Gas Production From Oceanic Class 2 Hydrate Accumulations

Abstract

Listen

Translate article

Abstract 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).