Study On Detection And Analysis Technology Of Natural Gas Hydrate-Bearing Core Samples

Natural gas hydrate,as a clean energy with huge potential,has attracted global attention.The development of exploration and investigation of submarine natural gas hydrate has come to a crucial stage in China.Through the analysis of hydrate-bearing sediments,we can establish the relationship between the physical properties of hydrate-bearing sediments and hydrate saturation.Moreover,the analysis of hydrate-bearing sediments also can provide important geophysical information for the development of efficient and safe production technology.Consequently,the acoustic characteristic of hydrate-bearing sediments and the occurrence law of hydrate in the pore space were studied.Additionally,the analysis of hydrate-bearing sediment core samples recovered from submarine reservoirs was conducted.In this paper,experiments were conducted to investigate the acoustic characteristic of hydrate-bearing sandy sediments and hydrate-bearing marine sediments.By introducing the effective medium theory model and the weighted equation,the occurrence law of hydrate in the pore space were studied.For hydrate-bearing sandy sediments,the impact of hydrate formation method on hydrate morphology was studied.The results show that hydrate formation with excess gas typically causes the preferential occurrence of hydrates at grain contacts,which show a grain-cementing morphology.Hydrates formed under an excess water condition usually show pore-filling morphology at low hydrate saturations(less than 10%).The transition of hydrate morphology from pore-filling to load-bearing is found to occur with increased hydrate saturation,and the grain-displacing hydrate formation is possible to occur when hydrate saturation exceeds ~40%.For hydrate-bearing marine sediments,the relationship between Pwave velocity and hydrate saturation under different water contents was discussed.It is found that the coexistence of multiple microcosmic hydrate morphology probably occurs during the hydrate growth.The hydrate cementation at marine sediments is low under the low hydrate saturation,but still improve the P-wave velocity.By changing the parameters of W and n,the predicted curve is matched well with the curve of measured P-wave velocities.And a simple experiential equation is proposed by simplifying the other equation parameters.Meanwhile,the handling and testing of pressurized core samples and non-pressurized core samples on board were conducted,including gas analysis,pore fluid analysis and host sediments testing.A pressure core ultrasonic test system for on-board analysis of gas hydrate-bearing sediments was developed.And a processing flow of hydrate-bearing sediment core samples was also proposed.The test results proved that the hydrate exists in the core samples and the type of hydrate is methane hydrate.The estimated hydrate saturations varied from 28.13% to 44.52%.In addition,the physical properties of the core samples in the different depths recovered from LW3-H4-1B site and LW3-H4-1C site were analyzed,including specific gravity,porosity,permeability,SEM image,X-ray CT image,particle size distribution and heat-transfer characteristic.