Electricity: measuring and testing – Particle precession resonance – Using well logging device
Reexamination Certificate
2001-04-20
2003-07-01
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using well logging device
Reexamination Certificate
active
06586931
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to apparatus and techniques for making nuclear magnetic resonance (NMR) measurements in boreholes, and to methods for determining magnetic characteristics of formations traversed by a borehole. Specifically, the invention relates to an NMR tool that has a large region of investigation within the formation as a result of measuring NMR spins at frequencies determined by the earth's magnetic field.
BACKGROUND OF THE INVENTION
A variety of techniques have been used in determining the presence and in estimating quantities of hydrocarbons (oil and gas) in earth formations. These methods are designed to determine formation parameters, including among other things, porosity, fluid content, and permeability of the rock formation surrounding the wellbore drilled for recovering hydrocarbons. Typically, the tools designed to provide the desired information are used to log the wellbore. Much of the logging is done after the wellbores have been drilled. More recently, wellbores have been logged while drilling of the wellbores, which is referred to as measurement-while-drilling (“MWD”) or logging-while-drilling (“LWD”). Measurements have also been made when tripping a drillstring out of a wellbore: this is called measurement-while-tripping (“MWT”).
One recently evolving technique involves utilizing Nuclear Magnetic Resonance (NMR) logging tools and methods for determining, among other things porosity, hydrocarbon saturation and permeability of the rock formations. The NMR logging tools are utilized to excite the nuclei of the fluids in the geological formations in the vicinity of the wellbore so that certain parameters such as spin density, longitudinal relaxation time (generally referred to in the art as “T
1
”), and transverse relaxation time (generally referred to as “T
2
”) of the geological formations can be estimated. From such measurements, porosity, permeability, and hydrocarbon saturation are determined, which provides valuable information about the make-up of the geological formations and the amount of extractable hydrocarbons.
A typical NMR tool generates a static magnetic field B
0
in the vicinity of the wellbore, and an oscillating magnetic field B
1
in a direction perpendicular to B
0
. This oscillating field is usually applied in the form of short duration pulses. The purpose of the B
0
field is to polarize the magnetic moments of nuclei parallel to the static field and the purpose of the B
1
field is to rotate the magnetic moments by an angle &thgr; controlled by the width t
p
and the amplitude B
1
of the oscillating pulse. For NMR logging, the most common sequence is the Carr-Purcell-Meiboom-Gill (“CPMG”) sequence that can be expressed as
TW−
90−(
t−
180
−t
−echo)
n
(1)
where TW is a wait time, 90 is a 90 degree tipping pulse, and 180 is a 180 degree refocusing pulse.
After being tipped by 90°, the magnetic moment precesses around the static field at a particular frequency known as the Larmor frequency &ohgr;
0
, given by &ohgr;
0
=&ggr;B
0
, where B
0
is the field strength of the static magnetic field and &ggr; is the gyromagnetic ratio. At the same time, the magnetic moments return to the equilibrium direction (i.e., aligned with the static field) according to a decay time known as the “spin-lattice relaxation time” or T
1
. Inhomogeneities of the B
0
field result in dephasing of the magnetic moments and to remedy this, a 180° pulse is included in the sequence to refocus the magnetic moments. This gives a sequence of n echo signals. These echo sequences are then processed to provide information about the relaxation times.
U.S. Pat. No. 4,350,955 to Jackson et al discloses a pair of permanent magnets arranged axially within the borehole so their fields oppose, producing a region near the plane perpendicular to the axis, midway between the sources, where the radial component of the field goes through a maximum. Near the maximum, the field is homogeneous over a toroidal zone centered around the borehole. With the Jackson arrangement, the axial extent of the region of examination is quite limited. As a result of this, the device can only be operated at relatively low logging speeds: otherwise, because of the tool motion during logging, the magnitude of the static field changes significantly within a fixed region of the formation with an accompanying degradation of NMR signals.
There are several devices in which the problem of limited axial extent of the basic Jackson configuration of permanent magnets is addressed. U.S. Pat. No. 4,717,877 to Taicher et al teaches the use of elongated cylindrical permanent magnets in which the poles are on opposite curved faces of the magnet. The static field from such a magnet is like that of a dipole centered on the geometric axis of the elongated magnets and provides a region of examination that is elongated parallel to the borehole axis. The RF coil in the Taicher device is also a dipole antenna with its center coincident with the geometric axis of the magnet, thereby providing orthogonality of the static and magnetic field over a full 360° azimuth around the borehole.
U.S. Pat. No. 6,023,164 to Prammer discloses a variation of the Taicher patent in which the tool is operated eccentrically within the borehole. In the Prammer device, NMR logging probe is provided with a sleeve having a semi-circular RF shield covering one of the poles of the magnet: the shield blocks signals from one side of the probe. The probe is provided with elements that press the uncovered side of the probe to the sidewall of the borehole so that signals from the uncovered side arise primarily from the formation.
A basic problem with currently used NMR logging tools and methods is that they are based on the generation of a strong magnetic field in the earth formation and operate at frequencies of the order of 1 MHz. The region of investigation is typically no more than a few centimeters inside the formation due to the decay of the static field away from the permanent magnet. A stronger magnetic field, besides being harder to generate, is self-defeating as this pushes up the Larmor frequency to a point where eddy current and dielectric dissipation of the NMR signals become significant. For some tools, the thickness of the region of investigation is of the order of a few millimeters at best because of the spatial variability of the magnetic field. As a result of this, logging is susceptible tool movement in direction perpendicular to borehole axis. For other tools the length of region of investigation along the longitudinal axis is short. As a result of this, logging speed is restricted.
The lack of penetration into the formation means that if a wireline is used for the NMR logging, the logging must be done a short time after drilling to avoid the development of a mudcake. Even if the wireline logging is done a short time after drilling or if a Measurement-while-Drilling method is used, there is still the possibility of invasion of the formation by drilling fluids, so that the logging tool does not measure properties of the virgin formation. Additionally, the drilling process itself usually alter rock formation properties in a close vicinity of a borehole by causing some mechanical damage.
An alternative approach is to use the earth's magnetic field for providing the static field for NMR measurements. The Larmor frequency f for protons at the earth's magnetic field is approximately 2.5 kHz. The signal level per unit volume for an NMR survey is approximately proportional to f
7/4
which means that the signal per unit volume at 2.5 kHz is reduced to about 25×10
−6
of the value for 1 MHz. However, the earth's magnetic field is uniform everywhere an NMR signal can be generated and received, so that the loss of signal intensity is more than made up by receiving a signal from a much larger volume: this makes it feasible to perform NMR logging wherein the precession frequency is determined by the earth's magnetic field.
U.S. Pat. No. 3,0
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