Formation density logging mwd apparatus

Abstract

A measurement-while-drilling (MWD) density tool is provided which incorporates a single gamma ray source and a pair of longitudinally displaced and mutually aligned detector assemblies (i.e., dual scintillation counters). A nuclear source is mounted in a pocket in the sub wall and partially surrounded by gamma ray shielding. The subwall section adjacent the source is expanded radially so as to define a lobe which displaces mud thus reducing borehole and mudcake effects. The two detector assemblies are mounted within a cavity or hatch formed in the subwall and enclosed by a detector hatch cover under ambient pressure. The detector assemblies are fixedly spaced from the source and are also partially surrounded by gamma ray shielding to provide good formation response. The hatch cover is secured to the sub by bolts and contains radiation transparent windows therethrough in alignment with the detector assemblies. A threaded-on fluid displacement sleeve is positioned on the sub and over the detector hatch cover. This sleeve has several important functions including the displacement of borehole fluids, reducing mudcaking which might have an adverse effect on the measurement and maintaining a relatively constant formatio to detector distance. The sleeve is full gage (with respect to the borehole diameter) and includes radiation transparent windows that are positioned over the hatch cover windows in a manner which optimizes the response of the tool. The sleeve includes blades which are configured such as to provide adequate flow area in the annulus between the density sub and formation wall.