FROZEN HEAT | Volume 2

3.1 INTRODUCTION

If economic and environmentally responsible production of gas hydrate resources proves achievable, the global conse- quences are potentially far-reaching. Natural gas emits sub- stantially less greenhouse gas thanmany other fossil fuels – up to 40 per cent less than coal or oil (EIA 2013). It has, therefore, been identified by many countries as a preferred energy source

over other hydrocarbons for the near future. Gas hydrates are thought to occur in relative abundance (in terms of the size of the resource) in select locations around the world. They occur in both marine and permafrost settings where methane gas and water co-exist at pressures and temperatures suitable for hydrate formation and stability (Figure 3.1).

Stability conditions for gas hydrates

Depth (metres) 0

Depth (metres) 0

gnittes eniraM

gnittes tsorfamreP

Sea surface

Ground surface

Ice freezing temperature

200

200

Stability zone

400

400

Stability zone

Base of permafrost

600

600

800

800

Sea oor

1000

1000

1200

1200

1400

1400

1600

1600

-30

-20

-10

0

10

20

30

0

01

20

3

0

Temperature ºC

Temperature ºC

Figure 3.1: Phase diagrams illustrating where methane hydrate is stable in marine (left frame) and permafrost settings (right frame). Hydrate can exist at depths where the temperature (blue curve) is less than the maximum stability temperature for gas hydrate (given by the hydrate stability curve in orange). Pressure and temperature both increase with depth in the Earth, and though hydrates can exist at warmer temperatures when the pressure is high (orange curve), the temperature in the Earth (blue curve) gets too hot for hydrate to be stable, limiting hydrate stability to the upper ~1km or less of sediment.

FROZEN HEAT 60