Compensation of errors in logging-while-drilling density measurements

A method for a determining density of earth formation penetrated by a borehole while drilling said borehole, the method comprising:

• (a) rotating and conveying a tool in said borehole on a drill string, wherein said tool comprises a source of nuclear radiation and a long spaced detector and a short spaced detector axially spaced at different distances from said source;
• (b) generating a long spaced detector response and a short spaced detector response in a plurality of azimuthal borehole segments wherein each response is indicative of nuclear radiation from said source interacting with said earth formation;
• (c) determining, for each said segment, a segment formation density by combining said long spaced detector responses and said short spaced detector responses within that segment;
• (d) correcting each said segment formation density for tool standoff thereby forming a plurality of corrected segment formation density values;
• (e) forming a two-dimensional image from formation density from said plurality of corrected segment formation density values, wherein said image encompasses the full periphery of said borehole;
• and
• (f) azimuthally averaging corrected segment densities of substantially constant value determined from said image to form a first one-dimensional log of said formation density with respect to an azimuthal reference vector.

A system for determining a property of earth formation penetrated by a borehole comprising:

• (a) a borehole tool comprising at least one sensor wherein said tool is conveyed and rotated within said borehole;
• and
• (b) a computer for combining responses of said at least one sensor in a plurality of azimuthal borehole segments to obtain a measure of said property so that:
• (i) a segment formation property is determined, for each said segment, from said sensor response within that segment,
• (ii) a two-dimensional image is formed from said segment formation property determined in a plurality of segments, and
• (iii) segment formation properties of substantially constant value are identified from said image and combining to form a one-dimensional log of said formation property with respect to an azimuthal reference vector.

A system for a determining density of earth formation penetrated by a borehole while drilling said borehole, the method comprising:

• (a) a tool mounted in a collar in a drill string, wherein said tool comprises a source of nuclear radiation and a long spaced detector and a short spaced detector axially spaced at different distances from said source and is rotating and conveying in said borehole by said drill string;
• (b) said long spaced detector and short spaced detector forming a response in a plurality of azimuthal borehole segments wherein each response is indicative of nuclear radiation from said source interacting with said earth formation defined by said segment;
• (c) a computer connected to said long and short spaced detectors for determining, for each said segment, said formation density by combining said long spaced detector responses and said short spaced detector responses within that segment wherein each said determined formation density for each segment is corrected for tool standoff thereby forming a plurality of corrected density values for each segment;
• and
• (d) said computer forms a two-dimensional image of corrected formation density from said plurality of corrected density values and azimuthally averaging regions of constant corrected density determined from said image to form at least one one-dimensional log of said formation density with respect to an azimuthal reference vector.