Formation density logging measurement-while-drilling apparatus

摘要

A measurement-while-drilling (MWD) density tool (10) 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 (88) 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 (132) 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 (42) 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 (42) is secured to the sub (27) by bolts (50) and contains radiation transparent windows (60) therethrough in alignment with the detector assemblies. A threaded-on fluid displacement sleeve (134) is positioned on the sub (27) and over the detector hatch cover (42). This sleeve (134) 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 formation to detector distance. The sleeve (134) is full gage (with respect to the borehole diameter) and includes radiation transparent windows that are positioned over the hatch cover windows (60) in a manner which optimizes the response of the tool (10). The sleeve (134) includes blades which are configured such as to provide adequate flow area in the annulus between the density sub and formation wall; and yet reduce to a minimum the amount of drilling mud between the source and detector assemblies on the one hand and the formation on the other.