Earth formation density measurement from natural gamma ray spectral logs

Claim 1

A method of natural gamma ray spectral logging of earth formations in the vicinity of a fluid filled well borehole to determine formation density, comprising the steps of: obtaining, with a borehole measurement system, an unknown gamma ray energy spectrum in a measurement borehole having unknown and possibly non-standard borehole conditions, said unknown gamma ray energy spectrum comprising gamma ray intensities measured in at least four energy bands, three of said energy bands corresponding to decay peaks of potassium, uranium and thorium unstable radioactive isotopes, said fourth energy band being selected to monitor changes in the shape of said gamma ray energy spectrum due to borehole and formation conditions; deriving from said unknown gamma ray energy spectrum by comparing said unknown spectrum with standard gamma ray spectra for unstable isotopes of potassium, uranium and thorium taken in standard boreholes, a measure of the total attenuation coefficient η for measured natural gamma rays occurring in earth formations in the vicinity of the measurement borehole; determining as a function of said measure of the attenuation coefficient η and predetermined measures of the wellbore fluid, casing program for the wellbore and properties of the cement between the casing and the wellbore, an indication of the density of the earth formations in the vicinity of the well bore; and repeating the previous steps at a plurality of depths in the borehole and recording said indications of density of the formations as a function of borehole depth, thereby to provide a well log of the formation density as a function of borehole depth.

Claim 2

A method of natural gamma ray spectral logging of earth formations in the vicinity of a fluid filled well borehole to determine formation density, comprising the steps of: obtaining, with a borehole measurement system, an unknown gamma ray energy spectrum in a measurement borehole having unknown and possibly non-standard borehole conditions, said unknown gamma ray energy spectrum comprising gamma ray intensities measured in at least four energy bands, three of said energy bands corresponding to decay peaks of potassium, uranium and thorium unstable radioactive isotopes, said fourth energy band being selected to monitor changes in the shape of said gamma ray energy spectrum due to borehole and formation conditions; deriving from said unknown gamma ray energy spectrum by comparing said unknown spectrum with standard gamma ray spectra for unstable isotopes of potassium, uranium and thorium taken in standard boreholes, a measure of the total attenuation coefficient η for measured natural gamma rays occurring in earth formations in the vicinity of the measurement borehole; determining as a function of said measure of the attenuation coefficient η and predetermined measures of the wellbore fluid, casing program for the wellbore and properties of the cement between the casing and the wellbore, an indication of the density of the earth formations in the vicinity of the well bore; and repeating the previous steps at a plurality of depths in the borehole and recording said indications of density of the formations as a function of borehole depth, thereby to provide a well log of the formation density as a function of borehole depth.