Electricity: measuring and testing – Particle precession resonance – Using well logging device
Reexamination Certificate
1999-08-04
2001-07-03
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using well logging device
C324S318000
Reexamination Certificate
active
06255818
ABSTRACT:
BACKGROUND
This invention relates to a method and apparatus for performing nuclear magnetic resonance (NMR) measurements, and more particularly, the invention relates to an arrangement for efficiently performing T1-based and T2-based measurements.
Nuclear magnetic resonance (NMR) measurements typically are performed to investigate properties of a sample. For example, an NMR wireline or logging while drilling (LWD) downhole tool may be used to measure properties of subterranean formations. In this manner, a typical NMR tool may, for example, provide a lithology-independent measurement of the porosity of a particular formation by determining the total amount of hydrogen present in fluids of the formation. Equally important, the NMR tool may also provide measurements that indicate the dynamic properties and environment of the fluids, as these factors may be related to petrophysically important parameters. For example, the NMR measurements may provide permeability and viscosity information that is difficult or impossible to derive from other conventional logging arrangements. Thus, it is the capacity of the NMR tool to perform these measurements that makes it particularly attractive versus other types of downhole tools.
Typical NMR logging tools include a magnet that is used to polarize hydrogen nuclei (protons) in the formation and a transmitter coil, or antenna, that emits radio frequency (RF) pulses. A receiver antenna may measure the response (indicated by received spin echo signals) of the polarized hydrogen to the transmitted pulses. Quite often, the transmitter and receiver antennae are combined into a single transmitter/receiver antenna.
There are several experimental parameters that may be adjusted according to the objectives of the NMR measurement and expected properties of the formation fluids. However, the NMR techniques employed in current NMR tools typically involve some variant of a basic two step sequence that includes a polarization time followed by an acquisition sequence.
During the polarization time (often referred to as a “wait time”) the protons in the formation polarize in the direction of a static magnetic field (called B
0
) that is established by a permanent magnet (of the NMR tool). The growth of nuclear magnetization M(t) (i.e., the growth of the polarization) is characterized by the “longitudinal relaxation time” (called T1) of the fluid and its equilibrium value (called M
0
). When the specimen is subject to a constant field for a duration t
p
, the longitudinal magnetization is:
M
(
t
p
)
=
M
0
(
1
-
e
-
t
p
T
1
)
Equation
1
The duration of the polarization time may be specified by the operator (conducting the measurement) and includes the time between the end of one acquisition sequence and the beginning of the next. For a moving tool, the effective polarization time also depends on tool dimensions and logging speed.
Referring to
FIG. 1
, as an example, a sample (in the formation under investigation) may initially have a longitudinal magnetization M
Z
10 of approximately zero. The zero magnetization may be attributable to a preceding acquisition sequence (for example), as described below. However, in accordance with equation 1, the magnetization M
Z
10 (under the influence of the B
0
field) increases to a magnetization level (called M(t
p
(1))) after a polarization time t
p
(1)after zero magnetization. As shown, after a longer polarization time t
p
(2) from zero magnetization, the magnetization M
Z
10 increases to an M(t
p
(2)) level.
An acquisition sequence begins after the polarization time. For example, an acquisition sequence may begin at time t
p
(1), a time at which the magnetization M
Z
10 is at the M(t
p
(1)) level. At this time, RF pulses are transmitted from a transmitter antenna of the tool. The pulses, in turn, produce spin echo signals 16. A receiver antenna (that may be formed from the same coil as the transmitter antenna) receives the spin echo signals 16 and stores digital signals that indicate the spin echo signals 16. The initial amplitudes of the spin echo signals 16 indicate a point on the magnetization M
Z
10 curve, such as the M(t
p
(1)) level, for example. Therefore, by conducting several measurements that have different polarization times, points on the magnetization M
Z
10 curve may be derived, and thus, the T1 time for the particular formation may be determined.
As an example, for the acquisition sequence, a typical logging tool may emit a pulse sequence based on the CPMG (Carr-Purcell-Meiboom-Gill) pulse train. The application of the CPMG pulse train includes first emitting a pulse that rotates the magnetization, initially polarized along the B
0
field, by 90° into a plane perpendicular to the B
0
field. A train of equally spaced pulses follows, whose function is to maintain the magnetization polarized in the transverse plane. In between the pulses, magnetization refocuses to form the spin echo signals
16
that may be measured using the same antenna. Because of thermal motion, individual hydrogen nuclei experience slightly different magnetic environments during the pulse sequence, a condition that results in an irreversible loss of magnetization and consequent decrease in successive echo amplitudes. This rate of loss of magnetization is characterized by a “transverse relaxation time” (called T2) and is depicted by the decaying envelope
12
of FIG.
1
. This may be referred to as a T2-based experiment.
Measurements of T1 are typically made using a method known as saturation recovery. In this approach, longitudinal magnetization is first destroyed, then allowed to recover for a length of time, t
p
, at which point it is monitored, using a radio frequency pulse or sequence of pulses, and the signal recorded in a receiver. The signal amplitude is proportional to the recovered magnetization at time, t
p
. By repeating the measurement for different t
p
values, the magnetization recovery profile, Mz(t
p
), is sampled and may be analyzed to determine the longitudinal relaxation time T1. This may be referred to as a T1 based experiment. If a sequence of pulses such as the CPMG sequence is used to monitor the magnetization recovery at time, t
p
, the initial amplitude of the echo decay envelope represents Mz(t
p
), while the echo decay profile,
12
, yields T2 information corresponding to this longitudinal magnetization, Mz(t
p
). Analysis of these experiments provides information concerning both T1 and T2.
In a CPMG pulse train with a spacing (called TE) between the pulses, applied to a sample containing a single type of fluid, an amplitude, A(n) of the nth echo may be described by the following equation:
A
(
n
)
=
M
(
t
p
)
e
nT
E
T
2
=
M
0
(
1
-
e
-
t
p
T1
)
e
-
nT
E
T
2
,
Equation
2
where t
p
is the polarization time.
The measured NMR signal, A(n), is governed by three quantities (M
0
, T1 and T2) that reflect physical properties of the fluids and the formation. The equilibrium longitudinal magnetization M
0
is used to compute the total porosity of formation, as described by the following equation:
φ
=
KM
0
HI
,
Equation
3
where HI is the hydrogen index of the formation fluid, and K is a calibration factor that accounts for several tool and external parameters. Relaxation times are related to permeability of the formation as well as the fluid properties and may be used to identify hydrocarbon types. Water relaxation times increase with increasing pore size. Thus, short T1 or T2 times indicate bound water, while long T1 and T2 times are associated with free fluid. For hydrocarbons in water wet rocks, the T1 and T2 times are determined by viscosity. The T1 time increases with decreasing viscosity over the entire hydrocarbon range from bitumen to methane gas. The T2 time follows a similar trend for heavy and medium oils. For lighter hydrocarbons, diffusion effects reduce the T2 time. The effect is most significant for gas. Because of the wide range of pore sizes found in rock formations and the chemical complexity of typical oils
Davies Dylan H.
Heaton Nicholas J.
Sezginer Abdurrahman
Sun Boqin Q.
Taherian M. Reza
Arana Louis
Jeffery Brigitte L.
Ryberg John J.
Schlumberger Technology Corporation
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