PROVEN RESERVES AND BASICS FOR DEVELOPMENT OF GAS HYDRATE DEPOSITS

Y. F. Makogon¹, S. A. Holditch¹, T. Y. Makogon²
¹Texas A&M University, College Station, TX
²BP America Inc., Houston, TX

Gas hydrates present the energy industry with both costly problems and enormous potential. As for the problems, the industry spends over $2 million a day to prevent hydrate plugs in pipelines and wells. However, there is enormous potential to produce natural gas from gas hydrate deposits in the coming decades. Natural gas is a clean burning hydrocarbon. There have been over 200 deposits of naturally occurring natural gas hydrates discovered to-date. The potential natural gas resource is estimated at 1.5*10¹⁶ м³ (Makogon, 1984) to 2.1*10¹⁶ м³ (Kvenvolden, 1989). If these potential is close to reality, then gas from gas hydrate deposits can supply the world's growing energy demands for 200 years.

There are two main types of hydrate deposits: the primary, which formed from the gas dissolved in formation water, and the secondary, which formed from free gas deposits after temperature and pressure changed over geologic times. Ninety-eight percent of the natural gas potential reserves found in the hydrated state are offshore in the upper several hundred meters of near-seabed sediments. Approximately 2% of hydrated gas deposits are on land in permafrost areas. The phase transitions, which characterize the secondary deposits, occur over geologic time scales with variations in temperature of earth's surface and climate changes. The secondary deposits were preserved only in presence of strata impermeable to gas over them. The primary deposits exist mainly near the deepwater shelf and oceanic slope at water depths of 300-700 m to 2-2.5 km. Absence of the sealing layers over the hydrates is characteristic of the offshore hydrate deposits, where hydrates may exist in a supercooled or an equilibrium state. The supercooled deposits don't have free gas. The deposits in equilibrium have free gas both in hydrated layers and in the underlying layers. The approach to the development of a specific deposit will depend on the type of deposit and how it was formed.

Development of a gas hydrate deposit is characterized by the need to convert gas from a solidified hydrate to a free state in-situ with a subsequent extraction of the gas by traditional means. The energy required to dissociate the hydrate and to extract the free gas from the gas hydrate deposit ranges from 10-15% to 60-80% of the total deposit's stored energy, depending on the geologic and thermodynamic conditions. When considering the technology of mastering the resource of gas in hydrate one should base the economic decision not on the potential reserve, but on the producible reserve of the free gas. Preliminary calculations indicate that on average, 17-20% of the potential reserves of natural hydrates can be commercially produced. Hydrate deposits become economically effective from 30-40% hydrate saturation of rock. The offshore limits for economic production of gas from hydrates are at 2-2.5 km water depth.

In the proposed work we review the factors that can facilitate or hinder the use of different techniques, depending on the geologic setting, with due consideration of health, technical safety and environmental stability aspects, which are very important during production of gas from hydrates. We also propose the steps that would help minimize or eliminate the duplication of effort and time spent studying natural hydrates, such as an International Coordinating Council on Hydrate Research and Production.

Figure 1. Thickness of Sediment and effective zone for development GH Deposits.