Improved cement sheath thickness measurement and its application in cement sheath variation monitoring

Abstract

Monitoring cement sheath thickness by the logging tool that utilizes Am–Be neutron generator as radiation source is crucial for the secure exploitation of oil and gas fields. Generally, a positive source distance probe is adopted to evaluate the cement sheath thickness by recording single energy window count, but the measurement result is greatly impacted by the working state of instrument. To get accurate changes about the thickness of cement sheath, this paper proposes a dual energy window combination method by photoelectric window and Compton backscatter window based on the scattering and attenuation theory of gamma ray in the medium. Subsequently, the relationship between the counting ratio of low window to high energy window and the thickness of cement sheath is established. By adjusting MCNP model settings, it can be concluded that the borehole fluid, the density of cement sheath, and the thickness of casing effects on the counting ratio of energy window A to energy window C for measuring cement sheath thickness can be disregarded. Besides, high-density formation and corrected borehole diameter are beneficial for evaluating the cement sheath thickness. According to these results, the simulation example shows that the thickness calculation error of the method at the overall cement sheath variations is less than 9%. The research result can provide reference for underground cement sheath monitoring and evaluation method.