Electricity: measuring and testing – Particle precession resonance – Using well logging device
Reexamination Certificate
2000-10-18
2002-05-14
Williams, Hezron (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using well logging device
C324S300000
Reexamination Certificate
active
06388441
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a method for measuring nuclear magnetic resonance properties of an earth formation traversed by a borehole, and more particularly, to a method for eliminating any ringing, such as magnetoacoustic ringing, and DC offset, during a nuclear magnetic resonance measurement.
2. Background of the Art
A variety of techniques are utilized in determining the presence and estimation of quantities of hydrocarbons (oil and gas) in earth formations. These methods are designed to determine formation parameters, including among other things, the resistivity, porosity and permeability of the rock formation surrounding the wellbore drilled for recovering the hydrocarbons. Typically, the tools designed to provide the desired information are used to log the wellbore. Much of the logging is done after the well bores have been drilled. More recently, wellbores have been logged while drilling, which is referred to as measurement-while-drilling (MWD) or logging-while-drilling (LWD).
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 liquids in the geological formations surrounding 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 measured. 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.
The NMR instrument also typically includes an antenna, positioned near the magnet and shaped so that a pulse of radio frequency (RF) power conducted through the antenna induces an RF magnetic field in the earth formation. The RF magnetic field is generally orthogonal to the field applied by the magnet. This RF pulse, typically called a 90 degree pulse, has a duration and amplitude predetermined so that the spin axes of the hydrogen nuclei generally align themselves perpendicularly both to the orthogonal magnetic field induced by the RF pulse and to the magnetic field applied by the magnet. After the 90 degree pulse ends, the nuclear magnetic moments of the hydrogen nuclei gradually “relax” or return to their original alignment with the magnet's field. The amount of time taken for this relaxation, referred to as T
1
, is related to petrophysical properties of interest of the earth formation.
After the 90 degree pulse ends, the antenna is typically electrically connected to a receiver, which detects and measures voltages induced in the antenna by precessional rotation of the spin axes of the hydrogen nuclei. The precessional rotation generates RF energy at a frequency proportional to the strength of the magnetic field applied by the magnet, this frequency being referred to as the Larmor frequency. The constant of proportionality for the Larmor frequency is known as the gyromagnetic ratio &ggr;
0
. The gyromagnetic ratio is unique for each different chemical elemental isotope. The spin axes of the hydrogen nuclei gradually “dephase” because of inhomogeneities in the magnet's field and because of differences in the chemical and magnetic environment within the earth formation. Dephasing results in a rapid decrease in the magnitude of the voltages induced in the antenna. The rapid decrease in the induced voltage is referred to as the free induction decay (FID). The rate of FID is typically referred to by the notation T
2
*. The FID decay rate consists of a first component referred to as “true T
2
”, which is due to internal nuclear environmental effects, and a second component resulting from microscopic differences in magnetic field and inhomogeneities in the earth formation. The effects of the second component can be substantially removed by a process referred to as spin-echo measurement.
One problem with analysis of NMR measurements is that the signal detected by the antenna includes a parasitic, spurious ringing that interferes with the measurement of spin-echoes. One source of the spurious signal is electromagnetic generation of ultrasonic standing waves in metal. The induced RF current within the skin depth of the metal interacts with the lattice in a static magnetic field through the Lorenz force and the coherent ultrasonic wave propagates into the metal to set up a standing wave. A reciprocal mechanism converts the acoustic energy, in the presence of the static field, to an oscillating magnetic field which is picked up by the antenna as a spurious, ringing signal.
Different types of magnetoacoustic interaction may produce a parasitic signal in the NMR antenna. Antenna wiring and other metal parts of the NMR logging tool can be affected by the static magnetic field and the RF field generated by the antenna. If the antenna is located within the strongest part of the magnet's field, when RF pulses are applied to the antenna, acoustic waves are generated in the antenna and the antenna sustains a series of damped mechanical oscillations in a process known to those skilled in the art as magnetoacoustic ringing. This ringing can induce large voltages in the antenna which are superimposed with the measurement of the voltages induced by the spin-echoes.
Another source of magnetoacoustic interaction is magnetorestrictive ringing which is typically caused when nonconductive magnetic materials, such as magnetic ferrite, are used in the antenna. If this magnetic material is located within the strong part of the RF field, application of RF pulses will generate acoustic waves in the magnet. The magnet will experience a series of damped mechanical oscillations upon cessation of the RF pulse. Magnetorestrictive ringing can also induce large voltages in the antenna which are superimposed with the measurement of the voltages induced by the spin-echoes.
One approach to reduce the effects of ringing has been to design the hardware to minimize the interaction between the electromagnetic fields and the materials in the device. For example U.S. Pat. No. 5,712,566 issued to Taicher et al. discloses a device in which the permanent magnet composed of a hard, ferrite magnet material that is formed into an annular cylinder having a circular hole parallel to the longitudinal axis of the apparatus. One or more receiver coils are arranged about the exterior surface of the magnet. An RF transmitting coil is located in the magnet hole where the static magnetic field is zero. The transmitting coil windings are formed around a soft ferrite rod. Thus, magnetoacoustic coil ringing is reduced by the configuration of the transmitting coil. Magnetorestrictive ringing of the magnet is reduced because the radial dependence of the RF field strength is relatively small due to use of the longitudinal dipole antenna with the ferrite rod. Further, magnetorestrictive ringing is reduced because the receiver coil substantially removes coupling of the receiver coil with parasitic magnetic flux due to the inverse effect of magnetostriction.
Another commonly used approach to reduce the effect of ringing is to use a so-called phase-alternated-pulse sequence. Such a sequence is often implemented as
RFA
±x
−&tgr;−n·
(
RFB
y
−&tgr;−echo−&tgr;)
−TW
(1)
where RFA
±x
is an A pulse, usually 90° tipping pulse and RFB is a pulse, usually a 180° refocusing pulse. The ± phase of RFA is applied alternately in order to identify and eliminate systematic noises, such as ringing and DC offset through subsequent processing. By subtracting the echoes in the − sequence from the pulses in the adjoining + sequence, the ringing due to the 180° is suppressed.
PCT publication WO 98/43064 of Prammer addresses the problem of ringing
Baker Hughes Incorporated
Madan Mossman & Sriram P.C.
Shrivastav Brij B.
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