Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Electrical signal parameter measurement system
Reexamination Certificate
2001-11-29
2004-05-18
Hoff, Marc S. (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Electrical signal parameter measurement system
C702S006000, C702S007000, C702S008000, C702S009000, C702S011000, C702S012000, C702S013000, C702S017000
Reexamination Certificate
active
06738720
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is directed toward measurement of density of material, and more particularly directed toward a system for measuring bulk density of material penetrated by a borehole, wherein the system comprises a source of neutron radiation and preferably two radiation detection spectrometers. The system can alternately be embodied to measure other material properties and to measure density of materials not penetrated by a borehole.
2. Background of the Art
Systems utilizing a source of radiation and a radiation detector have been used in the prior art for many years to measure density of material. One class of prior art density measuring systems is commonly referred to as “transmission” systems. A source of nuclear radiation is positioned on one side of material whose density is to be measured, and a detector which responds to the radiation is positioned on the opposite side. After appropriate system calibration, the intensity of measured radiation can be related to the bulk density of material intervening between the source and the detector. A second class of prior art density measuring systems is commonly referred to as “back scatter” systems. Both a source of nuclear radiation and a detector, which responds to the radiation, are positioned on a common side of material whose density is to be measured. Radiation impinges upon and interacts with the material, and a portion of the impinging radiation is scattered by the material and back into the detector. After appropriate system calibration, the intensity of detected scattered radiation can be related to the bulk density of the material.
Backscatter type systems have been used for decades to measure density of material, such as earth formation, penetrated by a borehole. The measuring instrument or “tool” typically comprises a source of radiation and at least one radiation detector which is axially aligned with the source and typically mounted within a pressure tight container.
Systems which employ the backscatter configuration with a source of gamma radiation and one or more gamma ray detectors are commonly referred to as “gamma-gamma” systems. Sources of gamma radiation are typically isotopic such as cesium-137 (
137
Cs), which emits gamma radiation with energy of 0.66 million electron volts (MeV) with a half life of 30.17 years. Alternately, cobalt-60 (
60
Co) is used as a source of 1.11 and 1.33 MeV gamma radiation with a half life of 5.27 years. The one or more gamma ray detectors can comprise ionization type detectors, or alternately scintillation type detectors if greater detector efficiency and delineation of the energy of measured scattered gamma radiation is desired.
The basic operational principles of prior art gamma-gamma type backscatter density measurement systems are summarized in the following paragraph. For purposes of discussion, it will be assumed that the system is embodied to measure the bulk density of material penetrated by a borehole, which is commonly referred to as a density logging system. It should be understood, however, that other backscatter density sensitive systems are known in the prior art. These systems include tools which use other types of radiation sources such as neutron sources, and other types of radiation detectors such as detectors which respond to neutron radiation or a combination of gamma radiation and neutron radiation.
A backscatter gamma-gamma density logging tool is conveyed along a well borehole penetrating typically earth formation. Gamma radiation from the source impinges upon material surrounding the borehole. This gamma radiation collides with electrons within the earth formation material and loses energy by means of several types of reaction. The most pertinent reaction in density measurement is the Compton scatter reaction. After undergoing typically multiple Compton scatters, a portion of the emitted gamma radiation is scattered back into the tool and detected by the gamma radiation detector. The number of Compton scatter collisions is a function of the electron density of the scattering material. Stated another way, the tool responds to electron density of the scattering earth formation material. Bulk density rather than electron density is usually the parameter of interest. Bulk density and electron density are related as
&rgr;
e
=&rgr;
b
(2(&Sgr;
Z
i
)/
M W
) (1)
where
&rgr;
e
=the electron density index;
&rgr;
b
=the bulk density;
(&Sgr;Z
i
)=the sum of atomic numbers Z
i
of each element i in a molecule of the material; and
MW=the molecular weight of the molecule of the material.
For most materials within earth formations, the term (2(&Sgr;Z
i
)/MW) is approximately equal to one. Therefore, electron density index &rgr;
e
to which the tool responds can be related to bulk density &rgr;
b
, which is typically the parameter of interest, through the relationship
&rgr;
b
=A&rgr;
e
+B (2)
where A and B are measured tool calibration constants. Equation (2) is a relation that accounts for the near linear (and small) change in average Z/A that occurs as material water fraction changes with material porosity, and hence changes with bulk density.
The radial sensitivity of the density measuring system is affected by several factors such as the energy of gamma radiation emitted by the source, the axial spacing between the source and one or more gamma ray detectors, and properties of the borehole and the formation. Formation in the immediate vicinity of the borehole is usually perturbed by the drilling process, and more specifically by drilling fluid “invades” the formation in the near borehole region. Furthermore, particulates from the drilling fluid tend to buildup on the borehole wall. This buildup is commonly referred to as “mudcake”. Mudcake, invaded formation and other factors perturbing the near borehole region can adversely affect a formation bulk density measurement. It is of prime importance to maximize the radial depth of investigation of the tool in order to minimize the adverse effects of near borehole conditions. Generally speaking, an increase in axial spacing between the source and the one or more detectors increases radial depth of investigation. Increasing source to detector spacing, however, requires an increase in source intensity in order to maintain acceptable statistical precision of the measurement. Prior art systems also use multiple axial spaced detectors, and combine the responses of the detectors to “cancel” effects of the near borehole region. This method is marginally successful since nuclear systems are inherently shallow depth of investigation. Depth of investigation can be increased significantly by increasing the energy of the gamma-ray source. This permits deeper radial transport of gamma radiation into the formation. Unfortunately, there are no isotopic sources emitting gamma radiation above 1.33 MeV which have a half-life sufficiently long for typical commercial use and which are reasonably inexpensive to produce. Accelerator sources have been used in the prior art to generate gamma radiation of energy greater than 10 MeV. These sources are, however, physically large, costly to fabricate, costly to maintain, and often not suited for harsh environments such as a well borehole.
SUMMARY OF THE INVENTION
This invention is directed toward a system for measuring density and other properties of material penetrated by a borehole. Alternately, the system can be embodied for material analysis in a variety of non-borehole environments. Configuration of the system is based upon the backscatter concept discussed in the previous section of this disclosure.
The sensor instrument or “tool” comprises preferably an axially spaced source of radiation and preferably two axially spaced radiation detectors, which discriminate energy of radiation impinging upon the detectors.
The source is preferably a neutron source which emits, or induces within material being measured, gamma radiation with energy greater than energy obtainable with isot
Odom Richard C.
Wilson Robert D.
Computalog U.S.A.
Hoff Marc S.
McCollum Patrick
Tsai Carol S
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