Use of redundant data for log quality measurements

Details Use of redundant data for log quality measurements Use of redundant data for log quality measurements

2000-04-27

2002-06-04

Lefkowitz, Edward (Department: 2862)

Electricity: measuring and testing

Particle precession resonance

Using well logging device

C324S300000

Reexamination Certificate

active

06400148

ABSTRACT:

FIELD OF THE INVENTION
This invention is related to the determination of log quality using redundant measurements.
BACKGROUND OF THE INVENTION
This invention is directed toward quality control in the measurement of geophysical parameters of earth formations penetrated by a borehole. Co-pending U.S. patent application Ser. No. 08/675,178, filed on Jul. 3, 1996 discloses a method for quality control of propagation resistivity techniques using spaced transmitters operating at different frequencies and a plurality of longitudinally spaced receivers. An electromagnetic wave is propagated from the transmitting antenna coil into the formation in the vicinity of the borehole and is detected as it pass the receiving antennas. The basic or “raw” parameters measured by the receivers are the phase and the amplitude of the passing wave. The downhole instrument is conveyed along the borehole making a plurality of raw measurements as a function of depth within the borehole from which geophysical parameters of interest are computed as a function of depth within the borehole. It is quite common in the prior art to first combine raw data measurement and then to compute parameters of interest from these process measurements. A typical example is the computation of apparent resistivity from the difference in phase of signals detected at receivers at different longitudinal spacings from the transmitter. A second example is the computation of apparent resistivity from the ratio of the amplitude of signals detected at the longitudinally space receivers. Such preprocessing or data combination is performed primarily to eliminate the gross effects of the borehole and is well known in the prior art.
The '178 application is directed toward the simultaneous measurement of a plurality of parameters associated with the formation and borehole environment, and a quantitative measure of the quality of such raw measurements or uncertainty associated with such raw measurements. Parameters of interest selected may include the resistivity of the formation from which hydrocarbon saturation is computed, invasion profiles of the drilling fluid which are indicative of the permeability of the formation, and perhaps physical characteristic of the well bore itself such as diameter, ellipticity, and rugosity.
In the '178 application, non-linear inversion techniques are used to determine the set of selected unknown parameters which, through the model, predicts a tool response which most closely matches the thirty two measured raw data points. The predicted tool responses and the measured tool responses will exhibit no discrepancies only if (a) there is no error associated with the measured data and (b) if the model represents without error the response of the instrument in every encountered borehole and formation condition. This is because there are more measured data points than unknown variable parameters in the model. Any degree of non-conformance or “mismatch” of the model data and the measured data is a measure of inaccuracy of either the data or the model or both the data and the model. In all cases the determined non-conformance is treated as a quality indicator for the determined parameters of interest. In other words, an uncertainty is attached to each parameter selected by the analyst based upon the goodness of fit between the model and the measured data. Obtaining formation parameters from observations at multiple frequencies and/or multiple source-receiver offsets involves the solution of an overdetermined system of equations.
A similar situation arises in obtaining formation parameters in induction logging techniques. For example, U.S. Pat. No. 5,452,761 to Beard et al, having the same assignee as the present application and 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. 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. The digitizing window is synchronized to a cycle of the oscillating current signal. 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 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 on 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. The Beard patent also teaches the use of analyzing data at multiple frequencies. These multiple frequencies may be obtained either by activating the transmitter at a plurality of frequencies, or, in a preferred embodiment, by a harmonic analysis of the data. As taught in the Beard patent, single frequency data modulated by a square pulse provides a signal that is rich in odd harmonics. Observations at multiple frequencies and solving for formation parameters gives an overdetermined system of equations, as in the '178 application.
U.S. Pat. No. 5,666,057 to Beard et al, the contents of which are fully incorporated herein by reference, teaches a multifrequency method of correcting for the so-called “skin-effect” and obtaining apparent conductivity of formations using induction logging tools. U.S. Pat. No. 5,889,729 to Frenkel et al having the same assignee as the present application, and the contents of which are fully incorporated herein by reference, discloses a method for 2-D inversion of induction logging data. Included therein is a step of 2-D forward modeling of induction logging data and the inversion of such data. U.S. Pat. No. 5,781,436 to Forgang et al, and U.S. Pat. No. 5,999,883 to Gupta et al., the contents of both of which are incorporated herein by reference, disclose the inversion of transverse induction logging data. A Transverse Induction Logging Tool (TILT) from which such data are obtained comprises a plurality of transmitters and receivers that have axes inclined to each other. Where the borehole axis is inclined to the bedding, such devices are able to determine apparent vertical and horizontal conductivities that are able to delineate resistive hydrocarbon bearing beds more accurately than conventional induction logging tools.
Solution of overdetermined systems of equations is also involved in Nuclear Magnetic Resonance (NMR) logging. This technique involves using NMR logging tools and methods for determining, among other things porosity, hydrocarbon saturation and permeability of the rock formations. The NMR logging tools are utilized to excite the nuclei of the fluids in the geological formations in the vicinity of the wellbore so that certain parameters such as spin density, longitudinal relaxation time (generally referred to in the art as “T
1
”), and transverse relaxation time (generally referred to as “T
2
”) of the geological formations can be estimated. From such measurements, porosity, permeability, and hydrocarbon saturation are determined, which provides valuable information about the make-up of the geological formations and the amount of extractable hydrocarbons.
U.S. Pat. No. 5,023,551 issued to Kleinberg discloses an NMR pulse sequence that has an NMR pulse sequence for use in the borehole environment which combines a modified inversion recovery (FIR) pulse sequence with a series of more than two, and typically hundreds, of CPMG pulses according to
[W
i
-180-TW
i
-90-(t-180-t-echo)
j
]
i
where j-1,2, . . . J

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