SPE 114163 Toward Production From Gas Hydrates: Current Status, Assessment of Resources, and Simulation-Based Evaluation of Technology and Potential

Gas hydrates 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 gas hydrate accumulations, the status of the primary international R&D programs, and the remaining science and technological challenges facing commercialization of production. After a brief examination of gas hydrate 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 are either not significant or are being addressed. We review the current body of literature relevant to potential productivity from different types of gas hydrate deposits, and determine that there are consistent indications of a large production potential at high rates over 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 gas hydrate deposits undesirable for production. Introduction Background. Gas hydrates are solid crystalline compounds in which gas molecules (referred to as guests) occupy the lattices of ice-like crystal structures called 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 G + NH H2O = G•NH H2O,..................................................................................................................(1) where NH is the hydration number. Hydrate deposits occur in two distinctly different geographic settings where the necessary conditions of low T and high P exist for their formation and stability: in the permafrost and in deep ocean sediments (Kvenvolden, 1988). The majority of naturally occurring hydrocarbon gas hydrates contain CH4 in overwhelming abundance. Simple CH4hydrates concentrate methane volumetrically by a factor of 164 when compared to standard P and T conditions (STP). Some modeling suggests that the energy needed for dissociation could be less than 15% of the recovered energy (Sloan and Koh, 2008). Natural CH4-hydrates crystallize mostly in the structure I form, which contains 46 H2O molecules per unit cell. Structure I hydrates have a NH ranging from 5.77 to 7.4, with NH = 6 being the average hydration number and NH = 5.75 corresponding to complete hydration (Sloan and Koh, 2008). Natural gas hydrates can also contain other hydrocarbons (alkanes CνH2ν+2, ν = 2 to 4), but may also comprise lesser amounts of other gases (mainly CO2, H2S or N2). Gas hydrates were first discovered in laboratory studies ca. 1800, but it was as late as 1965 that mankind first recognized that they may be common in nature, and that the age of some natural gas hydrate systems may be on the order of millions of years (Sloan and Koh, 2008). Although there has been no systematic effort to map and evaluate this resource on a global scale, and current estimates of in-place volumes vary widely (ranging between 10 to 10 m at standard conditions), the consensus is that the worldwide quantity of hydrocarbon gas hydrates is vast (Milkov, 2004; Klauda and Sandler, 2005). Given the sheer magnitude of the resource, ever increasing global energy demand, and the finite volume of conventional