利用背散射伽马射线确定套管厚度的模拟研究

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

根据伽马射线散射衰减原理,提出利用康普顿散射窗与光电窗的双能窗组合估算套管厚度的方法。利用数值模拟方法设置不同套管厚度计算模型,分析伽马能谱中不同能量的计数来源,对比得到最优能窗组合,并建立双能窗计数比与套管厚度响应关系;通过设置不同地层密度、水泥环密度和厚度参数模型研究水泥环和地层参数对确定套管厚度的影响程度。结果表明:利用能窗Ⅲ与能窗Ⅰ计数比评价套管厚度的相对灵敏度最高,其受地层密度、水泥环密度和水泥环厚度影响较小。模拟实例表明该方法在确定1 mm套管厚度时误差小于10%,研究结果可为井下套损评价方法提供参考。

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.