Impact of photoelectric effect on X-ray density logging and its correction

Abstract

This paper studied the impact of the photoelectric effect of X-ray density logging in order to improve the accuracy of X-ray measurement density. Firstly, the laws governing the changes of the Compton scattering and photoelectric effect of X-rays in the formation were analyzed based on the principle of X-ray attenuation in the formation and using the XCOM program. Secondly, the impact of photoelectric effect on X-ray density logging was studied on the basis thereof. Thirdly, a method for correcting photoelectric effect by calculating the formation bulk density using the counts in both lithology window and density window was proposed. Finally, the proposed method was verified through MCNP simulation. The results indicate that photoelectric effect was an important factor affecting the accuracy of X-ray density measurement, and the lower the X-ray energy was, the greater the impact of photoelectric effect on X-ray density measurement would be and the lower the accuracy of density measurement would be. As the γ-rays released from a Cs-137 source were at a high energy level, the impact of photoelectric effect was basically negligible when the formation bulk density was calculated using the counts in density window; on the contrary, the X-rays released from the X-ray source was at a low energy level, the impact of photoelectric effect could not be neglected when the formation bulk density was measured using X-ray. The method for calculating the formation bulk density proposed in this paper can effectively eliminate the impact of photoelectric effect and reach up to a measurement accuracy level of about 0.01g/cm3, providing a theoretical foundation for accurate density measurement during X-ray density logging.