A Novel Interpretation Model for Cased Hole Density Measurement

The application of thru-casing logging is essential to obtain formation parameters and control the operational risks under complex well conditions. However, thru-casing density measurement proves more challenging due to gamma’s weak penetrating power as compared to neutrons, etc. More importantly, the quality of cement, which is difficult to evaluate quantitatively downhole, casts a significant impact on formation density measurement. To obtain accurate thru-casing density in scenarios where cement information is incomplete or uncertain, a new interpretation model is introduced in this work. The model is implemented based on the recently introduced four-detector gamma density tool, and mainly includes three aspects: 1) A detector model and formation model whose solution targets at both formation density and cement density. 2) Cement invasion model consisting of mud, water and methane. It serves as a constraint to define mathematical limits to reduce the effects of cement uncertainty. 3) According to the prior information, the cement invasion model corresponding to each depth is automatically determined to constrain the formation model. The application of the proposed model in an oilfield is presented to attest its reliability. The thru-casing density obtained from the model matches well with open-hole density, and cement output is regarded reasonable judging from previous work on cement evaluation, which proves the applicability of the model in real oilfield logging.