Production of Sedimentary Methane Hydrates by Depressurization

Abstract

Abstract The voluminous amounts of hydrates found in nature are distributed across all continents and pose a huge possibility for future energy harvest. In fact, the total amount of energy stored in hydrates is predicted to be in the same range as conventional fossil fuels combined. Successful exploitation of this energy supply will serve as a vast and relatively clean source of fossil fuel for the remaining of the fossil era. Natural gas hydrates consist of guest molecules encapsulated inside cavities formed by hydrogen-bonded water molecules. The origin of the encaged gas may be biogenic or thermogenic with methane being the most common hydrate former in the earth. Methane hydrates are stabilized at elevated pressures and low temperatures and are mainly located in onshore permafrost regions and in marine sediments offshore. This paper review how production of methane from hydrates can be triggered by dissociation of the hydrate structure. Techniques leading to dissociation of hydrates are summarized by pressure depletion, thermal stimulation and injection of inhibitors. Depressurization is considered to be the most cost-effective method and is easily implemented in gas reservoirs with overlying hydrate layers. Examples and status of pressure depletion tests on field scale will be reviewed. In hydrate reservoirs not adjacent to gas zones, the success of pressure depletion is dependent on sufficient permeability to allow for pressure perturbations to reach within the hydrate reservoir and to allow for flow of dissociated gas. This effect has been investigated in this paper by performing controlled pressure depletions on hydrate-filled sandstone cores. Dissociation pressures at given temperatures have been quantified as well as recovery of methane as function of pressure-decrements below dissociation pressure. A 2D porous silicon wafer micromodel was used to gain insight on the mechanisms of hydrate dissociation. Direct visualization of hydrate melting induced by both depressurization and heating is reported from pores replicating authentic sandstone pores.