Research On Pulsed Neutron Carbon Oxygen Ratio Logging Method For Natural Gas Hydrate Reservoir

In view of the applicability of conventional logging methods for natural gas hydrate reservoirs,the accuracy and reliability of reservoir parameter calculations,this paper studies a pulsed neutron carbon oxygen logging method for natural gas hydrate reservoir which is suitable for while drilling environment.First,the structure and parameters of the pulsed neutron carbon oxygen ratio logging tool are optimized and designed according to the characteristics of the natural gas hydrate reservoir and the environment while drilling.Secondly,the tool is used to simulate the logging response law of natural gas hydrate reservoir carbon oxygen ratio.Then based on the theoretical relationship between the reservoir carbon oxygen concentration ratio and porosity and natural gas hydrate saturation,a logging interpretation model of natural gas hydrate saturation carbon oxygen ratio based on energy window count ratio is established and the model parameters are determined.Finally,a variety of stratigraphic models with different properties are established through design to simulate and verify the application effect of natural gas hydrate saturation carbon oxygen ratio logging interpretation model for sandstone and limestone reservoirs.The results of the study are as follows:Based on the characteristics of natural gas hydrate reservoirs and the environment while drilling,a carbon oxygen ratio logging tool capable of detecting natural gas hydrate reservoirs is designed.The instrument consists of a D-T pulsed neutron source,two gamma shields,and two gamma detectors.The lanthanum bromide(La Br₃)gamma detectors with the near and far source distances of 30cm and 52cm respectively,the near gamma shield of 6cm tungsten+2cm lead and the far gamma shield of 6cm lead are selected.The simulation results under different porosity,natural gas hydrate saturation,lithology,and borehole clearance show that the carbon oxygen energy window count ratio is a function of porosity and natural gas hydrate saturation,both of which are high formations.It is conducive to carbon oxygen ratio logging,and parameters such as lithology and borehole clearance are also important factors affecting the carbon oxygen energy window counting ratio.Based on the theoretical relationship between reservoir carbon oxygen concentration ratio and porosity and natural gas hydrate saturation and a large amount of experimental data,a logging interpretation model of natural gas hydrate saturation carbon oxygen ratio based on energy window counting ratio is established and determined Model parameters,and through the establishment of a variety of different types of stratigraphic models,simulations have verified the application effects of the interpretation model,and the results show that the interpretation model has small errors,high accuracy,and good application effects.This study enriches the logging evaluation methods for natural gas hydrate reservoirs,laying a theoretical foundation and providing technical support for the development of pulsed neutron carbon oxygen ratio logging tools and logging interpretation applications for natural gas hydrate reservoirs.The application of natural gas hydrate reservoir evaluation playes a good role in promoting.