Methane Hydrate Surface Morphology in a Dynamic Water Dominant Flowloop

Abstract

Methane hydrates are the world’s largest unexploited hydrocarbon resource, with the capacity to support the world’s current demand for energy and in particular, LNG. Methane hydrates are solid crystalline compounds, composed of a molecular cage of water molecules that trap methane under low-temperature and high-pressure conditions. The world’s first successful offshore production of methane hydrate occurred in the Eastern Nankai Trough of Japan in March 2013. The production process employed depressurisation of methane hydrates, followed by downhole separation of methane and water; independent methane- and water-continuous production lines connected subsea infrastructure to the surface. In favourable temperature and pressure conditions, the potential for hydrate reformation during this production process introduces a flow assurance risk to the operation; severe hydrate reformation may result in blockage of subsea equipment, which can be costly and dangerous to rectify. Most of the flow assurance research on hydrates has focused on oil-dominant systems, while this investigation focuses on hydrate blockage formation in water-dominant systems. This work has been part of “Research Consortium for Methane Hydrate Resources in Japan” (MH21) in accordance with “Japan’s Methane Hydrate R&D Program”. The flowloop consisted of 16.7 m of stainless steel tubing (10 mm diameter) with two high-resolution differential pressure transducers, two Platinum Resistance Thermometers, thermistor, mass flow meter, circulation pump and in-line camera. The flowloop operated at a temperature of 3 °C to simulate the conditions of the seafloor, where methane hydrate is stable at pressures above 35 bar. An in-line camera provided a 7000 by 9300 μm image at six frames per second. Images of three-phase flow (water/gas/hydrate) were captured for a range of hydrate volume fractions in the liquid slurry (1.5 to 20 %), as well as hydrate growth rates, under both laminar and turbulent flow conditions. This study focuses on morphological observations during hydrate growth, where the results illustrate that the hydrate crystal morphology depends on the degree of shear and degree of sub-cooling in the system. Hydrates preferentially form at the methane/water interface, where the behaviour of the resultant hydrate particles size on a gas bubble depends on the shear conditions of the system. The greatest differences observed between static (quiescent) systems and highly turbulent flow was the range of different hydrate morphologies that can co-exist in a dynamic system.

AAPG Datapages/Search and Discovery Article #90348 © 2019 AAPG Asia Pacific Region Geosciences Technology Workshop, Gas Hydrates – From Potential Geohazard to Carbon-Efficient Fuel?, Auckland, New Zealand, April 15-17, 2019