Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
2001-02-27
2003-07-08
McElheny, Jr., Donald E. (Department: 2862)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
Reexamination Certificate
active
06591194
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention is related generally to the field of interpretation of measurements made by well logging instruments for the purpose of determining the properties of earth formations. More specifically, the invention is related to a method for determination of anisotropic formation resistivity using multi-component resistivity data.
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. Nos. 4,837,517; 5,157,605 issued to Chandler et al, and U.S. Pat. No. 5,452,761 issued to Beard et al.
A limitation to the electromagnetic induction resistivity well logging instruments known in the art 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 which 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.
The effect of formation anisotropy on resistivity logging measurements have long been recognized. Kunz and Moran studied the anisotropic effect on the response of a conventional logging device in a borehole perpendicular to the bedding plane of t thick anisotropic bed. Moran and Gianzero extended this work to accommodate an arbitrary orientation of the borehole to the bedding planes.
Rosthal (U.S. Pat. No. 5,329,448) discloses a method for determining the horizontal and vertical conductivities from a propagation or induction well logging device. The method assumes that &thgr;, the angle between the borehole axis and the normal to the bedding plane, is known. Conductivity estimates are obtained by two methods. The first method measures the attenuation of the amplitude of the received signal between two receivers and derives a first estimate of conductivity from this attenuation. The second method measures the phase difference between the received signals at two receivers and derives a second estimate of conductivity from this phase shift. Two estimates are used to give the starting estimate of a conductivity model and based on this model, an attenuation and a phase shift for the two receivers are calculated. An iterative scheme is then used to update the initial conductivity model until a good match is obtained between the model output and the actual measured attenuation and phase shift.
Hagiwara shows that the log response of an induction-type logging tool can be described by an equation of the form
V
∝
i
L
3
(
-
2
ⅇ
ikL
(
1
-
ikL
)
+
ikl
(
ⅇ
ik
β
-
ⅇ
ikL
)
)
(
1
)
where
&bgr;
2
=cos
2
&thgr;+sin
2
&thgr; (2)
and
k
2
=&ohgr;
2
&mgr;(∈
h
+i&sgr;
h
/&ohgr;) (3)
where L is the spacing between the transmitter and receiver, k is the wavenumber of the electromagnetic wave, &mgr; is the magnetic permeability of the medium, &thgr; is the deviation of the borehole angle from the normal to the formation, &lgr; is the anisotropy factor for the formation, &ohgr; is the angular frequency of the electromagnetic wave, &sgr;
h
is the horizontal conductivity of the medium and ∈
h
is the horizontal dielectric constant of the medium.
Eq. (3) is actually a pair of equations, one corresponding to the real part and one corresponding to the imaginary part of the measured signal, and has two unknowns. By making two measurements of the measured signal, the parameters k and &bgr; can be determined. The two needed measurements can be obtained from (1) R and X signals from induction logs, (2) phase and attenuation measurements from induction tools, (3) phase or attenuation measurements from induction tools with two different spacings, or (4) resistivity measurements at two different frequencies. In the low frequency limit, ∈ can be neglected in Eq. (3) and from known values of &ohgr; and &mgr;, the conductivity &sgr; can be determined from k, assuming a value of &mgr; equal to the permittivity of free space.
Wu (U.S. Pat. No. 6,092,024) recognized that the solution to eqs. (1)-(3) may be non-unique and showed how this ambiguity in the solution may be resolved using a plurality of measurements obtained from multiple spacings and/or multiple frequencies.
Strack et al. (U.S. Pat. No. 6,147,496) describe a multicomponent logging tool comprising a pair of 3-component transmitters and a pair of 3-component receivers. Using measurements made at two different frequencies, a combined signal is generated having a reduced dependency on the electrical conductivity in the wellbore region.
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 determination of an initial estimate of 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 a one of the orthogonal axes which is also perpendicular to the instrument axis. A relative angle of rotation of the perpendicular one of the orthogonal axes is calculated from the receiver 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. 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 rotated magnitudes are rotated through a negative of the angle of inclination. Horizontal c
Fanini Otto
Kriegshauser Berthold
Xiao Jiaqi
Yu Liming
Baker Hughes Incorporated
Madan Mossman & Sriram P.C.
McElheny Jr. Donald E.
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