Electricity: measuring and testing – Particle precession resonance – Using well logging device
Reexamination Certificate
2001-11-30
2003-02-25
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using well logging device
C324S300000
Reexamination Certificate
active
06525535
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention is in the field of Nuclear Magnetic Resonance testing equipment. In particular the invention is an apparatus for NMR testing to be used for determining with greater accuracy the values of formation properties in borehole drilling.
2. Description of the Related Art
A variety of techniques have been used in connection with wellbore drilling to determine the presence of and to estimate quantities of hydrocarbons (oil and gas) in earth formations surrounding the wellbore. These methods are designed to determine formation parameters including, among other things, porosity, fluid content, and permeability of the rock formation. Typically, the tools designed to provide the desired information are used to log the wellbore. Much of the logging is done after the wellbore has been drilled. Removing the drilling apparatus in order to log the wellbore can prove costly in terms of time and money. More recently, wellbores have been logged simultaneously with drilling of the wellbores, which is referred to as measurement-while-drilling (“MWDV”) 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.
NMR well logging instrument typically include a permanent magnet to induce a static magnetic field in the earth formations and a transmitting 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 static magnetic field. After an RF pulse, voltages are induced in a receiving antenna by precessional rotation of nuclear spin axes of hydrogen or other nuclei about the static magnetic field. The precessional rotation occurs in an excitation region where the static magnetic field strength corresponds to the frequency of RF magnetic field. A sequence of RF pulses can be designed to manipulate the nuclear magnetization, so that different aspects of the NMR properties of the formation can be obtained.
For NMR well logging the most common sequence is the CPMG sequence that comprises one excitation pulse and a plurality of refocusing pulses. It is the intent of NMR methods that the region of interest, as defined by the placement of the magnetically induced fields, lies totally within the rock formation. These field lines can, however, lie within the borehole, thus producing erroneous signals. Due to differing geometries of boreholes, different methods of NMR logging have been devised. For a small axially symmetric borehole in which the probing device is centrally located, it is possible to obtain information from an axially: symmetric region within the rock formation.
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 on 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.
A “side-looking” NMR tool is sensitive to NMR excitation on one side of the tool and less sensitive to NMR excitation on the other side. The more sensitive side of the tool is typically pressed against the sidewall of a borehole adjacent a formation, thereby providing minimum separation between the NMR tool's RF field generating assembly and the formation volume of NMR investigation. The less sensitive side of the tool is thus exposed to the borehole. This operational NMR technique is most effective when the borehole diameter is much greater!than the diameter of the NMR tool.
Typically, side-looking NMR tools set up static and RF magnetic field distributions in a particular relationship to achieve maximum NMR sensitivity on one side of the NMR tool. These conventional side looking NMR techniques are well known in the art, as taught in the following patents: U.S. Pat. No. 4,717,877, Taicher et al., entitled Nuclear Magnetic Resonance Sensing Apparatus and Techniques, U.S. Pat. No. 5,055,787, Kleinberg et al., entitled Borehole: Measurements Of NMR Characteristics Of Earth Formation; U.S. Pat. No. 5,488,342, Hanley, entitled Magnet Assembly For NMR; U.S. Pat. No. 5,646,528, Hanley, entitled Magnet Assembly; and U.S. Pat. No. 6,0213,164, Prammer et al. entitled Eccentric NMR Well Logging Apparatus And Method.
The Kleinberg '787 patent teaches a side-looking NMR tool which generates a static magnetic field which results in a sensitive volume on only the front side of the tool. The sensitive region in front of this tool generates a field having a substantially zero gradient, while the region behind this tool has a relatively large gradient field. Consequently, the volume of the sensitive NMR region in front of the tool is much larger and contributes more significantly to the composite NMR signal, than does the NMR region behind the tool. The '787 patent technique, however, is only practical when the sensitive volume in front of the tool is very close to the tool. This condition therefore limits the available depth of NMR investigation. The '787 tool design also requires a substantially zero gradient in the sensitive volume. Such a zero gradient is not always desirable, however, in NMR well logging, as a number of associated NMR techniques depend upon having a finite, known gradient within the NMR sensitive volume.
The Hanley '342 patent teaches a NMR tool technique which provides a homogeneous region localized in front of the tool. The '342 tool design overcomes the disadvantageous requirement of the sensitive volume being undesirably close to the NMR tool. However, it suffers because the sensitive volume is not elongated along the longitudinal axis of the NMR tool or bore hole axis, causing unacceptable errors due to motional effects.
Hanley '528 discloses another variation of the Jackson device in which a shield of electrically conductive material is positioned adjacent to and laterally offset from the set of electrical coils whereby the magnetic field generated by the RF antenna is asymmetrically offset from the axis of the magnets. The region of uniform static field remains a toroid, as in the Jackson device. The Hanley '528 device may be operated eccentrically within a large borehole with a reduction in the borehole signal. Both of the Hanley devices suffer from the drawback that the axial extent of the region of examination is small, so that they cannot be operated at high logging speeds.
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 perman
Beard David R.
Reiderman Arcady
Baker Hughes Incorporated
Lefkowitz Edward
Madan Mossman & Sriram P.C.
Shrivastav Brij B.
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