Nuclear magnetic resonance well logging method and apparatus

Details Nuclear magnetic resonance well logging method and apparatus Nuclear magnetic resonance well logging method and apparatus

1999-03-25

2003-05-27

Lefkowitz, Edward (Department: 2862)

Electricity: measuring and testing

Particle precession resonance

Using well logging device

Reexamination Certificate

active

06570381

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to techniques and apparatus for determining characteristics of earth formations surrounding a borehole and, more particularly, to nuclear magnetic resonance borehole logging that utilizes pulse sequences which improve performance.
BACKGROUND OF THE INVENTION
General background of nuclear magnetic resonance (NMR) well logging is set forth, for example, in U.S. Pat. No. 5,023,551. Briefly, in NMR operation the spins of nuclei polarize along an externally applied static magnetic field, assumed to be in the z direction. The vector sum of the magnetic moment from individual nuclei is a macroscopic magnetic dipole called the magnetization, M
0
. The magnetization is normally aligned with the static magnetic field, but the equilibrium situation can be disturbed by a pulse of an oscillating magnetic field (e.g. an RF pulse generated by an RF antenna), which rotates the magnetization away from the static field direction. The length of the RF pulse can be adjusted to achieve a prescribed rotation angle, such as 90 degrees, 180 degrees, etc. After rotating, two things occur simultaneously. First, the spins precess around the static field at the Larmor frequency, given by &ohgr;
0
=&ggr;B
0
, where B
0
is the strength of the static field and &ggr; is the gyromagnetic ratio. For hydrogen nuclei, &ggr;/2&pgr;=4258 Hz/Gauss, so, for example, for a static field of 235 Gauss, the frequency of precession would be 1 MHz. Second, the spins return to the equilibrium direction according to a decay time T
1
, the spin lattice relaxation time. Also associated with the magnetization is a second relaxation called the spin-spin relaxation with a decay time T
2
.
A widely used technique for acquiring NMR data both in the laboratory and in well logging, uses an RF pulse sequence known as the CPMG (Carr-Purcell-Meiboom-Gill) sequence. As is well known, after a wait time that precedes each pulse sequence, known as polarization time, a ninety degree pulse rotates the magnetization to the x-y plane. The spins begin to precess around B
0
and dephase. After a certain time delay, a one hundred eighty degree pulse is applied to cause the spins which are dephasing in the transverse plane to refocus. Refocusing leads to an echo that is detected by the NMR instrument. By repeated application of one hundred eighty degree pulses, a series of “spin echoes” appear, and the train of echoes is measured and processed.
It has been recognized that “ringing” is a problem encountered when using pulsed nuclear magnetic resonance techniques. There are two types of spurious ringing in pulsed NMR. The first type is electronic ringing which arises from the transient effects of a resonance electronic circuit, and is determined by the characteristics of the resonance circuit. The electronic ringing can be substantially reduced using time-controlled hardware such as Q-switching approaches. The second type of spurious ringing arises from exciting the acoustic resonances in or around the RF antenna structure. [See A. A. V. Gibson and R. E. Raab, “Proton NMR and piezoelectricity in tetramethylammonium chloride,” J. Chem. Phys. 57, 4688-4693, (1972); M. L. Buess, and G. L. Peterson, “Acoustic ringing effects in pulsed magnetic resonance probes,” Rev. Sci. Instrum., 49, 1151-1155, (1978); E. Fukushima, and S. B. W. Roeder, “Spurious ringing in pulse NMR,” J. Mag. Resonance, 33, 199-203, (1979); and R. L. Kleinberg, A. Sezginer, D. D. Griffin, and M. Fukuhara, “Novel NMR Apparatus for Investigating an External Sample,” J. Mag. Res., 97, 466-485, (1992).] This is magnetoacoustic ringing, and can last up to several milliseconds. It appears whenever the frequency of the applied RF current matches at least one of acoustic resonance modes of the RF antenna or its surrounding structure. Both types of ringing are phase coherent with the applied RF pulse and therefore can not be canceled, unlike incoherent noise, by stacking repeated measurements. [Techniques for dealing with the problems of ringing in laboratory equipment are disclosed in U.S. Pat. No. 4,438,400 and in the following publications: I. P. Gerothanassis, “Methods Of Avoiding The Effects Of Acoustic Ringing In Pulsed Fourier Transform Nuclear Magnetic Resonance Spectroscopy”, Progress in NMR Spectroscopy, Vol. 19, pp. 276-329, 1987 (see Section 9.3 and see Note Added In Proof with regard to sequences of interest as used in laboratory spectrometry with single echo sequences); and S. Zhang, X. Wu, and M. Mehring, “Elimination Of Ringing Effects In Multiple-Pulse Sequences”, Chemical Physics Letters, Vol. 173, No. 5.6, pp. 481-484, 1990.]
The amplitude of the ringing signal can be large enough to saturate the receiver circuitry, making its response to the CPMG echo signal nonlinear. Magnetoacoustic ringing can be reduced by selecting proper material for the RF antenna and its surrounding structure (see, for example, U.S. Pat. No. 5,153,514), but it is very difficult to completely eliminate acoustic ringing by mechanical methods alone, particularly in well logging equipment that has design constraints relating to its adaptability for the borehole environment. This ringing can be a major obstacle for measuring parameters such as total porosity in magnetic resonance logging.
It is among the objects of the present invention to provide a technique and apparatus for substantially eliminating the effects of phase coherent acoustic ringing in nuclear magnetic resonance well logging.
SUMMARY OF THE INVENTION
In accordance with a form of the method of the invention, there is disclosed a technique for obtaining nuclear magnetic resonance measurements from formations surrounding an earth borehole, comprising the following steps: providing a logging device that is moveable through the borehole and through formations in which a static magnetic field is present; producing, from the logging device, a series of cycles of pulse sequences in the formations, each of the pulse sequences including an RF excitation pulse and several RF refocusing pulses; receiving, at the logging device, spin echoes from the formations to produce spin echo signals that may include spurious ringing signals from the excitation and refocusing pulses; and combining spin echo signals from corresponding spin echoes of each of the cycles of pulse sequences to obtain combined spin echo signals in which spurious ringing from the excitation pulses and refocusing pulses of the pulse sequences is substantially cancelled. The static magnetic field can be earth's magnetic field or a static magnetic field produced at the logging device.
In a preferred embodiment of the invention, the steps of producing cycles of pulse sequences and combining spin echo signals include manipulating the polarities of the excitation and refocusing pulses to obtain the substantial cancellation of the spurious ringing from the excitation and refocusing pulses.
Also in a preferred embodiment of the invention, the series of cycles of pulse sequences comprises four cycles of pulse sequences. In a form of this embodiment, the step of combining spin echo signals from corresponding spin echoes of each of the cycles of pulse sequences to obtain combined spin echo signals comprises combining corresponding spin echo signals from two of cycles and subtracting the spin echo signals from the other two of the cycles. Also in this form of the embodiment, all the spin echo signals of two of the four cycles have a polarity that is opposite to that of all the spin echo signals of the other two of the four cycles.
In a further embodiment of the invention, the step of producing a series of cycles of pulse sequences in the formations further includes producing an RF inverting pulse in some of the cycles of pulse sequences, and the step of receiving spin echoes from the formations to produce spin echo signals includes receiving spin echoes to produce spin echo signals that may includes spurious ringing signals from the inverting pulses, and the combining step includes combining spin echoes of each o

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