Laboratory Measurements on Fresh Terrestrial Gas-hydrate-bearing Sediment Cores

Physical properties (saturation, grain size distribution, porosity, permeability, resistivity, ultrasonic velocity and attenuation) of terrestrial gas-hydrate-bearing core samples from the Mallik 5L-38 gas-hydrate research well have been investigated in the field laboratory under simulated in situ conditions with a specially designed core analysis system (FLECAS). Twenty samples were prepared immediately after core retrieval and mounted into the pressure vessel at deep frozen conditions. Electrical resistivity, ultrasonic P-wave and S-wave velocities and amplitudes were recorded during the whole procedure, while the samples were brought to in situ pressure and temperature. A strong decrease of all parameters, especially of the P-wave and Swave amplitudes, could be observed at the melting point of ice. Smaller changes occurred later, apart from the loss of mechanical strength and a distinct recovery of the ultrasonic amplitudes, when the gas hydrate decomposition was initialized by the release of the pore pressure or by heating above the stability threshold. The gas-hydrate decomposition started instantaneously when the pore pressure was released, took about 20 min, and was accompanied by a temperature drop of about 3°C at the sample surface. Only small variations were found in the bulk parameters of the unconsolidated sand samples remaining after gas-hydrate decomposition. This explains the uniform behavior of all samples from the gas-hydrate zone of the Mallik well. This data set provides an experimental basis for formation evaluation and a reference for realistic studies with gas hydrates that are synthetically grown in sediments in the laboratory.