Study on density logging method based on γ-ray broad beam attenuation model

[Background] With more and more attention paid to low-density gas hydrate formation, higher requirements are raised for density measurement range and accuracy of density logging. [Purpose] This study aims to improve the accuracy of density logging in low-density formation by using board beam attenuation model. [Methods] Firstly, the attenuation law of γ-ray narrow beam used for density logging was analyzed. Secondly, a response model based on broad-beam γ-ray density logging was derived by combining with the broad-beam γ-ray attenuation law, hence the calculation formula of formation density was obtained by scaling. Finally, the Monte Carlo N Particle Transport Code (MCNP) was used to simulate the density logging response in low-density formation, meanwhile, the application scope and accuracy of two attenuation laws in density logging were compared and analyzed. [Results] The response relationship of gamma rays in the formation can be better described by the broad beam attenuation model in density logging, while the narrow beam response relationship is only a special case of broad beam. By using the formation density formula based on the broad beam γ attenuation model, the density measurement range is expanded to 1.00 ∼ 3.00 g · cm^-3. Furthermore, for gas hydrate formation with density ranging from 1.00 g · cm^-3 to 1.60 g · cm^-3, its average measurement error of the far detector is reduced from 0.092 g · cm^-3 to 0.009 g · cm^-3, and that of the near detector is reduced from 0.251 g · cm^-3 to 0.029 g · cm^-3. [Conclusions] The board beam γ-ray attenuation model can be used to greatly improve the accuracy and measuring range of density logging, and this provides theoretical and methodological basis for accurate measurement of gamma ray density logging in low-density formation in the future. (authors)