Downhole NMR tool having a programmable pulse sequencer

Details Downhole NMR tool having a programmable pulse sequencer Downhole NMR tool having a programmable pulse sequencer

1999-10-26

2002-06-04

Arana, Louis (Department: 2862)

Electricity: measuring and testing

Particle precession resonance

Using well logging device

C324S300000

Reexamination Certificate

active

06400147

ABSTRACT:

BACKGROUND
This invention relates to a downhole tool that includes a programmable pulse sequencer.
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, the typical downhole 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 petrdphysically important parameters. For example, the NMR measurements may provide information that may be used to derive the permeability of the formation and viscosity of fluids contained within the pore space of the formation. It may be difficult or impossible to derive this information 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 receives radio frequency (RF) pulses from a pulse generator of the tool and in response, radiates RF pulses into the formation. A receiver antenna may measure the response (indicated by a received RF signal called a spin echo signal) of the polarized hydrogen to the transmitted pulses. Quite often, the transmitter and receiver antennae are combined into a single transmitter/receiver antenna.
The NMR techniques employed in current NMR tools typically involve some variant of a basic two step technique that includes delaying for a polarization time and thereafter using 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).
An example of an NMR sequence is a Carr-Purcell-Meiboom-Gill (CPMG) sequence
15
that is depicted in FIG.
1
. By applying the sequence
15
, a distribution of spin relaxation times (T
2
times, for example) may be obtained, and this distribution may be used to determine and map the properties of a formation. 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 pulses an RF field (called 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 an 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
. At the end of the pulse
14
a
, the spins are rotated by 90° into the plane perpendicular to the B
0
field. The spins 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 pulses the B
1
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. 2
) that peaks at 2·T
CP
after the 90° tipping pulse
14
a
. 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 the amplitudes of each set of spin echo signals
16
may be used to derive a distribution of T
2
times.
Although it may be desirable to vary the characteristics of the measurement sequence to optimize performance to a particular formation, unfortunately, a conventional NMR tool may be specifically designed to perform a predefined NMR measurement sequence. Thus, the conventional tool may provide limited flexibility for changing the sequence, as the parameters that may be programmed into the tool may affect the global timing of the sequence without allowing the flexibility to change a particular portion of the sequence. For example, a conventional NMR tool may be programmed with the above-described T
CP
time, the time between the tipping pulse
14
a
and the first refocusing pulse
14
b
. However, this value also sets the time (2·T
CP
) between successive refocusing pulses
14
b
. Thus, although a time between refocusing pulses
14
b
other than 2·T
CP
may be desired to optimize performance of the tool, the tool may not provide the flexibility to change this time.
Thus, there is a continuing need for an arrangement that addresses one or more of the problems that are stated above.
SUMMARY OF THE INVENTION
In one aspect of the invention, an NMR measurement apparatus includes at least one antenna and a pulse sequencer. The pulse sequencer is coupled to said at least one antenna and is adapted to receive state descriptors that are indicative of states of an NMR measurement sequence. The pulse sequencer uses the antenna(e) to perform the NMR measurement sequence in a downhole formation in response to the state descriptors.
Other aspects of the invention as well as advantages of the invention will become apparent from the following description, drawing and claims.


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Copy of Combined Search and Examination Report under Sections 17 and 18 (3), Jun. 1, 2000.
Chandler, R.N. et al. “Improved Log Quality with a Dual-Frequency Pulsed NMR Tool,” SPE Paper 28365, presented at the 69thAnnual Technical Conference and Exhibition, New Orleans, Sep. 25-28, 1994.
Copy of Search Report (Rapport Betreffende het onderzoek naar de stand van de techniek).

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