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Methane hydrate as a “new energy” 253

7.4.1 Initial stress before MH dissociation started

Fig. 7.15 shows the stress paths followed for three different cases. In Case 1 there was
no initial shear stress, and the temperature was increased to simulate thermal recovery;
0
the mean principal effective stress was increased from a to a to simulate the depres-
surization method. In the case of depressurization, the back pressure was allowed to
increase again, after dissociation, to ensure that the postproduction equilibrium con-
ditions were being restored. In Case 2 an initial shear stress was applied before using
thermal recovery or depressurization. Following this the temperature was allowed to
0
increase at b; the back pressure was decreased from b to b , and from b back to b after
0
the dissociation of all the MH. Finally in Case 3 the stress path was taken into the
metastable zone between the failure envelopes for pure sand and MH-bearing sand.

15 Fig. 7.15 Initial stress before
MH dissociation started and
S MH = 53.1% image of MH dissociation [18].
Deviator stress q (MPa) 10 Point B Toyoura sand
Point C

5

Point A s c ′=5 MPa
T =5ºC
Volumetric strain e v (%)
B.P. = 10 MPa
0 10

10
0 10 20 30 40 50
(A) Axial strain e a (%)

Failure envelope of
Failure envelope of
MH sand
Toyoura sand
Depressurization
Thermal recovery
Deviator stress q Case 2 b) Water pressure recovery
c)
c')
Case 3
b')

Case 1 a) a')
(B) Effective mean principal stress p'