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It has been evidenced that shallow gas hydrate resources are abundant in deep oceans worldwide. Their geological background, occurrence, and other characteristics differ significantly from deep-seated hydrates. Because of the high risk of well construction and low production efficiency, they are difficult to be recovered by using conventional oil production methods. As a result, this paper proposes an alternative design based on a combination of radial drilling, heat injection, and backfilling methods. Multi-branch holes are used to penetrate shallow gas hydrate reservoirs to expand the depressurization area, and heat injection is utilized as a supplement to improve gas production. Geotechnical information collected from an investigation site close to the offshore production well in the South China Sea is used to assess the essential components of this plan, including well construction stability and gas production behavior. It demonstrates that the hydraulic fracturing of the 60 mbsf overburden layer can be prevented by regulating the drilling fluid densities. However, the traditional well structure is unstable, and the suction anchor is advised for better mechanical performance. The gas production rate can be significantly increased by combining hot water injection and depressurization methods. Additionally, the suitable production equipment already in use is discussed. [ABSTRACT FROM AUTHOR]

Acoustic mapping enables an understanding of the surface distribution of shallow gas hydrate (GH) and related products. Acoustically characteristic materials such as fluid-seepage-related methane-derived authigenic carbonate and/or shallow GHs, may be widely distributed beneath the shallow seafloor of the Sakata Knoll. High-amplitude reflectors over the knoll are the top of gas-bearing permeable layers and connect to the reverse fault at the foot of the knoll. Shallow GH and bacterial mats were observed at the high-amplitude layer cut by depression and/or the locally disturbed seafloor. Acoustic blanking zones observed on the sub-bottom profiler sections are current gas migration routes from the depth to the seafloor. Optical observations indicate that fluid seepage is not active in the current seafloor, and it is not necessarily observed above the acoustic blanking zones or shallow faults reaching the seafloor. In the Sakata Knoll, the tectonically formed reverse fault and gas-bearing permeable layers play more important roles in fluid migration from depth to the summit area of the knoll compared to acoustic blanking and shallow faults. The depression at the summit area of the Sakata Knoll was formed by the dissociation of a shallow GH at around the last glacial maximum. Limited fluid seepage is currently witnessed within and around the depression and it is less extensive than that in the past. Such knolls, with tectonically formed large faults and an anticline are abundant in the area and they can be good reservoirs for shallow GH along the eastern margin of the Sea of Japan. [ABSTRACT FROM AUTHOR]

We have collected two shallow gas hydrate samples at two sites having different geological settings off southwest Taiwan during the cruise MD214 in 2018. The first core site, MD18-3542, is on the South Yuan-An East Ridge at ~ 1200 m water deep, where a structural unconformity covered by fine-silt sediments appears at ~ 5.5 m below the seafloor. The second core site, MD18-3543, is close to the Good-Weather Ridge at ~ 1100 m water deep, where a gas-related pockmark structure and authigenic carbonates are present at shallow strata with fine-silt sediments near the seafloor. Sediment properties of core MD18-3542 are distinctively different above and below the layer corresponding to the unconformity. Both cores show obvious gaps or voids in the lower core halves. The core features could be linked to the dissociated methane upward migrating from deep strata. Core site settings with upwelling methane would favor the formation of shallow gas hydrates. At site MD18-3542, the shallow hydrate could be formed due to high concentration methane kept beneath the unconformity covered by fine-silt sediments. At site MD18-3543, the shallow hydrate could be formed due to an extremely high flux of upwelling methane trapped either beneath the authigenic carbonates or fine-silt sediments. [ABSTRACT FROM AUTHOR]

Previous studies of gas hydrate in the Dongsha area mainly focused on the deep-seated gas hydrates that have a high energy potential, but cared little about the shallow gas hydrates occurrences. Shallow gas hydrates have been confirmed by drill cores at three sites (GMGS2 08, GMGS2 09 and GMGS2 16) during the GMGS2 cruise, which occur as veins, blocky nodules or massive layers, at 8–30 m below the seafloor. Gas chimneys and faults observed on the seismic sections are the two main fluid migration pathways. The deep-seated gas hydrate and the shallow hydrate-bearing sediments are two main seals for the migrating gas. The occurrences of shallow gas hydrates are mainly controlled by the migration of fluid along shallow faults and the presence of deep-seated gas hydrates. Active gas leakage is taking place at a relatively high-flux state through the vent structures identified on the geophysical data at the seafloor, although without resulting in gas plumes easily detectable by acoustic methods. The presence of strong reflections on the high-resolution seismic profiles and dim or chaotic layers in the sub-bottom profiles are most likely good indicators of shallow gas hydrates in the Dongsha area. Active cold seeps, indicated by either gas plume or seepage vent, can also be used as indicators for neighboring shallow gas hydrates and the gas hydrate system that is highly dynamic in the Dongsha area. [ABSTRACT FROM AUTHOR]

Natural gas hydrates have been hailed as a new and promising unconventional alternative energy, especially as fossil fuels approach depletion, energy consumption soars, and fossil fuel prices rise, owing to their extensive distribution, abundance, and high fuel efficiency. Gas hydrate reservoirs are similar to a storage cupboard in the global carbon cycle, containing most of the world's methane and accounting for a third of Earth's mobile organic carbon. We investigated gas hydrate stability zone burial depths from the viewpoint of conditions associated with stable existence of gas hydrates, such as temperature, pressure, and heat flow, based on related data collected by the global drilling programs. Hydrate‐related areas are estimated using various biological, geochemical and geophysical tools. Based on a series of previous investigations, we cover the history and status of gas hydrate exploration in the USA, Japan, South Korea, India, Germany, the polar areas, and China. Then, we review the current techniques for hydrate exploration in a global scale. Additionally, we briefly review existing techniques for recovering methane from gas hydrates, including thermal stimulation, depressurization, chemical injection, and CH4–CO2 exchange, as well as corresponding global field trials in Russia, Japan, United States, Canada and China. In particular, unlike diagenetic gas hydrates in coarse sandy sediments in Japan and gravel sediments in the United States and Canada, most gas hydrates in the northern South China Sea are non‐diagenetic and exist in fine‐grained sediments with a vein‐like morphology. Therefore, especially in terms of the offshore production test in gas hydrate reservoirs in the Shenhu area in the north slope of the South China Sea, Chinese scientists have proposed two unprecedented techniques that have been verified during the field trials: solid fluidization and formation fluid extraction. Herein, we introduce the two production techniques, as well as the so‐called "four‐in‐one" environmental monitoring system employed during the Shenhu production test. Methane is not currently commercially produced from gas hydrates anywhere in the world; therefore, the objective of field trials is to prove whether existing techniques could be applied as feasible and economic production methods for gas hydrates in deep‐water sediments and permafrost zones. Before achieving commercial methane recovery from gas hydrates, it should be necessary to measure the geologic properties of gas hydrate reservoirs to optimize and improve existing production techniques. Herein, we propose horizontal wells, multilateral wells, and cluster wells improved by the vertical and individual wells applied during existing field trials. It is noteworthy that relatively pure gas hydrates occur in seafloor mounds, within near‐surface sediments, and in gas migration conduits. Their extensive distribution, high saturation, and easy access mean that these types of gas hydrate may attract considerable attention from academia and industry in the future. Herein, we also review the occurrence and development of concentrated shallow hydrate accumulations and briefly introduce exploration and production techniques. In the closing section, we discuss future research needs, key issues, and major challenges related to gas hydrate exploration and production. We believe this review article provides insight on past, present, and future gas hydrate exploration and production to provide guidelines and stimulate new work into the field of gas hydrates. [ABSTRACT FROM AUTHOR]

Highly active gas venting and outcrops of gas hydrates were recently found at the Umitaka Spur, on the eastern margin of the Japan Sea, through seafloor observations using a remotely operated vehicle (ROV). This was followed by sampling by using conventional coring techniques. In this study, we deployed a long-term osmotic fluid sampling system (OsmoSampler) to record geochemical changes in the shallow sedimentary environment over time. The OsmoSampler collected interstitial water at 30 cm below seafloor (cmbsf), providing us with a continuous daily record of the concentrations of SO 4 2−, Cl−, CH 4 , and C 2 H 6 dissolved in interstitial waters over one year. General mean concentrations of SO 4 2− and Cl− repeatedly showed synchronous increases and decreases over both long-term (i.e. dozens of days) and short-term (i.e. 3–5 days) periods. Such fluctuations relative to background seawater concentrations were likely caused by saline and fresh water generation due to gas hydrate formation and dissolution. The CH 4 concentration was significantly higher (>1 mM) during large fluctuations in SO 4 2− and Cl− over the first 3 months, which was attributable to gas-venting activity. This high venting activity of CH 4 could have promoted the rapid, large-scale formation of gas hydrate. The CH 4 concentration decreased gradually after the first 3 months, and was accompanied by a period of relatively low SO 4 2− and Cl− concentrations due to the predominance of gas hydrate dissolution around the sampling site. In later months, the C 2 H 6 concentration approached ∼1 μM and was accompanied by occasional, positive CH 4 spikes, reflecting the release of trapped gases associated with the dissolution of hydrates. These processes are essentially controlled by the fluctuations in gas concentrations along the gas migration path, and are well-characterized by shallow interstitial water geochemistry. • Shallow gas hydrates on the Umitaka Spur grew and dissolved repeatedly from 3 to 5 days up to dozens of days. • Ion concentrations were controlled by saline and fresh water produced by gas hydrate growth and dissolution. • High CH 4 concentrations caused large and rapid gas hydrate growth. • Gas hydrate growth/dissolution changed migration path and concentrations of gas. [ABSTRACT FROM AUTHOR]