A research on the effects of formation elements on the spatial distribution of D-D induced γ-ray source

The position and intensity of induced γ-rays are highly affected by formation factors during D-D controllable source density logging, resulting in a relatively low-level accuracy of density measurement. To improve the accuracy of density measurement, the laws governing the changes in the spatial distribution of induced γ-ray source from the viewpoint of the effects of formation elements are studied in this paper. On the basis of the fundamental theory of D-D controllable source density measurement, this paper first analyzed the generation process of D-D induced capture γ-ray source, then analyzed the effects of main formation elements involved in capture reaction on the spatial distribution of induced capture γ-rays, and, finally, analyzed and discussed the effects of various formation elements from the perspective of macroscopic formation factors. The results indicate that the spatial distribution of capture γ-rays is mainly affected by fast neutron deceleration, thermal neutron diffusion, and absorption, while the occurrence of these three processes is mainly related to the formation elements undergoing capture reaction. Among such formation elements, hydrogen has the greatest effect on the spatial distribution of capture γ-rays. Particularly, when hydrogen content increases from 0 to 5%, the induced γ-ray source changes from 100 cm to 25 cm in terms of its spatial position. The main reason is that when hydrogen content in the formation increases, the action of fast neutron deceleration becomes stronger, more γ-rays approach toward the neutron source, and the intensity of γ-rays also increases gradually at the same time; comparatively, other elements (especially chlorine) in the formation have smaller effects on the spatial positions of induced γ-rays and greater effects on the intensity of induced γ-rays. The main reason is that for different types and contents of formation elements, their thermal neutron capture cross-sections vary. For elements with large capture cross-sections, as their contents increase, more thermal neutrons will be captured and the intensity of γ-rays will also increase. As the sensitivity of induced γ-ray source to different formation elements varies, the spatial distributions of induced γ-ray source in formations with varying porosity, lithology, and salinity are also different. This research provides the theoretical basis for identifying the laws governing the spatial distribution of D-D induced γ-ray source and correcting the effects of formation elements on density measurement.