NMR sensor

Details NMR sensor NMR sensor

1999-01-19

2001-04-10

Patidar, Jay (Department: 2862)

Electricity: measuring and testing

Particle precession resonance

Using well logging device

C324S318000

Reexamination Certificate

active

06215304

ABSTRACT:

BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to an NMR sensor.
A measurement-while-drilling tool is described in EP-A-0581666 (Kleinberg) The tool comprises a tubular drill collar; a drill head positioned at an axial end of the drill collar; and an NMR sensor. The NMR sensor comprises a pair of tubular main magnets (which generate a static (B
0
) magnetic field) each located in an internal recess of the drill collar, and an RF antenna located in an external recess in the drill collar between the main magnets. The RF antenna recess is optionally filled with a magnetically soft ferrite to improve the efficiency of the antenna.
An NMR well logging system is described in U.S. Pat. No. 4629986 (Clow et al.). A pair of main magnets are separated by a gap in which a solenoid RF antenna is symmetrically disposed. The solenoid has a core of high permeability ferrimagnetic material (soft ferrite).
A problem with the prior art systems is that dimensional resonances can be induced in the ferrite by the RF electromagnetic field. This absorbs energy and reduces RF efficiency.
In accordance with the present invention there is provided an NMR sensor comprising a magnetic field generating assembly; an RF antenna; and a plurality of ferrite members which couple with RF magnetic fields transmitted or received by the RF antenna.
The ferrite members boost the Q of the RF antenna and compensate for the effects of eddy currents. Typically the ferrite members are soft ferrite members.
By splitting the ferrite into a plurality of separate members, dimensional resonance in the ferrite is minimised. In particular, this enables the ferrite members to each have a maximum dimension less than half the wavelength of the lowest order standing wave which could otherwise be set up in the ferrite.
The ferrite members may be separated by air gaps or by a suitable filler such as epoxy resin. The ferrite members may each comprise a separate particle in a single epoxy resin matrix. In a limiting case the ferrite members may be physically in contact. However at a microscopic level the members will only be in contact at points, and standing waves will still be substantially attenuated by the crystal discontinuities between the members.
Apart from minimising dimensional resonances, the provision of plural ferrite members provides an additional degree of freedom in the geometrical arrangement of the ferrite. Therefore the relative sizes and positions of the ferrite members can be selected to optimise the B
0
and RF field profiles. The effect of ferrite on the B
0
and RF field profiles has not previously been fully recognised in the prior art. It is important that the B field shape is optimised to maximise radial shell thickness to reduce susceptibility to lateral tool motions (such as vibration and whirl) whilst maintaining sufficient signal-to-noise ratio. In particular, unless care is taken in the design, the static magnetic field will tend to saturate the soft ferrite, reducing its relative permeability to unity and negating any improvements in RF efficiency. Similarly, the soft ferrite will modify the B
0
field profile, thereby changing the shape and position of the sensitive volume from which NMR signal arises. Both of these related effects must be considered in the design of a real sensor.
Various BO field profiles are achievable by adjusting the size and axial position of the soft ferrite members: it is possible to cancel the first and second order radial gradients to create a “radially optimised” field profile, as described by Hanley in U.S. Pat. No. 5471140, or alternatively to cancel the first order axial field gradient to generate an “axially optimised” field profile, as described in EP-A-0774671, or to shim the field for uniform BO magnitude for an “intermediate” field profile, as described by Slade in PCT/GB98/02398. Unlike this prior art, the BO field manipulation is achieved using the placement of soft ferrites only; no hard ferrite permanent magnet shims need be employed.
Furthermore, in a similar fashion adjustment of the soft ferrite members can be used to reposition the small crescent-shaped resonant regions, known as “borehole lobes” and shown in
FIG. 6
, which can produce unwanted NMR signal from the borehole region. The lobes can be moved until they are partially or wholly within the outside diameter of the tool. In this way they cannot generate a significant borehole NMR signal.
Typically the NMR sensor is an “inside-out” sensor which performs measurements on an external sample outside the space envelope of the magnetic field generating assembly and the RF antenna.
The sensor may be employed in a variety of applications. However typically the sensor is provided in apparatus for performing borehole measurements in a formation.
The apparatus may be a wireline tool which performs measurements after the borehole has been drilled. However in a preferred example the apparatus is a measurement-while-drilling (MWD) tool which is provided with a drill head at an axial end of a support whereby the apparatus can carry out NMR measurements during drilling of the borehole. The tool may be a logging-while-drilling (LWD) or formation-evaluation-while-drilling (FEWD) tool in which the NMR information relating to the formation is stored on in-board memory for retrieval when the tool is returned to the surface. Alternatively a telemetry system may be provided and the NMR information is used to control the drill in real time (i.e. steering).
The ferrite has the unavoidable effect of reducing the inner diameter of the working volume in comparison with similar sized logging tools using permanent magnet shims as described by Hanley in U.S. Pat. No. 5471140 and EP-A-0774671. This results in a loss of penetration depth. However this is less of a disadvantage in a MWD tool because the invasion of the formation by borehole fluids occurs slowly after drilling. The MWD tool generally arrives at the formation less than an hour after cutting, whereas a wireline tool can arrive days or weeks later. As a result there will be less borehole fluid in the formation under study and so the use of ferrite is particularly suited to a MWD tool.
Furthermore a typical MWD tool has a larger radius than a comparable wireline tool. Since the B
0
. strength scales approximately as the second power of the magnet mean radius, it is possible to space the main magnets farther apart in an MWD tool using larger diameter main magnets and thus regain some of the penetration depth.
The ferrite members may be axially spaced and/or spaced at right angles to the axis of the tool. A primary consideration in the design of an NMR MWD tool is making the NMR measurement insensitive to the effect of lateral tool motions, such as vibration and whirl. To a first approximation it is clear that it will not be possible to re-focus the NMR signal in the sensitive region if the tool is displaced laterally (i.e. in a direction parallel to the radius) during the pulse sequence by a distance which is a significant proportion of the radial thickness of the sensitive shell. It is therefore necessary to select a B
0
optimisation scheme and RF bandwidth such that the shell thickness is much larger than the maximum expected lateral displacement. Little is known about the precise motions of drilling tools down hole, but the typical range of displacement is from 1 to 10 mm at frequencies of a few Hz.
Rotation periods are between 1 and 3Hz. The typical NMR mea
0
surement lasts from 50 ms to 1s, so these motions are significant. However, the flexible nature of the sensor according to the present invention ensures that it is possible to design a tool with a sensitive shell thicker than the maximum expected motion. The tool described in the preferred embodiment has a shell with a radial thickness about 20 mm and axial length about 50 mm.
In comparison to a wireline borehole logging tool, an MWD tool has to be significantly stronger to support the drilling forces. In particular, as the sensor forms part of the drill collar, it has to be able to withsta

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