套管结垢对脉冲中子-中子测井的影响及校正OA

To address the issue that casing scale interferes with the calculation of formation macroscopic capture cross-section(Sigma)and reduces the accuracy of residual oil saturation evaluation in pulsed neutron-neutron(PNN)logging through cased holes,this paper establishes a three-dimensional numerical model of PNN instrument,wellbore and formation using the Monte Carlo simulation software.The simulation reproduces the slowing-down and capture processes of neutrons in the scaled borehole and formation.The effects of scale type,thickness,and composition on thermal neutron time spectra,neutron distribution,and measured Sigma are analyzed.A self-compensation correction method for the formation Sigma is developed and validated with field log data.The results show that:①Casing scale replaces borehole fluid,reducing the fast neutron slowing-down capability.Corrosion scale containing elements with high thermal neutron capture cross-section(e.g.,Cl)shortens the thermal neutron lifetime,leading to overestimation of the formation Sigma,whereas sedimentary scale weakens the thermal neutron capture capability,resulting in underestimation of Sigma.②The formation Sigma and the near-to-far detector thermal neutron count ratio increase linearly with the thickness of corrosion scale,while they decrease linearly with the thickness of sedimentary scale.The magnitude of this effect varies with formation porosity.For each 1 cm increase in corrosion scale thickness,formation Sigma increases by 3.37 c.u.in a formation with 8%porosity and by 3.69 c.u.in a formation with 15%porosity.③A higher NaCl content in the corrosion scale leads to a more pronounced influence on the formation Sigma.④The correction amount of the formation Sigma exhibits a linear relationship with the difference between the cased-hole neutron porosity and the compensated neutron porosity.The proposed self-compensation correction method achieves an average absolute error of only 0.29 c.u.after correction.It is concluded that casing scale significantly disturbs the logging response by altering neutron slowing-down and capture processes.The self-compensation correction method based on the thermal neutron count ratio and compensated neutron porosity effectively eliminates the scale effect,substantially improving the accuracy of formation Sigma calculation and the reliability of residual oil saturation evaluation,thus providing key technical support for dynamic monitoring of residual oil in cased holes.