Issue 45

Q.-C. Li et alii, Frattura ed Integrità Strutturale, 45 (2018) 86-99; DOI: 10.3221/IGF-ESIS.45.07 90 Conditions for hydrate stability Loh et al. investigated the dissociation of seawater hydrates (3.03 wt% NaCl) and gave the empirical phase boundary equation for methane hydrate in porous media, which is shown as Eqn. (7) [32]. The equation shows that the hydrate will dissociate when the temperature T exceeds the phase equilibrium temperature T eq under the corresponding pore pressure P p . ( ) 1.3 1.66exp 0.141 eq eq P T = + (7) Eqn. (7) can be used for determining whether the methane hydrate dissociate or not. F INITE ELEMENT MODEL efore the establishment of the FE model, some assumptions are made to simplify the investigation problem. Firstly, investigation of borehole stability while drilling in hydrate-bearing sediments can be simplified as the plane strain problem. Furthermore, hydrate reservoir is thought of as the isotropic porous media. Also, the temperature of the drilling mud is constant during the whole drilling operation. Finally, the effects of hydrate dissociation on both the temperature and the pore pressure of hydrate reservoir are neglected. However, no matter how the investigation problem is simplified, it is a complex physical process involving physical fields such as seepage, deformation and heat transfer. The established 2D FE model is displayed as Fig.3. As can be seen from Fig.3, the wellbore is assumed as the vertical open- hole one drilled in the clayey silt hydrate reservoir. Moreover, the diameter of the borehole is the same as that of the drill bit, and it is 8 1/2 inches. In order to ensure that the temperature and pore pressure at the outer boundaries are not affected during the entire simulation, both the length and width of the FE model are 20m. Herein, the widely used Mohr- Coulomb constitutive model has been used herein to describe the relationship between the stress and the strain of the porous media. Figure 3 : Schematic of borehole stability model during a drilling operation in hydrate reservoir. Boundary conditions and the initial conditions The entire simulation process can be divided into two steps, namely, the geostatic step and the borehole stability analysis step. In the first step, the normal displacements at the outer boundaries of the FE model are fixed. The borehole also needs to be fixed in this step to simulate the state that the hydrate reservoir has not been disturbed. Fig.4 shows the boundary conditions of the FE model in the first step. And, in order to clearly show the boundary condition of the borehole in this step and the detailed elements within the near-wellbore area, Fig.4b demonstrates the mesh model of the near-wellbore area. B