Electricity: measuring and testing – Particle precession resonance – Using well logging device
Reexamination Certificate
2000-04-18
2001-06-05
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using well logging device
C324S300000
Reexamination Certificate
active
06242913
ABSTRACT:
FIELD OF THE INVENTION
This invention is directed to a logging while drilling (LWD) and measuring while drilling (MWD) approach for obtaining nuclear magnetic resonance (NMR) data concerning petrophysical properties of a formation. More specifically, the invention is directed to a method and device for reducing the sensitivity of NMR measurements to tool motions.
BACKGROUND OF THE INVENTION
U.S. Pat. No. 5,280,243 to Miller discloses an NMR apparatus and method of use for geophysical examination of a bore hole as it is being drilled. The patented apparatus is connected to the drill bit and follows it through the bore hole as it is being formed. In operation, the apparatus generates a gradient static magnetic field in a region of the bore hole adjacent the apparatus. This static field extends radially with respect to the longitudinal axis of the apparatus and has a generally uniform amplitude along the azimuth with respect to that axis. Next, a pulsed radio frequency magnetic field is generated to excite nuclei in a substantially cylindrical shell around the tool that defines in the formation a sensitive region extending along the length of the tool and having thickness of about 1 mm. Due to this relatively narrow sensitive region, standard wireline NMR relaxation time measurements are difficult to perform with this tool because lateral vibrations during the measurement time would reduce the accuracy of the measurement.
U.S. Pat. No. 5,557,201 to Kleinberg et al. discloses a pulsed NMR device in which the accuracy of the measurement with respect to lateral tool vibrations is enhanced by providing a larger sensitive region. This is achieved by a special tool architecture shown in
FIGS. 2A-B
, using two tubular permanent magnets
22
with same poles facing each other, and an antenna
26
positioned in the recess between the two magnets. In operation, this tool architecture provides a sensitive region in the formation which is larger laterally, but is greatly reduced along the borehole axis, because of the presence of a single stationary point in the formation. It is expected therefore that vertical tool motions would affect the accuracy of the tool measurements.
Accordingly, it is perceived that there is a need for improved sensitivity of pulsed NMR measurements using pulsed NMR tools with respect to tool motions.
SUMMARY OF THE INVENTION
The present invention concerns a novel method and device for formation evaluation while drilling a borehole using pulsed NMR tools with magnetic fields that are rotationally symmetric about the longitudinal axis of the borehole.
In a preferred embodiment, the method of the present invention is based on NMR relaxation time measurements determining longitudinal relaxation times T
1
. In particular, the method comprises the steps of generating at least one radio frequency pulse covering a relatively wide range of frequencies to saturate the nuclear magnetization in a cylindrical volume around the tool; transmitting a readout pulse at a frequency near the center of the range of covered frequencies, the readout pulse following a predetermined wait time; applying at least one refocusing pulse following the readout pulse; receiving at least one NMR echo corresponding to the readout pulse; repeating the above steps for a different wait time to produce a plurality of data points on a T
1
relaxation curve; and processing the produced T
1
relaxation curve to derive petrophysical properties of the formation.
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Arana Louis
Numar Corporation
Pennie & Edmonds LLP
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