Estimating the upper limit of gas production from Class 2 hydrate accumulations in the permafrost: 1. Concepts, system description, and the production base case

Gas hydrates are solid crystalline compounds in which gas molecules are lodged within the lattices of ice crystals. The amounts of hydrocarbon gases (mainly CH4) trapped in natural hydrate accumulations are enormous, leading to a recent interest in the evaluation of their potential as an energy source. Class 2 hydrate deposits are characterized by a Hydrate-Bearing Layer (HBL) underlain by a saturated water zone (WZ), and are encountered in the permafrost and in deep ocean sediments. In this numerical study of long-term gas production from permafrost-associated (PA) Class 2 deposits, we use fine grids to analyze a base case that involves a conventional vertical well with a 5 m-long production interval in the WZ that begins at the HBL base. The production process follows a cyclical pattern, with each cycle composed of two stages: a long stage (months to years) of increasing gas production and decreasing water production, followed by a reduction in the fluid withdrawal rate. We determine that the base case can yield large volumes of gas that are produced at high rates (reaching a maximum of over 6 MMSCFD, and an average of 3.11 MMSCFD over the entire production period) for long times, while the corresponding water production declines continuously.