Pore scale study of the permeability anisotropy of sands containing grain-coating and pore-filling hydrates

Methane hydrate is a promising energy resource capable of meeting the future demand for green energy. Such hydrates consist of ice-like crystalline solids widely distributed in marine sediments or permafrost layers. Although several methods have been developed to extract natural gas from oceanic and permafrost sediments, commercial production of methane hydrate is still missing due to relatively low gas production rates globally. Notably, the hydrate reservoir permeability has a significant influence on the gas production rate. Thus, any factor affecting the permeability, such as hydrate formation, also affects the gas production rate.

Hydrate formation is generally random and inhomogeneous, resulting in permeability anisotropy in different directions. Moreover, due to their inhomogeneous distribution, the hydrates form different hydrate patterns, mainly grain-coating and pore-filling hydrates. Despite its significant influence on reservoir permeability and, subsequently, the gas production rate, there are limited studies on the permeability anisotropy of hydrate-bearing sediments. To date, numerous imaging techniques have been developed to facilitate the study and characterization of porous materials’ permeability and microstructural properties. Different techniques have different advantages and shortcomings that might either promote or limit their practical applications.

A combination of some of these techniques with image analysis has been extended to study the permeability properties of hydrate-bearing sediments. Nevertheless, most of the techniques used in previous studies have failed to accurately determine the influence of various hydrate patterns on permeability anisotropy. Moreover, previous studies were only limited to permeability measurement in two perpendicular directions. To overcome this issue, Dr. Cong Hu and Dr. Yonggang Jia from Ocean University of China together with Dr. Zhibo Duan from LML at the CNRS in France investigated the influence of grain-coating and pore-filling hydrate patterns on the permeability anisotropy of sands. A combination of X-ray computed tomography (CT) tests and image analysis via AVIZO was utilized. Their work is currently published in  Journal of Petroleum Science and Engineering.

The research team showed that the pore-filling hydrate had a relatively lower degree of anisotropy than the grain-coating hydrate. Both hydrate patterns, the highest and lowest permeability decreases due to hydrate formation, were observed in the y- and z-directions, respectively. The high degree of pore anisotropy resulted in a small absolute permeability. For instance, pore-filling hydrate formation produced more pore anisotropy than grain-coating hydrate. This suggested that decreasing the pore anisotropy ratio would significantly increase the permeability anisotropy.

Changes in pore anisotropy also affected pore tortuosity and shape factor, both of which influence permeability anisotropy. While the grain-coating hydrate mainly affected the pore size, pore-filling hydrate affected both pore shape and size. The largest increase in tortuosity was observed in the y-direction, while the smallest was in the z-direction. These observations agreed with the permeability variations. It was worth noting that pore-filling induced larger tortuosity ratio variation, further resulting in higher permeability anisotropy.

In summary, the authors successfully employed CT scanning test and image analysis to investigate the effects of hydrate pattern formation on permeability anisotropy. From the results, it was concluded that the pore-filling hydrate was responsible for inducing more significant permeability anisotropy compared with the grain-coating hydrate. In a statement to Advances in Engineering, first and corresponding author Dr. Cong Hu explained that their findings would improve our understanding of permeability anisotropy that would further advance the exploration and extraction of natural gas from marine sediments.