Neutron well logging technique for gas detection
AssigneeSchlumberger Technology Corporation
InventorsRichard M. Bateman, Christian M. Clavier
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Abstract
In accordance with an illustrative embodiment of the present invention, a technique is disclosed for determining a characteristic of an earth formation by irradiating the formation with a source of neutrons and measuring the resulting neutron population at two different spacings from the source. Each neutron population measurement is converted into a signal which is related to the formation porosity corrected for environmental effects. The two converted signals are then compared with the porosity derived from the ratio of the two neutron population measurements in order to obtain an indication of the presence of gas in the formation.
Claims19
A method for determining characteristics of an earth formation, comprising the steps of:
- producing a first measurement of the neutron population in said formation at a first spacing from a neutron source;
- converting said first measurement into a first signal which is related to the porosity of the formation and corrected for environmental effects;
- producing a second measurement of the neutron population in said formation at a second spacing from said neutron source;
- converting said second measurement into a second signal which is related to the porosity of the formation and corrected for environmental effects;
- and
- determining from said converted signals characteristics of the formation.
A method of calibration of a dual-detector neutron tool for determining the porosity of earth formations, comparing the steps of:
- deriving a value functionally representative of porosity by combining the responses of the two detectors of said tool;
- and
- combining said value with the response of only one of said detectors to obtain the relationship between porosity and the response of said one detector.
An apparatus for determining characteristics of an earth formation from first and second input signals representing the neutron population in said formation at two different spacings from a common neutron source, comprising:
- means for converting said first and second input signals into first and second signals respectively which are related to the formation porosity;
- means for combining said first and second input signals to produce a third signal which represents the formation porosity;
- and
- means for combining said first, second and third porosity signals to obtain characteristics of the formation.
A method for determining a characteristic of an earth formation, comprising the steps of:
- producing first and second measurements of the neutron population in said formation at two different spacings from a neutron source;
- converting said first and second measurements into first and second signals respectively which are related to the porosity of the formation;
- deriving from said first and second measurements a third signal which represents the porosity of the formation;
- and
- combining said first, second and third porosity signals to obtain an indication of the depth of invasion of the formation.
A method of calibrating a dual-detector neutron tool for determining the porosity of earth formations from the response of a selected one of the two detectors of the tool, comprising the steps of:
- combining the responses of the two detectors of the tool to derive a value functionally representative of porosity;
- and
- calibrating the tool by determining a detector sensitivity and source strength parameter for the selected one of the two detectors from said porosity value in accordance with a predetermined relationship.
A method of calibration of a dual-detector neutron tool for determining the porosity of earth formations from the response of only one of the two detectors of the tool through a relationship that includes a parameter related to the detector sensitivity and the strength of the neutron source, comprising the steps of:
- deriving indications of the porosity of earth formations from the responses of the two detectors of the tool;
- obtaining values of said strength source parameter by solving said relationship from said porosity indications;
- and
- determining the mean value of said parameter.
Dependencies are inferred from the claim text.