Development of a natural gas hydrate testing platform based on a segmented coating time-domain reflectometry probe and experimental study
[Objective]Dielectric permittivity and electrical conductivity are key parameters for characterizing the electrical properties of hydrate-bearing sediments and predicting hydrate saturation.Thus,petrophysical models based on the two electrical parameters for interpreting the dielectric and resistivity logging data need to be established.However,because of the peculiarity of marine sediments containing gas hydrates,effective petrophysical models for accurately evaluating hydrate saturation are lacking.Considering the high cost of obtaining real hydrate-bearing sediment samples from field drilling,researchers usually conduct simulation experiments in the laboratory to prepare hydrate-bearing samples and perform electrical tests.Time-domain reflectometry(TDR)has the advantage that both permittivity and conductivity can be derived from TDR responses.However,traditional TDR probes cannot be used in high-conductivity marine sediments because of the high attenuation of electromagnetic signals along the probe rods.Thus,the development of new TDR probes applicable to hydrate-bearing marine sediments with high porewater salinity is of great significance.[Methods]First,a new type of probe,called a segmented coating TDR probe,was designed.Then,a testing platform for hydrate simulation experiments in marine sediments and TDR testing was developed.To derive the permittivity and conductivity from the TDR responses,the TDR probe parameters,such as the real length of rods and geometric factor,were determined based on standard calibration tests.To correct the impacts of the segmented coatings on the derived electrical parameters,two groups of tests were designed and conducted.One group of tests was performed on standard materials with permittivity from 1 to 80,including air,ethanol,ethylene glycol,and deionized water.The other group of tests was performed on sodium chloride aqueous solutions with mass fractions up to 4％.Correction models were established based on the regression between the permittivity and conductivity derived from the TDR responses and reference values.The tetrahydrofuran hydrate simulation experiments in simulated sediments and TDR tests were conducted.The reliability of the testing platform was verified by analyzing the dynamic changes in the apparent permittivity and conductivity in TDR tests during the formation of the hydrates and by discussing the influences of hydrate saturation.Hydrate saturation evaluation models based on the apparent permittivity,conductivity,and their combination were established.[Results]Results showed that ① the hardware part of the testing platform mainly includes a sample container,TDR testing unit,and temperature measurement unit,whereas the software part includes two sets of software that are matched with the TDR testing unit and temperature measurement unit;② the real length of rods and geometric factor of the segmented coating TDR probe are 150.60 mm and 3.36 m-1,respectively;and ③ the root-mean-square errors of hydrate saturation predicted by the permittivity-based,conductivity-based,and joint models are 4.95％,6.62％,and 4.79％,respectively.[Conclusions]The testing platform developed based on the new TDR probe,i.e.,segmented coating TDR probe,can be used to measure the apparent permittivity and conductivity of high-conductivity marine sediments containing hydrates simultaneously.The joint model of hydrate saturation based on the combination of two electrical parameters,i.e.,apparent permittivity and conductivity,performs better than the permittivity-based and conductivity-based models.This result can be attributed to the incorporation of dielectric polarization and electrical conduction mechanisms into the joint model.This study provides a platform for the electrical testing of marine sediment samples containing natural gas hydrates and a petrophysical model for the interpretation of electrical logging data from hydrate reservoirs.
natural gas hydratetesting platformtime-domain reflectometrysegmented coating probehydrate saturation model
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