Gas hydrates are solid crystalline compounds in which gas molecules are lodged within the lattices of ice crystals. Natural hydrates in geological systems are composed mainly of methane. The amounts of hydrocarbon gases 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 zone of mobile water, and are encountered in the permafrost and in deep ocean sediments. The base of the HBL in Class 2 deposits may occur at the edge of, or within, the zone of thermodynamic hydrate stability. Because of the manner of their formation from pre-existing gas reservoirs, permafrost hydrate deposits are generally characterized by high hydrate saturations and are bounded by relatively impermeable strata.
In this numerical study of long-term gas production from permafrost Class 2 deposits, we investigate three different well configurations that involve different production intervals and combinations of depressurization (the main dissociation-inducing mechanism) with localized thermal stimulation. Using high-definition grids and realistic production scenarios, we determine that large volumes of gas can be produced at high rates (several MMSCFD) for long times using conventional technology. The production approach involves initial fluid withdrawal from the water zone underneath the HBL. 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 a short stage (days to weeks) that involves destruction of the secondary hydrate (mainly though warm water injection) that evolves during the first stage. A well configuration that initially involves heating of the outer surface of the wellbore and later continuous injection of warm water at low rates appears to yield the highest average rates over the period it takes for the exhaustion the hydrate deposit. We determine that gas production is affected by (a) the intrinsic permeability, (b) the initial hydrate saturation, (c) the fluid withdrawal rate, (d) the thickness of the water zone, and (e) the initial pressure and temperature of the hydrates.