| Scattering gamma logging in cased wells can provide well evaluation parameters for wells with poor well stability after casing,including providing density and porosity measurements for cased wells with no well logging data or wells with data quality problems.At the same time,cementing quality and casing corrosion can be evaluated by recording scattering gamma rays in cased wells while the traditional acoustic cement cementation evaluation technology encounters difficulties when micro-rings exist outside the casing.Thus it is necessary to carry out scattering gamma logging research in cased wells.In this paper,the Monte Carlo numerical simulation method is used to optimize the instrument structure parameters of scattering gamma ray cement density logging tool in cased wells,including the far and short spacing values,the size of the instrument outside diameter,instrument shielding materials,etc.Based on the logging response characteristics of the cement density logging tool,a log response relationship between the count rate of far and short spacing and the main influencing factors was established.Based on the linear function method,a forward model of count rate and the main influencing factors was established.Nonlinear weighted least squares method is then used to invert the established forward model.The inversion results show that relatively accurate casing thickness values can be obtained under different wellbore,cement sheath and stratum conditions.When the cement thickness exceeds 10mm,a more accurate cement density value can be obtained.When formation density is increased,the accuracy of the cement density is also improved.After formation density reaches 1.9g/cm~3,the cement density with higher accuracy can be obtained. |
