Experimental Study on the Difference of Fluid Flow between Methane Hydrate-Bearing Sand and Clay Sediments

Abstract

The gas−water seepage of natural gas hydrate sediments in the South China Sea significantly influences the efficiency of reservoir production of water/gas. However, there are significant differences between gas and water flow in methane hydrate-bearing sand and clay sediments. In this study, a series of permeability experiments were carried out in quartz and montmorillonite sediments in the presence of methane hydrate. The experiment results indicated that gas permeability decreases with increasing effective stress in montmorillonite and quartz in the absence of methane hydrate. The decrease in porosity has a higher impact on the permeability of quartz sand than that of montmorillonite. Due to the different effects of methane hydrate formation on the pore structure of sand and clay, with the increase in methane hydrate saturation in quartz sand and montmorillonite, the effect of effective stress on the permeability reduction of quartz sand is gradually weakened but the effect of effective stress on the permeability reduction of montmorillonite is gradually enhanced. Under the condition of constant total stress, the gas permeability decreases with the decomposition of methane hydrate. The permeability damage rate of montmorillonite caused by methane hydrate decomposition is much higher than that of quartz sand due to the swelling effect of montmorillonite particles. In addition, the water-phase seepage of quartz in the presence of methane hydrate is in line with the Darcy seepage characteristics. However, the water-phase seepage of montmorillonite in the presence of methane hydrate has non-Darcy seepage characteristics and the calculated water permeability is far less than the gas permeability.

Permeability Models of Hydrate-Bearing Sediments: A Comprehensive Review with Focus on Normalized Permeability

“…In recent times, most laboratory flow tests have been performed on coarse-grained sediments, but only very few flow tests have been conducted for the fine-grained host sediments [30,31,[144][145][146]. Therefore, due to the lack of reliable experimental permeability data, it is difficult to determine whether the existing permeability prediction models in the literature are suitable for fine-grained sediments or not.…”

Experimental Study on the Permeability of Hydrate-Bearing Silty-Clayey Sediments with Grain-Cementing and Pore-Filling Hydrate Morphologies

“…A series of investigations were undertaken by scholars to characterize hydrate-bearing sediments considering multiple factors from the perspective of permeability. One crucial recognition is that hydrate content is scattered in the pore space of sediments, which is rendered with negative correlation to permeability. , Furthermore, the decreasing tendency of effective permeability with increasing hydrate saturation is validated experimentally to be diverse with the functions of the pore-scale environment, such as sediments’ particle size, porosity, suffered effective stress, and hydrate morphology. − Typically, hydrate morphology is a vital permeability-related factor in view of hydrate reservoir formation and NGH exploitation, in which two typical forms represented by grain-cementing and pore-filling morphologies are introduced to hydrate permeability models, including the Parallel Capillary Model, Kozeny Grain Model, and Hybrid Model, and their modification. − However, the correlation between permeability and hydrate morphology is mostly established by model calculations based on computed tomography imaging or theoretical hypothesis, − and there are few laboratory experiments taking this into account. For in situ NGH deposited in natural hydrate-stabilized zones, massive references point out that pore-filling or fracture-filling hydrate-bearing silty-clayey sediments (HBSCSs) below the seabed occupy vast reserves in contrast to other hydrate reservoirs. − To this end, determining the seepage characteristics of HBSCSs with different hydrate morphologies in laboratory pore scale is essential for gaining insight into the numerical simulation and model verification of hydrate reservoirs, as well as provides gas and water recovery prediction over long-term hydrate production.…”