Gas Recovery and Temperature Response Characteristics of Methane Hydrate in Marine Sediments with High Water Content by Depressurization

Abstract

Marine hydrates represent a pivotal energy resource for the foreseeable future. However, experimental simulations pertaining to their production processes are rarely executed within real marine sediments, limiting the understanding of their formation and dissociation behaviors under natural conditions. In this study, real marine sediment cores, sampled from muddy reservoirs in South China Sea, were utilized to reconstruct hydrate-bearing sediment samples with a high-water content (42%~45%). Injecting water into high-pressure marine soil thrice notably boosts water content, hydrate saturation, and thermal conductivity in hydrate sediments, evident in temperature drops post-stabilization, with a 0.5 °C decrease after the third injection. Depressurization was conducted in marine muddy sediments (26.8%~29.3% hydrate saturation) to investigate the gas and water production behaviors. When the water content in marine sediments surpasses the liquid limit threshold of approximately 40%, the water production experiences a notable increase as the water content of the sediment samples augments. The gas production behaviors reveal a proportional relationship between the rate of hydrate dissociation and the rate of depressurization. Furthermore, sudden pressure drop in the hydrate reservoir results in a steeper temperature drop, exceeding 7 °C locally during gas recovery. In addition, while a high depressurization rate can stimulate the dissociation of hydrate, it may not necessarily facilitate the liberation of free gas trapped within marine sediments. The water adsorption by marine soil leads to muddy sediments entrapping hydrates and gases, which reduces gas permeability and triggers a pressure delay effect, where temperature drops by 0.4~1.3 °C post-pressure stabilization. Furthermore, temperature recovery and hydrate dissociation periods are approximately twice as long as those observed in gas-saturated reservoirs. The observed gas and water production behaviors, along with the associated thermal and permeability effects, offer critical insights into optimizing gas recovery strategies from marine muddy sediments rich in hydrates.

Keywords: Methane Hydrate, South China Sea sediments, High water content, gas production, Depressurization