Electricity: measuring and testing – Particle precession resonance – Using well logging device
Reexamination Certificate
1999-10-01
2001-01-30
Oda, Christine (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using well logging device
C324S322000
Reexamination Certificate
active
06181132
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a logging assembly for application in a wellbore formed in an earth formation. More specifically the invention relates to an assembly for NMR logging, whereby a magnetic field is induced in a selected zone of the earth formation, and an antenna system transmits radio-frequency (RF) signals into the selected zone and receives response RF signals therefrom. NMR logging tools are generally applied to obtain information about the hydrogen contents of the formation.
BACKGROUND OF THE INVENTION
In operation the magnetic field aligns the magnetic moments of the hydrogen nuclei, and the transmitted RF signals re-orient the magnetic moments of the hydrogen nuclei. The rate at which the magnetic moments return to their aligned position provides information about the pores in which the liquids are residing. NMR logging tools are typically operated on wireline in a lower, open hole section of the wellbore.
WO-9803767-A discloses a logging method wherein a string of logging tools is arranged within a lower end part of a casing, referred to as the casing shoe track, to be positioned in the wellbore. The logging tools are operated during lowering of the casing shoe track through the open hole section of the wellbore. The string of logging tools is retrieved to surface after the casing has been installed. A problem of the known method arises if an NMR tool is to be included in the string because in order to achieve adequate magnetic field strength a magnet of relatively large size has to be applied in the tool string which thereby becomes of unacceptably large diameter. Conversely, if a small diameter magnet is used in order to keep the string dimension acceptable, the induced magnetic field is relatively weak.
It is therefore an object of the invention to provide an improved NMR logging assembly which induces a strong magnetic field into the formation and which does not have the limitations referred to above.
SUMMARY OF THE INVENTION
In accordance with the invention there is provided a logging assembly for application in a wellbore formed in an earth formation, the assembly comprising
a casing suitable to be lowered through the wellbore and to be arranged in a selected part thereof; and
a NMR logging device comprising magnetising means for inducing a magnetic field into a selected zone of the earth formation, antenna means for transmitting radio-frequency (RF) signals into the selected zone and for receiving response RF signals from the selected zone, and data collecting means for collecting data representing the response RF signals, the data collecting means being removably arranged in the interior of the casing; wherein the means for inducing the magnetic field includes a magnetised section of the casing.
REFERENCES:
patent: 2838732 (1958-06-01), Brown
patent: 4656422 (1987-04-01), Vail et al.
patent: 4792757 (1988-12-01), Vail et al.
patent: 0 198 638 (1986-10-01), None
patent: WO 00/20897 (2000-04-01), None
International Search Report dated Nov. 24, 1999.
Oda Christine
Shell Oil Company
Shrivastav Brij B.
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