The Dynamic Oceanic Hydrate System: Production Constraints and Strategies

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Abstract
Potentially economic oceanic hydrate deposits contain enormous amounts of methane. These mainly occur between 1,700 m and 3,500 m water depth along continental margins. Deeper water drilling of conventional hydrocarbons is now moving into water depths where hydrates are found in the seafloor, and industry expertise and extractive equipment may need little modification to be used successfully for recovering methane. Drilling through hydrate and into gas zones with drill hole logging has been successfully accomplished without collapse of either the hydrate cap or the drill hole. Because the geotechnically weak sediment-gas zone offers difficult drilling conditions, it may be necessary to drill the long gas collector holes within the base of the hydrate, where material strength is higher and the geotechnical properties more predictable, with tapping of the gas zone by downward penetration. Water separation and drying the gas may be best accomplished on the seafloor where a vented recovery chamber can allow for water removal and gas concentration. Introduction Methane hydrate is a crystalline substance formed from methane and water that occurs in large enough concentrations to have potential economic value1,2. Hydrate formation forcesmethane molecules into closely packed lattice sites, effectively concentrating the methane. Methane hydrate is non-stoichiometric in that the crystal structure of the hydrate can be established without all the methane lattice sites being occupied. 1 m3 of fully saturated methane hydrate contains 164 m3 of methane (at STP) and 0.87 m3of water1. Although other hydrocarbon gases can form hydrates, biogenic methane has been found to predominate in the natural environment. Thermogenic gas production has been responsible for the production of the small amount of higher density hydrocarbon gasses, as well as some methane, that has been found in seafloor hydrate. Methane hydrate is naturally concentrated in two distinct types of deposit within its general area of pressure-temperature stability. Permafrost hydrates are restricted to Polar areas where they occur within continental shelves but mainly in land areas, whereas oceanic hydrates are concentrated in the seafloor (Fig. 1). Oceanic gas hydrate occurs in the Hydrate Stability Zone (HSZ), which is a zone of thermodynamic stability that extends downward into the marine sediment from the seafloor to some depth determined by temperature, which increases with depth. Where sufficient hydrate has formed in the lower part of the HSZ, the upward migration of further gas arriving from subjacent sources mal be blocked and large accumulations of gas may form3. Hydrate that dissociates at the base of the HSZ because of either lowering of pressure owing to sea level drop or ocean warming will also tend to pool beneath the hydrate rather than dissipate. Once trapped as hydrate, the methane will tend to stay concentrated in the HSZ, even though it may pass through periods of gasification. In a situation where sedimentation takes place, isotherms rise and warms the deposit. Dissociation of hydrate at the base of the HSZ takes place as the temperature increases. Gas released at the rising base of the HSZ4 tends to be trapped beneath the hydrate, which commonly will form a seal and trap, especially where there is a closure at the base of the HSZ.

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