The use of controlled X-ray sources instead of ¹³⁷Cs radioactive sources in density logging has become a new trend. The high voltage on the target substrate significantly affects the intensity of the X-ray source, and the density measurement uncertainty can be maintained at 0.01 g?cm^-3 when the high voltage is 350 kV.

This study aims to examine the depth-of-investigation characteristics and influence of a 350-kV high-voltage X-ray density logging instrument.

The depth of investigation of various source distance detectors in 20% water-bearing limestone formation was studied using the Monte Carlo method. By comparing the investigation characteristics of 350-kV high-voltage X-ray source and ¹³⁷Cs source density logging, the reasons for the differences in the depth of investigation among them were analyzed. Moreover, the contribution of mudcake and formation to the detector and the density deviations of various detectors were analyzed via simulation. Finally, the influence of mudcake on the density logging response of the well wall was explored.

The results indicate that the depth of investigation of X-ray density logging instrument increases with the augment of source distance. Compared to the ¹³⁷Cs source density logging, the scattered particles of the X-ray density logging are mainly concentrated at 1~3 cm from the bore wall, resulting in the depth-of-investigation differences between the two techniques. Furthermore, the contributions of mudcake and formation to different source distance detectors are different, and the detector density deviation decreases with the increase in source distance.

This study affords a theoretical basis for the depth-of-investigation characteristics and influence of 350-kV high-voltage X-ray density logging.