TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractGas hydrates are solid crystalline compounds in which gas molecules are lodged within the lattices of ice crystals. Vast amounts of CH 4 are trapped in gas hydrates, and a significant effort has recently begun to evaluate hydrate deposits as a potential energy source. Class 3 hydrate deposits are characterized by an isolated Hydrate-Bearing Layer (HBL) that is not in contact with any hydrate-free zone of mobile fluids. The base of the HBL in Class 3 deposits may occur within or at the edge of the zone of thermodynamic hydrate stability.In this numerical study of long-term gas production from typical representatives of unfractured Class 3 deposits, we determine that simple thermal stimulation appears to be a slow and inefficient production method. Electrical heating and warm water injection result in very low production rates (4 and 12 MSCFD, respectively) that are orders of magnitude lower than generally acceptable standards of commercial viability of gas production from oceanic reservoirs. However, production from depressurization-based dissociation based on a constant well pressure appears to be a promising approach even in deposits characterized by high hydrate saturations. This approach allows the production of very large volumes of hydrate-originating gas at high rates (> 15 MMSCFD, with a long-term average of about 8.1 MMSCFD for the reference case) for long times using conventional technology. Gas production from hydrates is accompanied by a significant production of water. However, unlike conventional gas reservoirs, the water production rate declines with time. The low salinity of the produced water may require care in its disposal.Because of the overwhelming advantage of depressurization-based methods, the sensitivity analysis was not extended to thermal stimulation methods. The simulation results indicate that depressurization-induced gas production from oceanic Class 3 deposits increases (and the corresponding waterto-gas ratio decreases) with increasing hydrate temperature (which defines the hydrate stability), increasing intrinsic permeability of the HBL, and decreasing hydrate saturationalthough depletion of the hydrate may complicate the picture in the latter case. IntroductionBackground. Gas hydrates are solid crystalline compounds in which gas molecules ("guests") occupy cavities within the lattices of ice crystals ("hosts"). Under suitable conditions of low temperature T and high pressure P, the hydration reaction of a gas G is described by the general equation:where N H is the hydration number. Natural hydrates in geological systems usually contain hydrocarbons (for example, CH 4 and other alkanes), but may also contain CO 2 , H 2 S or N 2 . Hydrate deposits occur in two distinctly different hydrogeologic settings: in the permafrost and in deep ocean sediments.While no systematic effort has been made to map and evaluate the size of this resource (and current estimates vary widely, ranging between 10 15 to 10 18 m 3 ), the consensus is that t...
Summary Gas hydrates (GHs) are a vast energy resource with global distribution in the permafrost and in the oceans. Even if conservative estimates are considered and only a small fraction is recoverable, the sheer size of the resource is so large that it demands evaluation as a potential energy source. In this review paper, we discuss the distribution of natural GH accumulations, the status of the primary international research and development (R&D) programs, and the remaining science and technological challenges facing the commercialization of production. After a brief examination of GH accumulations that are well characterized and appear to be models for future development and gas production, we analyze the role of numerical simulation in the assessment of the hydrate-production potential, identify the data needs for reliable predictions, evaluate the status of knowledge with regard to these needs, discuss knowledge gaps and their impact, and reach the conclusion that the numerical-simulation capabilities are quite advanced and that the related gaps either are not significant or are being addressed. We review the current body of literature relevant to potential productivity from different types of GH deposits and determine that there are consistent indications of a large production potential at high rates across long periods from a wide variety of hydrate deposits. Finally, we identify (a) features, conditions, geology and techniques that are desirable in potential production targets; (b) methods to maximize production; and (c) some of the conditions and characteristics that render certain GH deposits undesirable for production.
Gas hydrates are solid crystalline compounds in which gas molecules are lodged within the lattices of ice crystals. The vast amounts of hydrocarbon gases that are trapped in hydrate deposits in the permafrost and in deep ocean sediments may constitute a promising energy source. Class 2 hydrate deposits are characterized by a Hydrate-Bearing Layer (HBL) that is underlain by a saturated zone of mobile water. In this study we investigated three methods of gas production via vertical well designs. A long perforated interval (covering the hydrate layer and extending into the underlying water zone) yields the highest gas production rates (up to 20 MMSCFD), but is not recommended for long-term production because of severe flow blockage caused by secondary hydrate and ice. A short perforated interval entirely within the water zone allows long-term production, but only at rates of 4.5 - 7 MMSCFD. A new well design involving localized heating appears to be the most promising, alleviating possible blockage by secondary hydrate and/or ice near the wellbore) and delivering sustainably large, long-term rates (10-15 MMSCFD). The production strategy involves a cyclical process. During each cycle, gas production continuously increases, while the corresponding water production continuously decreases. Each cycle is concluded by a cavitation event (marked by a precipitous pressure drop at the well), brought about by the inability of the system to satisfy the constant mass production rate QMimposed at the well. This is caused by the increasing gas contribution to the production stream, and/or flow inhibition caused by secondary hydrate and/or ice. In the latter case, short-term thermal stimulation removes the blockage. The results show that gas production increases (and the corresponding water-to-gas ratio RWGCdecreases) with an increasing (a) QM, (b) hydrate temperature (which defines its stability for a given pressure), and (c) intrinsic permeability. Lower initial hydrate saturations lead initially to higher gas production and a lower RWGC, but the effect is later reversed as the hydrate is depleted. The disposal of the large amounts of produced water does not appear to pose a significant environmental problem. Production from Class 2 hydrates is characterized by (a) the need for confining boundaries, (b) the continuously improving RWGCover time (opposite to conventional gas reservoirs), and (c) the development of a free gas zone at the top of the hydrate layer (necessitating the existence of a gas cap for production).
Although significant hydrate deposits are known to exist in the Ulleung Basin of the Korean East Sea, their survey and evaluation as a possible energy resource has not yet been completed. However, it is possible to develop preliminary estimates of their production potential based on the limited data that are currently available. These include the elevation and thickness of the Hydrate-Bearing Layer (HBL), the water depth, and the water temperature at the sea floor. Based on this information, we developed estimates of the local geothermal gradient that bracket its true value. Reasonable estimates of the initial pressure distribution in the HBL can be obtained because it follows closely the hydrostatic. Other critical information needs include the hydrate saturation, and the intrinsic permeabilities of the system formations. These are treated as variables, and sensitivity analysis provides an estimate of their effect on production.Based on the geology of similar deposits, it is unlikely that Ulleung Basin accumulations belong to Class 1 (involving a HBL underlain by a mobile gas zone). If Class 4 (disperse, low saturation accumulations) deposits are involved, they are not likely to have production potential. The most likely scenarios include Class 2 (HBL underlain by a zone of mobile water) or Class 3 (involving only an HBL) accumulations.Assuming nearly impermeable confining boundaries, this numerical study indicates that large production rates (several MMSCFD) are attainable from both Class 2 and Class 3 deposits using conventional technology. The sensitivity analysis demonstrates the dependence of production on the well design, the production rate, the intrinsic permeability of the HBL, the initial pressure, temperature and hydrate saturation, as well as on the thickness of the water zone (Class 2). The study also demonstrates that the presence of confining boundaries is indispensable for the commercially viable production of gas from these deposits.
[1] Vast quantities of methane are trapped in oceanic hydrate deposits. Because methane is a powerful greenhouse gas (about 26 times more effective than CO 2 ), there is considerable concern that a rise in the temperature of the oceans will induce dissociation of oceanic hydrate accumulations, potentially releasing large amounts of carbon into the atmosphere. Such a release could have dramatic climatic consequences because it could amplify atmospheric and oceanic warming and possibly accelerate dissociation of the remaining hydrates. This study assesses the stability of three types of hydrates (case I, deep-ocean deposits; case II, shallow, warm deposits; and case III, shallow, cold deposits) and simulates the dynamic behavior of these deposits under the influence of moderate ocean temperature increases. The results indicate that deep-ocean hydrates are stable under the influence of moderate increases in ocean temperature; however, shallow deposits can be very unstable and release significant quantities of methane under the influence of as little as 1°C of seafloor temperature increase. Less permeable sediments, or burial underneath layers of hydrate-free sediment, affect both the rate of hydrate dissociation and methane transport to the seafloor but may not prevent methane release. Higher-saturation deposits can produce larger methane fluxes with the thermodynamics of hydrate dissociation retarding the rate of recession of the upper hydrate interface. These results suggest possible worst case scenarios for climate-change-induced methane release and point toward the need for detailed assessment of the hydrate hazard and the coupling of hydrate-derived methane to regional and global ecosystems.Citation: Reagan, M. T., and G. J. Moridis (2008), Dynamic response of oceanic hydrate deposits to ocean temperature change,
Summary The current paper complements the Moridis et al. (2009) review of the status of the effort toward commercial gas production from hydrates. We aim to describe the concept of the gas-hydrate (GH) petroleum system; to discuss advances, requirements, and suggested practices in GH prospecting and GH deposit characterization; and to review the associated technical, economic, and environmental challenges and uncertainties, which include the following: accurate assessment of producible fractions of the GH resource; development of methods for identifying suitable production targets; sampling of hydrate-bearing sediments (HBS) and sample analysis; analysis and interpretation of geophysical surveys of GH reservoirs; well-testing methods; interpretation of well-testing results; geomechanical and reservoir/well stability concerns; well design, operation, and installation; field operations and extending production beyond sand-dominated GH reservoirs; monitoring production and geomechanical stability; laboratory investigations; fundamental knowledge of hydrate behavior; the economics of commercial gas production from hydrates; and associated environmental concerns.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.