Abstract

Gas hydrates are solid crystalline compounds in which gas molecules are lodged within the lattices of an ice-like crystalline solid. The vast quantities of hydrocarbon gases trapped in hydrate formations in the permafrost and in deep ocean sediments may constitute a new and 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. 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. Both classes of deposits have been shown to be good candidates for exploitation in earlier studies of gas production via vertical well designs—in this study we extend the analysis to include systems with varying porosity, anisotropy, well spacing, and the presence of permeable boundaries. For Class 2 deposits, the results show that production rate and efficiency depend strongly on formation porosity, have a mild dependence on formation anisotropy, and that tighter well spacing produces gas at higher rates over shorter time periods. For Class 3 deposits, production rates and efficiency also depend significantly on formation porosity, are impacted negatively by anisotropy, and production rates may be larger, over longer times, for well configurations that use a greater well spacing. Finally, we performed preliminary calculations to assess a worst-case scenario for permeable system boundaries, and found that the efficiency of depressurization-based production strategies are compromised by migration of fluids from outside the system.

Introduction

Background. Gas hydrates are solid crystalline compounds in which gas molecules (referred to as guests) occupy the lattices of ice 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. Natural hydrates in geological systems usually contain hydrocarbons (mainly CH4 and other alkanes), but may also contain CO2, H2S or N2. Hydrate deposits occur in two distinctly different geologic 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.

Although there has been no systematic effort to map and evaluate this resource and current estimates vary widely1,2,3 (ranging between 1015 to 1018 m3), the consensus is that the worldwide quantity of hydrocarbon gas hydrates is vast. Even the most conservative estimate exceeds the total energy content of the known conventional fossil fuel resources. The sheer magnitude of the resource, ever increasing global energy demand, and dwindling conventional fossil fuel reserves, point to hydrates a promising energy source4,5 even if only a limited number of deposits might be suitable for production and/or only a fraction of the trapped gas can be recovered. The attractiveness of hydrates is further enhanced by the environmental desirability of natural gas (as opposed to solid or liquid) fuels. The production potential of gas hydrate accumulations demands technical and economic evaluation.

Gas can be produced from hydrates by inducing dissociation, which also releases large amounts of H2O (Eq. 1). The three main methods of hydrate dissociation are6:

  1. depressurization, in which the pressure P is lowered to a level lower than the hydration pressure Pe at the prevailing temperature T,

  2. thermal stimulation, in which T is raised above the hydration temperature Te at the prevailing P, and

  3. the use of inhibitors (such as salts and alcohols), which shifts the Pe-Te equilibrium through competition with the hydrate for guest and host molecules.

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