Pore scale modeling on dissociation and transportation of methane hydrate in porous sediments

Methane gas is commonly found in the water-ice lattice of natural gas hydrate (NGH) reserves. Methane hydrate (MH) is generally stable in low-temperature and high-pressure environments such as those found in ocean sediments and permafrost regions. MH dissociates at standard temperature and pressure conditions to release methane, which is generally more efficient and cleaner than fossil fuels. As a result, MH has been viewed as a bridge fuel between renewable energy sources and carbon-intensive fossil energy. Preliminary research has shown that extracting depressurization-induced methane gas is a preferred gas hydrate production technology. Thus, a thorough understanding of the underlying mechanism of MH dissociation and transportation is necessary to optimize this exploitation technology and prevent potential geohazards.

Most studies use chemical hydrate analogy or synthesized methane hydrate and very few studies are based on natural hydrates. This is because drilling stable natural hydrate samples is generally costly and time-consuming. The dissociation and transportation of these hydrates have been simulated in porous media. However, most studies assumed the homogenous nature of the hydrate distribution despite the evidence of their heterogeneous distribution. They also neglect the effects of the pore-scale distribution of the fluids (gas, hydrate and water) on the MH dissociation. Besides, the evolution of fluids distribution during dissociation in sandy sediments is challenging in complex pore structures.

Despite the availability of numerous techniques for investigating hydrate sandy porous media, most focus on single-phase flow while ignoring the mass and heat transfer associated with the MH formation and dissociation. The effects of heat and mass transfer on the fluid flow are also often ignored despite their implication for methane production. Therefore, to understand the heat and mass transfer in multiple thermal and physicochemical processes, knowledge of pore scale dissociation and transportation of MH in porous sediments is fundamental.

Herein, researchers at the Wuhan Institute of Rock and Soil Mechanics, Chinese Academy of Sciences: Professor Rui Song, Professor Jianjun Liu and Professor Chunhe Yang, together with Professor Shuyu Sun from King Abdullah University of Science and Technology, proposed a novel method for modeling the mass and heat transfer in phase-change process and multiphase flow of MH dissociation in porous sediments. This method combined the enthalpy-porosity technique and volume of fraction (VOF). The models, programmed in C language, were utilized as a subroutine for commercial FLUENT software. The distribution of fluid saturation, temperature and velocity in the MH dissociation were comparatively analyzed. The model was validated by comparing the theoretical results with experimental and numerical results in the literature. The work is published in the journal, Energy.

The research team showed that their model could characterize the complex MH dissociation and transportation process. Unlike in the previous studies, both the normalized absolute permeability of the porous media and normalized permeability of water were obtained at different hydrate saturation during the dissociation process. The predicted fluids saturation, temperature and velocity distribution in both 2D pore-filling and grain-coating hydrate models were presented, analyzed and discussed comparatively with the previous studies. Likewise, the surface reaction rate of the MH dissociation showed a good agreement with experimental data. Furthermore, the model enabled the calculation of the petrophysical properties of the hydrate dissociation and transportation process.

In summary, the authors presented a new pore-scale technique for modeling the MH dissociation and transportation in porous sediments. This was the first attempt to properly and effectively model the effects of phase change on the evolution of the pore structure, multiphase flow, kinetic reaction and heat and mass transfer processes in a porous media. In a statement to Advances in Engineering, Professor Rui Song stated that the novel approach proposed would allow accurate prediction of pore-scale MH dissociation and transportation mechanism in porous media.