Zhang Quanying, Zhang Feng, Yuan Chao, Deng Rui, Liu Guobin
Key Laboratory of Exploration Technologies for Oil and Gas Resources (Yangtze University), Ministry of Education, Wuhan, 430100, China; College of Geophysics and Petroleum Resources, Yangtze University, Wuhan, 430100, China.
School of Geosciences, China University of Petroleum, Qingdao, 266580, China.
Appl Radiat Isot. 2021 Jun;172:109672. doi: 10.1016/j.apradiso.2021.109672. Epub 2021 Mar 11.
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.
中子伽马密度（NGD）测井是传统密度测井（GGD）最具前景的替代方法，这对于解决石油工业中的辐射和安全问题具有重要意义。然而，由于氢指数（HI）校正方法不同，基于快中子、热中子和俘获伽马探测的多种NGD方法在测井领域并存，且在仪器规格上存在显著差异。为了厘清这些差异并指导NGD技术发展，选取了三种典型的NGD方法，即使用快中子计数比、热中子计数比和俘获伽马计数比的方法（简称为NGD-FC、NGD-TC和NGD-CC方法）作为代表进行对比研究。利用蒙特卡罗模拟，建立了一个综合的NGD工具模型，用于研究三种NGD方法在测井响应、数据处理方法和环境应用方面的差异。研究表明，尽管三种NGD方法具有不同的测量系统和数据处理方法，但这三种方法都能消除HI效应并获得准确的地层密度。井眼尺寸和井眼流体的变化对三种NGD方法有相似且显著的影响，并导致较大的密度误差，特别是在大尺寸井眼或井眼含气条件下。在不同岩性条件下，三种方法都有良好的表现，但NGD-FC和NGD-CC方法的密度误差比NGD-TC方法小。与其他两种NGD方法相比，NGD-FC方法在油层或气饱和地层中也具有出色的表现，而NGD-TC和NGD-CC方法在气饱和地层中存在极大误差。此外，NGD-FC方法几乎不受地层水矿化度的影响，NGD-TC方法受其影响较小，而NGD-CC方法受其影响较大。该研究可为NGD技术的仪器设计、数据处理和环境校正提供指导。
