Issue 45

Q.-C. Li et alii, Frattura ed Integrità Strutturale, 45 (2018) 86-99; DOI: 10.3221/IGF-ESIS.45.07 91 In the FE model, 4500 stress/pore pressure elements are obtained to simulate the borehole collapse in hydrate formation. Since the hydrate dissociation during drilling operation mainly occurs in the near-wellbore area (Fig.4b), the elements within the near-wellbore area are denser than other area. Figure 4 : The boundary conditions in the first step (the geostatic step). (a) : the whole model, (b) elements in the near-wellbore area. Just as the first step, Fig.5 demonstrates the boundary conditions of the FE model in the second step (the borehole stability step). In the borehole stability step, the boundary conditions are listed as the following items: (1) The fixed displacement of borehole should be removed first, which is an alternative to drilling operation; (2) Then, boundary conditions of both the temperature and pore pressure should be applied to the borehole; (3) Also, boundary conditions of both the temperature and pore pressure should be applied to the outer boundaries synchronously; (4) Besides, the bottom-hole pressure P m generated by the gravity of the drilling fluid needs to be applied to the borehole. Figure 5 : The boundary conditions in the second step (the borehole stability step). The initial conditions herein are the initial pore pressure and the initial temperature within the hydrate-bearing sediments. In addition, the in-situ stresses also need to be determined. The initial conditions and the boundary conditions can be determined by the logging data and/or the seismic data obtained from exploration activities.