US 6,686,738 B2Granted

Method for determining decay characteristics of multi-component downhole decay data

AssigneeBaker Hughes Incorporated

InventorCarl M. Edwards

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Abstract

Measurements from many logging instruments such as Gamma ray counts from a pulsed neutron instrument, and amplitudes of spin-echo signals from a NMR instrument, typically have a spectrum that contains a plurality of exponentially decaying components. The slowest decaying component is obtained by fitting a single exponential over the tail end of the data and selecting a beginning for the fitting window that minimizes the product of the chi-square and the standard error of the fit. The single determined component may be subtracted from the data and the process repeated to give additional components. The determined components are indicative of thermal neutron capture cross-sections (in the case of a pulsed neutron instrument) and of the distribution of relaxation times and fluids within the pores of a rock in the case of a NMR instrument.

Claims5

Dependent claims

A method for determining a parameter of interest of a subterranean formation comprising:

(a) conveying a nuclear magnetic resonance (NMR) logging tool into a borehole in said subterranean formation obtaining measurements related to the parameter of interest, said measurements comprising at least two exponentially decaying components corresponding to at least two relaxation times of the formation;
(b) for a plurality of starting times for analysis of said measurements, determining parameters of a fit to said measurements over a plurality of time windows between said starting times and a selected ending time, said selected ending time being greater than each of said plurality of starting times;
(c) determining a goodness of fit, a standard error of fit, and a product of said goodness of fit and said standard error of fit for said plurality of starting times;
and
(d) selecting a value of said plurality of starting times for which said product attains a minimum value.

2 more elements

Dependencies are inferred from the claim text.