Stand-off compensated formation measurements apparatus and method

Abstract

Apparatus and method for measuring density, porosity and other formation characteristics while drilling is disclosed. The apparatus, preferably housed in a drill collar and placed within a drill string, includes a source of neutrons and a source of gamma rays placed within a tubular body which is adapted to provide for the flow of drilling through it. Two sets of stabilizer blades are provided. One set, associated with the neutron source, includes secondary radiation detectors that are placed radially beyond the nominal outer radius of the body. Formation porosity measurement accuracy is substantially enhanced since the standoff of the detectors from the formation is substantially decreased. Another set, associated with the gamma ray source, includes one or more gamma ray detection assemblies in a single blade. Each of the gamma ray detector assemblies is also placed radially beyond the nominal outer radius of the tubular wall. Formation density or absorption coefficient accuracy is substantially enhanced since the standoff between the detection assembly and the borehole wall is decreased. In a particularly preferred embodiment, ultrasonic sensors incorporated either or both sets of blades provide a measurement of the borehole diameter and/or standoff of the detectors from the borehole. The bulk density measurement and neutron porosity measurement data can be corrected with information derived from the standoff or borehole diameter.