Patent ID: 7279677
Filing Date: 2007-10-09
Classification: E21B

Abstract:
1. A method for determining the diameter of a wellbore while the wellbore is being drilled by a drill string immersed in weighted mud, the weighted mud having a weight fraction of a heavy component, the drill string including a well logging instrument, the instrument rotating to define a transient interface with a facing portion of the wellbore wall, the instrument including a gamma ray source, an energy-sensitive short-spaced gamma ray detector, and an energy-sensitive long-spaced gamma ray detector, the method comprising: a) measuring gamma ray scattering in a first energy range of first gamma rays that cross a first interface, and in a second energy range of second gamma rays that later cross an opposite interface, in both a short-spaced detector and a long-spaced detector, at each of a series of axial locations along a wellbore, to produce a gamma ray count for each combination of first energy range and second energy range, first interface and opposite interface, and short-spaced gamma ray detector and long-spaced gamma ray detector; b) associating an assumed weight fraction with each of the series of axial locations; c) calculating, for each of the series of axial locations, a pair of first and second standoffs from the assumed weight fraction, and counts of gamma rays in first and second energy ranges, respectively; d) selecting the pair having least-squared difference between its standoffs; and e) determining wellbore diameter by setting wellbore diameter equal to a function of the calculated values of the selected pair.