Issue 45

Q.-C. Li et alii, Frattura ed Integrità Strutturale, 45 (2018) 86-99; DOI: 10.3221/IGF-ESIS.45.07 95 Figure 9 : Equivalent plastic strain (PEEQ) within the near-wellbore area during drilling operation in hydrate-bearing sediments. (a) : 1min, (b) : 10min, (c) : 30min, (d) : 1h, (e) : 2h, (f) : 3h Figure 10 : Comparison results of equivalent plastic strain at the last (drilling in hydrate formation for 3h) of the drilling operation between these two different models. (a) : PEEQ of Model 1, (b) : PEEQ of Model 2. Effect of mesh size on hydrate dissociation and borehole collapse Effect of mesh size on hydrate dissociation and borehole collapse are shown as Fig.11 and Fig.12 respectively. Herein, the Bias ratio (1 and 10 are investigated herein) has been defined a parameter to describe the size ratio between the maximum and the minimum elements within the near-wellbore region. It can be seen from Fig.11 that hydrates dissociate more when mesh quality of the FEM is not very good (Bias ratio is equal to 1). This is because hydrate dissociation within hydrate-bearing sediments during the analysis process is based on the temperature and pore pressure of each mesh element. The poor mesh quality makes the element interval within the near-wellbore area larger, resulting in faster hydrate