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Methane hydrate as a “new energy” 243

Fig. 7.6 Thermal recovery method for MH production.

7.2.2 Depressurization method

The depressurization method consists of lowering the pressure inside the well and
encouraging the methane hydrate to dissociate into methane gas and water, as illus-
trated in Fig. 7.7. At the end of the production, the water is injected back and the water
pressure recovered.
Promising progress has made in the MH gas production in both China and Japan
[14]. Despite the obvious advantages of physical methods, there are potential prob-
lems associated with these exploitation processes: these include the stress changes
caused by drilling, the settlement, landslide, and gas leakage. Due to these problems,
some accidents have happened in these processes such as submarine landslides, plat-
form foundation settlement, and failures of lifeline engineering projects [15,16].
Therefore, in order to safely extract the gas it is necessary to first understand the
mechanical and dissociation properties of the methane hydrate deposits.
To study the dissociation process, previous in situ tests [17] have investigated the
change of modulus by measuring the shear wave propagation velocities through the
MHBS, while other mechanical properties, such as stress-strain relationships, could
not be studied during the in situ MH dissociation. It is not practical to carry out lab-
oratory tests on undisturbed samples either, because it is too difficult to maintain the