Hydrate morphology: Physical properties of sands with patchy hydrate saturation

Abstract

.[1] The physical properties of gas hydrate-bearing sediments depend on the volume fraction and spatial distribution of the hydrate phase. The host sediment grain size and the state of effective stress determine the hydrate morphology in sediments; this information can be used to significantly constrain estimates of the physical properties of hydrate-bearing sediments, including the coarse-grained sands subjected to high effective stress that are of interest as potential energy resources. Reported data and physical analyses suggest hydrate-bearing sands contain a heterogeneous, patchy hydrate distribution, whereby zones with 100% pore-space hydrate saturation are embedded in hydrate-free sand. Accounting for patchy rather than homogeneous hydrate distribution yields more tightly constrained estimates of physical properties in hydrate-bearing sands and captures observed physical-property dependencies on hydrate saturation. For example, numerical modeling results of sands with patchy saturation agree with experimental observation, showing a transition in stiffness starting near the series bound at low hydrate saturations but moving toward the parallel bound at high hydrate saturations. The hydrate-patch size itself impacts the physical properties of hydrate-bearing sediments; for example, at constant hydrate saturation, we find that conductivity (electrical, hydraulic and thermal) increases as the number of hydrate-saturated patches increases. This increase reflects the larger number of conductive flow paths that exist in specimens with many small hydrate-saturated patches in comparison to specimens in which a few large hydrate saturated patches can block flow over a significant cross-section of the specimen.

Are Laboratory-Formed Hydrate-Bearing Systems Analogous to Those in Nature?

“…The hydrate morphology is governed by skeleton and capillary forces, which depend on the burial depth and grain size of the host sediment. 13 Although current estimates suggest that only 10 % or even less of the gas bound to methane hydrate can be found in sand/sandstone formations, 14 this type of reservoir is the focus of exploration for hydrates as energy resource. The high permeability of sands compared to that of the fine-grained sediments is very likely the reason that hydrate can accumulate to concentrations up to 90 % of the available pore volume.…”

“…Another modeling approach avoids the consideration of different hydrate habits using the patchy saturation concept 27 to estimate the influence of hydrate saturation on physical rock properties. 13 Dai et al 13 argue that in mature coarse grained hydrate systems Oswald ripening 28 of pore-filling hydrate will result in "patchy hydrate saturation" where patches containing 100 % hydrate in the pores are embedded in a hydrate-free water-saturated sand. For a detailed description of the modeling methodology see Dai et al 13 and references therein.…”

Analysis of the Physical Properties of Hydrate Sediments Recovered from the Pearl River Mouth Basin in the South China Sea: Preliminary Investigation for Gas Hydrate Exploitation

“…where the contact angle is assumed to be θ = 0 • , and the hydrate-water interfacial tension is γ hw = 0.032 ∼ 0.039 N/m [46]. The pore throat diameter (d th ) is determined by the fraction of fine sediment, taken to be ten percent of the finest grain size diameter here.…”

On the path to the digital rock physics of gas hydrate-bearing sediments – processing of in situ synchrotron-tomography data

“…1), but the approximations turned out to be still far from being satisfactory. None of the simplified models could accurately predict GH saturations from field electric resistivity or seismic data alone (Waite et al, 2009;Dai et al, 2012). This might now change with the advent of high-resolution in situ X-ray CT methods that open up new a possibility to explore various nucleation and growth paths of GH in a sedimentary matrix in three dimensions with a pixel resolution below 1 µm.…”

“…Different habits, distributions and saturation of gas hydrate crystals in the pore space affect the physical properties of the hydrate-bearing sediment (Priest et al, 2005;Waite et al, 2004). As the recovery of unperturbed natural methane hydrates is very difficult due to their fast decomposition under ambient conditions, a number of researchers have attempted to recreate the natural environment of gas hydrate in sedimentary matrices via laboratory experiments (Berge et al, 1999;Best et al, 2010Best et al, , 2013Dai et al, 2012;Dvorkin et al, 2003;Ecker et al, 2000;Hu et al, 2010;Li et al, 2011;Priest et al, 2006Priest et al, , 2009Spangenberg and Kulenkampff, 2006;Yun et al, 2005;Zhang et al, 2011). This collective effort eventually leads to a set of idealized micro-structural models (Fig.…”