Electricity: measuring and testing – Particle precession resonance – Using well logging device
Reexamination Certificate
1999-07-19
2003-05-20
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using well logging device
Reexamination Certificate
active
06566874
ABSTRACT:
BACKGROUND
The invention generally relates to inside-out nuclear magnetic resonance (NMR) measurements, and more particularly, the invention relates to detecting tool motion effects on NMR measurements of formation properties surrounding a borehole, such as measurements of the hydrogen content of the formation, for example.
Nuclear magnetic resonance (NMR) measurements 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 measurements 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 drill 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; Taicher et. al., U.S. Pat. No. 5,757,186, entitled, “Nuclear Magnetic Resonance Well Logging Apparatus and Method Adapted for Measurement-While-Drilling,” granted May 26, 1998; Jackson et. al., U.S. Pat. No. 4,350,955, entitled, “Magnetic Resonance Apparatus,” granted Sep. 21, 1982; or U.S. patent application Ser. No. 09,186,950, entitled, “Apparatus and Method for Obtaining a Nuclear Magnetic Resonance Measurement While Drilling,” filed on Nov. 5, 1998.
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 a generally thin resonance shell, or volume
20
a
(see FIG.
2
), and the resonance volume
20
a
may have a radial thickness that is proportional to the magnitude of an oscillating magnetic field and inversely proportional to the gradient of a static magnetic field. 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 motion that is attributable to the movement of the NMR tool
6
around the periphery of the borehole
3
, as further described below.
To perform the NMR measurements, the NMR tool
6
may include one or more permanent magnets to establish a static magnetic field called B
0
; a radio frequency (RF) coil, or antenna, to radiate the time varying magnetic B
1
field that is perpendicular to the 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
pulses the 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 B
0
field. Although not shown in detail, each pulse is effectively an envelope, or burst, of a radio frequency RF carrier signal. When the spins are rotated around B
1
away from the direction of the B
0
field, the spins immediately begin to precess around B
0
. The pulse is stopped when the spins are rotated by 90° into the plane perpendicular to the B
0
field. They continue to precess in this plane first in unison, then gradually losing synchronization. For step two, at a fixed time T
CP
following the excitation pulse
14
a
, the NMR tool
6
pulses the B
0
field for a longer period of time (than the excitation pulse
14
a
) to apply an NMR refocusing pulse
14
b
to rotate the precessing spins through an angle of 180° with the carrier phase shifted by ±90°. The NMR pulse
14
b
causes the spins to resynchronize and radiate an associated spin echo signal
16
(see
FIG. 5
) that peaks at a time called T
CP
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 to as many as several thousand, as an example) at the interval of 2.T
CP
. For step three, after completing the spin-echo sequence, a waiting period (usually called a wait time) is required to allow the spins to return to equilibrium along the B
0
field before starting the next CPMG sequence
15
to collect another set of spin echo signals. 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 lose irreversibly their unison precession 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 that is perpendicular to the static B
0
field, and the spins precess at a resonance frequency called the Larmor frequency for a perfectly homogeneous B
0
field. The Larmor frequency &ohgr;
L
may be described by the equation &ohgr;
L
=&ggr;B
0
, where &ggr; is the gyromagnetic ratio of the nuclei under investigation. However, the B
0
field is not really homogeneous, and the pulse excites spins roughly over the frequency range |&Dgr;&ohgr;|<&ggr;B
1
, with &Dgr;&ohgr;=&ggr;B
0
−&ohgr;
rf
being the off resonance frequency and &ohgr;
rf
being the carrier frequency of the RF pulses. So after excitation, the spins de-phase with T
2
* due to inhomogeneities in the static B
0
field. This decay is reversible and is reversed by the refocusing pulses
14
b
that produce the sin echo signals. In addition, irreversible de-phasing occurs (spin-spin relaxation) and is described by the T
2
time constant. This effect creates the decay of successive echo amplitudes according to the T
2
time constant. Thus, typically, only spins with T
2
>>T
2
* are measured.
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
SEPARATION
in
FIG. 6
) may be used to separate the T
2
distribution into two parts: one part including times less than the T
SEPARATION
time that indicate bound fluids and one part including times greater than the T
SEPARATION
time that indicate free, producible fluids.
Each T
2
time typically is computed by observing the decay of the magnitude of the spin echo signals
16
that are produced by a particular CPMG sequence
15
. Unfortunately, the drill string
5
(see
FIG. 1
) may move too rapidly for the NMR tool
6
to accurately observe this decay. However, the T
2
time is correlated with another time constant called a T
1
spin-lattice relaxation time. The T
1
time characterizes the time
Crary Steven F.
Poitzsch Martin E.
Speier Peter
Fetzner Tiffany A.
Lefkowitz Edward
McEnaney Kevin P.
Schlumberger Technology Corporation
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