--> Laboratory Research and 3-D DEM Investigation on Mechanical Properties of Gas Hydrate Sand Under Different Temperature, Confining Pressure, and Hydrate Saturation

AAPG ACE 2018

Datapages, Inc.Print this page

Laboratory Research and 3-D DEM Investigation on Mechanical Properties of Gas Hydrate Sand Under Different Temperature, Confining Pressure, and Hydrate Saturation

Abstract

Owing to its bright prospect in the exploration, gas hydrates have received increasing attention as one of potential future energy source. This study carries out triaxial compression tests on gas hydrate sand, to learn the effects of various factors on mechanical properties of gas hydrate sand. Discrete element method is adopted to simulate triaxial compression tests. Simulation results show good agreement with results from laboratory tests, which lays a foundation for future study on borehole stability and sand production during gas hydrate exploration.

Experiment methods: (1) man made unconsolidated sand mixed with powder ice were added into a triaxial cylinder of 50 mm in diameter and 100 mm in height, then the cylinder was kept for an hour at the pressure of 30 MPa and the temperature of -20°C; (2) at 0.5 MPa confining pressure and temperature of 2°C, methane gas was percolated into specimen with the rate of 0.3 MPa to 0.5 MPa/minute; (3) when pore pressure increased to 10 MPa and confining pressure to 12 MPa, gas hydrate sand was obtained. The ratio of released methane gas volume (at the pressure of 10 MPa and the temperature of 2°C) to specimen porosity is hydrate saturation. The amount of percolated methane gas would affect hydrate saturation of specimen. In order to achieve an efficient simulation, man made sand and pore-filling hydrate was modelled by particles with diameter of 0.1-0.4 mm and 0.15 mm respectively. Microscale contact parameters of the model was calibrated by the experiment results above.

Results: (1) peak strength increases with increasing confining pressure and hydrate saturation, decreases with rising temperature; (2) low confining pressure weakens strain softening while increasing hydrate saturation makes strain softening more obvious; (3) hydrate saturation has little impact on internal friction angle; (4) cohesion and secant modulus increase with increasing hydrate saturation, Poisson’s ratio at 50% failure decreases with increasing hydrate saturation; (5) a DEM model was set up to predict the mechanical properties of hydrate sand.

Contributions: (1) a comprehensive understanding was gained on the effects of confining pressure, hydrate saturation and temperature on the mechanical properties of gas hydrate sand; (2) microscale contact parameters of this model can be used to model other types of hydrate sand; (3) this model lays a foundation for future study on bolehole stability and sand production during gas hydrate exploration.