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Two-dimensional micromodel study of pore-throat clogging by pure fine-grained sediments and natural sediments from the 2015 National Gas Hydrate Program Expedition 2 (NGHP-02), offshore India

February 1, 2019

Fine-grained sediments, or "fines," are nearly ubiquitous in natural sediments, even in the predominantly coarse-grained sediments that host gas hydrates. Fines within these sandy sediments can be mobilized and subsequently clog flow pathways while methane is being extracted from gas hydrate as an energy resource. Using two-dimensional (2D) micromodels to test the conditions in which clogging occurs provides insights for choosing production operation parameters that optimize methane recovery in the field. During methane extraction, several processes can alter the mobility and clogging potential of fines: (1) fluid flow as the formation is depressurized to release methane from gas hydrate, (2) shifting pore-fluid chemistry as pore-fluid brine freshens as a result of pure water released from dissociating gas hydrate, and (3) the migration of gas/water interfaces, which are created as gas evolves from dissociating gas hydrate. In this study, 2D micromodel experiments were conducted on a selection of pure fines, natural sediments, pore-fluids, and micromodel pore-throat sizes to evaluate fines migration and changes in clogging behavior resulting from methane gas production and pore-water freshening during hydrate dissociation. Additionally, tests were run with and without an invading gas phase (carbon dioxide) to test the importance of a moving meniscus on fines mobility and clogging. The endmember fine particles chosen for this research included silica silt, mica, calcium carbonate, diatoms, kaolinite, illite, and bentonite (primarily made of montmorillonite). The pore fluids included deionized water, sodium chloride brine (2 molar concentration), and carbon dioxide gas. The microfluidic pore models, used as porous media analogs, were fabricated with pore-throat widths of 20, 40, 60 and 100 micrometers to cover the range of anticipated pore throat sizes sampled during NGHP-02. This dataset provides a clogging diagram showing how grain size, fines concentration, pore fluid chemistry and mobile interfaces define the clogging behavior of the pure fines. This fundamental properties diagram helps interpret the clogging behavior of three natural samples also tested for this dataset. The natural samples were collected during NGHP-02. This research shows that in addition to the expected dependence of clogging on the ratio of particle-to-pore-throat size, pore-fluid chemistry is also an important factor because the interaction between a particular type of fine and pore fluid influences that fine's capacity to cluster, clump together, and thereby increase the effective particle size relative to the pore-throat width. The presence of a moving gas/fluid meniscus increases the clogging potential regardless of fine type because the advancing meniscus tends to gather and concentrate the fines.