Electricity: measuring and testing – Particle precession resonance – Using well logging device
Reexamination Certificate
1997-03-26
2001-03-20
Arana, Louis (Department: 2857)
Electricity: measuring and testing
Particle precession resonance
Using well logging device
Reexamination Certificate
active
06204663
ABSTRACT:
FIELD OF THE INVENTION
The present invention concerns nuclear magnetic resonance (NMR) pulse sequences such as those used in evaluating earth formations. More specifically, the invention relates to pulse sequences and data acquisition methods which eliminate the effects of spurious signals caused by mechanical resonances within the measurement apparatus.
BACKGROUND OF THE INVENTION
Pulsed nuclear magnetic resonance (NMR) measurements alternate between transmitting high-powered radio-frequency (r.f.) pulses and receiving low-level response signals in a matter of a few ten or hundred microseconds. The combination of a strong static magnetic field and radio frequency pulses tend to excite mechanical resonances within the measurement apparatus, which resonances in turn cause an interference signal induced in the receiver system by a microphonic effect.
It has long been known that the interference arising from imperfect “refocusing” pulses can be canceled by repeating the measurement with the r.f. phase of the refocusing pulses inverted. This phase reversal does not affect the NMR signal, but inverts the phase of the interference. By acquiring both magnitude and phase of the compromised signals and by adding complex-valued measurements, the NMR signal is enhanced, while the “refocusing” interference is eliminated.
The above error cancellation scheme has become standard in practice, but it does not address interference problems arising from the “excitation” pulse, which typically is the first pulse in a long series of pulses. Changing the excitation phase would also change the phase of the NMR signal: excitation interference and NMR signal are always in phase with each other. Since often only the first data point (“echo”) is affected by excitation interference, it is customary to eliminate this first data point from the data set. The first data point, however, contains valuable information about fast time-dependent behavior of the NMR sample and therefore having to ignore this point is an unsatisfactory solution.
The method of the present invention, described in more detail below, uses a novel cycle of pulse sequences to reduce the effect of “excitation” interference, on the basis of changing the measurement frequency between certain pulse sequences. Naturally, the method is especially useful for NMR measurements in which small changes in frequency can readily be allowed or tolerated. For example, laboratory-type NMR machines typically operate in homogeneous fields with a single, well-defined frequency. Changes in frequency are employed either to follow fluctuations in the main magnetic field, or to enable magnetic resonance imaging (MRI). NMR machines built for wireline logging or similar industrial applications are much more robust with respect to small changes in frequency. Therefore, the proposed solution is well-suited for industrial NMR applications.
The method of the present invention uses prior art NMR apparatuses and logging tools to obtain previously unavailable data relating to the fast time-dependent behavior of an NMR sample. In particular, a novel pulse sequence is proposed and used to obtain improved NMR data by eliminating spurious signals corresponding to mechanical resonances in the measurement apparatus induced by the r.f. excitation pulse.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a method for improving the accuracy of borehole NMR logging measurements.
It is another object of the present invention to provide a method for improving the short time resolution of borehole NMR logging measurements.
It is yet another object of the present invention to provide a method for suppressing of magneto-acoustic artifacts in NMR data obtained from logging measurements.
These and other objects are accomplished in accordance with a preferred embodiment of the present invention by a novel cycle of pulse sequences and a data acquisition scheme that employ existing NMR logging instruments. The novel cycle of pulse sequences of the present invention is characterized by a change in the measurement frequency between pulse sequences. In a preferred embodiment of the present invention, the frequency change is chosen so that spurious signals induced by the excitation pulse may be significantly reduced by combining NMR signals from corresponding echoes received in response to each measurement frequencies.
In accordance with the present invention, one can determine petrophysical properties of a geologic formation more accurately by reducing the effect of spurious signals arising from the excitation pulse. In particular, significant errors in the first spin-echo are corrected in accordance with a preferred embodiment of the present invention, which therefore provides increased short time resolution and allows improved detection and quantification of components which are associated with short relaxation times such as clay-bound water. In turn, this more accurate measurement of the clay-bound water improves determination of the total porosity and use of the resistivity interpretation model.
More specifically, in a preferred embodiment of the present invention an NMR method for measuring attributes of a material is disclosed, comprising the steps of: (a) applying at least one first pulse-echo sequence having an associated measurement frequency F
1
; (b) applying at least one second pulse-echo sequence having an associated measurement frequency F
2
different from F
1
; (c) measuring NMR signals corresponding to the first pulse-echo sequence and the second pulse-echo sequence, these NMR signals representing spin-echo relaxation in the material, at least some of the measured NMR signals being corrupted by spurious signals; (d) combining measured NMR signals from the first pulse-echo sequence and from the second pulse-echo sequence to reduce the effect of said spurious signals; and (e) determining properties of the material on the basis of the combination of measured signals.
In another preferred embodiment of the present invention which is directed to borehole logging, a method for NMR borehole logging is disclosed, comprising the steps of: (a) providing at least one first pulse-echo sequence associated with a first measurement frequency F
1
; (b) providing at least one second pulse-echo sequence associated with a second measurement frequency F
1
different from F
1
; (c) measuring NMR signals corresponding to the first pulse-echo sequence and the second pulse-echo sequence, the NMR signals representing spin-echo relaxation of a geologic formation in the borehole, at least some of the measured NMR signals being corrupted by spurious signals; (d) combining measured NMR signals from the first pulse-echo sequence and from the second pulse-echo sequence to reduce the effect of said spurious signals; and (e) determining properties of the geologic formation in the borehole on the basis of the combination of measured signals.
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pat
Arana Louis
Numar Corporation
Pennie & Edmonds LLP
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