Gas Hydrate Deposits: Formation and Development
Authors: Makogon Y.F. , Holditch S.A. , Perry K.F. , Holste J.C. 
Published: October 2008

Naturally occurring hydrates were discovered only 39 years ago (Makogon, 1965). Gas hydrates that contain large volumes of hydrocarbon energy accumulated in a few hundreds of meters of sediments near the earth surface. The potential volume of gas-in-place in a hydrated state is estimated to exceed 1.5x1016 m3. A single m3 of natural gas hydrate contains about 150-160 m3 of gas. About 98% of all gas hydrate deposits are found offshore in deep water in upper sedimentary layers. The most prospective regions are in the oceanic slopes. The thickness of hydrate-bearing rock may reach hundreds of meters, and in some conditions exceed 1 kilometer. Large amounts of free gas or liquid hydrocarbon can accumulate under the impermeable hydrate roof. Such coexistence of deposits can be very efficient for hydrocarbon production.

There are many problems to be solved before we can produce gas from hydrate deposits. Natural gas hydrate research is currently being conducted in a number of countries, such as USA, Japan, Russia, and India. International collaboration will accelerate and increase the effectiveness of mastering these new resources of mineral energy accumulated in gas hydrate deposits. Knowledge of conditions of formation, stability and decomposition of hydrate and seawater and the mass ratios of gas and water during phase transition appears to be quite important.

Introduction

Natural gas in geological sediments can exist as free gas, gas dissolved in reservoir water, or gas in the solid state. The distribution of gas among these phases is determined by thermodynamic conditions and the composition of the gas and water. Natural gas hydrates are a metastable mineral whose formation, stable existence and decomposition depend upon pressure, temperature, composition and other properties of the gas and water. The volume of gas in the sediments depends upon the geologic conditions in the regions and changes in those geologic conditions over time.

The total potential resource of natural gas is over 3x1016 M3. This is further categorized as:Free gas - proved resource of natural gas - 1.56x1014 m3 (BP data, 2003).Solid hydrates - 1.5x1016 m3 (Makogon, 1984; Kvenvolden, 1988). Gas hydrates are clathrate inclusion compounds in which molecules ranging in size from 0.35 nm to 0.9 nm are hosted in a crystalline lattice formed by hydrogen-bonded water molecules. The formation, decomposition, and stable existence of hydrates depends upon the pressure, temperature, composition and other properties of the gas and water. Some hydrate properties have significant implications for energy production. One volume of water enclathrates 207 volumes of methane in the hydrate state, but because the specific volumes of hydrates are 26-32% larger than that of liquid water, one cubic meter of hydrate contains 164 m3 of methane at standard conditions. Hydrate formation is an exothermic process; therefore, hydrates release heat during formation and absorb heat during dissociation. Approximately 6-12 % of the energy contained within the gas hydrate is required to decompose that hydrate in natural conditions. Also, dissociation of methane hydrate to gas and water in a closed volume can generate very high pressures.