain. For the later the measured data would be transformed to the frequency domain for resistivity measurement data analysis, processing, inversion to define resistivity properties and structural characteristics of an earth formation resistivity model. Corresponding samples obtained in each cycle are cumulatively summed over a large number of such cycles. The summed samples form a stacked signal. Stacked signals generated for corresponding receiver coils are transmitted to a computer for spectral analysis. Transmitting to the surface the stacked signals and not all the individually sampled signals, reduces the amount of data that needs to be stored or transmitted. A Fourier analysis is performed of the stacked signals to derive the amplitudes of in-phase and quadrature components of the receiver voltages at the frequencies of interest. From the component amplitudes, the conductivity of the formation can be accurately derived.
A limitation to the electromagnetic induction resistivity well logging instruments such as that discussed in Beard et al. is that they typically include transmitter coils and receiver coils wound so that the magnetic moments of these coils are substantially parallel only to the axis of the instrument. Eddy currents are induced in the earth formations from the magnetic field generated by the transmitter coil, and in the induction instruments known in the art, these eddy currents tend to flow in ground loops which are substantially perpendicular to the axis of the instrument. Voltages are then induced in the receiver coils related to the magnitude of the eddy currents. Certain earth formations, however, consist of thin layers of electrically conductive materials interleaved with thin layers of substantially non-conductive material. The response of the typical electromagnetic induction resistivity well logging instrument will be largely dependent on the conductivity of the conductive layers when the layers are substantially parallel to the flow path of the eddy currents. The substantially non-conductive layers will contribute only a small amount to the overall response of the instrument and therefore their presence will typically be masked by the presence of the conductive layers. The non-conductive layers, however, are the ones that are typically hydrocarbon-bearing and are of the most interest to the instrument user. Some earth formations which might be of commercial interest therefore may be overlooked by interpreting a well log made using the electromagnetic induction resistivity well logging instruments known in the art.
U.S. Pat. No. 6,147,496 to Strack et al. teaches the use of an induction logging tool in which at least one transmitter and at least one receiver with orientation limited to orthogonal directions. By performing measurements with the tool with at least two different frequencies, it is possible to substantially reduce the effect of borehole and invasion and to determine the orientation of the tool to the bedding planes.
U.S. Pat. No. 5,999,883 issued to Gupta et al. (the “Gupta patent”), the contents of which are fully incorporated here by reference, discloses a method for determining the horizontal and vertical conductivity of anisotropic earth formations. Electromagnetic induction signals induced by induction transmitters oriented along three mutually orthogonal axes are measured. One of the mutually orthogonal axes is substantially parallel to a logging instrument axis. The electromagnetic induction signals are measured using first receivers each having a magnetic moment parallel to one of the orthogonal axes and using second receivers each having a magnetic moment perpendicular to one of the orthogonal axes which is also perpendicular to the instrument axis. A relative angle of rotation of this magnetic moment perpendicular to the orthogonal axes is calculated from the receiver signal including the signals measured perpendicular to the instrument axis. An intermediate measurement tensor is calculated by rotating magnitudes of the receiver signals through a negative of the angle of rotation corresponding to a first coordinate transformation. A relative angle of inclination of one of the orthogonal axes which is parallel to the axis of the instrument is calculated, from the rotated magnitudes, with respect to a direction of the vertical conductivity. The initially rotated magnitudes are rotated through a negative of the angle of inclination corresponding to the a coordinate transformation. The resistivity anisotropy evaluation is referenced to the principal axis of transverse anisotropy (in a simpler case) and the bedding plane. A similar procedure for a more general case could address the case of biaxial anisotropy in layered media where R
hx
differs from R
hy
. Horizontal conductivity is calculated from the magnitudes of the receiver signals after the second step of rotation. An anisotropy parameter is calculated from the receiver signal magnitudes after the second step of rotation. Vertical conductivity is calculated from the horizontal conductivity and the anisotropy parameter.
U.S. patent application Ser. No. 09/676,097 by Kriegshauser et al, the contents of which are fully incorporated herein by reference, discusses the use of a multi-component induction logging tool in which five components of the magnetic field are recorded. This tool, which is marketed under the name 3DEX™ by Baker Hughes Inc., measures three principal components H
xx
, H
yy
, H
zz
and two cross-components H
xy
and H
xz
. The measured data from
Fanini Otto N.
Merchant Gulamabbas A.
Baker Hughes Incorporated
Deb Anjan
Kinder Darrell
Madan Mossman & Sriram P.C.
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