Schlumberger Technology Corporation

William A Loomis

Jerome A Truax

Peter D Wraight

Kenneth Edward Stephenson

Bradley Albert Roscoe

Michael Lynn Evans

James A Grau

Keith A Moriarty

Benoit Couet

Wolfgang P Ziegler

Jacques M Holenka

William B Nelligan

Paul Albats, Jr

William R Sloan

Russell C Hertzog

David J Le Poire

S Zema Chowdhuri

Measurement-while-drilling apparatus includes a 14 MeV neutron accelerator, a near-spaced neutron detector which primarily senses source neutrons and whose output is proportional to source strength, one or more intermediately-spaced epithermal neutron detectors eccentered against the drill collar wall and primarily responsive to formation hydrogen concentration, and a third far-spaced radiation detector, either gamma ray or neutron, primarily responsive to formation density. The intermediately-spaced and far-spaced detector outputs, normalized by the near-spaced detector output, are combined to provide measurements of porosity, density and lithology and to detect gas. A thermal neutron detector and/or a gamma ray detector may also be provided at intermediate spacings to provide additional information of interest, such as standoff measurements and spectral analysis of formation composition. Tool outputs are related to the angular or azimuthal orientation of the measurement apparatus in the borehole.

Accelerator-based methods and apparatus for measurement-while-drilling

The first and second moments of an epithermal neutron slowing down time curve are determined and are combined in accordance with a predetermined empirical relationship to derive a raw measurement of the porosity of an earth formation surrounding a borehole. The raw porosity value is then combined with the second moment in accordance with further predetermined empirical relationships to derive a standoff measurement and a standoff-correction factor to be applied to the raw porosity measurement to produce a standoff-corrected porosity measurement. Where the raw porosity value changes appreciably from depth level to depth level, a differential spatial filter is applied to the raw porosity measurement and to the standoff-correction factor to provide a more precise standoff-corrected porosity measurement.

Methods and apparatus for epithermal neutron porosity logging

The energy-to-channel response of a bismuth germanate scintillator (BGO) gamma ray spectra measurement system is calibrated by use of the 10.2 MeV gamma ray peak originating from epithermal neutron capture within the BGO crystal as a calibration reference line. The 10.2 MeV peak is located by successively fitting the detected spectrum to the combination of a gaussian-shaped gamma ray peak and an exponentially-shaped background, beginning at the upper end of the spectrum and successively sliding the fitting window downwards until the peak is reached. Once the 10.2 MeV peak has been located, the spectral gain is adjusted to shift the peak to an assigned calibration channel location adjacent the upper end of the overall spectral window.

Methods and apparatus for calibration of BGO scintillator gamma ray

The porosities of subsurface earth formations surrounding a borehole are investigated using a logging tool run in the wellbore by repeatedly irradiating the formations with discrete bursts of high energy neutrons, measuring the populations of epithermal neutrons at near and far locations from the neutron source, and also measuring the rate of decay of the epithermal neutron population at a third location following each neutron burst and deriving therefrom a measurement of the epithermal neutron slowing down time. Formation porosity values derived from the near-and-far location population measurements are corrected for detector standoff effects by use, in accordance with a predetermined empirical relationship, of the epithermal slow down time measurement. Alternatively, formation porosities may be derived both from the near-and-far location measurements and from the epithermal slowing down time measurement, and the two differently-derived porosity values may be used independently to provide enhanced information of formation porosity or they may be compared to derive a standoff-corrected porosity value.

Method and apparatus for epithermal neutron porosity well logging

The porosity of an earth formation is derived in a plused neutron logging tool by use of a directionalized and borehole-shielded eptithermal neutron detector array and by processing detector count rate measurements of the epithermal neutron die-away curve in accordance with a multi-parameter fit to obtain as one parameter the epithermal neutron decay time of the formation and as a second parameter an indication of detector standoff (environmental) effects. The multi-parameter fitting procedure separates the formation and standoff effects on the measured count vs time curve, and yields a decay time measurement that is primarily representative of formation porosity and substantially free of environmental effects. To enhance count rate statistics, the neutron source and one or more of the epithermal neutron detector may be located at the same depth in the borehole. The neutron burst centroid is monitored an used to define a fiducial window for processing the die-way curve data.

Methods and apparatus for epithermal neutron logging

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Ridgefield, CT, United States of America

William A Loomis