Behavior of Methane Hydrate-in-Water Slurries from Shut-in to Flow Restart

Natural gas hydrates have attracted interest as a potential future energy resource to meet the expected growth in the global energy demand. One of the key challenges to be tackled for commercial production is gas hydrate re-formation in production lines. Such hydrate blockages have been a major concern of flow assurance in the oil and gas industries, and typically occur during start-up, shut-in, and restart operations. This study sheds light on the behavior of methane hydrate slurries in pure water systems under shut-in conditions and their transition to solid blockages during system restart: a key risk factor in gas hydrate production. Flowloop experiments were conducted to measure the critical stress required to restart flow in hydrate slurries, which were accompanied by visual observations using an in-line video camera. A transition from smooth restart to a critical stress requirement was observed at 4-5 vol % of hydrate; visually, this corresponded to complete occupancy of the flow cross section with porous aggregated hydrate particles in a loosely packed network. The critical stress increased with higher hydrate volume fractions per the behavior of yield stress for a general suspension: it was not affected by the shut-in period, suggesting that annealing was not a factor at the volume fractions measured. Further, hydrate blockage occurred in several restart operations at approximately 18 vol % of hydrate even though the video camera had captured partial yielding before the blockage occurred. These results show a progression in slurry behavior as a function of hydrate volume fraction, offering insights into varied mechanisms for blockage formation, and operational strategies to minimize blockage risk.