(449c) Pore-Scale Methane Hydrate Depressurization in Confined Pores for Efficient Methane Recovery

Natural gas is a cleanest-burning and energy resource with high energy density. Natural gas hydrate (NGH) is a crystalline compound formed by water molecules and natural gas molecules under low temperature and high pressure. It is estimated that 10¹⁴-10¹⁸ m³ natural gas (main component methane) is stored in NGH reservoirs, most of which are located 0-110m below seafloor. Massive oceanic methane reserves in NGH have attracted worldwide attention for utilizing this prospective future resource. It is therefore of importance to develop efficient, safe and economical exploitation method to recover methane from hydrate reservoir.

Depressurization, as the most feasible hydrate exploitation method, has been employed in many field tests. However, several technical issues need to be harnessed to facilitate its efficiency for commercial exploitation in the near future. One of the key issue is hydrate reformation. Hydrate dissociation is an endothermic process and depressurization may induce reservoir temperature to reduce. Loss of heat supply in hydrate-bearing sediments causes methane hydrate reformation near wellbore, which can significantly affect fluid mobilization, plug the pores in sediments and reduce methane gas productivity. Therefore, it is necessary to study the distribution habits and morphological features of secondary methane hydrate in sediments, with the purpose of avoiding or tackling hydrate reformation along with hydrate exploitation.

This work employed a micromodel etched with pores to investigate methane gas recovery from hydrate-bearing sandstone. The dynamic process of hydrate transitions and gas/water migration can be visualized directly in fixed field of view. Secondary CH₄ hydrate formation was initialized after multiple cooling & heating and instant gas/water production at constant-pressure. Depressurization exploitation on methane hydrate was conducted by constant-rate pressure depletion on micromodel. The focuses of this work were to: (1) confirm whether secondary CH₄ gas hydrate formation happened during gas/water production in hydrate-bearing sandstone; (2) how gas/water/hydrate varied in morphology during secondary formation and following constant-rate depressurization; (3) illustrate the effects of seepage properties, such as wettability and fluid saturation, on secondary hydrate formation and dissociation that affecting methane gas recovery efficiency.

The results showed that hydrate nucleation happened at gas/water interface simultaneously or 5-20 minutes after gas/water flow at 80 bar and 1℃. The hydrate nuclei then grew towards gas phase by consuming gas available, with the hydrate patterns varying from coarse to smooth totally in hydrophilic gas-rich pores and hydrophobic water-rich pores. During constant-rate depressurization on secondary methane hydrates, pore blockage with pressure differences of 3-45 bar existed until pressure relief existed at dissociation pressures of 28-45 bar. Abnormal methane hydrate dissociation and reformation emerged because of localized pressure variation triggered by gas/water migration in confined pores. These kinetic and morphology findings are beneficial to understand the mechanisms of hydrate transitions and fluid migration in confined pores, and thus provide insights of reducing secondary methane hydrate formation for efficient methane gas recovery through controlled depressurization.