Study on Hydrate Formation and Dissociation in the Presence of Fine-Grain Sand

Abstract

During the solid fluidization exploitation of shallow non-diagenetic NGHs (Natural Gas Hydrates) in the deep-water, hydrates together with mineral sand, natural gas, seawater and drilling fluids flow in the production pipeline. Natural gas released from hydrates during the process of solid fluidization will reform hydrates under the suitable conditions. Therefore, research on the formation and dissociation of methane hydrates in the presence of fine-grain sands is of great significance for ensuring the flow assurance of solid fluidization exploitation of shallow non-diagenetic NGHs in the deep-water field. In this paper, a high-pressure autoclave was used to carry out the experiments of hydrate formation and dissociation under different initial pressures and particle sizes of the fine-grain sand, for investigating into the hydrate induction time, formation amount, rate and dissociation affected by the presence of the fine-grain sand. Results indicated that hydrate formation kinetics in the presence of fine-grain sand was supposed to be also affected by mass/heat transfer, thermodynamics and kinetics. The fine-grain sand would be dispersed in the water phase under the effect of buoyancy, gravity and shearing force. Besides, the fine-grain sand at the gas-water interface would hinder the mass transfer of the methane gas into the water, inhibiting the nucleation of the hydrates, which was more obviously at the lower pressure. When the driving force for hydrate formation was larger, hydrate formation amount increased with the decrease of the particle size of the fine-grain sand. However, hydrate formation amount decreased with the decrease of the particle size of the fine-grain sand when the driving force for hydrate formation was lower. The average growth rate in the presence of fine-grain sand with 2.9 μm was larger than that of 9.9 μm. However, hydrates grew rapidly and subsequently tended to grow at a lower rate in the presence of fine-grain sand with 2.9 μm at 8.0 MPa initial pressure, which was assumed to be affected by the unconverted water wrapped inside the hydrate shell. The changing trends of gas emission during the dissociation process between the sand-containing system and the pure water system were nearly the same. The amount of gas emission reached a peak value within 15 minutes and then tended to stabilize. The difference in the amount of gas emission mainly depended on the formation amount before hydrate dissociation. Hydrates grew rapidly once methane hydrates nucleated in the presence of the fine-grain sand at the lower pressure, which would increase the plugging risk during the process of the solid fluidization exploitation. Further study of the fine-grain sand on flow assurance during hydrate dissociation process should be done in the future. The results of this paper provided an important theoretical basis and technical support for reducing the risk in the process of the solid fluidization exploitation of shallow non-diagenetic NGHs in the deep-water field.