Electricity: measuring and testing – Particle precession resonance
Patent
1994-09-23
1996-06-11
O'Shea, Sandra L.
Electricity: measuring and testing
Particle precession resonance
324303, 324318, 324319, 324322, G01V 300
Patent
active
055259049
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
It is well known that oil bearing rock characteristics such as porosity can be monitored by making use of nuclear magnetic resonance (NMR) techniques. Conventionally, test samples of rock cores from the well head are extracted and then sent to a remote laboratory for analysis. However, this is time consuming and there is a risk of damage to the rock core before it is analysed. Consequently, it has been proposed to carry out NMR analysis on rock cores at the well site. An example of apparatus of this type is described in U.S. Pat. No. 4,885,540. In this case, rock core samples are placed in individual troughs and fed by conveying rollers between the poles of an electromagnet and through a rf coil. The coil is pulsed in a conventional manner and used as a receiving coil to perform various conventional NMR analyses on the rock core samples.
2. Description of the Related Art
A number of problems arise with this known approach. One particular problem is that large fringe fields are generated by the magnet which is particularly undesirable when the apparatus is being used at the well head site. In this situation, there will tend to be large bodies of iron close to the equipment which experience the fringe field and as a result significantly reduce the uniformity of the field in the working region while there will also be sensitive instrumentation nearby which is affected by the fringe field.
Another problem which arises with the conventional approach is that the NMR analysis is carried out on the full cross-section of the rock cores which can lead to erroneous results due to the likelihood of damage to the surface areas.
SUMMARY OF THE INVENTION
In accordance with the present invention, apparatus for monitoring characteristics of rock cores comprises a clad permanent magnet for generating a magnetic field within a working volume which is sufficiently uniform to enable a NMR experiment to be performed; means for conveying rock cores along a path about which the magnet is arranged, the cores passing through the working volume of the magnetic field; and means for carrying out a NMR experiment on the portion of the rock core in the working volume.
In this new and improved approach, we have solved the problem of fringe field and provided apparatus which is capable of analysing small, central portions of the rock cores and which avoids the damaged outer regions.
Clad, permanent magnet structures for magnetic resonance imaging have been previously described in the art. The most compact and economical in materials usage are those described by E. Potenziani and H. A. Leupold (IEEE Transactions on Magnetics Mag-22, 1078-1080, 1986) which make use of "cladding magnets" to oppose the magnetomotive force (mmf) around the outside of the structure. This has the effect of preventing flux leakage so as to most effectively use the permanent magnet material and obtain good field uniformity in the working volume. These previous constructions, however have not been applied in the field of the present invention, namely rock core analysis, and it has certainly not been recognised that clad permanent magnets can provide both the advantages of small fringe field and the ability to achieve NMR analysis on central core regions.
In one example, the magnet has a box shape and this is particularly advantageous since it is possible to generate a magnetic field profile within the core of the magnet which has a saddle region extending along the magnet axis in the direction in which rock cores are conveyed, this saddle region being alignable with the surface of the rock cores and thus enabling the NMR to be performed on the central region of the rock core.
In other examples, the magnet may have a U or C-shape. In this case, the bight of the U may include a permanent magnet which generates 6a magnetic field whose flux passes through a working volume defined between the arms of the U, and wherein the dimensions of the assembly are chosen such that: ##EQU1## where: L.sub.b is the length of t
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IEEE Transactions On Magnetics, vol. 22, No. 5, Sep. 1, 1986, New York, pp. 1078-1080.
Hanley Peter
Hemsley David J.
McDougall Ian L.
Haynes Mack
O'Shea Sandra L.
Oxford Instruments Ltd.
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