Electricity: measuring and testing – Particle precession resonance – Using well logging device
Reexamination Certificate
1998-12-04
2002-12-10
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using well logging device
C324S300000, C324S318000
Reexamination Certificate
active
06492809
ABSTRACT:
BACKGROUND
The invention generally relates to preconditioning spins near a nuclear magnetic resonance (NMR) region.
Nuclear magnetic resonance (NMR) may be used to determine properties of a sample, such as body tissue (for medical imaging purposes) or a subterranean formation (for well logging purposes). For example, for the subterranean formation, NMR may be used to determine and map the porosity, formation type, permeability and oil content of the formation.
Referring to
FIG. 1
, as an example, NMR may be used in a logging-while-drilling (LWD) operation to map the properties of a subterranean formation
10
. In this manner, an axisymmetric NMR tool
6
may be part of a drill string
5
that is used to form a borehole
3
in the formation
10
. The tool
6
may be, as examples, one of the tools described in Sezginer et. al., U.S. Pat. No. 5,705,927, entitled “Pulsed Nuclear Magnetism Tool For Formation Evaluation While Drilling Including a Shortened or Truncated CPMG Sequence,” granted Jan. 6, 1998; Miller, U.S. Pat. No. 5,280,243, entitled “System For Logging a Well During the Drilling Thereof,” granted Jan. 18, 1994.
The NMR measuring process is separated by two distinct features from most other downhole formation measurements. First, the NMR signal from the formation comes from a small resonance volume, such as generally thin resonance volume
20
a
(see FIG.
2
), and the resonance volume
20
a
may have a radial thickness that is proportional to the magnitude of a {right arrow over (B)}
1
magnetic field (not shown). Depending on the shape of the resonance zones, the volume may extend, as an example, from as little as 1 millimeter (mm) in one direction and as long as several inches in another. Secondly, the NMR measurement may not be instantaneous. Both of these facts combined make the NMR measurements prone to tool motions, such as the NMR tool
6
moving around the periphery of the borehole
3
, as further described below.
To perform the NMR measurements, the NMR tool
6
may include permanent magnets to establish a static magnetic field called {right arrow over (B)}
0
(not shown); a radio frequency (RF) coil, or antenna, to radiate the time varying magnetic field {right arrow over (B)}
1
that is perpendicular to the {right arrow over (B)}
0
field; and an RF coil, or antenna, to receive spin-echoes from the formation in response to an NMR measurement, as described below. These two coils may be combined into a single transmit/receive antenna.
As an example, the NMR tool
6
may measure T
2
spin-spin relaxation times of hydrogen nuclei of the formation
10
by radiating NMR detection sequences to cause the nuclei to produce spin-echoes. The spin-echoes, in turn, may be analyzed to produce a distribution of T
2
times, and the properties of the formation may be obtained from this distribution. For example, one such NMR detection sequence is a Carr-Purcell-Meiboom-Gill (CPMG) sequence
15
that is depicted in FIG.
4
. By applying the sequence
15
, a distribution of T
2
times may be obtained, and this distribution may be used to determine and map the properties of the formation
10
.
A technique that uses CPMG sequences
15
to measure the T
2
times may include the following steps. In the first step, the NMR tool
6
transmits the {right arrow over (B)}
1
field for an appropriate time interval to apply a 90° excitation pulse
14
a
to rotate the spins of hydrogen nuclei (that are initially aligned along the direction of the {right arrow over (B)}
0
field) by 90°. Although not shown, each pulse is effectively an envelope, or burst, of an RF carrier signal. After the spins are rotated 90° from the direction of the {right arrow over (B)}
0
field, the spins immediately begin to precess in the plane perpendicular to the {right arrow over (B)}
0
field at first in unison, then gradually losing synchronization. For step two, at a fixed time T following the NMR pulse
14
a,
the NMR tool
6
pulses the {right arrow over (B)}
1
field for a longer period of time (than the NMR pulse
14
a
) to apply an NMR refocusing pulse
14
b
to rotate the precessing spins through an additional angle of 180° with its carrier phase shifted by ±90°. The NMR pulse
14
b
causes the spins to resynchronize and radiate an associated spin-echo
16
(see
FIG. 5
) which peaks at a time approximately equal to T, after the 180° refocusing NMR pulse
14
b.
Step two may be repeated “k” times (where “k” is called the number of echoes and may assume a value anywhere from several hundred to as many as several thousand, as an example) at the interval of t
e
(approximately 2·T). For step three, after completing the spin-echo sequence, a waiting period t
w
(usually called a wait time) is required to allow the spins to return to equilibrium along the {right arrow over (B)}
0
. field before starting the next CPMG sequence
15
to collect another set of spin-echoes. The decay of each set of spin-echoes is observed and used to derive the T
2
distribution.
The T
2
* time characterizes a time for the spins to no longer precess in unison after the application of the 90° excitation pulse
14
a.
In this manner, at the end of the 90° excitation pulse
14
a,
all the spins are pointed in a common direction perpendicular to the static B
0
field, and the spins precess at a resonance frequency called the Larmor frequency for a perfectly homogenous field. The Larmor frequency may be described by {right arrow over (&ohgr;)}
0
=&ggr;{right arrow over (B)}
0
, where &ggr; is the gyromagnetic ratio, a nuclear constant. However, the {right arrow over (B)}
0
field typically is not homogenous, and after excitation, the spins de-phase with T
2
* due to inhomogenieties in the static {right arrow over (B)}
0
field. This decay is reversible and is reversed by the refocusing pulses
14
b
that cause the echoes. In addition, irreversible de-phasing occurs (spin-spin relaxation) and is described by the T
2
time constant. This results in the decay of successive echo amplitudes in the CPMG sequence according to the T
2
time constant. With “inside-out” NMR, typically, spins are measured with T
2
>>T
2
*.
As stated above, the distribution of the T
2
times may be used to determine the properties of the formation. For example, referring to
FIG. 6
, the formation may include small pores that contain bound fluid and large pores that contain free, producible fluid. A T
2
separation boundary time (called T
CUT-OFF
in
FIG. 6
) may be used to separate the T
2
distribution into two parts: one part including times less than the T
OUT-OFF
time that indicate bound fluids and one part including times greater than the T
CUT-OFF
time that indicate free, producible fluids.
Each T
2
time typically is computed by observing the decay of the spin-echoes
16
that are produced by a particular CPMG sequence
15
. Unfortunately, the drill string
5
(see
FIG. 1
) may experience severe lateral motion. However, the T
2
time is approximately proportional to another time constant called a T
1
spin-lattice relaxation time. The T
1
time characterizes the time for the spins to return to the equilibrium direction along the {right arrow over (B)}
0
field, and thus, considering both the T
1
and T
2
times, each spin may be thought of as moving back toward the equilibrium position in a very tight pitch spiral during the T
1
recovery. Fortunately, the T
1
and T
2
times are approximately proportional. As a result, the T
2
distribution may be derived from measured T
1
times. In fact, the original work on establishing bound fluid cutoffs was done using T
1
. Those results were then expressed and used commercially in terms of T
2
. See W. E. Kenyon, J. J. Howard, A. Sezginer, C. Straley, A. Matteson, K. Horkowitz, and R. Ehrlich, Pore-Size Distribution and NMR in Microporous Cherty Sandstones, Paper LL (paper presented at the 30th Annual Logging Symposium, SWPLA, Jun. 11-14, 1989).
Polarization-based measurements may use either inversion recovery sequences or saturation recovery sequences. With the saturation recovery sequ
Ganesan Krishnamurthy
Speier Peter
Fetzner Tiffany A.
Jeffery Brigitte L.
Lefkowitz Edward
McEnaney Kevin
Ryberg John J.
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