MH growth and morphology - SXRCT&Optical Microcopy

For synchrotron XRCT, wet sand was compacted in the aluminium tube to reach an average porosity of 0.4. Residual air was evacuated before admitting methane gas at 7 MPa and the system was cooled down to the run temperatures in Table 1, which are 7-9 °C lower than the three-phase equilibrium temperature at 7 MPa. Four runs were performed, three with tap water and one test with brine (runs 1-4). The initial moisture content (mass ratio of water to sand determined before packing the sample tube) is either 10 % or 15 %, corresponding to average water saturation (fraction of the porosity occupied by water or brine) of 40 and 60 %, respectively. In the optical observations, dry sand grains were inserted in the capillaries, deionized water was added via capillary rise by using a micro-pipette connected to a vacuum line, so the water content could be adjusted (but not finely controlled) to exhibit capillary bridges between sand grains. Production of methane hydrate in a reasonable time in such small samples requires either strong supercooling to nucleate the hydrate directly, or formation of the hydrate from melting ice. Two runs were performed. In run 5, a methane pressure of 8 MPa was established prior to quenching to -16 °C to nucleate the hydrate. In run 6, ice was formed in the evacuated cell, by quenching to -25 °C. Then the temperature was raised to -5 °C and methane was admitted at 8 MPa. Finally, the temperature was raised to 2 °C to form methane hydrate from ice meltwater. The optical microscopy runs lasted a few hours.