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Dataset of diatom controls on the compressibility and permeability of fine-grained sediment collected offshore of South Korea during the Second Ulleung Basin Gas Hydrate Expedition, UBGH2

June 30, 2022

One of the primary goals of South Korea's second Ulleung Basin Gas Hydrate Expedition (UBGH2) was to examine the geotechnical properties of the marine sediment associated with methane gas hydrate occurrences found off the shore of eastern Korea in the Ulleung Basin, East Sea. Methane gas hydrate is a naturally occurring crystalline solid that sequesters methane in individual molecular cages formed by a lattice of water molecules. During UBGH2, concentrated gas hydrate was found in two sedimentary environments: thin, coarse-grained sediment layers interbedded with fine-grained sediment (fines, such as clays and muds) and as veins of essentially pure gas hydrate within beds of predominantly fine-grained sediment. This U.S. Geological Survey dataset includes physical property measurements of the fine-grained sediment associated with gas hydrate found during the UBGH2 expedition. Sediment samples were taken from the two sedimentary environments mentioned as part of a study looking into how the high diatom content of the UBGH2 sediment might affect the capacity to extract methane from UBGH2 gas hydrate reservoirs for use as an energy resource. Diatom refers here to the silica-based skeletal remains of microalgae. Diatom skeletons and skeleton fragments can get buried in marine sediment when the microalgae die. In the UBGH2 sediment, these diatoms and fragments can be up to 200 micrometers across, and larger than the sediment grains themselves (median grain size is about 10 micrometers for the samples tested as part of this study). Diatoms have the potential to alter how effectively methane can be extracted from gas hydrate as an energy resource. To extract methane from gas hydrate, a "production" well is drilled down into the gas hydrate-bearing reservoir. The gas hydrate reservoir can be depressurized by drawing pore water out of the sediment through the production well to reduce the reservoir's pore pressure. As the pore pressure falls below the gas hydrate stability limit, the solid gas hydrate breaks down, releasing gas and water, which then migrate toward the production well for collection. To understand how effectively methane can be extracted from a gas hydrate reservoir requires we know the compressibility and permeability of the bounding sediment (sediment in contact with the primary gas hydrate reservoir). If the bounding sediment is highly compressible, the reservoir depressurization process can cause the bounding layers to compact, putting stress on the production well walls; if the compacting part of the bounding layer is thick enough, the compaction-induced stress accumulates along the well wall and can cause the well to collapse and fail. Water migration through the bounding layers into the reservoir is affected by the compaction-dependent permeability of the bounding sediment. We concurrently measure permeability and compressibility of these diatomaceous sediments which is valuable for predicting pump rates needed to sustain gas hydrate dissociation. We conduct one-dimensional consolidation measurements on the bounding sediment with a stress-controlled oedometer cell (pictured in this data release). The pore-pressure response is measured over time during each loading step of the consolidation to estimate the sediment permeability by applying Terzaghi's equation for one-dimensional consolidation. In the fine-grained UBGH2 sediment studied, the high diatom content sediment (~22 to 45% diatoms by volume) has a high compressibility relative to typical coarse-grained gas hydrate reservoirs. The presence of diatoms also typically increases the permeability of fine-grained sediment. The permeabilities of the sediments tested in the study are still low enough relative to the reservoir permeability for the sediment to provide a reasonable barrier to fluid flow. As the gas hydrate-bearing reservoir is depressurized, the sediment compacts and permeability falls considerably, which indicates that if the production well is designed to handle the stress from compacting sediment, the bounding layers for the UBGH2 gas hydrate reservoirs will better seal the reservoir as it is depressurized, improving the methane recovery efficiency.

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