Estimate of transversal motion of the NMR tool during logging

Details Estimate of transversal motion of the NMR tool during logging Estimate of transversal motion of the NMR tool during logging

2001-05-01

2002-08-20

Arana, Louis (Department: 2862)

Electricity: measuring and testing

Particle precession resonance

Using well logging device

C324S300000

Reexamination Certificate

active

06437564

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention is related to the field of Nuclear Magnetic Resonance (“NMR”) apparatus and methods. More specifically, the invention relates to detecting and estimating the effect of transversal motion of the NMR tool used in oil well logging on the signal-to noise ratio by using both in-phase and out-phase measurements of spin echoes.
2. Description of the Related Art
NMR has applications in various fields from medical applications to oil well logging applications. In oil well testing, NMR is used to determine, among other things, the porosity of the material, the amount of bound liquid in the volume, permeability, and formation type, as well as oil content.
A current technique in wellbore logging employs an NMR tool to gather information during the drilling process. This technique is known as logging while drilling (LWD) or measuring-while-drilling (MWD) and requires the NMR tool to be included as part of the drilling bottom hole assembly. This process greatly increases speed at which information is gathered and consequently reduces the cost of acquiring downhole information. This tool can be, as an example, one that is outlined in U.S. Pat. No. 5,280,243, entitled, “System For Logging a Well during the Drilling Thereof”, granted to Miller. The device disclosed therein includes a permanent magnet which induces a static magnetic field into the surrounding volume. In addition, an antenna, which is aligned orthogonal to this magnet, has the purpose of introducing radio frequency (RF) pulses into the region. The same or another antenna is used to receive signals returning from the volume.
Typically, in the presence of only the permanent magnet, nuclear spins will align either parallel or anti-parallel to the static magnetic field, creating a net overall magnetic polarization, called a bulk magnetization. An electric RF pulse sent through this antenna induces another magnetic field in the region. If this induced magnetic field is perpendicular to the field of the permanent magnet, then the induced magnetic field pulse reorients the direction of individual spins perpendicular to the direction of the static field and to the direction of the induced magnetic field. Upon removing the RF pulse, the spins will relax by realigning to their original orientation, along the axis of the static field. The relaxation of the spins to their original orientation occurs over a characteristic time interval, which is known as the spin-lattice relaxation time, T
1
. This relaxation induces a voltage in the receiver antenna.
Spins oriented perpendicular to the static field undergo other motions which can be measured. The spin vector relaxes out of this transverse direction with a characteristic time known as the spin-spin relaxation time or transverse relaxation time, T
2
. Typically, a pattern of RF pulses can be used to determine T
2
. A commonly used pulse pattern is known as the Carr-Purcell-Meiboom-Gill (CPMG) sequence. The CPMG is comprised of one pulsed magnetic field applied in a direction orthogonal to the static magnetic field followed by several pulses applied at preset time intervals in a direction mutually perpendicular to both the direction of the first pulse and the direction of the static magnetic field. The first pulse of the CPMG sequence is known as the A-pulse, and typically occurs over a short time scale with respect to the relaxation time, T
2
. In response to the A-pulse, the spin vectors of the nuclei will align along a common direction in the plane that is perpendicular to the static magnetic field. When an individual spin vector is placed perpendicular to an applied external field, it will precess around the field with a frequency of precession known as the Larmor frequency, which is related to the strength of the applied field. Due to inhomogeneities in the magnetic field, some spins will precess faster while other spins will precess more slowly. Thus, after a time long compared to the precession period, and short compared to T
1
, the spins will no longer be precessing in phase. The diffusion of the phase of the precession takes place over a time scale T
2
*. For an acceptable observation, it is best to have T
2
>>T
2
*.
The B-pulse of the CPMG sequence lasts twice the duration of the A-pulse and is also short compared to precession periods and to relaxation time. Applying the B-pulse gives the nuclear spins an axial rotation of 180 degrees from their immediately previous orientation. In the new orientation after applying the B-pulse, the spins, which were previously diverging from their common orientation due to the A-pulse, are now returning towards this orientation. In addition, by inverting the spatial relation of leading and lagging precessors, the spins are now moving back into phase. As the spins realign, the cumulative effect of this alignment causes a spin echo. The sudden magnetic pulse of the spin echo induces a voltage in the receiving antenna.
Once the spins have realigned and produced the spin echo, they will naturally lose phase again. Applying another B-pulse flips the spin orientation another 180 degrees and sets up the condition for another spin echo. By a using a train of B-pulses, the CPMG pulse pattern creates a series of spin echoes. The amplitude of the train of spin echoes decreases according to the relaxation time, T
2
. Knowledge of T
1
and T
2
gives necessary information on the properties of the material being examined.
Measurements made for T
1
and T
2
require that the NMR measuring device remain stationary over the proper time period. However, a typical measurement period can last over 300 msec. Over a testing period that is sufficiently long, the measuring device will be susceptible to motion from its initial position. At the beginning of the testing period, the permanent magnet might polarize spins of nuclei remaining within a given volume, which can be seen in
FIG. 6
as the shaded volume
20
a.
It is necessary for a certain amount of time to lapse for these spins to polarize completely. If the NMR tool moves during this time, the volume
20
a
changes its position as shown in FIG.
7
. At this new position, the volume
20
a
contains only a portion of the original volume shown in
FIG. 6
, and the receiving antenna will necessarily record unsaturated spins from the new volume. Instead, the new volume contains spins that are not properly aligned to the static field. This effect is typically referred to as “moving fresh spins in” and is a source of error in the detection signal. As an example, the measurement may yield a bound fluid volume (BFV) that is higher than the amount that is actually present in the region.
Several methods have been proposed to detect motion in order to address the problems this motion introduces. Among these methods include use of strain gauges, an ultrasonic range finder an accelerometer, or a magnetometer. These arrangements are described in PCT Application Number PCT/US97/23975, titled “Method for Formation Evaluation While Drilling” filed Dec. 29, 1997. These motion detection devices help to set a threshold to establish the quality of the recorded data. However, they do not provide a means to make corrections which might maintain the quality of the data.
Another proposed device is detailed in European Patent Application 99401939.6, titled “Detecting tool motion effects on nuclear magnetic resonance measurements.” This application uses different geometries and magnetic gradients to measure tool motion. Given the same motion rates of the NMR tool, the signals from two regions of differing applied magnetic gradients will decay at different rates. In the application, setting up an apparatus with two magnetic field gradients makes it possible to obtain both signals and thereby determine the motion speeds and the necessary corrections. Similar information can be derived by measuring spin-echoes in two radially-adjacent regions.
Different magnetic field gradients are easily achieved by placing several permanent magnets in various spatial arrangements with

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