Boring or penetrating the earth – With signaling – indicating – testing or measuring – Indicating – testing or measuring a condition of the formation
Reexamination Certificate
2002-02-15
2003-12-23
Bagnell, David (Department: 3672)
Boring or penetrating the earth
With signaling, indicating, testing or measuring
Indicating, testing or measuring a condition of the formation
C166S254200, C166S066000, C250S254000, C250S269300, C073S152030, C175S325100
Reexamination Certificate
active
06666285
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. The system is embodied as a logging-while-drilling gamma ray back scatter density system. The system is configured to minimize the distance between active elements of the downhole logging tool and the borehole environs, to minimize material between source and one or more detectors, to maximize shielding and collimation efficiency, and to increase operational reliability and ruggedness.
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. This class of systems is not practical for borehole geometry since the borehole environs sample to be measured surrounds the measuring instrument or borehole “tool”. 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. This class of systems is adaptable to borehole geometry.
Back scatter type systems have been used for decades to measure density of material, such as earth formation, penetrated by a borehole. Typically density is measured as a function of position along the borehole thereby yielding a “log” as a function of depth within the borehole. The measuring 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 that employ the back scatter 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 back scatter 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 back scatter 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 back scatter gamma-gamma density logging tool is conveyed along a well borehole penetrating typically earth formation. Means of conveyance can be a wireline and associated surface draw works. This method is used to obtain measurements subsequent to the drilling of the borehole. Means of conveyance can also be a drill string cooperating with a drilling rig. This method is used to obtain measurements while the borehole is being drilled. 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
)/
MW
) (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 that “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”, and adversely affects the radial sensitivity of the system. Intervening material in a displacement or “stand off” of the tool from the borehole wall will adversely affect radial sensitivity of the system. Intervening material in the tool itself between the active elements of the tool and the outer radial surface of the tool will again adversely affect radial tool sensitivity. Typical sources are isotropic in that radiation is emitted with essentially radial symmetry. Flux per unit area decreases as the inverse square of the distance to the source. Radiation per unit area scattered by the formation and back into detectors within the tool also decreases as distance, but not necessarily as the inverse square of the distance. In order to maximize the statistical precision of the measurement, it is desirable to dispose the source and the detector as near as practical the borehole environs, while still maintaining adequate shielding and collimation.
In view of the above discussion, 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. It is also of prime importance to position active elements of the logging system, namely the source and one or more detectors, as near as possible to the outer radial surface of the tool while still maintaining collimation and shielding required for proper tool operation.
Generally speaking, the
Jones Dale A.
Mickael Medhat W.
Bagnell David
Dougherty Jennifer R.
McCollum Patrick H.
Precision Drilling Technology Services Group Inc.
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