Issue 45

Q.-C. Li et alii, Frattura ed Integrità Strutturale, 45 (2018) 86-99; DOI: 10.3221/IGF-ESIS.45.07 93 The relationship between Young's modulus of hydrate-bearing sediments E and hydrate saturation S h can be expressed by Eqn. (8) [34]. 125 1000 h E S = +  (8) The cohesion C also changes with hydrate saturation, and it can be expressed by the following equation 2.5 h C MPa S =  (9) In addition, change of dilation angle caused by hydrate can be expressed by Eqn. (10). sin 0.05 0.5 h S  = +  (10) Parameter Value Unit Parameter Value Unit Void ratio 0.8512 - Initial hydrate saturation, S h0 47 % Initial pore pressure 14.64 MPa Max. horizontal stress, σ H , (Y) 0.9 MPa Vertical effective stress, σ V 1.1 MPa Min. horizontal stress, σh , (X) 0.6 MPa Initial permeability, k 0 10 -14 m 2 Initial temperature, T 14.79 °C Initial porosity, ϕ 0 45.98 % Table 3 : Initial conditions of the FE model. Hydrate dissociation during the drilling operation In this case, the bottom-hole pressure ( P m ) is 15.5MPa, which is larger than the initial pore pressure (14.64MPa). Therefore, the investigation case can be defined as the overbalanced drilling operation (ODB). The properties of hydrate-bearing sediments listed in Tab. 2 and Tab. 3 have been as close as possible to that of the hydrate reservoirs in the South China Sea. Although it is ODB case, the conditions of drilling mud pressure (i.e., 15.5MPa) and its temperature (17.79 ℃ , which is higher than the phase equilibrium temperature calculated by drilling mud pressure) can result in the dissociation of seawater hydrates. Fig.7 shows the temperature distribution of at different drilling times along a path extending radially from the borehole. Figure 7 : Temperature distribution of at different drilling times along a path extending radially from the borehole.