Simulation study for borehole casing thickness determining method based on gamma ray backscatter

Key words:  casing thickness  gamma ray  dual energy windows combination  MCNP simulation  backscatter

Abstract:

The casing corrosion in oil and gas development wells is likely to cause fluid runout and leakage which will reduce the recovery rate, and an accurate downhole casing thickness detection method is beneficial to safe production. In this paper, a method to estimate casing thickness by combining Compton scattering window and photoelectric window based on the principle of gamma ray scattering and attenuation is proposed. The simulation method is used to set up different casing thickness calculation models, and the counting sources of different energies in the gamma ray spectrum are analyzed and studied. The response relationship between the counting ratio of dual energy windows and casing thickness is established based on the optimal energy window combination. The influence of the cement ring and formation parameters on determining casing thickness is studied by setting up different models of formation density, cement ring density and thickness. The results show that the relative sensitivity of the count ratio using energy window Ⅲ and energy window I to evaluate casing thickness is the highest, which is less influenced by formation density, cement ring density and thickness. The simulation results show that the error of this method is less than 10% in determining casing thickness of 1 mm. Our study provides a reference for evaluation of downhole casing corrosion.