Abstract

This paper examines hydrate stability against changes in seasonal weather, sediment slumps, sea-level fluctuations with time, glacial conditions, and topographic changes with time. The purpose is to determine longevity of hydrate accumulations with geological time and to determine their impact on geological regions and stability. The results indicate that stable geological conditions are not always the best locations for hydrates-dynamical changes can lead to higher hydrate production and retention. The significance is that the perturbation of potential hydrocarbon accumulation sites by hydrate appearance and disappearance, together with the variability of the potential worth of the hydrate accumulations themselves, may play a more important role than so far recognized in evaluation of economic worth.

I. Overview of General Hydrate Dissociation Conditions.

The presence of clathrates (gas hydrates) is well established observationally in the offshore region of the South Caspian Basin, as well as in many other regions of the world. Hydrates of given composition can exist under particular pressuretemperature conditions. However, under the impact of neotectonic processes, those conditions can change. In that case part, or all, of the mass of hydrates can be dissociated and released. The dissociation can take place gradually, or explosively, depending on how fast the pressure drops or temperature increases.

Five major variations of hydrate evolution are considered to illustrate possible patterns of behavior caused by variation of geological conditions:

  1. Variations in hydrate existence conditions due to sediment and mud redeposition. Due to earthquakes, volcanic eruptions or other processes, part of the sediments can be released from a sloping sea bottom and transported and redeposited in other places, thereby changing pressure conditions at the top of a hydrate layer.

  2. Variations in hydrate existence conditions due to glacial-interglacial conditions. Removal of ice cover not only decreases overlying pressure, but also allows water temperatures overlying the sediments to increase.

  3. Variations in hydrates due to sea level rise and fall.

  4. Enrichment of ethane in hydrates as a consequence of varying neotectonic conditions.

  5. Evaporation and reformation of hydrates in aeolian conditions due to winter cooling and summer heating.

Hydrates can cause an explosive hazard for exploration rigs, production platforms and pipelines, especially in deep water conditions. All of the above geological patterns of instability should, therefore, be kept in mind when potential hazards are assessed. In addition, flame initiation by dissociation is also examined as a potential hazard, as are hazards due to drill penetration, warm circulation mud, and drill-bit heating of hydrates.

In many areas of the world the prevalence of gas hydrates at the present-day, or inferred to have been present in the recent past, is one of the more influential factors in controlling geological sedimentation and attributes allied to the geological behavior. For instance, in the Green River Basin, Wyoming, the ENRON Corporation (1993) has estimated that a total gas (possibly some in hydrate form) reserve of 5,000 TCF exists; while in the eastern United States, offshore South Carolina, there are estimates available of major hydrates present, with associated contributions to dynamical instability (Lowrie, 1997).

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