US 4,066,892 AGranted

Coal logging system

AssigneeMobil Oil Corporation

InventorWyatt W. Givens

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Abstract

A borehole logging tool employs two neutron sources of differing energy outputs, two gamma-ray detectors, and a neutron detector in the radioactivity logging of a coal-bearing formation. A first of the gamma-ray detectors measures the gamma rays produced by the formation in response to the higher energy neutron source. A second of the gamma-ray detectors measures the gamma rays produced by the formation in response to the lower energy neutron source. The neutron detector measures thermal or epithermal neutrons produced by the formation in response to the neutron sources. These measurements are selectively transferred to a recording system employing a plurality of energy level discriminators, count rate meters, and comparators. The recording system, in response to these measurements, provides output signals representative of the carbon, sulfur, ash, and neutron moderating power of the coal in the formation being logged. The carbon and neutron moderating power provide a means of measuring the energy content of coal.

Claims15

Dependent claims

A system for logging a coal-bearing formation traversed by a borehole, comprising:

a. a borehole tool,
b. means for locating said borehole tool adjacent a coal-bearing formation,
c. a first neutron source located within said borehole tool for irradiating the coal-bearing formation with neutrons of energy sufficient to produce 4.43-Mev gamma rays from inelastic neutron scattering in the carbon component of the coal-bearing formation,
d. a first detector located within said borehole tool for measuring the gamma rays returning to the borehole in response to irradiation of the coal-bearing formation by neutrons from said first neutron source,
e. a second neutron source located within said borehole tool at a spaced-apart position from said first neutron source for irradiating the coal-bearing formation with neutrons of energy insufficient to produce a significant number of 4.43-Mev gamma rays from inelastic neutron scattering in the carbon component of the coal-bearing formation,
f. a second detector located within said borehole tool for measuring the gamma rays returning to the borehole in response to irradiation of the coal-bearing formation by neutrons from said second neutron source,
g. means for producing a first count of those gamma rays measured by said first detector that are of an energy level above the energy level of the gamma rays produced by the hydrogen component of the coal-bearing zone,
h. means for producing a second count of those gamma rays measured by said second detector that are of an energy level above the energy level of the gamma rays produced by the hydrogen component of the coal-bearing zone, and
i. means for determining the differential between said first and second counts as an indication of the carbon content of the coal-bearing formation.

6 more elements

A method of logging a coal-bearing formation traversed by a borehole, comprising the steps of:

a. irradiating the coal-bearing formation with a first group of neutrons of sufficient energy to produce 4.43-Mev gamma rays from inelastic neutron scattering in the carbon component of the coal and producing a first signal representative of the total number of gamma rays returning to the borehole from the coal-bearing formation,
b. irradiating the coal-bearing formation with a second group of neutrons of insufficient energy to produce 4.43-Mev gamma rays from inelastic neutron scattering in the carbon component of the coal and producing a second signal representative of the total number of gamma rays returning to the borehole from the coal-bearing formation,
c. identifying those portions of the gamma rays represented by said first and second signals that lie above the energy level of gamma rays produced by the hydrogen component of the coal, and
d. determining the differential between said portions of said first and second signals.

1 more element

Dependencies are inferred from the claim text.