Electricity: measuring and testing – Particle precession resonance – Using well logging device
Reexamination Certificate
2000-06-15
2002-12-24
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using well logging device
C324S313000, C324S314000
Reexamination Certificate
active
06498484
ABSTRACT:
FIELD OF THE INVENTION
The invention is related generally to nuclear magnetic resonance apparatus and methods. More specifically, the invention is related to methods and apparatus for reducing ringing in nuclear magnetic resonance instruments.
DESCRIPTION OF THE RELATED ART
Nuclear magnetic resonance (NMR) instruments have been adapted for use in wellbores drilled through earth formations. Generally speaking, NMR instruments used for analyzing earth formations include a magnet for inducing a static magnetic field in the earth formations to be evaluated, an antenna placed proximate to the formations to be analyzed, and circuitry adapted to conduct radio-frequency (RF) power pulses through the antenna to induce an RF magnetic fields in the same formations. The circuitry also includes a receiver adapted to detect signals induced in the antenna (or a separate receiving antenna). The induced signals are related to NMR phenomena induced in the formation of interest; by the combined action of the static magnetic field and the RF magnetic field.
Typically, measurement of NMR related phenomena in the earth formation is performed by allowing some time for the static magnetic field to polarize nuclei in the formation in a direction substantially along the direction of the static magnetic field. A first one of the RF pulses passed through the antenna has a magnitude and duration selected to reorient the nuclear magnetization by about 90 degrees from its previous orientation. After a selected time, successive RF pulses are passed through the antenna, each of these having a magnitude and duration selected to reorient the nuclear spin axes by about 180 degrees for their immediately previous orientations to enable the nuclear spin axes to “rephase” or realign with each other. The induced signals, known as “spin echoes”, are generally measured during the time interval between each successive one of the “180 degree” RF pulses. The succession of spin echo measurements is generally known as a “sequence”. The amplitude of the spin echo signals, and the rate at which the spin echo amplitudes change during a measurement sequence, are related to properties of interest of the earth formations, such as fractional volume of pore space (porosity) and the properties of fluids present in the pore spaces. The frequency of the RF magnetic field needed to reorient the nuclear magnetization, which is the frequency of the spin echo signals, is related to the amplitude of the static magnetic field and a factor, known as the gyromagnetic ratio &ggr;, which is unique to each isotope. For evaluation of earth formations, the static magnetic field amplitude and RF magnetic field frequency are typically selected to excite NMR phenomena in hydrogen nuclei, although other nuclei may be used for NMR evaluation of earth formations.
Exciting the antenna with RF power pulses in the presence of a strong static magnetic field causes mechanical excitation of the antenna. Mechanical excitation of the antenna leads to excitation of a signal, called “ringing”, in the antenna. The ringing is unrelated to NMR phenomena, and frequently has a very large amplitude. The amplitude of the ringing is often highest right after application of each RF pulse, and is of such a magnitude as to make it difficult to measure the amplitude of NMR induced signals. Reducing the effects of ringing on NMR measurement is very important in well logging applications, among others, because significant information about the properties of the earth formations are determined by the amplitudes of spin echoes occurring shortly after the RF pulses.
Several methods are known in the art for reducing ringing. One class of such methods includes creating a phase difference between the ringing signal and the NMR induced signals, and summing or “stacking” multiple sets of measurements to reduce the amplitude of the ringing signal in the output. One commonly used measurement sequence used in evaluation of earth formations is known as “phase alternate pairs” (PAPS). PAPS sequences include performing a measurement sequence as just described including a 90 degree RF pulse followed by successive 180 degree pulses. After a selected wait time, another such measurement sequence is performed, but with the polarity of the 90 degree pulse reversed. Stacking the two sets of measurements substantially cancels the ringing signal. Such a method is described, for example, in U.S. Pat. No. 5,596,274 issued to Sezginer and U.S. Pat. No. 5,023,551 issued to Kleinberg et al.
Another device for reducing ringing is to have the magnet arranged so as to dispose the antenna in a region having substantially zero static magnetic field amplitude. A NMR apparatus which has this arrangement is described, for example, in U.S. Pat No. 5,712,566 issued to Taicher.
Yet another device for reducing ringing is to provide separate antennas for inducing the RF magnetic field and detecting the NMR induced signals, where these two antennas are substantially orthogonal to each other. Ringing induced in the transmitting antenna is substantially undetected by the receiving antenna. See for example, the Taicher et al '566 patent referred to previously.
Still another device known in the art for reducing ringing is to select more than one RF frequency for exciting NMR phenomena and stacking the signals at the multiple frequencies to reduce the amplitude of the ringing. See for example International Patent Application No. WO 98/43064 filed by Prammer.
Still another method for reducing ringing is disclosed in U.S. pat. app. Ser. No. 09/102,719 filed Jun. 22, 1998 entitled
Method for Eliminating Ringing During a Nuclear Magnetic Resonance Measurement,
and assigned to the same assignee as this invention. The method described in the application includes measuring an NMR sequence including ringing and NMR signal, canceling the NMR signal from a part of the sequence to determine the ringing signal, measuring signal from a second part of the sequence which is substantially without NMR signal, and using the ringing thus determined in the second part of the sequence to correct the signal (ringing plus NMR signal) measured during the first part of the sequence for the effects of ringing and other spurious noise.
Still another method for reducing the effects of ringing on NMR signal detection is described in U.S. pat. app. Ser. No. 09/276,049 filed Mar. 25, 1999 entitled
Nuclear Magnetic Resonance Well Logging Method and Apparatus,
and assigned to the assignee of this invention. The method described in this application includes, practically speaking, measuring two sets of PAPS measurement sequences wherein the second PAPS sequence set induced in a manner such that the NMR signal (spin echo) polarity is reversed with respect to the first set. Subtracting time-corresponding signals in the second PAPS set from those in the first PAPS set effectively cancels the ringing signal attributable to the 90 degree pulse.
SUMMARY OF THE INVENTION
The invention is a method for reducing ringing in nuclear magnetic resonance measurements. The method includes inducing a static magnetic field in a sensitive volume to orient nuclear magnetic spins of nuclei in the sensitive volume. The nuclear spins are reoriented by a first selected angle. Typically this is performed by applying an oscillating magnetic field to the sensitive volume having an amplitude and duration selected to reorient the spins by the first selected angle. An amplitude of the static magnetic field is then adjusted to cause a first selected phase shift in a spin echo signal measured subsequently to the reorienting by the first angle. The spins are then reoriented by a second selected angle, and a first spin echo signal is detected. The reorienting by the second selected angle is also typically performed by applying an oscillating magnetic field to the sensitive volume, having a duration and amplitude selected to cause reorienting by the second selected angle. Exemplary values for the first and second selected, angles are 90 and 180 degrees, respectively. The oscillatin
Sun Boqin
Taherian Reza
Fetzner Tiffany A.
Jeffery Brigitte L.
Lefkowitz Edward
McEnaney Kevin P.
Schlumberger Technology Corporation
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