Electricity: measuring and testing – Particle precession resonance – Using well logging device
Reexamination Certificate
2001-05-22
2003-05-06
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using well logging device
C324S306000
Reexamination Certificate
active
06559640
ABSTRACT:
FIELD OF THE INVENTION
The invention is related to the field of Nuclear Magnetic Resonance (NMR) apparatus and methods. Specifically, the invention relates to NMR apparatus and methods using pulsed static magnetic fields.
BACKGROUND OF THE INVENTION
When hydrogen nuclei are placed in an applied static magnetic field, a small majority of spins are aligned with the applied field in the lower energy state, since the lower energy state in more stable than the higher energy state. The individual spins precess about the applied static magnetic field at a resonance frequency also termed as Larmor frequency. This frequency is characteristic to a particular nucleus and proportional to the applied static magnetic field. An alternating magnetic field at the resonance frequency in the Radio Frequency (RF) range, applied by a transmitting antenna to a subject or specimen in the static magnetic field flips nuclear spins from the lower energy state to the higher energy state. When the alternating field is turned off, the nuclei return to the equilibrium state with emission of energy at the same frequency as that of the stimulating alternating magnetic field. This RF energy is generating an oscillating voltage in a receiver antenna whose amplitude and electronic rate of decay depend on the physicochemical properties of the tissue and the magnetic environment of the nuclei. The applied RF field is designed to perturb the thermal equilibrium of the magnetized nuclear spins, and the time dependence of the emitted energy is determine by the manner in which this system of spins return to equilibrium magnetization. The return is characterized by two parameters: T
1
, the longitudinal or spin-lattice relaxation time; and T
2
, the transverse or spin-spin relaxation time.
There are at least two applications in which samples volumes are substantial and bulk material properties are of interest. One of these is logging of wells drilled for hydrocarbon recovery from earth formations and another is whole body fat determination.
Measurements NMR parameters of fluid filling the pore spaces of the earth formations such as relaxation times of the hydrogen spins, diffusion coefficient and/or the hydrogen density is the bases for NMR well logging. NMR well logging instruments can be used for determining properties of earth formations including the fractional volume of pore space and the fractional volume of mobile fluid filling the pore spaces of the earth formations.
Pulsed RF magnetic fields are imparted to the material under investigation to momentarily re-orient the nuclear magnetic spins of the hydrogen nuclei. RF signals are generated by the hydrogen nuclei as they spin about their axes due to precession of the spin axes. The amplitude, duration and spatial distribution of these RF signals are related to properties of the material under investigation. In the well logging environment, contrast is high between free and bound fluids based on their relaxation times, between oil and water based on their relaxation times and diffusion coefficient. In medical applications, tissue contrast is high between fat and muscle based on their relaxation times and can be further enhanced by application of certain RF sequences.
Methods of using NMR measurements for determining the fractional volume of pore space and the fractional volume of mobile fluid are described, for example, in
Spin Echo Magnetic Resonance Logging: Porosity and Free Fluid Index Determination
, M. N. Miller et al, Society of Petroleum Engineers paper no. 20561, Richardson, Tex., 1990. In porous media there is a significant difference in T
1
and T
2
relaxation time spectrum of fluids mixture filling the pore space. For example, light hydrocarbons and gas may have T
1
relaxation time of about several seconds, while T
2
may be three orders of magnitude smaller. This phenomenon is due to diffusion effects in the presence of gradients in the static magnetic field. The gradients may be external (from the applied static field) or internal. Internal magnetic field magnitude gradients are due to differences in magnetic susceptibility between the rock matrix of the formation and the fluids in the pores of the matrix.
Power requirements in NMR oil well logging have to be optimized for high efficiency operation. In order to perform a valid NMR experiment, a substance should be polarized for about 5 times the longest T
1
relaxation time, which is about 1 second long. Typical Carr-Purcell-Meiboom-Gill (CPMG) pulse sequences are about 0.5 to 1 second long. However, because of low signal-to-noise ratio (SNR), several repetitions of a CPMG sequence are required to bet an adequate SNR.
The earliest NMR logging instruments used the earth's magnetic field for providing the static field for NMR measurements. See, for example, U.S. Pat. No. 3,004,212 to Coolidge et al; U.S. Pat. No. 3,188,556 to Worthington; U.S. Pat. No. 3,538,429 to Baker; and U.S. Pat. No. 2,999,204 to Jones et al. The earth's magnetic field is approximately 60 &mgr;T at the poles with a Larmor frequency f for protons of approximately 2.5 kHz. The signal level per unit volume for an NMR survey is approximately proportional to f
7/4
. The early NMR logging instruments suffered from the problem of low resolution because signals from a large volume of the earth were required to get an acceptable SNR. When the earth's magnetic field is used for the static field, there is no problem in having a uniform static field over a large region, so that SNR is not a major problem; however, there are many applications in which high resolution is required. This is difficult to achieve using the earth's magnetic field as the static field for NMR experiments.
In order to achieve high resolution, NMR devices used in recent years for well logging operations use permanent magnets to generate the static magnetic field. These devices typically operate at 1 MHz corresponding to a magnetic field in the region of investigation of 0.0235T. Needless to say, this requires the use of permanent magnets with a strong magnetic field as part of the logging instrument.
For example, 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. 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 cf 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.
These, and others too numerous to mention, have been used for wireline logging wherein the logging tool is conveyed on a wireline into a borehole, as well as Measurement-While-Drilling (MWD) operations where the logging tool forms part of the drilling assembly. All of these tools typically have a region of investigation no more than a few centimeters into the formation and a few millimeters in thickness. Repeatability of the observations requires that the static magnetic field be predictable to a high level of accuracy. An unappreciated problem in NMR logging of earth formations using strong permanent magnets is that the static magn
Arana Louis
Baker Hughes Incorporated
Madan Mossman & Sriram P.C.
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