Application analysis on the different neutron gamma density (NGD) logging methods.

Neutron gamma density (NGD) logging is the most promising alternative to the traditional density logging (GGD), which is of significance for resolving the radiation and safety issues in oil industry. However, due to the different HI correction methods, multiple NGD methods based on the fast neutron, thermal neutron, and capture gamma detection coexist in the well-logging field, and show considerable differences in the tool specifications. To clarify these differences and guide the NGD development, three typical NGD methods using the fast neutron count ratio, thermal neutron count ratio, and capture gamma count ratio (abbreviated as NGD-FC, NGD-TC, and NGD-CC methods) are selected as representatives for comparative study. Using the Monte Carlo simulation, an integrated NGD tool model was established for studying the differences of three NGD methods in the logging responses, data processing methods, and environmental applications. Research shows that, although the three NGD methods have different measurement systems and data processing methods, the three methods can get rid of the HI effect and obtain accurate formation density. The changes of wellbore size and wellbore fluid have similar and significant impact on the three NGD methods and lead to large density errors, especially for the large-size wellbore or wellbore gas conditions. In the different lithology conditions, three methods have good performances, but the NGD-FC and NGD-CC methods have smaller density errors than NGD-TC method. Compared to the other two NGD methods, the NGD-FC method also has a perfect performance in the oil or gas-saturated formation, while NGD-TC and NGD-CC methods have extremely large errors in the gas-saturated formation. Besides, the NGD-FC method are hardly affected by the formation water salinity, the NGD-TC method is slightly affected, while the NGD-CC method is greatly affected. This study can provide a guidance for the tool design, data processing and environment correction of the NGD technology.