Electricity: measuring and testing – Particle precession resonance – Using well logging device
Reexamination Certificate
1998-06-09
2001-07-03
Oda, Christine (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using well logging device
C324S300000
Reexamination Certificate
active
06255817
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of well logging and, more particularly, to a method and apparatus for determining nuclear magnetic resonance logging characteristics of earth formations surrounding a borehole, either during or after the drilling of the borehole.
BACKGROUND OF THE INVENTION
In the evaluation of earth boreholes drilled in earth formations to produce hydrocarbons, determination of the porosity of the formations is considered essential for decision making. Nuclear magnetic resonance (“NMR”) provides a means of measuring total and producible porosity of earth formations. In certain conditions NMR well logging can provide important information on the pore size of formation rock and on the type of fluid contained therein. Measurement of nuclear resonance requires a static magnetic field {overscore (B)}
0
and a radio frequency (RF) magnetic field in the earth formation that is being probed. [As used herein, an RF field generally has a frequency in the range 2 KHz to 10 MHz.] Atomic nuclei with a nonzero nuclear magnetic moment and spin angular momentum precess about the static field {overscore (B)}
0
with an angular frequency &ohgr;
0
=&ggr;B
0
when perturbed from their thermal equilibrium. The constant &ggr; is the gyromagnetic ratio of the resonating nucleus, most commonly the hydrogen nucleus. For hydrogen nuclei, the gyromagnetic ratio is 2.675198775×10
8
radian/second/Tesla. To manipulate the spin state of the particles, for example, to perturb the thermal equilibrium, a radio frequency (RF) magnetic field {overscore (B)}
1
is needed. The frequency of the RF field {overscore (B)}
1
should be close to &ohgr;
0
and substantially perpendicular to the static field {overscore (B)}
0
in the region of investigation. Magnetic resonance is observed by detecting the oscillating magnetic field produced by the precession of the spins. Typically, but not necessarily, the same coil that produces the RF field {overscore (B)}
1
is used for detection. In pulsed-NMR, repeated pulses are applied to the coil and spin-echoes are detected in between the transmitted pulses. Reference can be made, for example, to U.S. Pat. Nos. 5,376,884, 5,055,788, 5,055,787, 5,023,551, 4,933,638, and 4,350,955 with regard to known nuclear magnetic resonance logging techniques.
In logging-while-drilling, the measurement apparatus is mounted on a drill collar. Drill collars are long, tubular pieces of a strong material, typically nonmagnetic stainless-steel. Drill collars and drill pipes transmit the torque from the surface apparatus to the drill bit. During drilling, the drill collars typically rotate about their axes, which are substantially aligned with the axis of the borehole. The rates of rotation of the drill collars and the drill bit are the same in rotary drilling, and can be different if a downhole mud motor is used. In either case, the drill collar is subject to rotation. For logging-while-drilling NMR logging, the magnitudes of {overscore (B)}
0
, {overscore (B)}
1
, and the angle between them should be substantially invariant of the rotation angle in the region of investigation. This does not preclude the possibility that the directions of {overscore (B)}
0
and {overscore (B)}
1
may depend on the rotation angle. The foregoing invariance is required because magnetic resonance measurements take on the order of 0.01 to 1 seconds during which the drill collar may rotate by a substantial angle. Consistent preparation and measurement of spin states are not possible without the rotational invariance.
Directional drilling involves the drilling of a well bore along a deviated course in order to reach a target region at a particular vertical and horizontal distance from the original surface location. Directional drilling is employed, for example, to obtain an appropriate well bore trajectory into an oil producing formation bed (or “pay zone”) and then drill substantially within the pay zone. A horizontally drilled well can greatly increase the borehole volume in the pay zone with attendant increase in oil production. Recent advances in directional drilling equipment and techniques have greatly improved the accuracy with which drilling paths can be directed.
Nuclear magnetic resonance logging systems have previously been proposed for logging-while-drilling applications. If an NMR logging device of a logging-while-drilling system has an axially symmetric response, the NMR characteristics measured by the logging device will tend to average the signals received circumferentially from the formations. For example, when drilling a near-horizontal well along the boundary between two formation beds with dissimilar producible porosities, such a logging device would give indication of an intermediate porosity. It would be very advantageous to be able to use NMR to better delineate the presence, locations, and characteristics of the formation beds in this type of a situation.
It is among the objects of the present invention to address limitations of the prior art with regard to nuclear magnetic resonance logging techniques and apparatus.
SUMMARY OF THE INVENTION
The invention described in the copending parent application hereof, U.S. application Ser. No. 08/880,343, provides the capability of azimuthally resolved nuclear magnetic resonance logging. That invention and the invention hereof can both be used in so-called wireline logging, but the inventions are particularly advantageous in achieving azimuthally resolved NMR logging-while-drilling measurements.
A form of the invention set forth in said copending U.S. application Ser. No. 08/880,343 is directed to an apparatus and method for determining a nuclear magnetic resonance property of formations surrounding a borehole while drilling the borehole with a rotating drill bit on a drill string. An embodiment of the method of that invention includes the following steps: providing a logging device in the drill string, the logging device being rotatable with the drill string or a portion of the drill string, the logging device having a rotational axis; producing a static magnetic field and an RF magnetic field at the logging device, the static and RF magnetic fields having mutually orthogonal components in an investigation region in the formations surrounding the logging device, the magnitudes of the static and RF magnetic fields in the investigation region being substantially rotationally invariant as the logging device rotates around its axis; receiving nuclear magnetic resonance spin echoes at at least one circumferential sector on the logging device; and determining a nuclear magnetic resonance property of the formations, for different portions of the investigation region, from the received nuclear magnetic resonance spin echoes. [It will be understood that the static and RF magnetic fields are defined as having “mutually orthogonal components” if they are not parallel. Typically, but not necessarily, the static and RF magnetic fields will be substantially perpendicular in the investigation region.]
In another form of the invention set forth in said copending Application, the receiving of nuclear magnetic resonance spin echoes is implemented at a plurality of different circumferential sectors on the logging device and comprises providing a plurality of arcuate receiver segments around the logging device and detecting nuclear magnetic resonance spin echoes in signals received by the individual receiver segments.
In embodiments hereof, in order to make an azimuthally-resolved NMR measurement, the receiving radio-frequency antenna has a non-axisymmetric response pattern. The transmitting (pulsing) antenna can be either the same non-axisymmetric antenna, or a separate non-axisymmetric antenna, or a separate axisymmetric antenna. If the azimuthal measurement is to be performed while the tool is rotating (as is typically the case in MWD), the static magnetic field ({overscore (B)}
0
) should be axisymmetric, at least in terms of its magnitude. In certain embodiments hereof, the rf antennas employ ax
Chang Shu-Kong
Ganesan Krishnamurthy
Goswami Jaideva C.
Poitzsch Martin E.
Speier Peter
Jeffery Brigitte L.
Oda Christine
Ryberg John J.
Schlumberger Technology Corporation
Shrivastav Brij B.
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